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Ecological gait dynamics: stability, variability andoptimal designC.J. Palmer a b c , R.E.A. van Emmerik a & J. Hamill aa Department of Kinesiology , University of Massachusetts , Amherst , MA , USAb Center for the Ecological Study of Perception and Action , University of Connecticut , CT ,USAc Natick Soldier Research , Development, and Engineering Center (NSRDEC) , Natick , MS ,USAPublished online: 17 May 2012.

To cite this article: C.J. Palmer , R.E.A. van Emmerik & J. Hamill (2012) Ecological gait dynamics: stability, variability andoptimal design, Footwear Science, 4:2, 167-182, DOI: 10.1080/19424280.2012.666271

To link to this article: http://dx.doi.org/10.1080/19424280.2012.666271

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Footwear ScienceVol. 4, No. 2, June 2012, 167–182

Ecological gait dynamics: stability, variability and optimal design

C.J. Palmerabc, R.E.A. van Emmerika and J. Hamilla*

aDepartment of Kinesiology, University of Massachusetts, Amherst, MA, USA; bCenter for the Ecological Study ofPerception and Action, University of Connecticut, CT, USA; cNatick Soldier Research,

Development, and Engineering Center (NSRDEC), Natick, MS, USA

(Received 30 September 2011; final version received 8 February 2012)

Variability–stability relationships are the very foundation of flexible biological movements. For footwear design,stability is best defined as pattern stability at the level of segmental relationships with regard to goal-orientedperformance. Footwear design should seek to optimize the variation in segmental relationships while maintainingthe overall locomotion pattern, providing a system that is both adaptable to upcoming events and stable tounanticipated perturbations. As footwear is designed for a variety of uses other than forward, continuousrunning on a flat terrain, an examination of the fundamental assumptions and applicability of traditional gaitdynamics is necessary. Expanding evaluation of gait dynamics to non-forward, non-continuous motion acrossdifferent terrains would seem appropriate, given the ubiquity of these movements in everyday life. Optimizingfootwear design for different tasks requires trade-offs between design and material selection and a principledscientific approach to understanding these trade-offs is required. Evaluation of these non-traditional gaitdynamics and incorporation of goal-oriented performance requires consideration of the behavioural conse-quences of design and perception–action coupling. Understanding variability–stability relationships remains atthe center of any attempt to understand footwear performance and design implications in these expanded gaitdynamics. The basis for changing historical perspectives on variability–stability relationships is discussed withregard to maintaining and changing coordinative patterns, information–movement relationships, injuryprevention, and optimizing performance through footwear design. A principled approach to building a footwearperformance space from empirical data is provided, and it is suggested that such an approach offersobjective metrics for making appropriate trades in footwear design to optimize performance across categories ofintended use.

Keywords: segmental phase relationships; information–movement relationships; footwear performance;functional adaptability; goal-oriented; perception–action coupling

1. Introduction

Optimizing human performance through footweardesign requires explicit knowledge of the scenariosand environments in which performance is expected.Designing footwear for running, climbing or protec-tion from extreme weather requires a principledscientific approach to understand the general require-ments and specific contextual constraints on design tooptimize human–environment interaction. For exam-ple, soccer requires flexibility for cutting whereasmountaineering boots require rigidity for cramponattachment and vertical ascent; yet they must performsimilar fundamental roles during locomotion. The rolemost explicitly acknowledged for footwear is protec-tion from injury due to interactions with the environ-ment such as large contact forces, ankle ‘stabilization’and protection from extreme weather or terrain

features. Although the current understanding of gait

dynamics is fundamentally sound for continuous

forward locomotion, our understanding of non-

continuous, non-forward locomotion over different

terrain is less robust. A move to expand gait dynamics

beyond traditional laboratory scenarios must be made,

as these conditions are most encountered in daily life

and during athletic performance (Chiel and Beer 1997,

Dickenson et al. 2000). Footwear provides the means

to move over different terrain in a variety of ways,

while maintaining stable gait patterns and speed of

movement.A more implicit role for footwear is the need to

transmit accurate information about properties of the

terrain (Robbins et al. 1995, 1997) regarding what the

ground–foot relationship affords for adaptable move-

ment. In some situations, such as technical climbing or

*Corresponding author. Email: jhamill@kin.umass.edu

ISSN 1942–4280 print/ISSN 1942–4299 online

� 2012 Taylor & Francis

http://dx.doi.org/10.1080/19424280.2012.666271

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for those with peripheral neuropathy, reduced infor-mation exchange can have severe consequences interms of performance or catastrophic injury. Thisimplicit requirement to know about what the ground–foot interaction allows with regard to upcomingmovement is important for athletic performance,occupational safety and clinical rehabilitation. Forexample, athletes must know during the ongoingstance phase if they can immediately change directionson a muddy surface (Fajen et al. 2009). Informationalrequirements for successful action within the human–environment relationship are fundamental to the eco-logical approach to human movement. Ecological taskanalysis (Van Emmerik 2007) provides an approach toexpanding the understanding of traditional gaitdynamics to more realistic scenarios involving naviga-tion across various terrains, locomotion patterns andrealistic task performance. These locomotion styles arenot traditionally examined to the same extent asforward continuous gait on flat terrain, but are thosemost often encountered in daily life. These includenavigation around objects, transitions to leaping,changing direction, starting and stopping on a varietyof terrains, and moving in correspondence to others ingaming situations.

Information exchange at the base of support playsanother significant role in maintaining a locomotivepattern under different task and environmental con-straints (Fitch et al. 1982, Riccio 1993). From a morerecent perspective, maintaining appropriate segmentalphase relationships during locomotion is not contin-gent on the brain controlling every motor unit ormuscle separately, but requires local haptic informa-tion to autonomously maintain functional patterns ofmovement (Fitch et al. 1982, Turvey 2007). The hapticsystem is not the only perceptual system that helpsensure performance, as the visual system plays asignificant role in ‘pretuning’ segmental relationships(Greene 1972) based on the complexity of terrain andupcoming events (Lee and Lishman 1977, Van Essenet al. 1992, Warren 1995). Segmental relationships andhuman–environment interactions must be consideredwhen examining stability of performance in activitiesof daily living and to avoid trips, slips and falls. Assuch, assessment of individual joint kinematics andsingle measures of joint dynamical stability (e.g.Lyapunov exponent) offers relatively little practicalinsight into footwear design, as it is the variability ofsegmental coordination that underlies pattern stability.

The need to pick up visual information constrainsthe relationships within lower extremity kinematicsand head–trunk relations (Mulavara and Bloomberg2002, Mulavara et al. 2002, Peters et al. 2006, Marigoldand Patla 2008). This underlies the basic functions of

orientation and navigation through complex environ-ments, such as cluttered industrial workplaces andsporting events (Gibson 1958, 1986, Lee and Lishman1977, Warren 1995). Thus, the consequences of visionon the tuning of lower extremity segmental relation-ships and the ability of the kinetic chain to maintainvisual performance are reciprocal. Both must beachieved simultaneously by the entire kinetic chainacting as a single functional unit, coordinated towardsimmediate and longer-term goals (Mulavara andBloomberg 2002). Footwear design must seek tooptimize the flow of accurate information across theshoe while providing a platform for dissipating kineticenergy during heel strike and allowing adaptable andflexible movement within the environment and taskconstraints for which it was designed (Figure 1). Thisrequires an expansion of traditional gait studies toparadigms that examine different patterns of locomo-tion in different environments and the consequences onvision for navigation that underlie adaptable perfor-mance and ensure safe movement through differentcomplex environments.

The traditional understanding in motor control,that invariance in segmental relationships provides thestability required for repeatable performance, isreflected in footwear designs that seek to provide

(1)

(2)

(3)

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(SP)

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(SP)

(A)

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Perception (P)

Action (A)

(A)

(1)

(2)

(B)

Figure 1. Nested functional units (see also Table 1): (1) thedistal functional unit (DFU), (2) the visual functional unit,and (3) the orienting functional unit. Visual information (A)about terrain complexity prospectively ‘tunes’ stiffness anddamping parameters (B) of the DFU such that stability ofpattern is maintained within task and environmental con-straints. Relationships between action (A) and perception (P)cycles (A–B) are reciprocal because, without this relation-ship, shock propagation (SP) to the visual organs resultingfrom an ‘untuned’ lower extremity kinematics may precludethe pick-up of the very information necessary to tune lowerextremity stiffness prospectively for upcoming actions. Thisexample demonstrates the inability to separate ‘perception’from ‘action’ in any principled way that is meaningful forrealistic movement through the environment.

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stability by increasing material constraints. A morerecent perspective suggests that this invariance insegmental coordination cannot be equated with move-ment stability in biological systems. Instead, stabilityof performance under varying levels of perturbation orunder different constraints comes from the degree towhich interacting processes and parts are allowed tovary within the pattern itself (Bernstein 1967, Reed1986, Newell and Corcos 1993, Davids et al. 2003).This understanding was initially attributed to NicholaiBernstein (1896–1966), who examined repetitivemotion to find that, while the goal of hammer strikeon a particular spot on the anvil was maintained, thetrajectories and kinematic relationships varied fromstrike to strike (Bernstein 1967). These findings ledmovement scientists to explore variability within seg-mental relationships as a fundamental way to under-stand overall movement pattern stability and adaptivemovements (Scholz and Schoner 1999), such as main-taining a consistent gait pattern under changingenvironmental and task constraints. From this per-spective, variability in segmental and/or joint motionscannot unequivocally be equated with movementstability, nor can the use of single dynamic systemsmeasures (e.g. entropy or Lyapunov exponent) providethe necessary empirical data to determine unstable gaitpatterns, as has been implied by some researchers(Kurz and Stergiou 2003, Myers et al. 2009).

As biomechanics and motor control become moreintegrated, the finding that variability in segmentalrelationships underlies adaptive and stable perfor-mance has been well established across human andanimal locomotion models (Newell and Corcos 1993,Sparrow and Newell 1998, Dickenson et al. 2000).Variability within and between segmental relationshipsis fundamental to the functional ground–shoe–lowerextremity relationships (the distal functional unit orDFU; see Figure 1), and this variability must beexamined with regard to its impact on stable perfor-mance of the wearer (Davids et al. 2003). Footwearscientists, then, should seek to optimize the variation insegmental relationships while maintaining the overalllocomotion pattern; thereby providing a system that isboth adaptable to upcoming events and stable tounanticipated perturbations. This type of variabilitycan be measured by different analytical tools includingvector coding (Chang et al. 2008), continuous relativephase (LaFiandra et al. 2002, Seay et al. 2011) anddiscrete relative phase (Hamill et al. 2000), dependingon the specific research question and task beingevaluated (for a review of dynamical systemsapproaches to stability–variability relationships, seeHamill et al. 2000, Van Emmerik and van Wegen2000). This variability in segmental phase relationships

can only be achieved when perception is understood asan implicit requirement for optimized footwear design.Reduction in haptic information may increase uncer-tainty and require a shift to gait dynamics of a moreexploratory nature (Riccio 1993, Buckley et al. 2008)and may change segmental relationships, patterns ofcoordination, or increase the risk of injury (Robbinset al. 1995, 1997). For example, footwear designs thatdo not afford information about terrain could poten-tially increased mediolateral movement to pick up thisinformation, potentially leading to ankle injury.Alternatively, lacking this information could cause aconservative approach to movement under uncertainconditions (Buckley et al. 2008), reducing the adapt-ability and flexibility of the system through increasedstiffness. These types of information–movement rela-tionships within the context relevant to footweardesign are not understood, and broadening empiricalstudy in this area is required.

Based on the previous paragraphs, three funda-mental constraints on the design of footwear seem toexist. First, footwear must provide physical protectionand allow information exchange between the groundand the lower extremities. Second, understanding theconsequences of different locomotion styles for visualperception would appear necessary for designing foot-wear for athletic, clinical or industrial contexts giventhe ubiquity of navigating through complex environ-ments. Third, the specific goals and environmentalcontexts for which footwear is designed must beconsidered. Variability–stability relationships cannow be defined specifically with regard to thoseinvolved in footwear design and human performance.Variability within coordinative phase relationshipsunderlies the ability to modify gait within stablelocomotive patterns to meet the current environmentaland task constraints. The 2-torus in Figure 2 providesa simplistic visual representation of phase relationshipsbetween two coupled oscillators, and the availabletopography of the torus itself represents the patternstability. Of course, in real systems not all of theavailable space is available or used, as each oscillatordoes not necessary move through 360� during move-ment due to morphological, environmental and taskconstraints. Coordinative phase relationships describethe ongoing relationships between oscillating segmentswithin a particular stable locomotion pattern that areable to vary in different ways while maintainingpattern stability. Although intricately related, stabilityand variability are not the same. Stability at one levelof analysis (gait pattern stability; such as running vs.walking vs. shuffling) can be characterized by thevariability at the underlying level of analysis (segmen-tal phase relationships within the pattern) and must be

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related to the context and goal of the intendedmovement. As these goals generally require visualsystem performance for navigation in complex envi-ronments, segmental relationships of interest mustinclude the head and trunk for a complete understand-ing of ecological gait dynamics.

The use of dynamical systems theory to providemeasures of stability has increased over the years in anattempt to exploit advanced analysis tools for betterunderstanding of different systems of interest (e.g.Dingwell et al. 2001). However, single measures such asLyapunov exponents of joint or segmental motionsthat attempt to capture the dynamic stability of anentire system have two significant shortcomings: (1) itis often not clear that the single measure chosenadequately represents the system, and (2) these mea-sures cannot provide a measure of stability or vari-ability that is useful for understanding the necessarysegmental relationships for the footwear or appliedscientist (Dingwell et al. 2001). This is not to say thatthese dynamic systems tools are not important for

understanding stability of human movement: our noteof caution is directed towards universally equatingresults from these measures with gait stability. If,however, nonlinear measures such as the Lyapunovexponent can be appropriately examined in the contextof whole-system performance and in conjunction withchanging segmental phase relationships, they mayprovide novel insights into how segmental variabilityand overall system stability are linked. This wouldrequire empirical data, and the particular manner inwhich these singular dynamic systems measures ofstability are related to the coordinative phase relation-ships of interest. One might suggest that the goal ofany branch of movement science that treats humanmovement as a dynamical system would need tounderstand the relationships between these levels ofanalysis, but analytical tools and empirical datasupporting this understanding are not yet developed.In the meantime, in our view, the most useful definitionof stability–variability relationships for footwear sci-entists is at the level of segmental relationships.

The purpose of this paper was to broaden theperspective of gait dynamics and the understanding ofvariability–stability relationships when optimizingfootwear performance outside laboratory conditions.Although much has been revealed about gait dynamicsunder these conditions, scientists should be circumspectabout the applicability of this knowledge to footweardesigned for other circumstances. This ecological per-spective involves examining gait dynamics that are non-continuous, non-forward, over terrain, and in relationto objects and people with whom one must interact oravoid, and acknowledges the specific contexts underwhich footwear is designed (sports, safety, clinical).This approach will provide scientists and footweardesigners with a broader understanding of the globaland local constraints that require consideration whendesigning footwear specific to the task and environ-ment. This should help to design footwear that allowsthe variation in segmental relationships that provide thestability in gait patterns required for the specific tasks,and a system that is both adaptable to upcoming eventsand stable to unanticipated perturbations. An intro-duction to Bernstein’s degrees of freedom (DoF)problem provides the background for understandingthe importance of variability–stability relationships,information–movement relationships, and the controlof movement for contextualized action.

2. The DoF problem

The DoF problem, as originally posed by Bernstein(1967, 1996), refers to the task of integrating the many

1θθ2

2-torus (1:1 coordination)

2-torus (2:1 coordination)

θ1θ2, 1

θ 2, 2

(a)

(b)Absolute 1:1Coordination

Relative 1:1Coordination

Figure 2. (a) The 2-torus represents all possible phaserelationships between two coupled oscillators (e.g. bodysegments). Morphological and task constraints could resultin a smaller space within which trajectories can be observedpractically. 1:1 coordination patterns are portrayed when atrajectory around the 2-torus makes a complete rotationaround both axes (left; seen in lower extremity segmentrelationships during locomotion), and 2:1 coordination whenone axis is circumvented two times for every one rotation ofthe other oscillator (right; seen between leg–arm coupling atlower locomotion speeds; Wagenaar and van Emmerik 2000).(b) Absolute coordination is typified by segmental phaserelationships with very low variability (left; see alsoFigure 3a) whereas relative coordination is characterized byvarying coordinative phase relationships within a specificpattern (right). The abstraction to the n-torus (Tn) allows theconceptualization of more than two coupled oscillatorsrelating in a similar lawful manner. Pattern stability (suchas maintaining a running pattern in bipeds) is the mainte-nance of movement on the 2-torus (n-torus in higherdimensional space with more than two oscillatory relation-ships involved in the pattern of interest).

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individual parts (motor units, muscles, joints, seg-ments, etc.) into relationships that achieve the taskspecific goals (Turvey et al. 1982, Latash 1993).Bernstein’s finding that segmental and end-pointtrajectories were not invariant within the sameperson, task or initial conditions provides a basis forexamining the way variability is used in biologicalsystems to stabilize coordinative patterns (Fitch et al.1982, Reed 1986). There are many combinations withina kinematic organization that are available to performa given task and the range of combinations has beendescribed as a performance manifold of segmentalrelationships that achieve the goal (Scholz and Schoner1999, Scholz et al. 2000, Latash et al. 2002). Availablerelationships, however, are constrained based on theactivity of the animal (e.g. human locomotion aspredominantly sagittal anti-phase relationshipsbetween legs, and sagittal in-phase flexion and exten-sion within segments) and its morphology (e.g. limitedknee add/abduction in humans). In general, constraintscome from three interacting sources: the animals’morphology, mass, etc.; the environmental terrain;and the task (Turvey et al. 1978, Sparrow and Newell1998). Recognizing the DoF problem and the broadernesting of the runner within the environment furthersupports the notion that gait dynamics are betterunderstood in terms of functional relationships ratherthan in terms of individual segmental or joint motions(Bernstein 1935, Rosen 1988, Scholz et al. 1999).

The availability of different DoF provides animalswith variability at one level of the system (metricalprescription) as a measure of adaptability withinspecified stable patterns (structural prescription;Turvey et al. 1978). Structural prescriptions are thoserelationships between segments within a coordinationpattern that remain invariant within a task or activity(e.g. the topography of the 2-torus in Figure 2).Metrical prescriptions are those relationships that areable to change (e.g. joint range of motion, stridefrequency, etc.) and that allow adaptation to differenttask and environmental constraints within the samecoordination pattern (Turvey et al. 1978). For exam-ple, across uneven terrain one leg may step up onto alog or down into a rut. Here, the structural prescrip-tion of anti-phase relationships between lower extrem-ity segments and in-phase relationships within thosesegments are maintained (stable coordination pattern)whereas metrical prescriptions (e.g. limb frequency,range of joint motion) can vary with constraints of theenvironment and task. In general, additional con-straints reduce the variability within the performancemanifold and reflect a reduction in the adaptability ofthe overall system to respond to perturbations orchanging environmental contexts (Latash et al. 2002,

Van Emmerik 2007). The relative coordinationobserved in less constrained phase relationships isgenerally viewed as more structurally stable to unan-ticipated perturbations (Figure 2b), and related to thenotion of ‘meta-stable’ systems that are highly adaptive(Kelso 1995). For locomotion, these additional con-straints may be terrain complexity, interactive require-ments with others (team sports), informationrequirements through the footwear (technical climb-ing), and unintended footwear design restrictions.

This reduction in the available DoF or limitationsto the range of available performance manifold (the 2-torus in the simplest case) may also occur: (1) withover-engineered footwear designed with mechanical‘stability’ in mind; (2) when running over morecomplex terrain; (3) when visual information is reduced(low-light conditions or objects obscuring the path); or(4) under heavy load carriage (LaFiandra et al. 2002).Determining the appropriate relationships of interest,however, is not a trivial task and must consider thewhole body, specific task requirements, and theexpected range of environmental conditions. Duringreal movement in complex situations, visual scanningor visual acuity requirements may require examinationof the relationships between the DFU capability andthe head–trunk dynamics that support visual perfor-mance (Peters et al. 2006). Those interested in footweardesign for other than laboratory and flat-forwardrunning need to consider interactions across the wholebody, not just the lower extremity.

2.1. Building a footwear performance space

From a physical perspective, variability within theDoF during unconstrained locomotion provides thecapability to dissipate energy in any number of ways,while satisfying segmental phase relationships neces-sary for the maintenance of stable locomotion. Thisvariability in the phase relationships has been shown todecrease when constraints on gait increases, such asthose due to injury (Hamill et al. 1999, Heiderscheitet al. 2002, Pollard et al. 2005, Seay et al. 2011).Additionally, the reduced variability observed in moreconstrained conditions may also be related to thedegree to which repetitive traumatic injury occurs as aresult of less varying movement dynamics (Hamill et al.1999, O’Connor and Bottum 2009, Seay et al. 2011)(Figure 3b); as greater variability broadens the surfacecontact area and repetitive torsional stress on softtissue that helps to maintain healthy tissue (Pollardet al. 2005). This switch from relative coordination to amore absolute coordination (Figure 3a) suggests thatlocomotion patterns become less variable and less

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variable and less adaptable with increased constraints,

and may provide an objective basis for deselection of

footwear during performance assessments of different

designs. Footwear that overconstrains the oscillatory

dynamics during gait and increases potential for soft

tissue injury would be poor designs when highlyadaptive movement is required. However, in clinical

situations it may be advantageous for injury recovery

to start patients in more restricted gaits and slowly

unconstrain the movements available to avoid recently

repaired soft tissue, for example. Using the above

definition of stability–variability relationships, clinical

footwear could be empirically and objectively tested

for its ability to slowly move patients from absolute to

relative coordination, based on the clinician’s prescrip-

tion and assessment of recovery.Although energy dissipation within the DFU is an

important aspect of what footwear must provide,

energy not dissipated by the DFU must be dissipated

by recruiting other DoF within the kinematic chain

(McNitt-Gray 1993, Kelso 1995). As recruiting addi-

tional DoF such as the hip, pelvis and trunk may

change other segmental relationships across the system

for performance, it is important to understand how

footwear design related to DFU capabilities affects the

overall performance of interest. For example, it has

been shown that the kinetic chain is able to attenuate

shock generated from foot strike to the head during

forward locomotion on flat surfaces (Hamill et al.1995) and that lower extremity kinematics and head–

trunk relationships change during locomotion when

dynamic visual acuity is required (Mulavara and

Bloomberg 2002, Peters et al. 2006). However, in

more ecological conditions where participants do not

only move on flat surfaces, and stop or change

direction, the shock from foot strike does reach the

head (Brizuela et al. 1997, Zhang et al. 2008). This

finding, coupled with the sensitivity of eye and head

movement in picking up spatial information (Lee et al.

1997, Rucci et al. 2007), suggests that visual perfor-mance must be considered for movement in complex

environments (Peters et al. 2006, Fajen et al. 2009).Understanding gait dynamics as nested relation-

ships within the overall task constraints of the system is

important for determining how transfer and dissipa-

tion of energy between or across functional units may

2.3 m•s-1 3.8 m•s-13.3 m•s-12.8 m•s-1

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(least adaptive)Relative Coordination

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Figure 3. (a) Coordination between two segments [phase angle (’¼ �1 – �2); see also Figure 2]; the frequency of coupling about acertain phase relation (’) provides an indication of the strength of the coupling and the adaptability of the coordination pattern.(b) Exemplar graph from Seay et al. (2011) demonstrating the constraints on pelvic-trunk coordination (continuous relativephase variability) imposed on runners with low back pain (LBP), those recovered from low back pain (RES) and controls (CTR).Reduced variability in LBP and RES reflects more absolute coordination and has implications for further injury as well as limitedadaptability during running (e.g. an observation of more relative and adaptable phase relationships in controls and moreabsolute coordination in those recovered from and with ongoing LBP).

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have consequences for behavioral performance. The

understanding of relationships within the DFU is not

enough to optimize footwear design and performance

because specific relationships within the entire system

must be considered relative to task and environmental

constraints. The determination of how to conceptualize

the functional units of the system is dependent on the

questions of interest and contexts of footwear use.

Table 1 provides a functional decomposition that

could help in understanding the relationships between

functional units, as well as the simultaneous perfor-

mance requirements for footwear within the DFU.

Functional footwear requirements to be considered

are: (1) energy management (negative at heel strike,

positive towards toe-off); (2) variability–stability rela-

tionships (adaptability and flexibility to environment

and task constraints); (3) information pick-up (percep-

tion); and (4) physical protection specific to footwear

type. Empirical determination of this footwear perfor-

mance space (Figure 4) for various footwear categories

would seem to provide the necessary scientific rigor for

a principled approach to decision making in design and

performance.Executing the necessary science to build the foot-

wear performance space in Figure 4 would provide the

understanding of those relationships that are most

important to particular activities from a footwear

design perspective, and could be combined with current

approaches to design optimality. Current design guid-

ance comes predominantly from traditional gait

dynamics (non-ecological) and input from subject

matter experts such as professional athletes for sports

footwear. Although the use of experts can be helpful for

design, combining their qualitative information with

quantitative metrics provides a more objective, verifi-

able and rigorous scientific approach to understanding

what is being specified by these experts and would

strengthen footwear science. For example, the non-

forward, non-continuous gait dynamics in tennis pro-

duces very different gait dynamics (e.g. sideways

locomotion) with significant requirements for dynamic

visual acuity and constraints on trunk–head orientation

Table 1. Functional unit decomposition and required interactions for goal achievement.

Nested units Relationships Required functions Required from other functional units

Visual functionalunit (VFU)

Eyes, head, andneck

Scans environment and informationpick-up (head and eyes), naviga-tion path determination, tuneslower extremity, dissipates energy(neck; last link in kinematic chainto be able to do so)

Stability within the OFU for vision,energy dissipation from the OFUand DFU for visual scanning andinformation pick-up

Orienting func-tional unit(OFU)

Pelvis, trunk,upperextremities

Orients main body towards goal,dissipates energy between theDFU and VFU, provides a stableplatform for the VFU and upperextremity movements (reachingfor ball)

Navigation guidance from the VFUto orient trunk and arms towardsgoal, tuning from the VFU forflexible and adaptable movementwithin goal (e.g. trunk–pelvis cou-pling), energy dissipation from theDFU, haptic and exproprioceptiveinformation from the DFU forflexibility within orientation (e.g.during cutting or stopping theOFU may need to become moreadaptable, given the constraints onthe DFU in achieving some goals)

Distal FunctionalUnit (DFU)

Ground, foot-wear, foot, andshank

Dissipates energy from heel strike,information pick-up (pattern reg-ulation through haptic and expro-prioceptive information),adaptively adjusts to changes inpattern (cutting, stopping), physi-cal protection from environment(terrain, weather, or other)

Navigation guidance from the VFUto orient the DFU towards goal(direction of feet), tuning from theVFU for stiffness versus adapt-ability trade under terrain andenvironment constraints, Proprio-and exproprioceptive informationfrom the OFU regarding con-straints on DFU boundary condi-tions (e.g. centre of mass, centreof pressure relationships) specificto goal

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for hitting the ball. Feedback from experts, whencoupled with quantitative data on the various perfor-mance attributes of the DFU during sideways, stop-and-go style locomotion, is more beneficial than eitheralone. It is safe to assume that the more a specificactivity deviates from traditional gait dynamics, the lessscientifically based the footwear design may be foroptimized performance and injury prevention. Giventhe lack of empirical data in non-traditional gaits(argued here as being more ubiquitous than ‘traditionalgaits’), there is a wide gap in footwear science for thevast majority of footwear design and performanceoptimization.

2.2. Information–movement relationships1

Informational constraints during real-world navigationthrough a complex environment require the integration

of perceptual systems at several nested time-scales.This is best exemplified by the relationship betweenvisual information pick-up and the prospective controlof movement (Fitch et al. 1982, Gibson 1986, Turvey1992) through tuning of ‘local’ joint stiffness and forcescaling in task performance (Warren et al. 1986,Sidaway et al. 1989). Information within the opticflow pattern directly specifies the direction of move-ment, point of orientation within the environment,speed of movement, changing relationships within thevisual field, and time to contact different surfaces orobjects (Gibson 1958, 1986, Warren 1995, Duchon andWarren 2002).

It is, in fact, this flow of information thatconstrains the forces that must be generated tosuccessfully navigate the world (Kugler and Turvey1987, Turvey 1992). At this time, we can determinehow forces are transformed to flows (by Newtonian

Adaptability-Pattern Stability (Relative Coordination)-Metrical Limits on Range of Motion-Information Exchange (Sole)

Energy Management -Shock Attenuation-Energy Return-Bottoming Flexibility

Physical Protection-Thru Sole Trauma Protection-Thermal Protection-Special Protection

(Soling, Shank, Steel toe, Ankle Bracing)

Sub-Space Models

FootwearPerformance

Space(Empirically-based)

Adaptability

Physical Protection

Energy Management

A

B

A

A

A

BB

B

Figure 4. Footwear performance space: building the performance space through empirical testing of footwear in the differentsubspaces that contribute to DFU capability. Subspace metrics are placed in the same coordinate system (Physical Protection,Adaptability, Energy Management) and individual metric values create a vector value of performance in that subspace. Thesubspace vector is then used as the ordinate value within the footwear performance space, allowing direct comparison betweendesigns and performance. Footwear of different categories can then be tested in field studies for their ability to meet functionalgoals, and the Footwear Performance Space can be assessed for its ability to separate different footwear requirements specific totask and environment. Footwear in the ‘B’ area would be highly adaptive, provide moderate protection, but little energymanagement capability (e.g. technical climbing shoe). Footwear in the ‘A’ category offers high physical protection, high energymanagement, but little adaptability (e.g. Cold Weather Boots).

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dynamics) but have less understanding on how infor-mational flows constrain forces (Kugler and Turvey1987). Prospective control of action requires movementfor information (Reed and Jones 1982, Riccio 1993),from the scale of microsaccadic eye movement andphysiological tremor (Riccio 1993, Rucci et al. 2007)through movement of the entire body for informationpick-up in more dynamic tasks (Michaels and Carello1981, Gibson 1986).

The visual information provided in optic flow hasbeen shown to tune nested dynamics in the systemwithin the confines of task requirements in humans(Warren et al. 1986, Duchon and Warren 2002). Forexample, timing the ‘turning on’ of muscles duringtransition from movement to upright stance is specifiedby time to contact with the impending surface, directlyavailable from the optic flow (Sidaway et al. 1989).At the ground–shoe–lower extremity interface, hapticand proprioceptive flow provides information to con-strain local pattern achievement commensurate withupcoming events through visual information or uncer-tainty of visual information (Santello et al. 2001,Buckley et al. 2008, Cowie et al. 2008, Magalhaes andGoroso 2009, Timis et al. 2009). When visual uncer-tainty increases, the system ‘turns on’ the plantarflexors earlier so as to be prepared for contact at anytime (Sidaway et al. 1989, Santello et al. 2001,Magalhaes and Goroso 2009). These data suggestthat, in more complex situations, the emergence ofankle stiffness and damping characteristics is likely tobe a trade-off between: (1) prospective regulationthrough information contained within the opticalarray related to the terrain complexity over which aperson is moving and (2) the need to optimizeadaptability during movement (e.g. reduced stiffnessallowing a player to cut to avoid opposing players).

Increased variability within a required coordinationpattern also provides greater information to the hapticand proprioceptive systems, in terms of the availableperceptual information (Riccio 1993, Carello andTurvey 2004, Van Emmerik 2007). When the variabil-ity in pattern over a single stride or a number of stridesis reduced through constraints, the flow of informationacross the sole of the foot may also be reduced.Imagine the loss of haptic, proprioceptive and expro-prioceptive information (information about the posi-tion and orientation of the foot relative to theenvironment necessary for navigation; Lee andLishman 1977) if only the centre of pressure wasperceived, rather than the pressure distribution acrossthe sole of the foot. Information transmission mayinvolve not only the sole of the foot but also all contactpoints of the footwear as well as the myofascialconnections across the lower kinematic chain

(information flow by mechanotransduction; Levin2002, Ingber 2008, Scarr 2008). Reductions in footand ankle joint variability (about all three axes) mayhave consequences for segmental relationships up thekinetic chain through a loss of information bymechanotransduction across myofascial arrays withinwhich mechanoreceptors are embedded (Turvey andFonseca 2009). The more footwear constrains theavailable movement (joint movements within the footand between foot-shank, etc.), the less the DFU iscapable of picking up information necessary foroptimal pattern maintenance. Although it is generallyassumed (de facto) that current soling designs andmaterials allow adequate information to the foot, thereis no empirical basis for this assumption because thefocus of soling design has traditionally been aboutslippage, shock attenuation, durability and traction.Moreover, Robbins et al. (1995, 1997) found thatsoling thickness and hardness had significant impactson the ability to perceive foot orientation, inclinationof terrain and overall performance during a beam-walking task, adding support to the notion thatinformation–movement relationships are necessary toconsider for optimized footwear design.

2.3. Optimizing information flow throughfootwear design

As discussed above, increases in stiffness at the foot–ankle may emerge in response to uncertainty instepping or terrain. Similarly, increases in exploratorydynamics upon contact can emerge in response touncertainty in visual information when vision isreduced (Santello et al. 2001, Magalhaes and Goroso2009). In this connection, the old debate aboutpotential benefits of ‘high-tops’ may be reconceptua-lized in terms of the information flow from the groundthrough the footwear to the lower extremity. Rigidankle support integrated into the sole and wrappingaround the ankle in some alpine trekking footwear, forexample, may be as important for information pick-upat the shank about the ground relationships as for theproposed support it provides. Given the time availableto perceive exproprioceptive information during thestance phase during locomotion, it would seem advan-tageous to allow information about ground contactand terrain conditions over a wider anatomical area.However, the rigid constraints on movement in thistype of ankle support may also reduce the variability(by design) necessary for the flow of informationacross different perceptual fields, resulting in poorerperformance (Brizuela et al. 1997). This is a goodexample of where information–movement and

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variability–stability relationships in the available

design trade space could use empirical data from

footwear scientists for optimal performance. We need

only review the difficulties in locomotion for patients

with peripheral neuropathy (Dingwell et al. 2000, 2001,

Dingwell and Cavanagh 2001) or the consequences of

losing mechanoreceptors in the soles of the foot with

age (Cauna 1965, Robbins et al. 1995) to gain insight

into the importance of maintaining accurate perceptual

information for movement.Despite underlying support for the information–

movement perspective provided in terms of general

research (Riccio 1993, Van Emmerik 2007, Magalhaes

and Goroso 2009, Palatinus et al. 2011), there are very

limited data on the consequences of reduced informa-

tion flow across the base of support during non-

traditional locomotion and none involving the assess-

ment of coordinative variability. Although the data

from quiet stance and other tasks using dynamic touch

provide a basis for conceptualizing this question

(Riccio 1993, Riley and Turvey 2002, Van Emmerik

2007, Palatinus et al. 2011), specific paradigms must be

developed for footwear design. As this information

provides a basis for the accuracy of terrain information

and what the terrain affords at that instance for

adaptable performance, it is important to understand

fundamental information–movement relationships

across different gaits and environments for footwear

design.It seems that the technology to evaluate some

aspect of this footwear information flow is available

because foot pressure measurement systems have been

around for quite some time. This technology, com-

bined with vector field mechanics, could provide an

initial attempt to ascertain differences in information

flow across the ground–shoe–lower extremity interface

by placing the pressure sensing technology under both

the footwear sole and foot. This technique would no

doubt require refinement, but it could provide a first

pass at the information exchange within the DFU in

different bottoming designs and terrains in relation to

performance. The pressure vector field provided by

these technologies, usually condensed to centre of

pressure movement, could be used to understand

potential consequences of materials and design within

or between footwear performance options, and the

designing of footwear for those with perceptual deficits

in the feet (peripheral neuropathy, diabetes, ageing,

etc.). This approach and understanding may be espe-

cially useful when changes in terrain (hiking boots,

extreme racing), starting–stopping transitions (racket

sports) and cutting transitions (soccer, football) are a

regular locomotive style within a performance category

and are related to the dexterity required for successfulperformance (Ellis et al. 2004, Orendurff et al. 2005).

3. Changes in coordination pattern

Although the emphasis in the preceding sectionshighlights the importance of maintaining patternstability (structural invariance) through the ability tovary the segmental phase relationships (metricalchanges, coordinative variability), the ability to rapidlytransition between different patterns of movement isequally important. Using Figure 2, this change incoordination pattern could be either moving from the2-torus structure to another, or changing the segmentsor the way the segments are relating. For example,changing from sagittal anti-phase relationships of thethighs in forward locomotion to in-phase frontal phaserelationships of the thighs would indicate a transitionto sideways shuffling. In these situations there is alsoevidence that supports the benefits of variability withinpattern coordination. The less variable a pattern ofcoordination, the less flexible the organization is withregard to its ability to switch between differentcoordinative patterns (Kelso et al. 1988, Schoner andKelso 1988, Scholz and Kelso 1990, Lamoth et al.2009). The ‘depth of well’ metaphor (Figure 5)provides an abstract conceptual diagram, where theball in the bottom ‘well’ is very stable because it isrestricted by the depth and contact with walls of thewell (absolute coordination), and the ball in the top‘well’ is less stable and unrestricted by the well in whichit resides (relative coordination) but much more able totransition to other coordinative patterns, or ‘switchbetween wells’. The wells in this example and theposition of the ball in the wells reflect differentmovement patterns and stability of those patterns(depth), such as walking vs. running, forward andsideways running, etc. The ability for an animal to bemaximally adaptive to ongoing relationships within itsenvironment has to do with its ability to stand on the‘cusp of instability’ to the maximal extent practical(meta-stability), rather than to be highly stable(Gilmore 1981, Conrad 1986, Strogatz 1994, Kelso1995, Van Emmerik 2007). The issue for footwearscientists is to create a functional performance spacethat provides the trade-offs for optimal performanceacross categorical requirements of the footwear inrelation to the other functional units (Table 1,Figure 4) in goal achievement.

Changes in coordination pattern within an activityare ubiquitous in everyday tasks (walking, stopping,turning) and sporting events (running, jumping, stop-ping, leaping and cutting). Changes in coordination are

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important for safety when firefighters or soldiers haveto stop short of dangerous areas and for athleticperformance when transitions from semi-static pos-tures or quick direction changes are necessary. Theconsequences of footwear design on transition betweencoordination patterns have not been as extensivelyexamined as traditional forward gait, yet the work thathas been done suggests that it may aid in understand-ing what is necessary for optimal footwear and humanperformance in more realistic conditions. In one studythat specifically examined the functional implicationsof an ‘ankle support’ shoe design (high-top, rear-lacingand substantial heel counters; Brizuela et al. 1997),these design constraints were found to reduce theperformance in the vertical jump, lengthen obstaclecourse completion time, and increase frequency con-tent at the head through reduction of specific ranges ofmotion. These findings demonstrate the need tounderstand footwear design in terms of ongoingDFU requirements, stability–variability trade-offs,and how the constraints on the DFU have conse-quences for behaviour and achievement of perfor-mance goals. In another study, Zhang et al. (2008)showed that transitions from movement to uprightstance (landing) may impact global task performance,as these abrupt transitions caused increased shockpropagation to the head and eyes. Combined with theresults of Brizuela et al. (1997), these data suggest thatfootwear design and movement style may have

consequences for visual performance, which is neces-sary for identifying the rapidly changing affordancesfor action (Fajen et al. 2009). Determining how thisinteraction occurs within relevant situations for foot-wear design and resulting behavioural constraintsbecomes the task of footwear scientists. Given theubiquitous nature of these fast transitions duringcrucial situations (in sports, fire/rescue maneouvres,or in clinical populations trying to avoid a fall) and thepotential reduction of visual information during thesetransitions, it would seem important to unfold thesevisual–footwear–movement dynamics.

3.1. Footwear performance space, design andcoordinative transitions

Currently, fast transitions (such as cutting man-eouvres) are studied predominately with regard tofootwear or bracing design and preventing injuries atthe ankle or knee (Ellis et al. 2004, Cloak et al. 2010).Reducing injury is certainly important, but constrain-ing segmental phase relationships by potentially over-engineered footwear designs must be considered morebroadly as prevention of injury is certainly not the onlyrequirement for footwear. The impact of these designsmust also be understood in terms of the otherrequirements for the DFU (adaptability, informationabout terrain, etc.) for appropriate trades to be made

Figure 5. Variability within coordinative patterns (‘wells’) provides the basis for adaptable performance within and betweencoordinative patterns (switching between ‘wells’). Increased variability (side-to-side or up-down movement within the wells)provides the perceptual information necessary for knowing what coordination patterns are available in current ground–shoe–lower extremity relationships (exploratory–performatory relationships). The two different potential wells clearly show thatvariability cannot be equated with stability; in the shallow (top) example, a certain level of variability can result in loss of patternstability (i.e. transition from forward to sideways running). A similar amount of variability in the steep well (bottom) will notlead to this transition and loss of pattern stability.

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in optimal design and human performance. Again, acomplete mapping of the footwear performance spaceand understanding the variety of simultaneous roles itplays for successful goal achievement are necessary foroptimizing footwear in these circumstances (Figure 4).The ability to place all the different requirements forfootwear into a single performance space and evaluatethe trade-offs within that space specific to the goalallows a principled approach to footwear design, andwould be a significant step forward in footwearscience.

One potential consequence to the restrictions frombracing or footwear would be seen in the availability ofa full range of motion for adaptable performance(metrical space and available trajectories within struc-tural patterns), as demonstrated by the work ofBrizuela et al. (1997). A potential behavioural conse-quence of this might be a modification to the strategyfor changing direction by ‘cutting’ to ‘steering’, as thelatter may better protect the particular joints frominjury. In this case ‘cutting’ is defined as a quickchange of direction whereas ‘steering’ is a morecontinuous change in direction that does not requirea change in coordinative pattern. Rather, this type ofsteering is achieved through changes in metrical scalingacross the entire system reflected in slight changes inthe centre of mass towards the direction of interest(Full and Koditschek 2000). Clearly, these changes inglobal movement strategies after injury may be con-servative approaches during recovery and have conse-quences for the adaptability of the performer.Evaluation of these relationships between potentiallyrestrictive footwear design and behavioural modifica-tion could provide a basis for assessing the capabilitiesof performers under different DFU relationships(footwear design, terrain type, movement style). Thiswould potentially allow the selection of an optimizedfootwear design that minimizes physical constraintsand the risk of injury, while maximizing performancefor those with previous injuries. This approach couldalso provide the necessary performance trades inphysical protection for firefighters and soldiers whoalso require great dexterity in crucial situations(Bernstein 1996). Fundamentally, the trade-space map-ping using an empirically based footwear performancespace could provide a basis for categorization ofcapability constraints understood explicitly and objec-tively defined through principled scientific study.

4. Summary

Natural movement through the environment in sport,recreation and work requires navigation around

obstacles or other people, moving and orientingtowards certain surfaces, abrupt changes in direction,and potential changes in terrain. Achieving successfulcoordinative relationships for these movements isbased on the constraints on stability–variability rela-tionships within the human–environment system.These relationships are across structural–metrical rela-tionships within the DFU, as well as between the DFUand other functional units within whole-body move-ment. As such, neither singular time-series data norsingle joint focus is able to capture the nature of thevariability–stability duality necessary for footwearscientists. Traditional gait dynamics, although fairlywell understood, are limited to a very constrained typeof locomotion where terrain is consistent, locomotionis forward and continuous, and obstacles are notusually encountered (Chiel and Beer 1997). Theseconstraints significantly reduce the requirement forinformation from the visual, proprioceptive and hapticperceptual systems for performance. Alternatively,most recreational activities, sports, and some workenvironments (construction workers, firefighters, etc.)are unconstrained in at least one of the above ways.Strong foundations in traditional gait mechanics, whencombined with ecological task analysis (Van Emmerik2007), provide a basis by which an expanded under-standing of footwear design and task performance maybe considered. When designing shoes for ‘stability’,then, we must consider the type of movement involvedin the specific task and seek to understand whichcoordinative relationships are most important for goalachievement.

Different types of locomotion style, suited to theenvironment and task in which performance takesplace, must be examined more explicitly if footwear isto be optimized for its intended use (e.g. sideways gaitin tennis with non-continuous locomotion).Unconstraining gait dynamics to more ecologicalscenarios involving navigation with obstacles, terrainchanges, starting and stopping, and different locomo-tion styles will certainly benefit footwear design andoptimal performance regardless of the specific purposeof the footwear. In this light, ‘stability’ footweardesigned for improvement of static balance, for exam-ple, may not be optimized for stability of phaserelationships during movement and may hinder per-formance in some way. A principled scientificapproach that embeds the human in specific task–environment relationships requires a broader under-standing of the relationships between variability andstability within various locomotion styles, visual per-formance, and the degree to which information acrossthe footwear optimizes purposeful action. Determiningthis performance space is important for understanding

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the relationships between the simultaneous functionalrequirements of the DFU and design of ‘stable’footwear that optimizes behavioural outcomes forsuccessful performance.

Defining the footwear performance space and itsrelationship to goal achievement in categorical activ-ities (cutting, stopping, visual acuity, etc.) will help tooptimize footwear design, and stability–variabilityrelationships remain at the centre of this understand-ing. Moreover, this trade-space can provide a context-specific understanding when protection of some sortmust be traded within the functional requirements of atask, such as successful navigation through high-riskenvironments and the potential for injury. From thisperspective, traditional gait dynamics provide a soundfoundation upon which to explore the non-traditionalgait dynamics in the more ubiquitous ecologicalconditions commonly encountered. The segregationof the nested functional units provided above in noway represents the only appropriate functional decom-position. A variety of tools can be exploited withindynamic systems theory, and the human system can befunctionally decomposed in specific ways to uncoverthe relationships necessary for goal achievement, withspecific research questions driving which functionalrelationships are important. Given the focus on foot-wear, the functional decomposition in Table 1 providesan initial attempt to categorize functional relationshipswithin and between the human–environment–taskrelationships for optimal footwear design. The specificsegmental relationships that define stability may not bethe same for all footwear categories, but the level ofanalysis that provides insight into this stability forfootwear science certainly begins with coordinativevariability and must be related to goal-orientedoutcomes. Given that footwear scientists must considermore than just coordinative stability, development ofthe footwear performance space provides other impor-tant aspects of design that can be evaluatedsimultaneously.

Injury prevention has been one of the primarydrivers of footwear design for many years, and theinjury rates justify this (Clement et al. 1981, Messierand Pittala 1988, Layne and Pollack 2004). However,without understanding the constraints that injury-prevention design places on the other necessaryfunctions of footwear, we should question our abilityto optimize footwear and make scientifically princi-pled decisions across the different roles it must playoutside prevention of injury alone. Traditionalattempts to reduce injury usually entail reducing theavailable range of motion, increasing the shockattenuation capabilities, changing sole design andfrictional characteristics, or adding other material

solutions outside footwear such as ankle braces.These material solutions may have significant conse-quences for performance in that they seem to reducethe available DoF and impair information–movementrelationships (Robbins et al. 1995, 1997, Brizuelaet al. 1997). Recent research on motor control andbiomechanics suggests that the traditional under-standing of variability–stability relationships is oppo-site what was once considered in sport and motorperformance. This newer duality relationship betweenthe right kind of variability and stability of perfor-mance suggests that variability in the coordinationpattern provides: (1) increased adaptability/flexibilitywithin larger nested tasks, resulting in overall greaterstability of task performance; (2) increased informa-tion flow; and (3) reduced soft tissue/bone repetitivedeformation. These findings again support the need tounderstand the trade-space for optimal footweardesign, even if the primary intent is to increasephysical protection. To design for stability andprotection, the contribution of variability and infor-mation–movement relationships through the footwearmust be part of the trade-space to provide a trulyoptimized design for performance specific to environ-ment and task constraints.

Our understanding of gait dynamics in realisticbiological movement is very limited, but there areanalytical tools available for the pursuit of under-standing ‘non-traditional’ locomotion. A necessaryfirst step to expanding our understanding to full gaitdynamics (across locomotion style and environmentrelationships) is the need to unconstrain the loco-motion patterns to broader situations. Understandingthe notion of stability is important for footweardesign, and the stability ‘of what’ relationships and‘for what’ goal becomes central to determining whichvariables should be considered. Dynamic systemstheory offers tools that allow the building of therequired footwear performance space, based onrelationships within and between the nested func-tional units of importance to footwear science. Thesetools provide a principled scientific approach tounderstanding the many roles that footwear playswithin the DFU and across the system in providingthe necessary coordinative variability that underliesstable phase relationships and performance. Buildingthe footwear performance space can help in under-standing which segmental phase relationships areimportant in providing stable patterns of perfor-mance within task and environmental constraints.These segmental stability–variability relationshipsprovide the adaptability to respond to upcomingevents and stability to unanticipated perturbationsnecessary for performance.

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Declaration of interest: The views, opinions and/or findingscontained in this report are those of the authors and shouldnot be construed as official Department of Army position,policy or decision, unless so designated by other officialdocumentation.

Note

1. The term ‘information’ used throughout this article isGibsonian information, not Shannon informationrelated to information theory. Gibsonian informationis information about specific relationships between thehuman–environment system that directly specify what ismeaningful to the animal for prospective control ofaction (affordances) through perception of ambientenergy distributions in the environment (e.g. light).

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