3. ADHERENCE AND POSTURALCONTROL: A BIOMECHANICAL

ANALYSIS OF TRANSIENT PUSH EFFORTS

Simon Bouisset

Laboratoire de Phlsiologie du Mouuement, Uniuersitl Paris-Sud, 91405 ORSAY France

Serge Le Bozec

Laboratoire de Physiologie du Mouuement, Uniuersitd Paris-Sud, 91405 ORSAY France; U731 INSERI,I / UPMC

Christian Ribreau

Laboratoire de Biomlcanique et Biomatlriaux Ostlo-Articuhires, Uniuersitl Paris 12 - Val de Marne,

94010 CRETEIL, France

AbstractThis chapter focuses on the question of the inter-face between the body and its physical environment,namely adherence and friction. First, a short surveyof literature is presented and some basic statementson adherence reviewed. They help de6ne the adher-ence constraints associated with different motor tasks.

Then, a new paradigm is presented, the transient pushparadigm, which offers manifold facilities. In partic-ular, it makes it possible: i) to exert transient externalforce in the absence of external.movemenu ii) to di-vide the body into a focal and a postural chain; and iii)to manipulate the surface contacts between the bodyand its supports, without perturbing body balance.

The chapter is documented with recent results ontransient isometric pushes performed under two con-ditions of surface contact. A biomechanical model is

presented. Based on an experimental recording of themain terms of the model, it is concluded that tran-sient muscular effort induces dynamics of the posturalchain. These observations support the view that thereis a postural counter-perturbation, which is associated

with motor acts. Changing ischio-femoral contact has

been proven to modify postural chain mobiliry whichappears to be a key factor of performance.

The influence ofadherence was considered from theadherence ratio, that ir, p - R1/RN (with p beingthe adherence ratio, R1 and RNr, the instantaneoustangential and normal reactions at the contact sur-face). It was found to evolve, during the course ofthe effort, up to a certain value, which is close to thecoefficient of friction to within a security margin, atthe seat contact surface, at least. Lastly, the adher-ence effects on motor programming are highlighted,and the possibiliry ofconsidering the centre ofpres-sure as the postural control variable is discussed. It is

proposed that the instantaneous adherence ratio, withreference to the coefficient of friction, might be one ofthe rules for controlling muscle activation to accom-plish voluntary efforts, when there is the risk ofloosingbalance.

Keywords: Postural dynamics; ramp push efforts; ad-herence, motor control.

t{4ren they move, human, as well as animals, have tocomply with mechanical rules, known as laws of dy-namics ("Newtont laws"). The forces taken into con-sideration are those which are external to the system.

For example, when the human body is considered as a

whole, the external forces are limited to graviry and the

28 I. CONTROL OE MO\GMENTAND POSTURE

reactions are developed at the interface with the phys-ical environment, primarily the ground reaction if themovement is performed on the earth tfack. Moreover,it is well known that the ground reaction, and, moregenerally, the reactions induced by the contact areas,

depends on their physical properties, that is rigidityand adherence.

The aim of this chapter is to highlight the inter-actions berween postural dynamics and adherence,and to discuss their effects on motor control. It is

documented with recent results on the transient pushparadigm, which is considered a "pure" kinetic motoract, in that there is no hand movement, even thoughmuscular effort varies at each instant.

l. Adherencq Friction andPostural DynamicsA short survey of literature will be usefirl before re-viewing some basic statements on adherence and thearticulated body chain.

1. 1. ADHERENCE AND MOTOR ACTS:A LITERATURE SURVEYThe problem of adherence has been considered ac-cording to rwo main biomechanical viewpoints. Thefirst set of research was more practical. It included grossbody movements, such as exertion of push/pull force(Gaughran and Dempster, 1956; \X/hitney, 1957;Kroemer, 1974; Grieve, 1979), walking (Carlscici,

1962, Lanshammar and Strandberg, 1981; Strand-berg, 1983; Tisserand, 1985), running (see Nigg,1986, for a review) and ice skating (de Koningand Van Ingen Schenau, 2000). Most of the stud-ies on push/pull forces focused on maximal force ex-ertion, that is static conditions, and aimed at defin-ing the most efficient ones. Studies on locomotionconsidered necessarily dynamic conditions. Those onwalking were conducted with the aim of measuringfloor/shoe slip resistance in order to prevent slip-ping, and to prevent fall-related injuries, and those onrunning and ice skating, with the aim of improvingperformance.

The second set of research focused on the mecha-nisms controlling the contact forces at the hand, dur-ing both "static" and "dynamic" efforts (Johansson and\Testling, 1984; see \X/ing, 1996, for a review). Themanual efforts under consideration included transientgrip force paradigms. Prehension forces are limited tothe grip force and the load force (that is the object'sweight). The results stressed the influence of frictionon the motor act, and acknowledged the importance ofa safety margin, in order to prevent slipping (Flanagan

and 'Wing, 1995; lf'estling and Johansson, 1984).More precisely, the grip force would have to be cali-brated in relation to the load force. It was concludedthat the coefficient of friction might be implementedin the motor program.

The prehension studies focused on the efforts ex-erted at the hand level, contrary to the gross bodymoyement studies, which considered every reactionforces at the interface between the subjects and thephysical environment. In order to study the question,a series of experiments on transient push efforts was re-cently initiated. Biomechanical (Bouisset et aL.,2002;Le Bozec et al., 1996; 1997; Le Bozec and Bouisset,2004) and EMG (Le Bozec et a1.,2001; Le Bozec andBouisset, 2004) data were considered, and a biome-chanical model was elaborated (Bouisset et al., 2002).The main biomechanical results are presented lateron, with the aim of stressing their contribution to themotor control approach.

1.2. COEFFICIENT OF FzuCTION ANDADHERENCE RAIIOThe Coefficient of Friction (CoF) is defined at theslipping limit by the well-known relationship:

R*1 : 1t-*R*p (1)

where p* is the coefficient of friction, R*r the tan-gential reaction (or friction force), and R*N the nor-mal reaction at the contact surface. The coefficientof friction varies according to the properties of theinterface, and the risk of slipping increases as p*decreases.

In order to evaluate adherence, and consequentlythe risk of slipping, an Adherence Ratio (AR) can be

defined, which is:

Rr: pRN Q)

where p is the adherence ratio, and R1 and Ry, theinstantaneous tangential and normal reactions at thecontact surface (Fig. 1).

During locomotion, AR was also called "frictionuse" by Strandberg (1983), and was defined by theratio berween the tangential and vertical ground re-actions. However, during prehension, the inverse ofAR was usually considered, that is, the ratio of thegrip force (that is, normal force) to the load force(that is, tangential force). It was called the "slip ratio"(Johansson and Vestling 1987).

Adherence and friction are close companions, be-cause the coefficient of friction is the boundary markof the adherence ratio (p < p*). However, AR is not a

measure of CoF since, by this very fact, it varies under

3. ADHERENCE AND POSTUML CONTROL 29

".....'.........r":::Il'.'........,......... 1I

aa

Focal chain :

Postural chain : Upper body

Lower body

ffiII

FIGURE 1. Coefficient of friction and adherence ratio. R*1,tangential reaction (or friction force), and R* 51 , normal reac-tion at the contact surface, are the reactions at the slippinglimit; g* is the friction angle. R1 and Ry are the actualtangential and normal reactions at the contact surface; gis the actua.l angle ofadherence Adherence ratio (that is p)reaches the limit of slipping (that is p*) when Ry increases,

and/or when R51 decreases. There is no slipping as long as

as (p < (p*.

CoF until slipping occurs. However, AR reflects howthe CNS takes into account the contact forces be-rween the body and its physical environment in orderto perform the motor act efficiently. In addition, itcan be assumed that the highei the CoF, the higherthe AR, that is, the more rhe contact forces are pur inroPlry.

More generally, for a given interface, the CoF valueappears to delimit two motor behaviours: it separatesthe domain where voluntary acrion can proceed inaccordance with the primary intent, from the domainwhere it is perturbed by unexpected slipping and a

possible fall.

1.3. REACTION FORCES AND POSTURAICFTAIN D\NAMICSFrom a biomechanical viewpoint, the skeletont struc-ture allows the modelling of the human body as anarticulated chain ofrigid solids, which are actuated inrelation to each other. The forces (and torques) aretransmitted between the segment(s) the subject inten-tionally mobilizes and the distal one(s) and berween

FIGURE 2. Focal and postural chains. The partitioning ofthe body between a focal and a postural chain is illustratedin pushing (left) and pointing (right) tasks.

these and the physical supports. As a consequence, anintended movement involves a perturbation of bodybalance, as has been suggested by several neurologistssince the turn of the last century (see, for example,Andr6-Thomas, 1940).

This is why it has been proposed that the articu-lated body chain be divided into rwo functional parts(BouissetandZattara, 1981 and 1983). One, thefocalchain, would be directly in charge ofvoluntary move-ment, that is, ofthe task movement rhe subject intendsto perform. The other, the postural chain, includesthe rest of the body. It would be responsible for thestabilizing action, which must be opposed to the bal-ance perturbation provoked by voluntary movemenr.This counter-perrurbation is necessary in order to per-form the task efficiently (Bouisset and Zattara, 7981;Friedli et al., 1988).

For example (Fig.2), when pointing at a targe t withthe upper limb, this limb clearly represents the focalchain. Similarly, when pushing on a bar, the intendedpush force originates from the shoulder muscles andis transmitted to the bar through the upper limbs,which constitute the focal chain. The chain locatedberween the shoulders and the ground is the posturalchain. Again, it is easy to divide the postural chain intotwo parts, particularly when the efforr is performed ina sitting posture: the upper body, which is located

30 I. CONTROL OF MOVEMENTAND POSTURE

between the shoulders and the seat, and the lowerbody, located between the seat and the ground.

During push efforts, in addition to the push force,external forces include body weight and reacrions atthe support surface contacts. These reactions originatefrom the ground ifthe subject is standing, and fromseat contact as well, if the subject is sitting. It is aimedto consider the role played by reacrions at rhe supporrsurface contacts, unlike the grip studies, which focusedon local efforts on objects, and to consider rhe way inwhich the postural chain contribures to the motoract.

2. The Transient Push ParadigrnA transient push paradigm was considered in orderto explore in greater detail how the postural chaincontributes to the motor acr.

This paradigm has been used in the past to studythe control of motor responses under isometric con-ditions, in order to minimize several problems, whichcomplicate experimental analysis. Rapid force im-pulses produced at a distal joint, like the elbow,were usually considered (see, for instance, Ghez andGordon (1987), Gordon and Ghez (1987) orCorcos et al. 1990).In these studies, stops were usedto preyent any body movement. In contrast, multi-joint pull and push tasks, performed by free-standingsubjects, were chosen by authors like Vhitney (1957)and Grieve (1979), or Cordo and Nashner (1982) andLee and Patton (1997).

In this research, seated subjects were ins-tructed toexert horizontal bilateral pushes on a bar, as rapidlyas possible, up to their maximal force, and to main-tain it for 5 seconds. They were asked to sit upright,and the appararus was set to ensure their thighs werehorizontal, their legs vertical, their upper limbs hor-izontally extended, and their hands gripping the bar.As the body was in contact with rigid surfaces (seat

and footrests), making hand and foot movements im-possible, the articulated body chain is said to be aclosed chain. But the postural chain was not pre-vented from moving, as no additional support wasused at the shoulder and trunk levels.

This paradigm offers many advantages: i) the mus-cular effort varies, but there is no movement of the ex-tremity of the focal chain: as there are no hand move-ments, there are no "focal" kinematics; ii) since thesubjects are in quasi-static conditions, the dynamicsshould be located in the postural chain (i.e. betweenthe feet and the shoulders), which is divided into twoparts: the upper and lower body; iii) since the subjectsare seated, the mobiliry of the postural chain is easy

to manipulate through a change in the ischio-femoral

contact with the seat, without perturbing body bal-ance. In this view, full ischio-femoral contact (100 BBwith BP for Bilateral Push) and a one-rhird conracr(30 BP) were considered, the former being known toinduce lesser lumbar spine and pelvis mobiliry thanthe latter.

2.I. BIOMECHANICAL MODELLINGA biomechanical model was elaborated in order tospecify the role plaltd by postural dynamic phenom-ena and to evaluate the effect ofadherence ar the con-tafi level berween the subject and the seat, as well as

the footrests, in the course oftransient efforts (Bouissetet a1.,2002).

To this end, the general equations of the mechanicswere applied to the system. The subjectt body wasconsidered to be an isolated mechanical system. Con-sequendy the forces applied to the system include thereaction forces originating from the body contact sur-faces, in addition to body weight (Fig. 3).

In the Galilean coordinates system of the labora-tory the nro dynamic scalar equations for the Cen-

:::,"t".r"t* (CoG) movement in the sagittal plane

mjic-F.+&mzc:(&-\0+f,

(3)

In these equations, ii6, 26 are the coordinaes of CoGacceleration, I7 is the weight of the subject and mhis/her mass; -F, and -F, are rhe antero-posteriorand vertical external forces exerted by the bar on thesubject (conversely, the forces exerted by the subjecton the bar are equal to within the sign); \ and R, arethe antero-posterior and vertical components of thereaction forces.

Furthermore, it can be written:

& : Rs*.1Ra

&: Rs, * Re @)

where fu* and Rg are the reaction forces along theantero-posterior axis at the seat and foot levels respec-tively, R5, and Rg, the same reaction forces along thevertical axis.

The variation 6y(G) of angular momenum (bodyangular acceleration times the moment of inertia) ofthis planar system is deduced from the moments offorces about the origin, O, of the laboratory referenceframe as:

6y(G) + mii6z6 - mzcxc : x5!7 - aF, + hF,

-xp& * bRs,

3. ADHERENCEAND POSTURAI CONTROL 3r

FIGURE 3. Biomechanical modelling. The diagram of exter-nal force vectors corresponds to a two-handed push exerted

on a bar by a seated subject. Horizontal and vertical reactionforces, F, and F,, exerted on the stbject; R5* and Rs,, Rcand Rg: antero-posterior and verticai reaction forces at theseat and foot levels; \7: is the weight ofthe subject, actingthrough the CoG line; xp, x6: x coordinates ofCoP (P) and

CoG (G) according to origin O; h: vertical distance fromthe dynamometric bar (A) to the footrest plane; a: horizon-tal distance ofthe bar to O; b: vertical distance between seat

and foot levels.

The quantities a, b and h are parameters of the ex-

perimental set-up which are adjusable according tothe subject's anthropometrical data (Fig. 3). The x-coordinates of the Centre of Pressure (CoP) at theseat and feet are denoted respectively as xp5 and xp6.

The x-coordinate xp of the global CoP is given by:

Rr;-'l- xor

-,&

At the end of the push effort, a new mechanical equi-librium occurs, and the equations ofbalance can be

deduced from (3) and (5), that is:

&: -F*R. - \(/: -F,

and:

xc - xp : tG" - \7)/\71("p - a) + (Rs./W

x (h - b)h + (Ra/\Oh (B)

This equation becomes simpler if Rr, and R6 are negli-gible in comparison to Rs, and Rs" respectively, whichwill be proven later (section 2.2.2.2):

xG - xp : [(Rs, - \0/\4(rp - a) + (Rs"/Wft - b)

(e)

(9) can be rearranged in order to get push

-F* : (R5, - V1(" - xp)/(h - b)

+\rok-xP)/(h-b) (10)

Hence, -F" increases as a function of xp, xG and R5,.

In particular, ifx6 is negligible, and (R5, - Vf is con-stant at the end of push effort, -F" is proportional tothe CoP baclcrvard displacement.

Furthermore, equation (9) can be rewritten, takingthe adherence ratio into account:

(*p - *c) + [(Rs, - V1/\7](rp - ")+(psRs,/\xDG-b):0 (11)

Equation (11) relates CoP displacement to verticalreaction forces and adherence ratio (ps, at the seat

level). However, it does not result in a cause and effectrelation berween these three factors.

An experimental protocol was designed to measurethe various terms of the model (Bouis set et al., 2002).To this end, the subjects were seated on a custom-designed device (Lino, 1995). Three rectangular forceplates, linked by a rigid frame, measured reactionforces and positions of the centre of pressure at thefoot and seat levels. The CoG coordinates (*c, ,c),along the antero-posterior and vertical axes, were de-duced from the CoG acceleration (equations (3) by adouble integration. fu the x origin was taken at theglobal CoP at rest, the x coordinates measured the xdisplacement. Force transducers measured the antero-posterior and vertical forces exerted by the bar on thesubject, and conversely.

2.2. TRANSIENT PUSH INDUCESPOSTURA]- DYNAMICS

2.2.1. Tiansient Push Force. As the subjects wereasked'to push horizontally, the horizontal external

EquationForce:

(6)RS,

XD:XDc-. ,"R,

(7)

32 I. CONTROL OF MOVEMENTAND POSTURE

150

zrL 100

zrL

-50

-100

-150

l-H-{

l2Time(s)

FIGURE 4. Tiansient push force. t-eft: F" and F, refer to the antero-posterior and vertical forces applied by the bar az thesubject. Mean curve calculated over seven trials performed by the same sub.iect. The arrow indicates the onset of push force.Right: Absolute peak force values. Means and standard deviations were calculated over seven subjects. ***, p = 0.00 1 (highlysignificant).

FzFx

force (F*) developed during tle transient push eftortwas a measure of the intended act. It could be ob-served that the corresponding force exerted by the baron the subject was negative (Fig. 4). Also, negative ver-tical (F,) forces were developed during the transientpush effort. According to the sign conventions, thepush effort on the bar was directed upwards, as wellas forwards. Both components, F* and F,, increasedprogressively. They displayed the same well-knownforce-time shape (Wilkie, 1950) when they are plottedagainst time, and the horizontal force, F*, peaked at amean value which is almost double F,(-153 + l-24N, as compared to -71 + /-14 N). The F, vs. F*

relationship was exponential (F, : a(t - e-bF')).Thus, task achievement included two force com-

ponents. One, the horizontal force, measured the taskperformance, while the other, the vertical force, ap-peared to be a "by product" ofthe motor act. In accor-dance with previous studies (for a review, see Bouissetand Le Bozec, 2002), F, which is an input ofthe motorsystem, can be considered to provoke a perturbationofbody balance.

2.2.2. Body Dynamics. The equations resultingfrom the biomechanical model included global quan-tities, as well as local ones, measured at the seat andfoot levels. Their time variations yielded during thepush effort were considered and evaluated from ex-perimental data.

2.2.2.1. GL}BALDINAMICS. The time course of the re-sultant reactions originating from the supports (R",R, along the antero-posterior and vertical axes), that

of CoP and CoG displacements (xp and x6 alongthe antero-posterior axis), as well as the reaction (F*)to the horizontal push force (-F*), are displayed inFig. 5.

All the time courses show the same sigmoid profile,to within the sign. It can be observed that when thepush force increased, the reactions originating fromthe supports, \ and R", increased as well, that is

the subject exerted downward and backward effortson the supports. It was also observed that F* (and

&), as well as F, (and R ), displayed opposite signs,in agreement with the action-reaction law (equation(3)): the perturbation applied on the body at the handlevel was instantaneously counter-acted by the reac-tions at the seat and foot levels. Simultaneously, xpdecreased, showing that the CoP moved baclavard.More precisely, CoP unlike CoG displacement wasfound to be great: (- 108 + /-94 mm as comparedto -5 * /-2 mm), and xp - x6 decreased progres-sively, showing that CoP withdrew from CoG.

It was also observed that the onsets of \ (-50 + I-5 ms), R, (-60 + l-7 ms) andxp (-62 * /-6 ms)preceded highly significantly (p < 0.001) the onset ofthe push force increase. In other words, there wereAnticipatory Postural Adjustments (APAs).

In addition, parametric relations were considered(& .rs. F*, fu vs. F", xp vs. F* and p : fu/R, vs.F*). The relationship berween F* and R* establishedthat R* was approximatelyproportional to F- (Fig. 6),as was the relationship berween & "nd

F,. How-ever, systematic, though minor, deviations from thebisector line were observed. In accordance with the

3. ADHERENCE AND POSTURAL CONTROL 33

A 1003xG50

2xlt

EE

G,x;x

2-, 800G,

-50

-100

-150

850

7fi

150

-50

-100

ITlmeF)

ITfitp (s)

FIGURE 5. push force, global reaction forces and centre of pressure time courses. 1" row: Left column: horizontal reaction

io prrffo... (F,); right"column' global reaction forcesat the seat and foot contacts (\ along the antero-posterior axis). 2nd

.o*, f-.f, .ol,r-1, gl"obrl ..".tio.r"forces at the seat and foot contacts (R, along the vertical axis); right column: global CoP

Jirpl"".-.rrr, ("p ird *c along the antero-posrerior axis). The vertical arrow indicates the onset of push force Mean curve

calculated over seven trials performed by the same subject.

equations (3), these discrepancies benveen the actual

profile and the linear one result from inertial forces,

ihat is, body link acceleration (inertial forces : sub-

jectt mass times CoG acceleration). The same result

was obtained when xp was plotted against F*. Thediscrepanry from linearity could plso be attributedto inertial force effects, that is, angular momentumvariations in this instance, according to equation (5).

In other words, inertial forces flowing throughoutthe body chain underlie dynamic phenomena. More

specifically, it can be said that the articulated body

chain was in a state of dynamic equilibrium.Moreover, the subject was in a fixed posture, with

his upper limbs outstretched and his hands grasping

the bai. Therefore, body link accelerations could onlyoriginate from the rest of the body, that is, from the

postural chain (Le Bozec et al., 1997) . The role played

Ly the postural chain was confirmed by the backwarddisplacement of the centre of pressure, correspondingto hip extension. This displacement requires a modi-fication in the distribution of reaction forces between

the body and its supports, which local biomechanics

help to specifr.2.2.2.2. LocAL D\.NAMICS. The Partitive dynamic

method allows a more precise statement of the postu-ral counter-perturbation. To this end, local dynamicswere assessed, that is reaction forces and CoP positionsat the seat and foot levels (Fig.7).

It appeared that the local curves displayed the same

sigmoid profile as the global ones. Howevel this sim-

ilarity held true only to within the sign. Indeed, the

vertical force variations at the seat and foot levels (R5,

and Rs) yielded opposite signs (Fig. 7, middle row)'unlike antero-posterior variations at the same levels

(R5. and Rs).More precisely, vertical reaction forces increased at

the seat level, whereas they decreased at the foot level,

that is, the upper body was pushing on the seat dur-ing the transient push effort' In other words, there is

a transfer ofthe reaction forces to the seat suPPort up

to the end of push, resulting in progressive anchor-

ing of the upper body to the seat. The vertical foot

34 I. CONTROL OF MO\T,MENTAND POSTURE

& = l.cl + o.nl?. Fr

,.o.oq l=o.cop<0.0001

r00

x6EE50sG

x6Ee. 50

sE

E,*ENg.xc.E50N&iG,

5o

x6Efs0;e

ox

50

F, & f. rnax)

50

F, (% Frrna)

50

F" {% F, rnar}

100

Rr, R2= 0.118. O.e8 .Fr

r.o-gs: r2d-!8a

50

Fr(%Frra)r00

1(x)

100

lm

FIGURE 6. Parametric relationships during maximal ramp pushes. The regression lines are represented as a broken line;r: Bravais-Pearson coefficient of correlation. Mean curves calculated over seven trials performed by the same subject, lstrow: Left column: & (global reaction forces albng the antero-posterior axis) plotted against horizontd push force (F-); rightcolumn: R" (global reaction forces along the vertical axis) plotted against vertical push force (F,) 2nd row: Left column:xp (global CoP displacement along the antero-posterior axis) plotted against horizontal push force (F"); right column: p(adherence ratio) plotted against horizontal push force (F-). All the quantities are expressed as a percenrage ofrheir maximalvalue.

reactions favour forward body destabilization, and alsocontribute to CoG antero-posterior acceleration. Inaddition, because they yield an opposite sign to theupper body vertical reactions, lower limb dynamicscontribute to upper body verticd force productionand favour pelvis rotation.

Consequently, the increase in upper body verticalreaction forces and the decrease in lower body forcesreinforce the abiliry to counteract the perturbation in-duced by the push effort, that is, it enhances Posturo-Kinetic Capacity (PKC) (Bouisset and Zattara,1983;Bouisset et al., 2002). In other words, there \Mas a co-ordinated action ofthe upper and lower body.

In addition, R5, and R5, peak values were highlysignificandy greater than the Rp and Rs peak values.AIso, the global reaction forces (( and R,) were nearlyequal to the reactions at the seat contact surface (Rg*

and Rs,). In particular, R5, rras almost equal to R,.Hence the CoP backward displacement at the end oftransient effort, xp, (- I 08 */ - 30 mm) was very closeto xps (-94* I -17 mm). As a consequence, it appearsthat the push effort entails a transfer of the global CoPto the upper body CoP.

It was also observed that the onsets ofR{i e64+l-4 ms), RE (-61+/-7 ms) andxps (64+l-2 ms) preceded very significantly

Rz= 5.07 t O.9O7 , F2

r=0.96r; r2=O.gao

rp= ll.3 + 0.Gl8, Fx

r.o.e8q 12=0.095p.o.061

3. ADHERENCEAND POSTURAT CONTROL 35

^ tsa,zc ,oo

zd,i

c- 50

G

2 600

a

ZGrpui{D

a,

* 2rx!

ox

eoE

E' -soxiB

1 -lmAx

qR-l--. --{

,-r-{

R"

&H-{

q

o lt'o+-l+

tm*ilB J.

1.

i--i--{

.-lxE

ITirne (s)

FIGURE 7. Local reaction forces and centre ofpressure time courses. Left column: Reaction forces and centre ofpressure timecourses. From top to bottom: global reaction forces (R" along the antero-posterior axis) and local reaction forces at the seat(R5.) and foot (R6) levels; global reaction forces (&) and local reaction forces at the seat (R5,) and foot (Rs) levels along thevertical axis; globa1 and local centres ofpressure along the antero-posterior axis (xp, xp5 and xp6)). Mean curve calculated overseven trials performed by the same subject. Middle column: Peak values for the same reaction forces and centres of pressure.Means and standard deviations wore calculated for all seven subjects. ***, p = 0.001 (highly significant). Right column Aand B: Direction of the efforts exerted by the subject on the seat and footrests; C: Displacement of the centres of pressure atthe seat and loot levels.

io

c

(p < 0.0 I ) the push force increase. They also precededvery significantly the onset of Rs-(-61*/-5 ms),Rs,(-60+/-7 ms) and xpsG62+l-2 ms). There-fore, there were Anticipatory Postural Ad;'ustments(APA$, and the APA sequence started at the foot level.Moreover, there was no significant difference berweenthe onsets ofR5,, R5. and xp5.

To summarise: i) upper and lower body actions arecoordinated; ii) upper body dynamics appear to play a

major role in postural stabilization; iii) APAs proceedaccording to a bottom-up sequence.

i. Postural Chain Mobility, A Key Factor

for PerformanceThis paradigm made it possible to manipulate thesurface contacts berween the body and its physicalsupports. For instance, it was easy to reduce the ischio-femoral contact with the seat, from complete con-tact (100 BP) to one-third contact (30 BP), withoutperturbing balance. Indeed, this modification did notchange the overall support contour: the support base

perimeter remained the same (Fig. 8).

*,l"^<-.+-+

i*

eiooillml

s€al lwel

A

.ro I. CONTROI, OF MOVEMENTAND POSTURE

IOO BP 30 BP

e!

8dni

G

cro

z -r*

L -r$

*@E&tx -10ix3ro

2ct$

G

4, 100

E$

---,IIIBP

&-rh-

&-*- -*

&-ta*-

flnp I,BP 'lx,P

30p 'rm :IP

L"...fr-

4l-*3F

ll***-

r@ m tooP rBP tooBP 3Ap {NBP 3)BP 1{XBP IP IUEP 3O8P

FIGURL S. Infuence of postural chain mobiliry on biomechanical variables. Top inset : Schematic representation of complete

and one-third ischio-femoral conracts. 100 BP: complete ischio-femoral contact; 30 BP: one-third ischio-femoral contact.

Bottom : Peak values Left column: Horizontal push forces (F. ) exerted on the subject (first row); antero-posterior g1obal

(() and local reaction forces at the seat (R5-) and foot (R*) levels (second row). Right column: vertical global (&) and

local reaction forces at the seat (R5,) and loot (Re,) levels (first row); global (xo ) and local centres ofpressure along the

antero-posterior axis (xps, xp6) (second row). Means and standard deviations were calculated lor all seven subjects. 100 BP:

compleie ischio-femoral conracr; 30 BP: one-third ischio-femoral contact **: p < 0.01 (very significant); ***: p < 0'001

(highly significant).

'\7hen ischio-femoral contact was reduced, the peak

push force, F*, was very significantly increased (Fig. 8).

This might appear surprising, though only at firstglance. Indeed, performance enhancement was as-

sociated with significant dynamics increases, which

were proven by the maximal values of the globalbiomechanical variables under consideration (\, fuand xp). The local biomechanical variables at the

seat and foot level (Rs,, Rr,; Rs*, Rr,; xps and xpr)

were also increased and yielded the same general

3. ADHERENCE AND POSTUML CONTROL 37

features, when ischio-femoral contact was limited(Fis. 8).

It is known that pelvis mobility is modified bya reduction in the seat contact area from 100 BP to30 BP Indeed, when ischio-femoral contact is limited,such as in the 30 BP posture, the pelvis can rotatewith respect to the seat about an axis passing throughthe contact of the ischiatic tuberosities with the seat,

and, with respect to the thighs about an axis passingthrough the femoral heads (Vandervael, 1956). On theother hand, when ischio-femoral contact is complete,that is in the 100 BP posture, the thighs are in close

contact with the seat and cannot be displaced: thepelvis can only move about an axis passing throughthe femoral heads. Therefore, pelvis mobility is less

in the 100 BP than in 30 BP condition. As a con-sequence, CoP displacement is greater in the 30 BPcondition.

Moreover, according to the PKC theory (Bouisset

and Zattara,, 1983; Bouisset and Le Bozec, 2002),if movement induces a dynamic perturbation, thecounter-perturbation must be dynamic as well. Now,given that transient push ef[orts induce dynamics, thepostural counter-perturbation must also be dynamic,in order to attain the intended performance. Conse-quendy, if postural chain mobiliry is constrained inone way or another, fewer postural segments can beaccelerated, counter-perturbation is limited, and per-formance reduced. In other words, the increased mo-biliry of the postural chain favours postural dynamics,and hence PKC, which produces greater force at theend ofthe effort.

These results genera)ize to ramp efforts those ob-tained by Lino et al. (1992) for pointing movementsperformed under the sarne two support conditions.-{4ren ischio-femoral contact is reduced, performance(that is, maximal velociry in the pointing moyement)increases significantly, in parallel to dynamic postu-ral phenomena. Thus, it does not matter whether theeffort is, according to the physiological terminology,"dynamic" as in the Lino et al. ( I 992) study, or "static"(but "anisotonic") as in this one. In both conditions,the perturbing effect on balance is associated with a

variation of muscular force. tWhen the contact areais reduced, that is, when postural chain mobiliry isgreatet performance is enhanced. In terms of biome-chanics, it can be said that transient efforts are neces-sary for the body system to proceed from the initialto the final mechanical equilibrium, which has beenakeady defined (equations (7) and (8)).

In conclusion, postural compensation to the per-turbation provoked by an effort depends not only onthe support base perimeter, that is the stability area,

but also on postural chain mobiliry that is on the heeplay ofpostural joints. In this study, it is a function ofpelvis and lumbar column mobility. fu a consequence,postural chain mobiliry appears to be a key factor inPKC.

4. Global and Local Adherence RatiosThe adherence ratio has been defined as "frictionuse" (see section l-2).It reflects how the CNS takesinto account the contact forces between the bodyand its physical environment in order to perform themotor act efficiently. In addition, by this very fact,AR corresponds to the ratio of tangential to nor-mal reaction forces at the contact surfaces, and con-sequently to the actual angle of adherence (Fig. 1).Adherence ratios were considered globally, that is, ina whole, or locally, that is, at the foot and seat surface

contacts.

4.1. TMNSIENT PUSH INDUCES ACONTINUOUS INCREASE IN FzuCTION USEIn the earliest instants of push, the global AR (p :&/&) was almost nil, and then increased sharply, upto the peakvalue displayed at the end ofpush (Fig. 9,first row): there was a continuous increase in AR, thatis AR got closer and closer to CoF. Similar results werefound when the local ARs at the seat (ps : Rs*/Rs,)and the foot (p6 : Rn/Ra) supports were considered(Fig. 9, second and third rows).

In addition, the global peak Adherence Ratios(pAR) were highly significantly greater when theischio-femoral contact with the seat was changed,from complete (0.18 +l- 0.03) to one-third (0.21

+ /- 0.02) contact (Fig. l0). The increase was relatedto increases in reaction forces at the seat and foot levels(Fig. 8). Therefore, there was increased "friction use"when postural chain mobiliry was enhanced. Similarresults were reported in the study of pointing tasks inthe same postural conditions (Lino, 1995).

Local pARs yielded the same feature. Indeed, thepAR values at the seat support were also highlysignificantly higher for 30 BP (0.20 * l- 0.02) thanfor 100 BP (0.17 +l- 0.03) (Fig. l0). These val-ues were lower than the coefficient of friction (0.25),which was measured directly at the seat (and footrests)fabric-wood interface. Therefore, a safery margin canbe assumed, in accordance with Johansson and'West-ling (1984). On the other hand, the pARvalues at thefoot (0.29 + I - 0.06 for 30 BP and 0.23 + l- 0.llfor 100 BP) were not significantly different (Fig. 10),and were so close to the CoF, that slipping cannot

38 I. CONTROL OF MOVEMENTAND POSTURE

lOOBP 3OBP

s&.

Ellf

sE.

E,lt

3.

sN

E,\G.I:L

st\Elt

0,20

0,10

0,00

0,20

0,10

0,00

0,20

0,10

0,00

0,50

0,40

0,30

0,20

0,10

0,00

0,20

0,r0

0,00

0,50

0,r()

0,30

0,20

0,10

0.00

s,l!t

tril{

*!

E.

Eilf

2N

150

11ooG. 50

0

2N

150

'- lootr50

0

700 8m750

R".(N)

7W 7*Ro (N)

8m

FIGURE 9. Instantaneous adherence ratio variations. 1" row: Global adherence ratio (fr: &/&, in o/o) as a function oftime.2"d row: adherence ratio at the seat contact surface (p5: Rs,/Rs, in o/o) as a function of time. 3'd row: adherence

ratio at the foor conract surface (p6 : Ri-/Rr,, in o/o) as a function of time. 4d row: tangential component at the seat contactsurface (R5*) plotted against the corresponding normal component (R5,). Mean -f / - one standard deviation was calculatedover seven trids performed by the same subject. 100 BP: complete ischio-femoral contact; 30 BP: one third ischio-femoralcontact The hatched line indicated by an arrow corresponds to the CoF value (0.25).

3. ADHERENCEAND POSTURAI CONTROL 39

0,4

.9

e 0,3oocoo-c 02!t.YGoo

0,1

1OOBP 3OBP

V,7% st.=R,IR.

N p"=R.,/Rs,

ffi pr= R,,/R,,

FIGURE 10. Global and local peak adherence ratios. Global (p : &/R,) and local (ps : Rs*/Rs, and p, : Rr-/Rr,)adherence ratios at the seat (subscribe S) and foot (subscribe f) levels. The coefficient offriction berween the subjects andthe seat (and the footrests) was 0.25. Means and standard deviations were calculated for all seven subjects. 100 BP: completeischio-femoral contact; 30 BP: one-third ischio-femoral contact. ': p > 0.05 (non signi6canr); *: p < 0.05 (significant); **:

p < 0.01 (very significant); ***' p < 0.001 (highly significant).

be excluded at the very end of the push effort, atleast for some subjects, as exemplified in Fig. 9 (thirdrow).

Consequently, the data obtained ar rhe foor sup-port might suggest that the safery margin would berespected only under certairiconditions. Indeed, suchpossibilities could occur when the orders regardingposture were not compatible with the intended task-movement performance and/or with body stabiliry.One can wonder whether this is not the case in theseexperiments, as a lack of contact with the footrestswas shown to favour maximal push force (Gaughranand Dempster, 1956). Moreover, slipping at the footlevel might not be a problem: given that the subjectwas holding the bar, global posture was not insecure.Therefore, local ARs could be supposed to be man-aged with reference to the CoF, but in a different wayaccording to the intended performance and the effectof local slipping on body stability.

4.2. ADHERENCE MIIO INCREASE RESULISFROM SIMULT,{NEOUS INCREASE OFREACTION FORCE COMPONENTSAccording to equation (2), AR is the ratio of & to &,that is, of the instantaneous horizontal ro the verri-cal reactions at the contact surfaces. Consequently, anincrease in AR could result from simultaneous or inde-pendent variations in \ and R,. Simultaneous varia-tions have been reported in various tasks, such as walk-ing (Strandberg, 1983), prehension (Johansson and\flestling, 1984) and pointing (Lino, 1995). Resultson global and local body dynamics (see sections 2.2.2and 2.2.3, as well as Fig. 5 and 7) were in favour ofsuch an assumption.

In order to deepen the question, the instantaneousvariations of the reaction forces ar rhe sear level (R5*

and R5,) were considered, given the major role devotedto the reaction forces at the seat, and consequentlyto upper body dynamics (section 2.2.2.2; FigT and

40 I. CONTROLOF MOVEMENTAND POSTI]RE

Fig. 8). Indeed, global reaction forces (& and &)were found to be nearly equal to the reactions at theseat contact surface (R5* and R5). In addition, theglobal pARwas found to be approximatelyone percentgreater than the local pAR at the seat support for bothischio-femoral conracr conditions (0.18 as comparedto 0.17 for 100 BP; 0.21 as compared to 0.2b for30 BP).

At the beginning of push, the vertical componentR5, increased faster than the horizontal componenrRs" (Fig. 9, fourth row). After the infexion point,both fu* and Rs, conrinued ro increase, but the slope(dRs"/dRsJ decreased. Subsequently, th. Rs* increasewas greater than the R5, one. Hence, the vertical andhorizontal force components displayed sirnultaneousincreases. Therefore, the continuous increase in ARoriginated mainly from a simultaneous and continu-ous increase ofthe reaction forces at the seat (R5* andR5,) during the push effort (Fig. 7). Similar resultshave been reported for pointing tasks by Lino (1995),suggesting that it is not a particular feature. More gen-erally, as simultaneous vertical and horizontal reactionforces were observed, it can be surmised that the stabi-lizing reactions imply that they are modulated in sucha way that the maximal push force is developed withthe aim ofpreventing slipping at the end ofpush, thatis under the guidance of AR.

Lastly, it is interesting to keep in mind that pAR, as

well as maximal external force, were enhanced whenthe coefficient of friction was increased (Kroemer,1974; Grieve, 1979; Gaudez et al.,2003). Therefore,in order to enhance pAR and maximal force, there aretwo possibilities: increase postural chain mobility (see

section 3) andl or increase the CoF ar rhe support sur-faces. In other words, the Maximal Voluntary Force(MVF) does not depend only on the prime movensmaximal force, that is of those muscles that are pri-marily responsible for the intended movemenr. MVFis also limited by the CoF value, which in turn limitsthe AR maximal value, to within a possible Securitymargin. For a given CoR it also depends on posturalconditions, such as the mobiliry of the postural chainand support base perimeter. In other words, posturalfactors limit the maximal effort that the muscles canexert: the capacity to oppose the perturbation pro-voked by the voluntary effort, that is Posturo-KineticCapaciry modulates the intensity of the voluntary ef-fort in order to prevent slipping.

To summarize: i) continuous global as well as localAR increases were observed in the course of the pusheffort up to values which were close to CoF; ii) thevertical and the horizontal reaction forces yieldedsimultaneous increases; iii) the risk of slipping onthe supports during the effort was bounded by the

postural chaint capaciry to afford convenient AR val-ues, insofar as adherence is required to make the pusheffort possible; iv) & variations are assumed tt bemodulated under AR control, that is, in such a way asto prevent slipping at the end of the push.

5. Postural Control and AdhterenceIt is well known that there are manyways to approachmotor control, and thu the complexity of the pro-cess leads to some speculations. This experimental ap-proach provides new data ofa biomechanical order.It is interesting to examine how they help clarify cer-tain aspects of motor control, and in particular theadherence effects on motor programming.

The biomechanical data allowed a description ofthe motor sequence, taking place between initial andfinal static equilibrium. They establish that a contin-uous increase of body dynamics is associated with thecontinuous increase of the push effort: the posturalchain is in a state of dynamic equilibrium. Body dy-namics originate at the footrest level and proceed upto the hand level, according to a bottom-up sequence.A continuous dynamic increase at ischio-femoralcontact is associated with rear pelvis rorarion and CoPdisplacement. In this process, upper body dynamicsappear to play a major, .ho"gh not exclusive, rolefor postural stabilization during the effort. Posturalstabilization depends on postural chain mobiliry thatis, on the free play of postural joints (pelvis and'lowerspine mainly, in these conditions). The adherenceratio increases continuously during the effort, up to avalue, which appears to correspond to the coefficientof friction to within a safery margin, ar least ar rhe seatlevel.

5.1. RAIE OF FORCE RISE, AS API-{NNED VARIABLEAs reviewed by Macpherson (1991), several authorshave proposed that motor act parameters are con-trolled hierarchically. The higherJevel parameterscould be assumed to be global, usually mechanicallydefined, and related to the goals of the movement.They would participare in determining the values ofthe more local lower level variables in any given solu-tion of a motor problem.

According to Bernstein (1935; Amer. tanslation,1967), a motor task evolves a volunrary movement,and is planned in terms of kinematics in the externdCartesian space, that is, in ihe task space. In otherwords, the goal of the planned movemenr is expressedin terms of its path, that is, the displacement of thedp of the distal segment (usuallycalled'tnd-point" or"working point"). To this end, the system should be

3. ADHERENCE AND POSTURAL CONTROL 4t

able to perform an internal simulation of a plannedmovement, where its actual parameters are taken intoaccount. Then, the commands would lead to changesin the activation of the muscles controlling the jointsmobilized by the voluntary movement. \7hile thisviewpoint has been widely adopted, the authors dif-fer as to the relative role devolved to spinal reflexesand central command (see Latash 1993 for a detailedreview).

In this study, there were no "focal" kinematics.Hence, the possible internal simulation could not fol-low any relation between the end-point kinematicvariables. Consequently, one might envision a rela-tion berween some of the variables characterizing theexternal force exerted at the end-point, which couldbe considered as the planned variable. As isometricforces are developed as quickly as possible up ro themaximum, the parameter of the planned motor actwould be the rate of force rise, in accordance withGordon and Ghez (1987).Indeed, these authors haveshown that peak isometric force is achieved by a pro-portipnal modulation of the rate of force rise, whichhas been confirmed by Corcos et al. (1990). Parallelvariations of the peak force and the rate of force risewere also found in transienr push efforts (Le Bozecand Bouisset, 2004).

Even if the postural chain were free ro move in theseexperiments, contrary to the single-joint paradigmsconsidered by Gordon and Ghez (1987) and Corcoset al. (1990), there is no reason to exclude that themotor act is planned in terms of rate of fdrce rise.

5.2. CENTRE OF PRESSURE, AS A POSTURALCONTROLVARIABLEHowever, the role of the postural chain cannot be ig-nored. Indeed, it has also been proposed by Gelfandet al. (1965), revisiting Bernsteint ideas (1935), thatmotor tasks include a focal and a postural compo-nent, one referring to the body segments that aremobilized in order to perform voluntary morremenrdirectly, and the orhe! ro the rest of the body whichis involved in the stabilizing reactions. These defini-tions suggest that the two parts must be conceived as

functional. They transcend simple anatomical parti-tioning, and are assumed to cope in motor control.In this context, the possibiliry of postural control isjustified.

Various postural control variables have been pro-posed in literature, mainly CoG, CoP and Rx, whichwere assumed to be at a lower hierarchical level. Sev-eral authors have suggested that CoG and CoP arepostural control variables for postural tasks (for a re-view, see Horak and MacPherson, 1995). The role ofone or the other is still under discussion (Lacquaniti

and Maioli, 1994), and it is very likely that it will de-pend on the task conditions. On the other hand, au-thors have claimed that the contact forces at the feet,namely the tangential ones, are high-order control pa-rameters, at least for quadruped posture (MacPherson,1988,1991).

In the biomechanical model, which has beenproposed above (equations (3) and (5)), five mainquantities appear to be involved in push efforts (F,, \,R,; xp and x6). These quantities canbe a prioriiderti-fied as control variables, given that the horizontal pushforce, F,, is the planned variable. They are linked bythe three independent equations (3) and (5). In orderto limit the risk of slipping, there is a complementaryinequation, which expresses the no slipping conditionp < p* (relation (2)).

In these experiments, the CoG displacement wasfound to be negligible in contrast to CoP displace-ment (Fig. 5). However, the postural constraints rowhich the subjects have to comply limit the numberand amplitude of the anatomical degrees of freedom,suggesting that the CoG displacement might be onlyvery limited. Therefore, CoP displacemenr appears tobe a better candidate than CoG as a postural controlvariable. In addition, the only possibility for the pos-tural chain to develop a counrer-perturbation to thebalance perturbing push force ( and consequendy roexert a significant push force), originates from a CoPdisplacement, in accordance with the comments onequation (10). Such a contenrion is reinforced by theEMG data: the activation of the pelvis extensor mus-cles (Gluteus Maximus in cooperation with BicepsFemoris), which provokes pelvis backward rotation, isin relation with CoP rear displacement (LeBozec et al.,2001;Le Bozec and Bouisset,2004). In this context, itis interesting to observe that transient push force andCoP displacement presented the same sigmoid timecourse profile (Fig. 5), and that their relationship wasapproximately linear (Fig. 6), which could simpli$,the command.

Once it is admitted that CoP is a postural con-trol variable, the question is to determine the role in-duced by Newtont law and the no slipping conditionon the other three biomechanical quantities (&, &and F,). It has been shown that CoP rear displace-ment results from a coordinated action of the lowerand upper parts ofthe postural chain. A rough out-line of the question points to the major role playedby the pelvis, that is, to rear pelvis rotation. Such a

rotation has been shown to induce an increase in \(Fig. 5), and primarily an increase in R5*, that is, at theseat contact surface (Fig. 7). This increase constirutesa necessary counrer-perrurbation to the destabilizinghorizontal push force, F,, according to equation (3).

42 I. CONTROL OF MO\GMENTAND POSTURF,

Moreover, pelvis rotation is associated with an increasein R" (mainly R5,), whose destabilizing effect wouldbe compensated by F,. In addition, AR is kept un-de^r the CoF (mainly [r5, at rhe seat level), taking thesafery margin into accounr (Fig. 9). This suggests-thatthere is a pairing of the horizontal and vertical reac-tion forces, in order to prevenr slipping at the end ofpush.

If it is admitted that the R5, increase is the result ofpelvis rotation, and that the simultaneous R5, increaseis a biomechanical consequence of this rotation, theps value could be one of the rules for controlling Copdisplacement (equation 11). Hence, the actual ioeffi-cient of friction value might be implemented in themotor program, as it has generally been supposed sincerVestling and Johansson (1984).In this context, it isinteresting ro observe that the relationship benveenglobal AR and transient push force (and consequentlyCoPdisplacement) was approximately linear (Fig. 6),which could simplify the command

Finally, the stabilizing reacrions are actuated in or-der_to integrare sensory information originating inthe body contact surfaces. The forces .*.n.d on thesesurfaces are assumed to be calibrated so as to respecrthe adherence limit. Of the information taken i.rroaccounr, it is generally considered that haptic infor-marion plays a major role (see Wing et al., 1996, for areview). Unfortunately, there are presendy very little,if any, physiological data on ischio-femoral afferenthapdc signals. For the successful elaboration of a mo-tor task, QNS control processes may use feed-forwardmechanisms, which are based on internal models, thatnot only program the action, but also predict devia-tions induced by perturbations, and appropriate re-sponses to resrore the initial plan (Ghez et al., 1995).The APAs, which were reported in this study, confirmfeed-forward postural control. But they do not makeit possible ro semle in favour of rwo parallel controlsresponsible for the intended task movement and re-lated balance stabilization (A-lexandrov rr a1.,2001), ora single control process for a whole-body movement,leading to these two distinct peripheral patterns clas-sified as focal and postural (Latash, 1993; Aruin andLatash, 1995).

To summarize, in the context of a hierarchical orga-nization, it could be proposed that the planned vari-able of the isometric transient effort is the rate of thevoluntary force increase. The displacement of CoP issuggested as being the postural control variable, whichis at a lower hierarchical level. The limit of adherenceratio, with reference to the coefficient offriction, couldbe one of the rules for controlling CoP displacementand muscle activarion in order to accomplish volun-tary ramp effort.

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