Posture, Dynamic Stability, and Voluntary Movement
Posture, Dynamic Stability, and Voluntary Movement
Posture, Dynamic Stability, and Voluntary Movement
REVIEW/MISE AU POINT
KEYWORDS Summary This paper addresses the question of why voluntary movement, which induces a
Voluntary movement; perturbation to balance, is possible without falling down. It proceeds from a joint biomechanical
Postural stabilization; and physiological approach, and consists of three parts. The first one introduces some basic
Postural chain; concepts that constitute a theoretical framework for experimental studies. The second part
Newton’s laws; considers the various categories of ‘‘postural adjustments’’ (PAs) and presents major data on
Dynamic stability; ‘‘anticipatory postural adjustments’’ (APA). The last part explores the concept of ‘‘posturo-
Anticipatory postural kinetic capacity’’ (PKC) and its possible applications.
adjustments (APAs); © 2008 Elsevier Masson SAS. All rights reserved.
Postural
programming;
Posturo-kinetic
capacity (PKC);
Foco-kinetic capacity
(FKC);
paraplegics;
Lower limb amputees
Résumé Cet article aborde la question de savoir pourquoi le mouvement, qui induit une per-
MOTS CLÉS turbation de l’équilibre corporel, s’avère possible sans entraîner de chute. On se situera dans
Mouvement une double perspective associant approche biomécanique et physiologique. La première partie
volontaire ; introduit quelques concepts de base servant de cadre théorique aux études expérimentales. La
Stabilisation seconde considère les diverses catégories d’« ajustements posturaux » et présente les princi-
pales données concernant les « ajustements posturaux anticipateurs » (APA). Enfin, la troisième
∗ Corresponding author.
E-mail address: simon.bouisset@u-psud.fr (S. Bouisset).
0987-7053/$ – see front matter © 2008 Elsevier Masson SAS. All rights reserved.
doi:10.1016/j.neucli.2008.10.001
346 S. Bouisset, M.-C. Do
posturale ; partie explore le concept de « capacité posturo-cinétique » (CPC, ou PKC en anglais) et ses
Chaîne posturale ; possibles applications.
Lois de Newton ; © 2008 Elsevier Masson SAS. All rights reserved.
Stabilité dynamique ;
Ajustements
posturaux
anticipateurs (APAs) ;
Programmation
posturale ;
Capacité
posturocinétique
(CPC) ;
Capacité
fococinétique (CFC) ;
Paraplégiques ;
Amputés de membre
inférieur
Figure 2 Hess’s model. The model includes three components, each being represented by a figure: the jumper (1) is standing
on the bearer’s shoulders (2), and the bearer is secured by a supporter (3) during the jump. The jumper represents the voluntary
movement, while the bearer and the supporter represent the ‘‘static’’ and ‘‘dynamic’’ equilibration reactions. Left column (a, b,
c): the task is performed efficiently, owing to adequate postural dynamics, and a ‘‘static’’ posture is recovered at the end of the
jump. Right column (d, e, f): the task is not performed efficiently, as the subject falls at the end of jump: postural stabilization is
not efficient enough to counteract transient disequilibrium (from [43]).
tion between internal and external forces in such a way that where the vertical forces resultant is applied. Now, CoG hor-
performance fulfils task-movement constraints. izontal acceleration has been shown to be proportional to
Support base and support reaction forces. Contrary to the difference between CoP and CoG horizontal displace-
gravity, reaction forces originating from the ground (and ments [21]. Therefore, CoG horizontal acceleration may be
more generally from the physical environment) vary as a reduced when the support base perimeter is limited and
function of the forces developed by the muscles during the when the CoP initial position is close to the support base
motor act. Therefore, a clear understanding of the postural boundaries (Fig. 4).
phenomena associated with voluntary movement requires
taking the external forces into account, and the support The human body, as a system of rigid solids
reaction forces in particular. This necessity requires con- The human body is composed of anatomical ‘‘segments’’,
sidering the ‘‘support base’’, i.e. the body areas in contact including bones and the living tissues around them, such as
with the physical support. the arm or the leg. From a biomechanical point of view,
Support base characteristics depend on two factors, the it can also be considered a system of rigid solids, termed
physical properties of the support and the interface between ‘‘links’’ [28], which are movable in relation to each other,
the body and the support. The first depends on the proper- i.e. they form an ‘‘articulated chain’’ and interact. The
ties of the materials (elasticity, friction, etc.), as well as on mobility of an articulated chain is a function of the num-
geometrical parameters (flatness or curvature, inclination, ber of the degrees of freedom and of the range of individual
etc.) and design (steadiness or oscillations, translations, joint movements.
etc.). The second depends on the subject’s posture (stand- Postural maintenance is an ‘‘active’’ process. A certain
ing, on tiptoe, on the entire foot, on one foot or both feet, equilibrium state, often termed ‘‘static’’, is associated to
seated, with or without a backrest, etc). They also depend given posture maintenance. According to Newton’s laws, it
on the support base perimeter. corresponds to the condition when the resultant reaction
Biomechanical principles indicate that support base char- and the resultant torque of the external forces are nil. This
acteristics have an effect on the reaction forces applied necessity implies two sets of requirements.
to the body (and on their torque). For example, by con- The first are global requirements: they are the same
struction, the support base perimeter delimits Centre of as if the body were an invariant structure, i.e. as if the
Pressure (CoP) displacement, insofar as the CoP is the point body segment configuration were mechanically constrained.
Posture, dynamic stability, and voluntary movement 349
Figure 3 Illustration of Newton’s laws in shoulder abduction performed in two support base conditions. A. Top inset: Newton’s
laws, (1): ‘‘dynamic resultant law’’, (2) ‘‘kinetic torque law’’. Eq. (1) relates the external forces resultant to the dynamic resultant,
which is the center of gravity (CoG) linear acceleration times body mass (m). Eq. (2) relates the resultant moment of external forces
to the kinetic moment, which is the angular acceleration times the body moment of inertia (JG ) with respect to the CoG. A: lower
inset: when the subject is at rest (˙ Fe = 0 and ˙ M G (Fe ) = 0), for instance during quiet standing, he is subjected to gravity (P) and
ground reaction (R) forces. One is applied to the center of gravity (Go), and the other to the center of pressure (Io). The center of
pressure (CoP) is the barycenter of the minute ground reaction forces applied to the subject’s contact areas, such as Ri . B (middle
inset), upper part: the feet are in contact with the ground. During shoulder abduction, there is a displacement of the CoG, from Go
to G1, and of the CoP, from Io to I1. B. Lower part: external forces balance sheet: during the movement (˙ Fe = G (Fe ) =
/ 0 and ˙ M / 0),
the subject is always subjected to gravity (P) and ground reaction (R) forces. However, the ground reaction forces display a vertical
(RN ) and a horizontal (RT ) component, which vary at each instant. C. Right inset, upper part: the feet are no longer in contact
with the ground and gravity force is nil (as in a space lab), the upper limb and the rest of the body move in opposite directions.
The body’s center of gravity no longer moves (G1 = Go) and there is no longer a center of pressure, as there is no contact between
the subject and the ground. Right inset, lower part: external forces balance sheet: the subject is no longer subjected to any force
(from [11]).
They result in the usual contention according to which gen- mechanical action exerted on any body point is transmit-
eral body balance is secured when the projection on the ted with no time lag to all the others. In particular, the
ground of the centre of gravity is situated within the support forces (and torques) developed during the movement are
base perimeter. The second are local requirements, which transmitted from the segment(s) that the subject intention-
are usually understated. They result from the articulated ally mobilizes to others, and from the distal one(s) to the
structure of the human body: each body part must also be support surface(s), according to the ‘‘action and reaction
balanced with respect to the underlying ones. Owing to the principle’’.
configuration of the joint contact surfaces and the shape of The initial conditions of movement are defined by body
bones, purely mechanical factors cannot fulfill this require- posture, which is usually ‘‘static’’, such as standing, sitting,
ment. This is why appropriate muscular forces, originating crouching, etc. Therefore, the dynamic phenomena implied
from equilibration reactions, are necessary even in a nat- by movement—–i.e. inertial forces (and torques)—–tend to
ural upright posture, where the soleus is the emblematic perturb balance.
muscle.
Therefore, postural maintenance proceeds from an Equilibrium state and dynamic stability
‘‘active’’ process, insofar as it results from muscle con- According to Martin [58,64], ‘‘posture denotes first the
traction, in addition to inanimate ‘‘passive’’ forces, such shape of the body, i.e. the geometrical relationship of the
as gravity. different parts to each other’’. In order to prevent any risk
Voluntary movement is a perturbation to posture. Fur- of misunderstanding, ‘‘posture’’ would be limited to the
thermore, the body segments are rigid, and each one ‘‘static’’ whole body configuration, i.e. to postural main-
is closely linked to the adjacent ones. Therefore, every tenance.
350 S. Bouisset, M.-C. Do
Figure 6 Shoulder flexion kinetics time courses. Left inset: Schematic representation of the shoulder flexion paradigm. Right inset:
From left to right: OUF, unilateral shoulder flexions with no additional inertia; IUF, unilateral shoulder flexions with an additional
inertia; BF, bilateral shoulder flexion. From top to bottom: Rx, Ry and ARz, antero-posterior, lateral and vertical components of the
resultant force (Rz = Rz — P, with P being the weight of the subject; plus sign corresponds respectively to forward, right-to-left and
upward forces); Mz, resultant torque about the vertical axis (positive sign corresponds to a moment which tends to rotate the body
from the right side to the left); Awi, tangential acceleration of the upper limb recorded at wrist level; to and te correspond to the
onset and the end of the upper limb movement. Force platform data recorded on one subject performing upper limb movements,
under three experimental conditions. For each type of movement, records of five trials were superimposed by synchronizing them on
the onset of Awi. Dynamic phenomena can be observed before the onset (APAs) and after the end (CPAs) of the voluntary movement,
that is, before and after upper limb acceleration. Moreover, there is a morphological difference between the shape of the upper
limb acceleration time course and whole body kinetics (that is, center of gravity acceleration). If the body were rigid, a complete
and instantaneous transmission of the forces created by the voluntary movement would be observed. This hypothesis can be ruled
out since the time courses of the body’s biomechanical variables were different from that of the upper limb, which ascertains SPAs
(from [17]).
port base allows interpretation of the results without any APAs: movements which precede movement
risk of misinterpretation related to unexpected support base In order to present APAs, it is easier to refer first to unilateral
effects. shoulder flexions, performed with the upper limb stretched
out and on a single support base. Indeed, as it is a very simple
APA measurement paradigm, one can suppose that the postural mechanisms,
As it is well-known, the CNS triggers muscle excitation, which are involved, can guide the interpretation of more
which in turn induces muscle activation, and hence mus- complex experimental situations. Nevertheless, a compar-
cle contraction after a certain time (‘‘electro-mechanical ison with locomotion can be helpful, in that locomotion is
delay’’: EMD). Then, movement can occur and biomechani- a lower limb movement, including a periodic support base
cal variables can be recorded. transfer.
Therefore, there are various possibilities for measuring A given sequence of EMGs was observed in the lower limbs
APAs. APAs can be defined with respect to EMGs, i.e. they are and trunk muscles before the onset of the Anterior Del-
measured by the time interval between postural and focal toïdeus EMG, which is the PPM (Fig. 7). First, the ipsilateral
muscle EMG onset. APAs can also be defined with respect Soleus is deactivated 50 to 100 ms prior to PPM onset. Then
to biomechanical variables, i.e. the time interval between there is a sequence of phasic activations and deactivations,
global dynamics (or postural movement) onset and focal starting with activation of the contralateral Tensor Fasciae
movement onset. Of course, it is also possible to define APAs Latae and Rectus Femoris. This sequence is reproducible and
by the time interval between the onset of a biomechani- specific to the task under consideration [85].
cal quantity and an EMG onset. This method raises various A sequence of segmental accelerations corresponds to
problems, such as the location and parameter of the two the EMG sequence. Thus, before the onset of upper limb
signals. Also, the influence of electro-mechanical delay on acceleration, i.e. PMS onset, the lower limbs, pelvis, trunk
the results, which differs according to the muscles and the and shoulder display anticipatory accelerations (refer later
task parameters, cannot be disregarded. on to Fig. 9): the existence of accelerations indicates that
Last, there is a technical problem, which cannot be anticipatory activities are actually movements. More pre-
underestimated: how to estimate postural and focal onset. cisely, there is a given order in anticipatory accelerations,
Indeed, special care must be taken in determining EMG starting with the contralateral lower limb and hip, the
onset and cessation. Also, it is less accurate to consider dis- ipsilateral ones, and ending with the ipsilateral shoulder:
placements rather than accelerations, because of the slower it follows a bottom-up progression, as the postural seg-
variation rate at the onset of movement. ment accelerations proceed from the support surface. The
Posture, dynamic stability, and voluntary movement 353
Figure 7 EMG patterns in upper limb and lower limb movements. Top inset: electrodes location. Left inset: EMG activities of
lower limbs and pelvis recorded for shoulder flexion. From left to right: OUF, unilateral flexions with no additional inertia; IUF,
unilateral flexions with additional inertia; BF, bilateral flexions. From top to bottom: activities of anterior portion of Deltoideus
(DA), Erectores Spinae (ES), Gluteus Maximus (GM), Tensor Fasciae Latae (TFL), Semitendinosus (ST), Rectus Femoris (RF), Vastus
Lateralis (VL), Tibialis Anterior (TA), Soleus (SOL). Ipsilateral (i) and contralateral (c) activities with respect to the moving limb are
displayed in opposite directions. EMG activities are recorded for one subject performing the three types of movements. For each
type of movement, rectified and smooth EMG activities of five trials were superimposed, by synchronizing records on the onset of
DAi activities (dotted line). (From [94]) Right inset: EMG activities of lower limbs and pelvis recorded for gait initiation. From top
to bottom: activities of anterior portion of Soleus (SOL), Tibialis Anterior (TA), Rectus Femoris (RF), Semitendinosus (ST). Right (R)
and left (L) activities (gait is initiated with the right foot). TS: Soleus deactivation onset; tTA: Tibialis Anterior activation onset;
tE: ipsilateral Soleus burst onset; tRF: ipsilateral Rectus Femoris burst onset; tHO: onset of heel-off; tC: heel contact; tTO: onset
of the second step. Rectified and smooth EMG activities of five trials were superimposed by synchronizing recordings on the onset
of heel-off (tHO) (from [20]).
accelerometric sequence is also reproducible and specific anticipatory accelerations. The anticipatory sequence starts
to the task under consideration. Since these results are at the stance leg, then at both hips and at the ipsilateral
consistent with EMG ones, the EMGs could be related to shoulder, and afterwards at the contralateral shoulder [19].
local accelerations: the local movements are not ‘‘passive’’ Neck and head displacements in addition to trunk displace-
movements. ments and their corresponding EMGs have also been reported
In gait initiation, the anticipatory EMG sequence (Fig. 7) [81]. Finally, all the local accelerations indicate that the
includes Soleus deactivation, followed by activation of both movement is accelerated forwards and towards the stance
Tibialis Anteriors 60 to 110 ms later. Then, the Soleus of foot.
the forthcoming swing limb, which is the PPM of the heel- Here again, the sequence follows a bottom-up progres-
off (HO), is activated. Thereafter, EMG bursts occur in the sion. But, as the postural segment accelerations proceed
Rectus Femoris of the swing limb and in the Semitendi- from the support surface, it seems more appropriate to con-
nosus of the stance limb [20]. In addition, before heel-off sider that the progression order follows a ‘‘posturo-focal
(HO) (i.e. PMS onset), the pelvis, trunk and shoulder display gradient’’, i.e. proceeds from the support base (ground,
354 S. Bouisset, M.-C. Do
seat, etc... according to the initial posture) to the postural is executed by the contralateral limb (Fig. 9). This char-
chain, and then to the focal one. acteristic is called ‘‘dynamic asymmetry’’ as opposed to
Finally, APAs are postural movements. They precede vol- ‘‘dynamic symmetry’’, corresponding to bilateral movement
untary movement. And like voluntary movement, they are a [16]: in terms of posturo-kinetic programming, it is not
dynamic phenomenon. equivalent to raise both upper limbs simultaneously, or one
or the other upper limb. Therefore, since by definition,
APAs occur before the onset of voluntary movement, it can
APAs and postural programming be said that dynamic asymmetry and dynamic symmetry
According to Bernstein’s ideas [9], it is assumed that PAs are ‘‘pre-programmed’’. The organization of the postural
constitute a part of the motor program. In other words, it sequence according to the forthcoming focal limb(s) is a gen-
is admitted that there is a ‘‘postural task’’ associated with eral feature. For example, gait initiation displays a mirror
the focal one. APAs constitute a valuable tool in identifying organization according to the stepping leg, i.e. a dynamic
task parameters, which are programmed. Indeed, since by asymmetry pattern, whereas it is a dynamic symmetry pat-
definition, APAs precede the onset of focal movement, they tern in standing jump initiation.
cannot result from re-afferentation triggered by the focal Finally, APAs are dynamic, polarized, and task specific.
movement: they are ‘‘pre-programmed’’. If APAs occur in They are programmed in relation not only to the focal
the postural chain, it can be said that the postural chain is movement parameters per se, but more generally to task-
programmed. If APAs vary in relation to task parameters, it movement parameters. In addition, they are ‘‘adaptable’’,
can be assumed that the postural chain is programmed in as they depend on the functional state of the motor system,
relation to these parameters. which will be discussed later on.
Different specific parameters have been identified, such
as ‘‘velocity, load, direction’’, and orientation in space’’
(for a review, see [13]). For instance, in gait initiation, APAs Dual role of APAs
are graded as a function of movement peak velocity (Fig. 8). APAs as a counter-perturbation. The role of APAs may
A similar result has been found in shoulder flexion, and the be proposed on the basis of an analysis of forces acting
curvature of the relationship increases with additional iner- at the shoulder level and at the body’s centre of gravity
tia [14]. Moreover, APAs depend on ‘‘postural conditions’’, (Fig. 10), whose directions are opposite. From this analysis,
as will be discussed later on. it can be assumed that ‘‘APAs tend to create inertial forces,
In addition, it must be stressed that APAs are not only which, when the time comes, will counterbalance the dis-
a function of the specific task-movement parameters. They turbance to postural equilibrium due to the intentional
also depend on its location with respect to the body’s axes forthcoming movement’’ [15]. According to this hypothesis,
of symmetry. Indeed, the role played by the homonymous the intentional movement is considered a ‘‘perturbation’’,
muscles and segments is inverted, when the movement in accordance with the ideas put forward by several neu-
Figure 8 Gait initiation. Left part: Biomechanical quantities time course. From top to bottom: XG: forward (F) antero-posterior
CoG displacement (dotted line) and Xp: backward (B) antero-posterior CoP displacement (solid line) with ax being the peak
amplitude; Yp and YG: CoP and CoG lateral displacements to the left (L) and to the right (R); ŸG : right (R) and left (L) CoG
lateral acceleration; X G : forward (F) and backward (B) CoG antero-posterior acceleration; X’G : CoG antero-posterior velocity. to:
onset of kinetic phenomena; tHO; heel off; tV: peak velocity progression (V) instant. The movement is executed at spontaneous
speed with the left foot. It can be observed that CoP and CoG displacements precede tHO. In particular, the CoP first moves backward
and toward the forthcoming stepping foot, while the CoG moves in the opposite direction. More precisely, there is a rapid unloading
of the forthcoming swing limb with an equivalent loading of the stance limb. (From 21). Right part: relationship between APA indices
and progression velocity. APA peak amplitude (ax) and duration (tHO) are plotted against the peak of progression velocity (V) (from
[21]).
Posture, dynamic stability, and voluntary movement 355
Figure 9 Dynamic asymmetry and symmetry. Left part: Local accelerations in shoulder flexions. From left to right: OUF, unilateral
flexions with no additional inertia; IUF, unilateral flexions with additional inertia; BF, bilateral flexions. From top to bottom: Awi,
tangential acceleration of the upper limb measured at wrist level (plus sign corresponds to the acceleration phase of the movement);
Ash, Atr, Ah, At and As, antero-posterior accelerations measured at level of shoulders, trunk, hips, thighs and shanks (plus sign
corresponds to forward acceleration); i and c, ipsilateral and contralateral accelerations with respect to the moving limb. Antero-
posterior local accelerations are reported from one subject performing the three types of movement. For each type of movement,
accelerations of five trials were superimposed, by synchronizing records on the onset of Awi (dotted line). (From 94). Right part:
direction of anticipatory local accelerations. From left to right: unilateral flexions performed by the right upper limb (UFR), the
left upper limb (UFL) and both upper limbs simultaneously (BF). Notice that APA direction is inverted when the forthcoming moving
upper limb is the left instead of the right (from [84]).
rologists (see, for instance [1]), which were first revisited In lower limb movements involving a transient change
by Belenkii et al. [7]. In other words, APA represents a of the support base perimeter, such as lower limb flexion,
‘‘counter-perturbation’’, which would be responsible for APAs have been shown to be related not only to the counter-
the stabilizing action. perturbation of the forthcoming movement, but also to
In this context, it was proposed that one consider that the ‘‘body weight transfer’’ onto the forthcoming stance foot
postural perturbation, associated with voluntary movement, [32]. This transfer can be interpreted as being related to
as well as the counter-perturbation, might be characterized postural stabilization, as well as to the necessary unloading
by two biomechanical factors [16]. They correspond to the of the forthcoming moving foot.
two vectors to which the perturbation force system may be In locomotion, there is a succession of balance losses and
reduced, i.e. its resultant and its torque (in reference to the recoveries, which corresponds to the periodic support base
body’s centre of gravity, for example). One is linear, while transfer. It has been demonstrated that during gait initia-
the other is rotational. Both may include three orthogonal tion, the greater the forward fall of the centre of gravity,
components. the faster the progression velocity [21]: APAs induce postural
However that may be, as APAs are triggered by a feed- destabilization, which is necessary to initiate gait. In addi-
forward command and are specific to the characteristics of tion, the initiation phase also includes body weight transfer
the motor task, they have to be determined from previous towards the forthcoming stance foot, which can be inter-
knowledge of its perturbing effect [84]. More precisely, APAs preted as being related to postural stabilization.
are programmed according to the expected perturbation and APAs as a perturbation and a counter-perturbation.
not to the actual one [77]. Finally, it cannot be excluded that APAs play a dual role
APA as a perturbation. The contention according to which simultaneously, i.e. to perturb balance and to counteract
APAs represent a counter-perturbation has been argued the perturbation, as suggested by gait initiation: acceler-
within the framework of segmental movements performed ations occur at the ipsilateral hip at heel-off, the forward
on a continuous support base. But, the support base can also direction of which is opposite to the hip perturbation at this
change transiently, as in lower limb movements, or peri- instant [30].
odically, as in locomotion. These movements will provoke Indeed, just as in shoulder flexion initiation, lower-limb
a perturbation of the whole kinematic chain, but the cor- forward displacement at heel-off will induce a perturbation
responding APAs have been shown to play a more complex at the hip joint, which is directed backwards. It must be
role. compensated in order to prevent the hip from being pushed
356 S. Bouisset, M.-C. Do
In the second experiment, the voluntary movement vated simultaneously, at the expense of greater forces to
(a flexion or a flexion-extension of the lower limb) was be developed during the movement itself. In other words,
performed while various initial and final postures were the lack of APAs suggests that postural stabilization is
considered [32,66]. The general trend was an increase in a major constraint instead of muscular energy minimiza-
APA duration and a decrease in maximal velocity when tion.
the initial and final postures were less stable: PKC was
reduced in relation to postural stability. Further, APAs were PKC and the functional state of the motor system
absent when the subjects stood on one foot, from the As stated above, performance results from a hierarchi-
beginning of the lower limb movement to the end [66]: cal process, associating three different components, i.e.
as APAs induce dynamics, they can potentially perturb the central nervous, muscular, and bone and joint systems
body balance, and are no longer present when the pos- (Fig. 1). More, or less, efficient performance depends on
ture is excessively unstable. Therefore, in conditions of their functional state, which is not easy to assess, either
high stability demand, the CNS may reduce and even sup- clinically or scientifically. Nevertheless, it is agreed that it
press APAs, as protection against their possible destabilizing varies according to impairment or rehabilitation, condition-
effects. Then, the focal and the postural chains are acti- ing or fatigue, development or ageing [51].
Figure 12 EMG pattern in paraplegic (Pa), as compared to able-bodied (Va) subjects, performing a pick-and-place task. Top
inset Schematic representation of the experimental situation: Biomechanical traces of able-bodied and paraplegic subjects. Rz*:
vertical component of the normalized resultant of forces (vertical acceleration of the center of gravity); plus sign (+) indicates
upward acceleration; b1: first peak amplitude; tz: onset time. Rx*: horizontal component of the normalized resultant of forces
(horizontal acceleration of the center of gravity); plus sign (+) indicates forward acceleration; a1 and a2: first and second peak
amplitudes; tx: onset time; d1 and d2: duration of the first and second phase of voluntary movement. to and te: onset and end of load
transport. Notice that the load transport duration is greater for Pa than for Va. Superimposition of five records (from [31]). Bottom
inset: Location of bipolar surface electrodes for recording EMG activities. From top to bottom: EMG activities of Latissimus Dorsi
(LD), Serratus Anterior (SA), Trapezius Medialis (TM), Trapezius Superior (TS), Erectores Spinae (ES), which cannot be activated by
Pa patient; subscript R and L indicate right and left respectively. Rx* is the antero-posterior component of the normalized resultant
of forces. to and te are the onset and the end of the forward load transport. Superimposition of five records of rectified EMGs.
Posture, dynamic stability, and voluntary movement 359
For instance, it has been reported that APAs last longer in velocity was slower than in the able-bodied controls, but
the elderly [46,49,57,75] and in hemiplegics [46], whereas that APAs lasted much longer [31]. Thus, PKC is less in para-
maximal movement velocity is reduced. Also, APAs van- plegics: PKC = 3.4, as compared to PKC = 9 for able-bodied
ish when people are bedridden for several weeks [40], subjects.
and in Parkinsonians [5,29,18]. Moreover, postural patterns Postural synergy is also different: other scapular girdle
are changed in Parkinsonians, hemiplegics, etc. Therefore, and trunk muscles are activated, especially on the contralat-
sensory-motor impairments diminish the functional state, eral side (Fig. 12). If it is assumed that the primary synergy
i.e. ‘‘sensory-motor capacity’’, which is the resultant of PKC corresponds to an optimization criterion, the transition from
and FKC. one synergy to another can be assumed to correspond to a
In other words, PKC assessment in sensory-motor impair- less economical process.
ments requires consideration of those impairments, which In other words, paraplegics, whose equilibrium is more
result in a restriction of joint mobility, regardless of the unstable than able-bodied subjects, could not develop PAs
exact origin (bones, joints, tendons, muscles or nervous sys- adapted to the perturbation provoked by the motor task
tem). It is particularly interesting to discuss paraplegics and without an excessive balance risk: the perturbation, and
lower-limb amputees. hence the required performance, should be reduced. There-
Paraplegia and PKC. In paraplegics performing a pick- fore, due to PKC reduction, voluntary movement is less
and-place task, it was established that maximal movement efficient.
Figure 13 Gait initiation by above-knee amputees. Top: biomechanical quantities time course. From left to right: above-knee
amputee subject initiating gait with the prosthetic limb (A) and with the sound limb (B), and able-bodied subject (C). Ground
reaction forces were recorded from two force platforms. to: onset of biomechanical quantities variation; FO: time of foot-off;
APA: anticipatory postural adjustment phase; EXE: swing phase; tVm: time of maximum velocity, Fx: antero-posterior ground reac-
tion force recorded on the side of the stepping limb (dashed line) and on the side of the stance limb (solid line); ˙Fx: sum of
antero-posterior reaction forces. f: forward; b: backward. (From 68). Bottom: PKC cost in above-knee and below-knee amputees
and controls. From left to right: Amputation (Amp.) level: above-knee (AK), below-knee (BK); performance: maximum speed of
progression; APA duration; PKC: ratio between speed and APA duration. It can be noted that PKC depends on the level of amputa-
tion. When the prosthetic limb is the stance limb, PKC is lower in the AK amputee, and PKC is higher in the lower amputation (BK
amputee). Moreover, when the stance limb is the sound limb, PKC is comparable between AK and BK amputees, but still lower than
controls. This gap suggests that other factors could contribute to the PKC (from [61] results).
360 S. Bouisset, M.-C. Do
Lower limb amputation and PKC. Similarly, unilateral [9] Bernstein N. Coordination and regulation of movements. Perg-
lower limb amputees showed many kinetic changes in gait amon Press; 1935 (Amer. translation, 1967).
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as in amputees when the stance limb is the sound one
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