Do fast voluntary movements necessitate anticipatory postural adjustments even if equilibrium is unstable?

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<0.0001). The local motor organization could be analyzed from the E M G recordings (Fig. 2). For Fbu, the first muscle to be activated was the SOLi. It preceded to by 204+40 ms (individual means: 263_+54 ms, 141_+33 ms) and the onset of the lateral CG acceleration (~G) by 38+18 ms. The SOLi E M G profile showed 2 levels of activation, the first of low magnitude, and the second of high magnitude occurring approximately 30~,0 ms prior to to. The early SOLi activation was associated with postural requirements, while the second was correlated with the initiation of voluntary movement. The TAc was activated 149_+67 ms prior to to (ranging, 239_+46 ms to 62_+50 ms). The strong activation was followed by relative silence just after to. The TFLi was activated just after TAc, on average 131_+57 ms prior to to (individual means: 214 + 24 ms, 55-+ 11 ms). The initial low activity was followed by a

Fuu

Fbu tO

tO

TFLc

TFLi I

I 1 mv SARi

TAc

I I

SOLi I

I m

200 ms

200 ms

Fig. 2. Average (10 trials, 1 subject) of rectified EMG activity of lower limb muscles in lower-limb flexion (same subject as in Fig. 1). Left fig.: Fbu, flexion with initial bipedal stance and final unipedal stance. Right fig.: Fuu: flexion with initial and final unipedal stance. TFLc, TFLi, SARi, TAc, SOLi: ipsilateral (i) and contralateral (c) EMG activities of Tensor Fascia Latae, Sartorius, Tibialis anterior and Soleus. Ipsilateral is the moving limb. to: onset of thigh acceleration, i.e. onset of voluntary movement.

strong sustained activity. Then SARi was activated 94___39 ms prior to t o (individual means: 150+42 ms, 53+ 19 ms). The TFLc was the last muscle to be activated, 43_+31 ms prior to to (individual means: 65_+46 ms; 18_+9 ms). These results suggest that both contralateral and ipsilateral muscles are involved as well in postural dynamics, given that the electromechanical delay was usually 20-40 ms. For Fuu, the muscles were activated just prior to or at the same time as the intentional movement. Approximately 30 ms prior to to, the TFLc, TFLi and SARi (one postural muscle and 2 movement muscles) were activated (TFLc: 27_+14 ms; TFLi 22+11 ms and SARi: 20-+8 ms). The SOLi was activated later (7+13 ms prior to to) and TAc was the last to be activated (0-+19 ms with respect to to). The early activation of TAc and SOLi did not occur in Fuu, and the SOLi activation level was reduced and more steady. The main result was that APA were large when the lower limb flexions were performed from an initial bi-

pedal posture and absent when they were performed from an initial unipedal posture. The muscular pattern, and markedly, the biomechanical pattern were significantly modified in Fuu. Moreover, the results for Fbu [4] stressed that APA in movements involving modification of support base perimeter are not only related to the intentional movement (more precisely, the velocity of lower limb flexion), but also to the transfer of body weight towards the future stance foot, that is to the stability. The absence of APA has been reported for very slow movement [8, 16] and when the trunk is initially inclined toward its final position [11, 13]. Even if the movement velocity in Fuu represents 60% of the control value (Fbu), Fuu still corresponds to rather fast movement, inducing inertial forces, as confirmed by the thigh acceleration profile (ATi, Fig. 1). Consequently, this lower limb dynamic may significantly perturb body balance [15]. There are 2 possibilities. Either APA existed in Fuu but have not been observed, or there are actually no APA when the initial posture is unipedal. Indeed, long-lasting anticipatory postural adjustments (termed preparatory postural adjustments in ref. 5) have been reported by refs. 3 and 7. They include body inclination in the direction of the forthcoming CG displacement and precede the dynamic anticipatory postural adjustments. Thus slow trunk forward displacements could occur in Fuu so as to enhance the equilibrium stability. According to preliminary data (Nouillot et al., to be published), the trunk accelerations in Fuu started at the same time as the onset of the intentional movement (i.e. there are no APA). In contrast, there are forward and leftward (towards stance foot) anticipatory accelerations in Fbu. Therefore, there are no APA in Fuu. But, as APA depended on the velocity of movement [15] and on postural stability [4], APA should be expected in Fuu, because of the velocity of movement. As this is not the case, a change in postural strategy can be assumed. This could be induced by the precariousness of the initial equilibrium state [6]. As has been suggested elsewhere [1], APA are themselves perturbation to balance and their presence would be hazardous when balance is poor. In this case, another strategy could be developed, as in compensatory adjustment to platform manoeuvre [9]. The absence of APA in Fuu suggests that the postural command is time-locked to the focal one. In so far as the temporal reciprocal sequencing of postural and focal command is shifted in Fuu as compared to Fbu, these commands are probably organized in a parallel process as suggested by [10].

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