Postural dynamics as a function of skill level and task constraints

S Slobounov, K M Newell

The Pennsylvania State University, State College, PA, USA

Summary

An experiment is reported that was set up to examine the interactive effects of the base of support (number of feet; sole versus toes), visual information and skill level on the stability of adults in certain postural balance tasks. The findings showed that the role of vision in decreasing the motion of the centre of pressure becomes more significant as the base of support is reduced and the lower the skill level of the performer. The skilled subjects more systematically used a smaller set of compensatory movement strategies to regain or maintain balance in the impoverished physical and informational support conditions. The findings are consistent with the proposition that there are interactive effects of environmental, organismic and task constraints on postural performance and they suggest that learned compensatory search strategies are used to maintain balance in the face of reduced stability regions of postural support.

Key words: Centre of pressure, compensatory movement strategies, postural dynamics

Gair & Posfu~ 1994, Vol. 2, 85-93, June

Introduction

The movements used to realize actions are consequences of organismic, environmental and task contraints’. This is the case even in postural activities where the goal of the task is often to minimize movement in a particular body segment or segments. Posture is one of the most studied activities in motor control but most theoretical orien- tations to posture rarely give recognition to all three of the major sources of constraint to action (although see Riccio and Stoffregen2). In this paper we experimentally manipulate a variable from all three constraint categories and show the interactive effects of visual control, skill level and base of support on certain aspects of the dynamics of human upright stance.

The degree of postural sway in upright bipedal stance is typically very small. Indeed, to the eye of the natural observer, normal healthy young adults rarely appear to exhibit postural sway at all. However, upright bipedal posture is a dynamic activity and force platform techno- logy has, for example. long revealed the evidence of

Received: 19 May 1993 Accepted: 22 December 1993 Correspondence and reprinr requests IO: K. M. Newell, The Pennsylva- nia State University, Calder Way Building Suite 301, 248 Calder Way, State College, PA 16804. USA

C‘J 1994 Butterworth-Heinemann Ltd 096&6362/94/020085-09

motion of the centre of pressure at the base of support even in normal bipedal stance (cf. Goldie et al.‘).

Early studies of the role of vision in adult posture showed that the spontaneous postural oscillations increased by up to 50% when subject’s eyes are closed4,5. The role of vision in stance appears to be influenced by the relative degree of stability provided by the base of support in that the effect of withdrawing vision is magni- fied in stances with reduced base of support6,7. However, the effects of visual manipulations on postural stability are not entirely consistent (e.g. Ashmead and McCartyx), in large part due to the subtle influence of experimental manipulations, such as indeterminable cues available in the visual surround, the ambiguity to the subject of the task goal, and so on.

Reducing the area of the base of support generally decreases the stability region of the postural coordina- tion mode9. The area of the base of support can be reduced in several ways, including switching the task constraint from tw.o legs of support to one leg’, using the complete sole of the foot as opposed to only a portion of itlo, changing the compliance of the surface of support”. and reducing the extent of the surface of the base of support12. The reduction of the area of the base of sup- port generally leads to increased motion of both the centre of pressure and centre of mass of the body.

The role of organismic factors such as age, strength and skill level all probably contribute to the degree of

86 Gait & Posture 1994; 2: No 2

postural sway under a variety of task conditions. For example, it has been shown that the motion of the centre of pressure of elderly individuals is enhanced in contrast to their younger counterparts13J4. These organismic con- straints are usually viewed as reflections of changes in physical properties of the system that support posture (such as strength) but organismic contributions to enhanced postural sway in aging could also arise from the functional changes in reflex controlls. It is also the case that the relative degree of learning through differen- tial experiential effects is another instance of organismic constraints that could influence postural control. Learn- ing factors are contributors to the development of chil- dren’s posture, (Slobounov & Newell, unpublished manuscript) but they can also arise in adults due to differential practice effects on certain tasks. For example, certain sports tasks demand practice at particular pos- tural conditions, and indeed postural control is in essence a task criterion in tasks such as gymnastics and diving. Thus, individuals that have had a considerable degree of practice and obtained a high level of profi- ciency in these tasks may control posture differently through more fully exploiting the dynamics available in the interaction of the organism and the environment in the pursuit of task relevant goal@.

Posture like any other action is a consequence of the search for the appropriate dynamics that support the realization of a given task goal”. This view suggests that performance is not merely a consequence of the intrinsic dynamics that arise from the organism-environment interaction’*J9, but in addition the exploratory activity engaged in by the performer to create and change the equilibrium regions formed at the interface of the percep- tion-action cyclei7J0. The loss of postural stability through reducing the physical and informational base of support induces a search process on the part of the subject for attainment of a new stable coordination mode.

The role of exploratory activity is particularly relevant in posture when the base of support for postural stability is reduced. In this situation the subject is likely to approach the stability boundaries for the usual bipedal postural coordination mode*,9, and is now required to find new coordination dynamics to continue to realize the task goal of minimizing postural sway. This search for the introduction of appropriate compensatory move- ment strategies must take place simultaneously within the more global context of preserving the general inte- grity of the dynamics of the system*’ - an example of what Feld’baum*l has labelled the dual control problem.

How the subject introduces compensatory dynamics to standing posture under reduced postural base of sup- port will be a function of the skill level of the subject and the supporting environmental information available. It seems reasonable to project that general experience in postural activities contributes to the use of different search strategies in the face of changing postural con- straints. Recently we have found some preliminary evi- dence for this proposal in contrasting the compensatory balance strategies of 3- and 5-year-old children under

varying task contraints, (Slobounov 8~ Newell, unpub- lished manuscript). These balance strategies were deter- mined through a categorization of the qualitative properties of the postural kinematics.

In summary, we examine in this study the postural dynamics of the centre of pressure and the qualitative properties of the compensatory body kinematics as a function of skill level, the availability of visual infor- mation, and the task constraints of different bases of support. It was anticipated that these three factors would interact in influencing postural stability and the strategy by which subjects sought to maintain stability in the face of a reduced base of postural support. Such findings would support the general propositions that perfor- mance is a consequence of organismic, environmental and task constraints to action and the search processes used to exploit the dynamics of the perceptual-motor workspace.

Method

Subjects

There were two groups of adult subjects recruited from the University of Illinois community. One group con- sisted of six male subjects (non-collegiate athletes) who had a mean age of 20.68 years (SD 1.24). The second group consisted of six male athletes from the university gymnastic and diving teams with a mean age of 20.42 years (SD 1.24).

Apparatus

The dynamics of posture were recorded with a force platform from Advanced Mechanical Technology Inc. (AMTI). The platform records three force components along the lateral (Fx) and anterior-posterior (Fy) hori- zontal axes and the vertical axis (Fz), together with the three respective moment components (Mx, My and Mz). The signals were amplified through an AMTI model SGA6-4 amplifier (six-channel). A maximum gain of 4000 was used with a low-pass filter of 10.5 Hz. Bridge excitation was set to 10 volts. All six channels were factory calibrated. The unit of measurement for the three force components (Fx, Fy, Fz) was in Newton (N). The unit of the measurement for the three moment compo- nents was in Newton-metres (N m). Videotape recording of all postural task conditions was conducted with a video camera Camcord ‘Hitachi’ VHS Movie 6000A. The standard AMTI software was used to collect and analyse the data. This program uses the Herron tech- nique to calculate the area of the centre of pressure.

Tasks and procedures

Each subject performed 10 postural balance tasks that were presented in random order. In all task conditions subjects were standing in the middle of the force plat- form with bare feet. The tasks required subjects to stand as still as possible under the following task conditions.

Slobounov and Newell: Posture and skill 87

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Double leg stance, feet flat, with eyes open. Double leg stance, feet flat, with eyes closed. Double leg stance, on toes, with eyes open. Double leg stance, on toes, with eyes closed. Single leg stance, foot flat, on dominant leg, with eyes open. Single leg stance, foot flat, on dominant leg, with eyes closed. Single leg stance, on toes, on dominant leg, with eyes open. Single leg stance, on toes, on dominant leg, with eyes closed. Single leg stance. foot flat, on non-dominant leg, with eyes open.

Single leg stance, foot flat, on non-dominant leg, with eyes closed.

Each subject performed five trials at each condition in a blocked fashion. In the eyes open conditions subjects were requested to fix their gaze on a marked spot on the wall which was at eye height and 8 m away from the platform. No instructional constraint was imposed on how the subjects used their arms in each task, although a!! subjects held their arms to the side of the body at the beginning of each trial. Each trial was of 15 s duration and the force platform data were collected at a sample frequency of 100 Hz.

Each trial was videotaped to afford qualitative analy- sis of the dynamics of the torso and limbs while executing the postural tasks. A video camera was placed to the front of the subject and 45 degrees off the sagittal plane of the subject as he stood on the force platform. There was sufficient focal length to the videocamera image to afford the recording of whole-body action. The video- tape of each subject was analysed on completion of the experiment to determine the compensatory movement strategies employed to realize the task demands in each experimental condition.

Results

In this experiment a!! subjects but one completed al! trials without violating the task constraints. One non- athlete subject touched the platform with the non-sup- porting foot in one trial of the single stance toes eyes open condition and one trial of the single stance toes eyes closed condition. The following analyses are based on five trials per condition for each subject except for the missing trial in each of the above conditions.

Cmtre qf pressure

Several dependent variables are traditionally derived from the centre of pressure time-series’. Typically there is a high correlation between a number of these variables and this was the case in this experiment. For example, over all subjects in all conditions the correlations were: length and area of centre of pressure, r = 0.955; length and displacement along both axes, r = 0.990; area and displacement along both axes, r = 0.974. As a conse-

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Figure 1. The position of the centre of pressure (cm) in two sample trials: a DSFO - double stance flat feet eyes open condition; and b DSTO - double stance toes eyes open condition.

quence we have selected one centre of pressure variable. namely area within centre of pressure trace, as a rep- resentative variable on which to analyse group and task condition effects. The analysis of the area of centre of pressure was conducted with a MANOVA in light of the within- and between-subject design. Figure I shows two sample configurations of the centre of pressure within a trial from the double stance hat feet eyes open (DSFO) condition (Figure la) and the double stance toes eyes open (DSTO) condition (Figure 1 b).

Analysis of the effects of leg dominance in the one foot conditions revealed no significant main effect for domi- nance or the interaction of leg dominance with the other factors (all Ps > 0.05). Accordingly, in the following analysis of the centre of pressure the leg dominance factor is not considered and the single leg stance perfor- mance is based upon the dominant leg data.

The mean area of centre of pressure as a function of skill group, visual condition, one versus two legs, and feet flat versus on toes is shown in Figure 2. There were significant main effects for vision, F (1. 102) = 34.59. P

cO.01, feet flat versus toes, F(1. 102) = 31.33. P<:O.O!.

skill level, F (I, 102) = 12.60, P< 0.01, and one versus two leg stance, F (1, 102) = 42.07, PCO.01. Overall. there was a smaller area of centre of pressure on two feet as opposed to one foot, in the eyes open as opposed to eyes closed condition, in the skilled group as opposed to the unskilled group, and on two legs as opposed to one leg. These main effects need, however, to be considered in the light of several interactions between the respective independent variables.

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Figure 2. The mean area of the centre of pressure (cm*) as a function of postural condition: OSFO - double stance flat feet eyes open; DSFC - double stance flat feet eyes closed; DSTO - double stance toes eyes open; DSTC - double stance toes eyes closed; SSFO - single stance fiat feet eyes open; SSFC - single stance flat feet eyes closed; SST0 - single stance toes eyes open; SSTC - single stance toes eyes closed. H Skilled athletes; 0 non-athletes.

There were three significant double interactions in terms of the area of the centre of pressure: feet x base F (1,102) = 4.89, P<O.O5; feet x vision F (1,102) = 10.32, P~0.05; and base x vision F (1,102) = 5.53, P < 0.05. Post-hoc analyses confirmed the impression conveyed by the data displayed in Figure 2. Namely: (a) reducing the base of the support from the sole of the foot to the toes produced a much greater increase in the area of the centre of pressure in the single stance as opposed to the bipedal condition; (b) withdrawing vision had a much greater effect on increasing the area of the centre of pressure in the single stance as opposed to the bipedal condition; and (c) withdrawing vision had a much greater effect on increasing the area of the centre of pressure in t.he reduced toes base of support as opposed to the full sole of foot base of support condition. These double interactions are all consistent with the position that reducing the number of feet supporting posture and reducing the area of the base of support from the sole of the foot to the toes have additive effects on the stability of posture.

The informational constraint of withdrawing vision had a much more pronounced detrimental effect on pos- tural stability when the physical base of support was reduced. This effect was enhanced in the non-athlete group as revealed in a quadruple interaction between vision, feet, base, and skill level, F (1,102) = 7.83, P < 0.05. Post-hoc analyses revealed that this interaction was due to the skilled athlete group not increasing the area of the centre of pressure as much as the non-athlete group when vision was withdrawn in the single stance toes as opposed to double stance toes conditions.

Qualitative analysis of videotapes

Qualitative analyses of the kinematics of the torso and limbs were conducted by analysing the videotape of each trial. A set of distinct movement strategies were identi- fied that subjects used to compensate for the loss of stability in balance tasks. The compensatory movement strategies reflect unique qualitative features of the rela- tive motions of the kinematics. These movement categor- ies had been shown previously to be sensitive to the different compensatory movements used by 3- and 5- year-old children in the maintenance of balance under different task constraints. (Slobounov & Newell, unpub- lished manuscript). The identification of a movement strategy required the subject to produce at least three consecutive cycles of the strategy within a given trial condition. The list of the 13 movement categories used in the analysis is shown in Table 1. One experimenter coded all the postural trials according to the categories listed in Table 1. A second experimenter, who was unaware of the group each subject, coded 25% of the trials from each group using the same category list. There was a 92% consistency in recognition for the skillled athlete group trials and a 89% consistency in recognition for the non- athlete trials.

The prevalance of these compensatory movement stra- tegies as a function of experimental condition is shown in Figures 3-5. In these figures the frequency of a given moment strategy is expressed as the number of trials on which the strategy occurred. There were no discernible compensatory movements in either the skilled athlete group or non-athlete group in both the double stance flat foot eyes open and double stance flat foot eyes closed conditions. Accordingly, these conditions were not included in the figures.

There are several group trends in the compensatory movement strategies apparent in the remaining con- ditions where the base of support was reduced and/or visual information was not available. These trends relate to both the number and type of compensatory movement strategies employed and the number of occasions that such strategies were used. First there were significant main effects on the absolute number of trials on which

movement strategy &1&O) = 20.21,

was invoked for group, PcO.01, base of support,

F(l,BO) = 170.17, PCO.01, toe versus sole of foot, F(l,SO) = 112.63, P~0.01; and the availability ofvisual information, F (1,80) = 39.25, P-C 0.01. The number of trials with compensatory movements was less in the skilled versus unskilled group, the vision versus no vision condition, two feet versus one foot base of support, and full sole of the foot support versus toes only. There was a triple interaction of group x base of support x vision, F(l,SO) = 5.56, P~0.05 which showed that the with- drawing of the physical and informational base of sup- port had less influence on the skilled athletes as opposed to the non-athletes group. These finding are consistent with the enhanced motion of the centre of pressure of these group and task conditions.

There was also a significant effect on the number of

Slobounov and Newell: Posture and skill 89

Table 1. Postural movement categories used in videotape analysis of movement during stance tasks

1. Lateral torso sway (i.e. oscillatory categories used in videotape analysis of movement during stance tasks) 2. Anterior-posterior torso sway (i.e. oscillatory torso motion back and forth, ‘hip strategy’) 3. Knee bending and extension (i.e. knee strategy) 4. Ankle movements (i.e. Lateral sliding motion of the supported leg, heel elevation, eversion abduction, inversion abduction) 5. Head movement (i.e. forward head tilt, turns from side to side) 6. Arms lateral sway (i.e. periodic abduction and adduction with observable amplitude) 7. Arms rotation (i.e. circular arm motion in the frontal or sagittal plane) 8. Shoulder movements (i.e. shoulder elevation) 9. Step (i.e. quick and short duration touch of the platform by the unsupported leg)

10. Jump (i.e. hops on unsupported leg) 11. Anterior-posterior arm sway (i.e. aperiodic back and forth arm motion in the sagittal plane) 12. Movement by unsupported leg (i.e. bending and extension of knees, lateral, anterior-posterior and circular motion on

the unsupported leg) 13. Non-discriminative activity (i.e. variety of aperiodic, whole body movements, usually followed by loss of balance)

1 2 3 4 5 6 7 8 9 10111213 Movement patterns

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Figure 3. The number of movement strategy trials as a function of postural condition, group and category of movement pattern. Condition abbreviation as outlined in Figure 2. Upper pair, skilled athletes; lower pair, non-athletes; left pair, SSFO task; right pair, SSFC task.

compensatory movement strategies used as a function of information. F(1,80) = 49.91, P<O.Ol. There were

group, F (1.80) = 23.16, P-c 0.01, two feet versus one several double interactions in regard to the number of

foot, F (I .80) = 83.03, PC 0.01; sole of foot versus toes, compensatory movement strategies used: number of feet

F (1.80) = 158.59. PC 0.01; and the availability of visual x base of support, F (1.80) = 1 1.4 I. P < 0.0 I ; vision x

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Figure 4. The number of movement strategy trials as a function of postural condition, group, and category of movement pattern. Condition abbreviation as outlined in Figure 2. Upper pair, skilled athletes; lower pair, non-athletes; left pair, DSTO task; right pair, DSTC task.

base of support, F (1,80) = 21.23, P~0.01, group by number of feet, F (1,SO) = 7.64, PC 0.01. These findings point to the conclusion that the trained group had a small set of preferred compensatory movement strategies through which subjects sought to preserve postural stabi- lity, but that the relative size of this effect varied as a function of task condition. The area of the base of sup- port, particularly as manipulated through the toes versus sole of foot conditions, had a strong influence on the number of movement strategies used.

Discussion

The findings of this experiment show that the postural stability of subjects in the balance tasks studied was clearly a function of the interaction of features of the base of support, the availability of visual information, and the skill level of the subjectsQ. The interactive effect of these constraints to posture was also evident in the

number, type, and prevalence of movement strategies employed to maintain or regain postural stability under the more difficult task conditions. These findings have a number of theoretical implications for the coordination of posture which will now be discussed.

There was no difference in the area of the centre of pressure between the skilled athletes and non-athletes in the double stance flat feet conditions (both eyes open and closed). That is, in a postural support condition that more closely approximates the constraints that occur in a number of everyday activities of daily living the training experience in the particular sports of gymnastics or div- ing did not reduce the motion of the centre of pressure. In fact, the centre of pressure was actually higher in the skilled athlete group, a finding that may be a reflection of this group’s enhanced exploration of the equilibrium region of the perceptual-motor workspace. It appears that the stability margins provided by the physical base of support in normal bipedal stance are so broad that the

Slobounov and Newell. Posture and skill 91

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Figure 5. The number of movement strategy trials as a function of postural condition, group, and category of movement pattern (condition abbreviation as outlined in Fiaure 2). Uooer pair, skilled athletes; lower pair, non-athletes; left pair, SST0 task; right pair, SSTC task.

_ , . .

withdrawal of visual information does not produce a performance decrement. The role of perceptual thres- holds in subjects exploring the layout of the perceptual- motor workspace (Newell et al.“) has yet t? be examined systematically in posture or other tasks.

The centre of pressure data reveal that the decrement of postural stability is generally related to the reduction in the base of support and the withdrawal of visual information. The single stance toes eyes closed condition produced the highest amount of motion of the centre of pressure. This task proved to be very dificult for even the skilled athlete group and they produced approximately three times as much motion in this condition as they did in any other condition. The postural training of gym-

nastic and diving athletes does not normally include non- visual conditions and so the withdrawal of vision in postural control was a relatively unpractised situation for even this group. How the type and degree of training can reduce motion of the centre of pressure in the non-

vision conditions remains an empirical question. Theore- tically, one can hypothesize that the general skilled ath- lete versus non-athlete effect on the motion of the centre of pressure is due to the enhanced practice of the skilled athlete group changing the nature of the attractor dynd- mic supporting posture7J3.

There was a trend for the amount of motion of the centre of pressure to be similar in the double stance toes and the single stance flat feet conditions were identical. This finding raises the question as to whether the base of support issue is strictly one of amount of area of the surface of support, or whether one versus two leg neuro- muscular control is a contributing factor to the motion of the centre of pressure. The area of support of two toes condition approximates that of one full flat foot con- dition and thus the centre of pressure data from this study suggests that absolute area of support is the critical issue. Further research is required to tease out more precisely the relative effects to postural control of the

92 Gait 81 Posture 1994; 2: No 2

area of support and one versus two leg neuromuscular organization,

The non-athlete group introduced a broader range of compensatory movement strategies into the postural tasks than the skilled athlete group in all the reduced postural support conditions, This finding is consistent with the centre of motion data and suggests that there is an association between degree of motion of the centre of pressure and the number of compensatory movements. The data also suggest that experience is a contributor to the adaptive use of compensatory movement strategies in posture, although the design of this study does not rule out the possibility that other individual differences con- tribute to this performance effect. The skilled athlete group more consistently used a smaller set of preferred compensatory movement strategies than the non-athlete group.

This finding complements data from our earlier work that showed 5-year-old children had a smaller set of preferred movement strategies than 3-year-old children in similar postural tasks (Slobounov & Newell, unpub- lished manuscript). Synthesizing from the data of this study of children’s posture and the current study it appears that the role of experience engenders a certain sequence to the introduction of compensatory movement strategies in posture. Initially, young children do not introduce systematic compensatory movements to the loss of stability in postural tasks. Their response to reduced base of support in posture is to freeze the body segments to operate as an inverted pendulum. This strategy is followed in time by the increased probability of a broad range of compensatory strategies being used in a non-systematic way, and subsequently this set of compensatory movements is narrowed further to a small set of preferred strategies by general experience and par- ticular balance training. This profile of the effects of experience on the use of compensatory movements in posture parallels Bernstein’s’6 ideas on the stages of learning to coordinate redundant biomechanical degrees of freedom in movement control.

A central question to the degrees of freedom issue in movement coordination and control is why there tends to be a certain order to the way in which particular degrees of freedom are introduced into the coordination mode16J7+2V, This general question can be rephrased in the context of this study to ask why it is that there is a particular order to the way in which compensatory movement strategies are introduced in regaining and maintaining postural stability? Presumably these move- ment strategies are initially introduced as a reaction to the loss of stability, although once introduced into the coordination mode they can also contribute toward creating both instability and stability. There are several hypotheses available to relate motion of the centre of pressure with the introduction of a particular compensa- tory movement strategy. One hypothesis is a mechanical one and suggests that the relative mass of each body segment is a significant factor in determining the order to the introduction of compensatory movement strategies to regain postural stability, in the same way that this

factor influences the unfreezing of the degrees of freedom in skill acquisitionrQ4. Another related hypothesis is a functional one and suggests that each particular compen- satory strategy has a particular functional consequence on the confluence of accelerations produced by the limb and torso body segments*5. Our data are consistent with the functional hypothesis in that there is a specific pre- ferred compensatory movement strategy that is most probable given a particular task constraint and organi- smic constraint (e.g., skill level of the subject). More generally, the data support the position that the con- straints to action are interactive in channelling the dyna- mics of posture and movementr~20.

It also seems reasonable to propose that the stability of the centre of mass is essential to the preservation of the integrity of the system in all physical activities, and is a significant part of the dual control problem** in posture. There may, however, be multiple criteria to the evalu- ation function in posture*, so that the motion of the centre of mass may not be the only variable regulated. We could not link in real-time the initiation of these compensatory movements to the loss of stability as re- flected in the force platform measures but the findings from this preliminary study encourage a more formal dynamical analysis of the introduction of compensatory movements in posture.

In summary, our findings show that there are signifi- cant differences between skilled athletes and non-athletes in the degree to which ground reaction forces are required to maintain postural stability, in addition to differences in the type and prevalence of movement stra- tegies used to regain postural stability. Our data encour- age a more formal analysis of the postural dynamics of regaining postural stability. This preliminary analysis of the movement strategies used in the regaining of postural stability suggests that they may be consonant with the principles of the acquisition of coordination.

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Currently in other Butterworth-Heinemann journals

Journal of Electromyography and Kinesiology

Gait analysis of myopathic children in relation to impairment level and energy

cost D Trias, M Gioux, M Cid, C L Bensch; JEZectromyog Kinesiol, 1994; 4: 67-82

Estimation uncertainty in ensemble average surface EMG profiles during gait R F M Kleissen, G Zilvold; JElectromyog Kinesiol, 1994; 4: 83-94

Power spectrum analysis of the rectified electromyogram during gait for normals

and patients J Nielsen, L Arendt-Nielsen, A Pedotti; JElectromyog Kinesiol, 1994; 4: 105-l 15

Clinical Biomechanics

Gait following lateral collateral ligament repair S Poulis, R W Soames; Clin Biomech, 1994; 9: 220-224

Relationship between alignment, kinematics, and kinetic measures of the knee of normal elderly subjects in level walking H Wang, S J Olney; Clin Biomech, 1994; 9: 245-252

Influence of prosthesis alignment on the standing balance of below-knee amputees E Tsakov, J Mizrahi, 2 Susak, N Hakim, Clin Biomech, 1994; 9: 258-262