Neuroscience Letters, 147 (1992) 1-4
1
© 1992 Elsevier Scientific Publishers Ireland Ltd. All rights reserved 0304-3940/92/$ 05.00
NSL 09075
Do fast voluntary movements necessitate anticipatory postural adjustments even if equilibrium is unstable? P. Nouillot, S. Bouisset a n d M.C. Do Laboratoire de Physiologie du Mouvement, Universit~Paris-Sud, Orsay (France) (Received 9 April 1992; Revised version received 14 August 1992; Accepted 19 August 1992)
Key words: Anticipatory movement; Posture; Balance Anticipatory postural adjustments (APA) were studied in maximum velocity flexion of lower limb from two initial postures, a bipedal stance (Fbu) and unipedal stance (Fuu). In Fbu, the dynamics of center of gravity (CG) and ankle and hip muscle EMG activity showed large APA. In contrast, in Fuu there were no APA, the CG dynamics and the ankle E M G activity started at the same time as the intentional movement while the hip EMG activity started some 30 ms before the thigh flexion. The knee flexion velocity was lower in Fuu than in Fbu (7 rd/s versus 12 rd/s). These results suggest that fast voluntary movements do not require APA when the postural equilibrium is unstable, and that an alternative strategy is used. The absence of APA in Fuu, in contrast to the presence of APA in Fbu, suggests that the postural command and the focal one are time-locked and organized in a parallel process.
Intentional movement is known to be preceded, accompanied and followed by postural phenomena. The most widely studied of the phenomena are the anticipatory postural adjustments (APA) for several categories of intentional movements (cf. refs. 1 and 12). In particular, it has been shown that APA depend on movement parameters such as velocity or load and localization. They also depend on postural parameters. APA are reduced or absent when an external support is given to the subject and increased when the support surface is continuously oscillating, i.e. when more postural stability is required [2]. APA are also increased when the support base perimeter is reduced [18]. To explain these results, it has been suggested that intentional movement is a perturbation of balance, and that a counter-perturbation must be developed to allow efficient movement. The influence of the initial and/or final postures on APA have been studied in movements which involve a transient postural base, such as rising onto toe tips or rocking on heels [11, 13]. The APA are reduced when the subjects are given additional support, and when they are initially inclined forward. Therefore, it has been postulated that APA serve to minimize the subsequent postural destabilization [13]. For lower limb movements, APA depend not only on movement dynamics [4, 15] or Correspondence: M.C. Do, Laboratoire de Physiologie du Mouvement, U.R.A. CNRS 631, Universit6 Paris-Sud, Orsay, France.
balance at the end of the movement [4], but also on experience [14]. This paper examines whether APA depend on balance stability in both the final and initial postures. As APA are themselves a perturbation to the initial equilibrium, the question is whether APA occur even when initial equilibrium is particularly unstable. The subject, in upright posture, performed 2 series of flexion of the right lower limb. The final posture was unipedal, but the initial one was varied. In the first series of tests, the initial posture was bipedal (Fbu), while it was unipedal (Fuu) in the second series, i.e. both the initial and final postures were unstable [6]. In the latter series (Fuu), the right lower leg was slightly flexed so that the foot was just off the ground. The thigh was elevated to the horizontal position as fast as possible following an auditory signal. It was held in the final position for 2-3 s. As in a previous paper [4], the biomechanical variables were recorded together with electromyographic activity (EMG).The components of the center of gravity (CG) acceleration and center of foot pressure (CP) displacement were recorded with a force platform. The amplitude (O) of the knee flexion was recorded with a goniometer and its velocity (O) was obtained by derivation. Preliminary tests have showed that the hip flexion velocity and knee flexion velocity were proportional. The onset of intentional movement, i.e. lower limb movement, was recorded using a mono-axial accelerometer.
The E M G activity of the ipsilateral soleus (SOLi), contralateral tibialis anterior (TAc), ipsilateral sartorius (SARi) and ipsilateral and contralateral tensor fasciae latae (TFLi, TFLc) muscles were recorded by surface bipolar electrodes. The ipsilateral side referred to the moving limb. Because of its flexor actions at both hip and knee joints, SARi can be considered as the prime mover of lower limb flexion, i.e. of the intentional movement [4]. The biomechanical and E M G variables were digitized at a sampling rate of 1 kHz and stored on a hard disk for subsequent analysis. Six subjects performed the experiment under their informed consent. Each series of movements consisted of 10 trials. The rest period between trials was at least 15 s. The time-courses of the biomechanical variables in lower limb flexion from a bipedal stance were similar to those described previously [4, 15]. Global dynamic phenomena, i.e. APA, (Fig. 1, Fbu) occurred prior to the onset of the intentional movement (to), indicated by acFbu to i Ati
f
~G
f b
Fuu tO
20 m/s/s
J "yG
I r
! b
yp
I r
lm/s/s
I
"zG u d
xP
I
2 m/s/s
.10 m
200 'ms
200--'--ms
Fig. 1. Average (10 trials, 1 subject) of the mechanical traces of lowerlimb flexion. Left fig.: Fbu, flexion with initial bipedal stance and final unipedal stance. Right fig.: Fuu, flexion with initial and final unipedal stance. ~G, ~G, ZG: components of the acceleration of center of gravity following the antero-posterior, lateral and vertical axis. xP, yP: components of the displacement of center of pressure following the anteroposterior and lateral axis. Ati: ipsilateral thigh acceleration (anteroposterior directed) to: onset of thigh acceleration, i.e. onset of voluntary movement, f, b, 1, r, u, d: forward, backward, left, right (moving limb), upward and downward, direction of trace variations.
celeration of the thigh. The largest APA occurred in the frontal plane. A clear-cut sequence between biomechanical variables was observed, including an early displacement of CP towards the moving foot (yP trace), and an upward (2G) and leftward (~G) (i.e. towards the stance foot) CG acceleration and then, an antero-posterior CP displacement (xP) and an antero-posterior CG acceleration (~G). The mean APA duration, measured from yP, was 160_+44 ms (individual means: 135+25 ms, 198+29 ms).
The mean peak knee velocity was 11.8_+2.6 rd/s (individual means: 8+0.5 rd/s, 15.8_+0.9 rd/s). This velocity was similar to the velocity reported for the flexion-extension tasks [4]: 11.8+2.3 rd/s for FEbb (initial and final bipedal stances) and 12.1+1.8 rd/s for FEbu (initial bipedal and final unipedal stances). In contrast, when lower limb flexion was performed from a unipedal stance (Fuu), the time-course of biomechanical variables was different and there were no anticipatory dynamic phenomena. The biomechanical traces started at the time of, or after the onset of thigh acceleration (Fig. 1, Fuu). The CG was accelerated forward during the early phase of the intentional movement, in contrast to the Fbu condition. The lateral CG acceleration was very low during the same phase and its time course varied greatly between subjects. Following the vertical axis, the CG (2G) was accelerated upwards and its amplitude was greater than in Fbu. The start of the CP was no longer anticipatory. The xP direction during the early phase was negative, unlike to Fbu. The lateral CP displacement, yP, was also extremely limited during the course of the movement (18+4 mm in Fuu as compared to 98-+21 mm in Fbu). Finally, the mean value of the knee velocity was 6.9-+ 1.9 rd/s (individual means: 3.7+0.4 rd/s, 8.6_+0.8 rd/s). ANOVA indicated significantly lower knee velocity in Fuu than in Fbu (F 1L5:=127, P
1
© 1992 Elsevier Scientific Publishers Ireland Ltd. All rights reserved 0304-3940/92/$ 05.00
NSL 09075
Do fast voluntary movements necessitate anticipatory postural adjustments even if equilibrium is unstable? P. Nouillot, S. Bouisset a n d M.C. Do Laboratoire de Physiologie du Mouvement, Universit~Paris-Sud, Orsay (France) (Received 9 April 1992; Revised version received 14 August 1992; Accepted 19 August 1992)
Key words: Anticipatory movement; Posture; Balance Anticipatory postural adjustments (APA) were studied in maximum velocity flexion of lower limb from two initial postures, a bipedal stance (Fbu) and unipedal stance (Fuu). In Fbu, the dynamics of center of gravity (CG) and ankle and hip muscle EMG activity showed large APA. In contrast, in Fuu there were no APA, the CG dynamics and the ankle E M G activity started at the same time as the intentional movement while the hip EMG activity started some 30 ms before the thigh flexion. The knee flexion velocity was lower in Fuu than in Fbu (7 rd/s versus 12 rd/s). These results suggest that fast voluntary movements do not require APA when the postural equilibrium is unstable, and that an alternative strategy is used. The absence of APA in Fuu, in contrast to the presence of APA in Fbu, suggests that the postural command and the focal one are time-locked and organized in a parallel process.
Intentional movement is known to be preceded, accompanied and followed by postural phenomena. The most widely studied of the phenomena are the anticipatory postural adjustments (APA) for several categories of intentional movements (cf. refs. 1 and 12). In particular, it has been shown that APA depend on movement parameters such as velocity or load and localization. They also depend on postural parameters. APA are reduced or absent when an external support is given to the subject and increased when the support surface is continuously oscillating, i.e. when more postural stability is required [2]. APA are also increased when the support base perimeter is reduced [18]. To explain these results, it has been suggested that intentional movement is a perturbation of balance, and that a counter-perturbation must be developed to allow efficient movement. The influence of the initial and/or final postures on APA have been studied in movements which involve a transient postural base, such as rising onto toe tips or rocking on heels [11, 13]. The APA are reduced when the subjects are given additional support, and when they are initially inclined forward. Therefore, it has been postulated that APA serve to minimize the subsequent postural destabilization [13]. For lower limb movements, APA depend not only on movement dynamics [4, 15] or Correspondence: M.C. Do, Laboratoire de Physiologie du Mouvement, U.R.A. CNRS 631, Universit6 Paris-Sud, Orsay, France.
balance at the end of the movement [4], but also on experience [14]. This paper examines whether APA depend on balance stability in both the final and initial postures. As APA are themselves a perturbation to the initial equilibrium, the question is whether APA occur even when initial equilibrium is particularly unstable. The subject, in upright posture, performed 2 series of flexion of the right lower limb. The final posture was unipedal, but the initial one was varied. In the first series of tests, the initial posture was bipedal (Fbu), while it was unipedal (Fuu) in the second series, i.e. both the initial and final postures were unstable [6]. In the latter series (Fuu), the right lower leg was slightly flexed so that the foot was just off the ground. The thigh was elevated to the horizontal position as fast as possible following an auditory signal. It was held in the final position for 2-3 s. As in a previous paper [4], the biomechanical variables were recorded together with electromyographic activity (EMG).The components of the center of gravity (CG) acceleration and center of foot pressure (CP) displacement were recorded with a force platform. The amplitude (O) of the knee flexion was recorded with a goniometer and its velocity (O) was obtained by derivation. Preliminary tests have showed that the hip flexion velocity and knee flexion velocity were proportional. The onset of intentional movement, i.e. lower limb movement, was recorded using a mono-axial accelerometer.
The E M G activity of the ipsilateral soleus (SOLi), contralateral tibialis anterior (TAc), ipsilateral sartorius (SARi) and ipsilateral and contralateral tensor fasciae latae (TFLi, TFLc) muscles were recorded by surface bipolar electrodes. The ipsilateral side referred to the moving limb. Because of its flexor actions at both hip and knee joints, SARi can be considered as the prime mover of lower limb flexion, i.e. of the intentional movement [4]. The biomechanical and E M G variables were digitized at a sampling rate of 1 kHz and stored on a hard disk for subsequent analysis. Six subjects performed the experiment under their informed consent. Each series of movements consisted of 10 trials. The rest period between trials was at least 15 s. The time-courses of the biomechanical variables in lower limb flexion from a bipedal stance were similar to those described previously [4, 15]. Global dynamic phenomena, i.e. APA, (Fig. 1, Fbu) occurred prior to the onset of the intentional movement (to), indicated by acFbu to i Ati
f
~G
f b
Fuu tO
20 m/s/s
J "yG
I r
! b
yp
I r
lm/s/s
I
"zG u d
xP
I
2 m/s/s
.10 m
200 'ms
200--'--ms
Fig. 1. Average (10 trials, 1 subject) of the mechanical traces of lowerlimb flexion. Left fig.: Fbu, flexion with initial bipedal stance and final unipedal stance. Right fig.: Fuu, flexion with initial and final unipedal stance. ~G, ~G, ZG: components of the acceleration of center of gravity following the antero-posterior, lateral and vertical axis. xP, yP: components of the displacement of center of pressure following the anteroposterior and lateral axis. Ati: ipsilateral thigh acceleration (anteroposterior directed) to: onset of thigh acceleration, i.e. onset of voluntary movement, f, b, 1, r, u, d: forward, backward, left, right (moving limb), upward and downward, direction of trace variations.
celeration of the thigh. The largest APA occurred in the frontal plane. A clear-cut sequence between biomechanical variables was observed, including an early displacement of CP towards the moving foot (yP trace), and an upward (2G) and leftward (~G) (i.e. towards the stance foot) CG acceleration and then, an antero-posterior CP displacement (xP) and an antero-posterior CG acceleration (~G). The mean APA duration, measured from yP, was 160_+44 ms (individual means: 135+25 ms, 198+29 ms).
The mean peak knee velocity was 11.8_+2.6 rd/s (individual means: 8+0.5 rd/s, 15.8_+0.9 rd/s). This velocity was similar to the velocity reported for the flexion-extension tasks [4]: 11.8+2.3 rd/s for FEbb (initial and final bipedal stances) and 12.1+1.8 rd/s for FEbu (initial bipedal and final unipedal stances). In contrast, when lower limb flexion was performed from a unipedal stance (Fuu), the time-course of biomechanical variables was different and there were no anticipatory dynamic phenomena. The biomechanical traces started at the time of, or after the onset of thigh acceleration (Fig. 1, Fuu). The CG was accelerated forward during the early phase of the intentional movement, in contrast to the Fbu condition. The lateral CG acceleration was very low during the same phase and its time course varied greatly between subjects. Following the vertical axis, the CG (2G) was accelerated upwards and its amplitude was greater than in Fbu. The start of the CP was no longer anticipatory. The xP direction during the early phase was negative, unlike to Fbu. The lateral CP displacement, yP, was also extremely limited during the course of the movement (18+4 mm in Fuu as compared to 98-+21 mm in Fbu). Finally, the mean value of the knee velocity was 6.9-+ 1.9 rd/s (individual means: 3.7+0.4 rd/s, 8.6_+0.8 rd/s). ANOVA indicated significantly lower knee velocity in Fuu than in Fbu (F 1L5:=127, P
