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Are there anticipatory segmental adjustments associated with lower limb flexions when balance is poor in humans?
Neuroscience Letters 279 (2000) 77±80 www.elsevier.com/locate/neulet
Are there anticipatory segmental adjustments associated with lower limb ¯exions when balance is poor in humans? P. Nouillot, M.C. Do*, S. Bouisset Laboratoire de Physiologie du Mouvement, Universite Paris-Sud, 91405 Orsay, France Received 3 August 1999; received in revised form 15 November 1999; accepted 25 November 1999
Abstract For a leg raising task performed in a sagittal plane, it has been shown that body balance instability can suppress anticipatory postural adjustments (APAs). The aim of this study was to determine whether the global (centre of mass) postural adjustments were replaced by local (segmental) ones, which were compensating each other and resulting in a lack of global APAs. Six healthy subjects must perform a lower limb ¯exion from two initial postures, corresponding to a bipedal (Fbu) and an unipedal (Fuu) stance. Kinematics of postural adjustments were recorded with accelerometers. The results showed that in Fbu the kinematics have large durations of APAs, contrary to Fuu where there are none. They showed also that during the voluntary movement the magnitudes of the segmental postural accelerations were equal or superior in Fuu than in Fbu on the anteroposterior and lateral axes, where balance is poor. Also while, on the contrary, the magnitudes are reversed on the vertical axis. These results suggest that ®rstly: (1) the absence of APAs can correspond to a strategic response for weak postural base con®guration and secondly; (2) the local postural accelerations, depending to the axes, are linked to two different functions: to maintaining the balance and to performing the focal movement. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Anticipatory postural adjustment; Balance; Motor synergy
It is well established that anticipatory postural adjustments (APAs), which are dynamic phenomena, depend on type of movement and postural parameters. For example, the APA durations or magnitudes, associated with upper or lower limb movements, increase with velocity of the forthcoming voluntary movement [3,12,18] or when the support base perimeter is reduced [19] or unstable [8,13]. In lower limb movements, APAs depend on the initial [15] and ®nal postural [10] balance, and also on experience [14]. To explain these results, it has been suggested that the dynamics of the intentional movement is a perturbation to postural equilibrium, and that a counterperturbation must be developed to allow ef®cient movement. Thus, it has been hypothesized that APAs serve to minimize the subsequent postural destabilization [4]. However, it was recently established that APAs lacked [15] when lower limb ¯exion is performed from an unipedal stance. To interpret this result, it has been suggested that APAs could perturb postural equilibrium because they are dynamic phenomena [5]. Thus, their absence could represent an adaptive adjustment by * Corresponding author. E-mail address: [email protected] (M.C. Do)
the central nervous system in movements performed in postural instability as on a narrow support area [1], in Parkinson's disease [2], or in unipedal stance [17]. In these studies the APAs were characterized by global biomechanical variables, i.e. dynamics of centre of feet pressure and/or centre of mass. Therefore, it could not be excluded that APAs could occur at some local level [15]. However, if there are no local anticipatory postural phenomena then to preserve equilibrium an other segmental postural organization might be supposed. This assumption is supported by postural synergies evidenced resulting from perturbation of upright standing on normal or altered ground [13], or in studies where a transient modi®cation of the postural base occurs [3,14]. Segmental motor strategies were reported to change, at the hip and trunk levels. However, in these studies APAs were still observed at the trunk level. Now, when lower limb is involved in the intentional movement such as in lower limb ¯exion or gait initiation, the perturbation to body balance is ®rst applied to the postural hip through the pelvis [7,9], i.e. to the lower part of the trunk. As trunk is an important element for global body equilibrium due to its inertia, its control yields a major factor in the posturo-kinetic organization. It was why it seemed
0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 9 9) 00 94 7- 7
78
P. Nouillot et al. / Neuroscience Letters 279 (2000) 77±80
worthwhile to consider conjointly trunk and postural hip movements. The aim of this research was to determine whether an unstable body balance, i.e. body balance in an unipedal stance, entails a different postural organisation when lower limb ¯exion is performed. In other words, ®rst: (1) are APAs present at the trunk level even when global APAs are lacking, and is a different local postural organization associated to lower limb ¯exion when the initial support base is poor? and second; (2) as the APAs serve to minimize the subsequent destabilization, the magnitude of postural kinematic must be increased if they are absent. To this end, the posturo-kinetic organization of trunk and hip was assessed by the accelerations. Six subjects performed the experiment under their informed consent. The subject, in upright posture and with upper limbs along the body, performed two series (2 £ 10 trials) of right lower limb ¯exions (focal movement) at maximal velocity following an auditory signal. The lower limb ¯exion was to elevate the thigh to a horizontal position and to hold it for 2±3 s. In the ®rst series, the initial posture was bipedal and the ®nal posture was unipedal (Fbu). In the second one, the initial and ®nal postures were unipedal (Fuu). In Fuu, the right lower leg was initially slightly ¯exed so that the foot was just off the ground. The local kinematics was recorded by three accelerometers, two tri-axial and one mono-axial (ENTRAN, ECG D, ^5 g). One tri-axial accelerometers was ®xed accurately to the trunk at the xyphoõÈd point, and the other at the contralateral stance hip, accurately at the coxo-femoral level. The mono-axial accelerometer was ®xed on the right thigh, above the knee, and was used to date the onset of the lower limb movement. All the accelerometers were mounted on appropriately shaped splints. The biomechanical variables were digitized at a sampling rate of 1 kHz and stored on a PC hard disk for subsequent analysis. The statistic methods used were paired t-test. The signi®cant differences were considered when P , 0:05. When lower limb ¯exion was performed from a bipedal stance (Fbu), local anticipatory postural accelerations occurred prior to the onset of the lower limb ¯exion, t0 (Fig. 1), as had been shown previously for global dynamics. The earliest APAs occurred at trunk level along the lateral axis (Atry) (Table 1), whose aim was to project the body mass towards the supporting foot and to release the moving lower limb from its postural function [15,16]. In contrast, in the Fuu stance there were no more anticipatory local phenomena. The onset of the trunk and hip patterns of accelerations started simultaneously with the onset of thigh acceleration (Table 1). Regardless of the presence or lack of APAs, the patterns of trunk's accelerations are similar in the three axes for the two conditions, whereas it is not the case for the stance hip. It was also observed that the signs of anticipatory (Fbu) and early (Fuu) trunk' accelerations according to the three reference axes are similar to the corresponding features of the
Fig. 1. Accelerometric traces of the trunk and the contralateral hip. Left. Fbu: ¯exion with initial bipedal stance and ®nal unipedal stance. Right. Fuu: ¯exion with initial and ®nal unipedal stance. Ati: ipsilateral thigh acceleration (dating to onset of focal movement). Atrx, Atry and Atrz: anteroposterior, lateral and vertical trunk accelerations. Ahcx, Ahcy and Ahcz: anteroposterior, lateral and vertical contralateral hip accelerations. t0: onset of thigh acceleration, i.e. onset of voluntary movement. t1: end of thigh acceleration, i.e. end of voluntary movement. f, b, l, r, u and d: forward, backward, left, right (moving limb), upward and downward, direction of trace variations. (average of seven trials for one subject).
centre of mass accelerations, as it has been previously reported [15]. Such results cannot be considered surprising, owing to the important mass of the trunk which amounts (head and upper limbs included) to more than 65% of the whole body mass. For the stance hip, on the lateral axis, the direction of accelerations in the beginning were in the opposite direction between the two tasks. It was leftward accelerated, i.e. towards the forthcoming stance foot, for Fbu, whereas it was systematicaly rightward oriented for Fuu, i.e. towards the moving foot. At last, on the antero-posterior and vertical axes, the patterns of stance hip accelerations were variable between conditions and between subjects. On the vertical
P. Nouillot et al. / Neuroscience Letters 279 (2000) 77±80
79
Table 1 Kinematic APAs durations a
Atrx Atry Atrz Ahcx Ahcy Ahcz
Fbu (mean ^ SD) (ms)
Fuu (mean ^ SD) (ms)
t-test (n 6)
271 ^ 34 2127 ^ 35 2137 ^ 5 2129 ^ 28 246 ^ 68 2105 ^ 43
15 ^ 13 17 ^ 26 17 ^ 20 21 ^ 9 114 ^ 19 116 ^ 7
P , 0.01 P , 0.01 P , 0.001 P , 0.001 NS P , 0.01
a The values correspond to the durations between the local acceleration onsets and the voluntary movement onset (t0). The negative sign signi®es that the local acceleration precedes t0. Atr, Ahc: respectively, accelerations of the trunk and contra-lateral hip; x, y, z: respectively antero-posterior, lateral and vertical directions. Fbu: ¯exion with initial bipedal stance and ®nal unipedal stance; Fuu: ¯exion with initial and ®nal unipedal stance. The comparisons between the two conditions are obtained with the paired t-test. Mean (of subjects) and standard deviations (SD) are expressed in milliseconds.
one, in the early part of the acceleration, after a very weak magnitude displayed a large peak which was oriented upwards, i.e. in the direction of focal movement. During the voluntary movement, with the aim of accounting for representative parameters of imbalance, the peak to peak magnitudes of segmental accelerations were measured. A large difference between subjects was observed. Consequently, the comparison between the two conditions was made on the difference of magnitude, for each of the three perpendicular axes. The only signi®cant differences between the two conditions were observed on the anteroposterior axis for the contralateral hip (*Ahcx) and the vertical axis for the hip and trunk (*Ahcz and *Atrz) (Fig. 1; Table 2). In other words, the accelerations in Fuu were larger than in Fbu on the anteroposterior axis for the contra lateral hip, contrary to the vertical axis on which they were smaller. The ®rst main result is the absence of local APAs when the lower limb ¯exion was performed from an initial unipedal posture (Fuu), whereas they were present when it was performed from a bipedal posture (Fbu). Thus, local kinematics data showed similar results to those of global kinematics previously reported [15]. Therefore, it is established that there are really no local APAs and the hypothesis according to which there are local APAs compensating each others can be discarded. It had been previously argumented [10] that APAs in Fbu are necessarily programmed, because they varied according to the ®nal balance. In Fuu, in so far as the onset of global and local postural kinematics were simultaneous of intentional ¯exion movement, it could be supposed that these onsets of postural adjustments were also programmed, at least for the ®rst 20±25 ms [2]. The second main result concerns the magnitude of postural accelerations accompanying the voluntary movement. We had hypothesized that, if the APAs were absent, the magnitude of postural kinematics would be increased when the stance was unipedal (Fuu), in comparison to bipedal (Fbu). The results showed that it was partly the case. Indeed, the amplitude of the local postural accelerations were equivalent in the two postural conditions or greater in Fuu on the lateral and antero-posterior axes, i.e. in the
horizontal plane where balance is more precarious, whereas the voluntary movement being performed in a para sagittal plane the perturbing forces had to be oriented along the three axes. The signi®cant difference is located at the stance coxo-femoral joint on the antero-posterior axis. The results concerning the equivalence of magnitude between the two postural conditions can be explained from the performance of the voluntary movement. The velocity of the voluntary movement is signi®cantly lower by 40% in Fuu comparatively with Fbu [15]. As APA durations or magnitudes vary with velocity [3,12,18], one can think that the reduction of the velocity can have as effect the reduction of the postural perturbation, because of inertial forces which are likely much less. Thus, we can hypothesize that the lack of APAs in Fuu excludes the possibility to minimize in advance the postural perturbation. In other words, the lack of APAs do not allow the limitation of the local postural accelerations on the plane of imbalance. However, on the vertical axis, on the contrary to the other axes, the magnitudes of the accelerations are greater in Fbu. We can suppose that on this axis no balance requirement needs to be preserved, because, for example, the displacements of the centre of mass remain on the support area. In an experiment presented by [11], concerning a movement of an elbow ¯exion, the results strengthen this hypothesis. In this Table 2 Difference of local acceleration magnitudes between Fbu and Fuu a
Atrx *Ahcx Atry Ahcy *Atrz *Ahcz
Mean ^ SD (ms 22)
t-test
0.27 ^ 2.59 4.32 ^ 3.20 0.97 ^ 3.46 0.28 ^ 1.54 8.64 ^ 7.96 7.23 ^ 3.37
NS P , 0.05 NS NS P , 0.05 P , 0.01
Fbu , Fuu Fbu . Fuu Fbu . Fuu
a Atr: accelerations of the trunk; Ahc: accelerations of the contra-lateral hip; x, y, z: respectively, antero-posterior, lateral and vertical directions. The comparisons between the two conditions are obtained with the paired t-test. Mean (of subjects, n 6) and standard deviations (SD) are expressed in ms 22.
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P. Nouillot et al. / Neuroscience Letters 279 (2000) 77±80
study, it was shown that the accelerations of the voluntary movement were compensated on the antero-posterior axis but not on the vertical one. In the task of lower limb ¯exion, we can be hypothesized that on the vertical axis the accelerations are not likely linked to the perturbations of balance, but be linked rather to the performance. In conclusion, the results strengthen the hypothesis which had been previously proposed [5]. As APAs are dynamic phenomena, they can perturb balance and might be absent when balance is poor, i.e. when the postural base con®guration does not allow to the body a suf®ciently stable balance [6]. [1] Aruin, A.S., Forrest, W.R. and Latash, M.L., Anticipatory postural adjustments in conditions of postural instability. Electroenceph. clin. Neurophysiol., 109 (1998) 350±359. [2] Balzagette, D., Zattara, M., Bathien, N., Bouisset, S. and Rondot, P., Postural adjustments associated with rapid voluntary arm movements in patients with Parkinson's disease. Adv. Neurol., 45 (1986) 371±374. [3] BeÂraud, P. and GaheÂry, Y., Relationships between the force of voluntary leg movements and the associated postural adjustments. Neurosci. Lett., 194 (1995) 170±177. [4] Bouisset, S. and Zattara, M., A sequence of postural movements preceedes voluntary movement. Neurosci. Lett., 22 (1981) 263±270. [5] Bouisset, S., Relation entre support postural et mouvement intentionnel: approche biomeÂcanique. Arch. Int. Physiol. Biophys., 99 (1991) A77±A92. [6] Bouisset, S., Do, M.C. and Zattara, M., Posturo-kinetic capacity assessed in paraplegics and parkinsonians, XIth International Symposium of the Society for Postural and Gait Research, Portland. In M. Wollacott and F. Horak (Eds.), Posture and Gait: Control Mechanisms, Vol. II, University of Oregon Books, Oregon, 1992, pp. 19±22. [7] Cappozzo, A., The forces and couples in the human trunk during level walking. J. Biomech., 16 (1983) 265±277. [8] Cordo, P.J. and Nashner, L.M., Properties of postural move-
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ments related to a voluntary movement. J. Neurophsiol., 47 (1982) 287±303. Dietrich, G., BrenieÁre, Y. and Do, M.C., Organization of local anticipatory movements in single step initiation. Human Mov. Sci., 13 (1994) 195±210. Do, M.C., Nouillot, P. and Bouisset, S., Is balance or posture at the end of voluntary movement programmed? Neurosci. Lett., 130 (1991) 9±11. Friedli, W.G., Coen, L., Hallett, M., Stanhope, S. and Simon, S.R., Postural adjustments associated with rapid voluntary arm movements. II. Biomechanical analysis. J. Neural. Neurosurg. Psychiatry, (1988) 232±243. Horak, F.B., Esselman, P.E., Anderson, M.E. and Lynch, M., The effects of movement velocity, mass displaced and task certainty on associated postural adjustments made by normal and hemiplegic individuals. J. Neurol. Neurosurg. Psychiatry, 47 (1984) 1020±1028. Horak, F.B. and Nashner, L.M., Central programming of postural movements: adaptation to altered support-surface con®gurations. J. Neurophysiol., 55 (1986) 1369±1381. Mouchnino, L., Aurenty, R., Massion, J. and Pedotti, A., StrateÂgies de controÃle simultane de l'eÂquilibre et de la position de la teÃte pendant l'eÂleÂvation d'une jambe. C.R. Acad. Sci. Paris, 312 (1991) 225±232. Nouillot, P., Bouisset, S. and Do, M.C., Do fast voluntary movements necessitate anticipatory postural adjustments even if equilibrium is unstable? Neurosci. Lett., 147 (1992) 1±4. Rogers, M. and Pai, Y.C., Dynamic transitions in stance support accompanying leg ¯exion movements in man. Exp. Brain Res., 81 (1990) 398±402. Troop, H. and Odenrik, P., Postural control in single limb stance. J. Ortho. Res., 6 (1988) 833±839. Zattara, M. and Bouisset, S., In¯uence de la vitesse d'exeÂcution du mouvement volontaire sur les acceÂleÂrations locales anticipatrices. In ReÂsume descommunications, 8 (1983) 113±114. Zattara, M. and Bouisset, S., Posturo-kinetic organization during upper limb movement performed from varied foot positions. Neurosci. Lett., 22 (1987) 1969.
Are there anticipatory segmental adjustments associated with lower limb ¯exions when balance is poor in humans? P. Nouillot, M.C. Do*, S. Bouisset Laboratoire de Physiologie du Mouvement, Universite Paris-Sud, 91405 Orsay, France Received 3 August 1999; received in revised form 15 November 1999; accepted 25 November 1999
Abstract For a leg raising task performed in a sagittal plane, it has been shown that body balance instability can suppress anticipatory postural adjustments (APAs). The aim of this study was to determine whether the global (centre of mass) postural adjustments were replaced by local (segmental) ones, which were compensating each other and resulting in a lack of global APAs. Six healthy subjects must perform a lower limb ¯exion from two initial postures, corresponding to a bipedal (Fbu) and an unipedal (Fuu) stance. Kinematics of postural adjustments were recorded with accelerometers. The results showed that in Fbu the kinematics have large durations of APAs, contrary to Fuu where there are none. They showed also that during the voluntary movement the magnitudes of the segmental postural accelerations were equal or superior in Fuu than in Fbu on the anteroposterior and lateral axes, where balance is poor. Also while, on the contrary, the magnitudes are reversed on the vertical axis. These results suggest that ®rstly: (1) the absence of APAs can correspond to a strategic response for weak postural base con®guration and secondly; (2) the local postural accelerations, depending to the axes, are linked to two different functions: to maintaining the balance and to performing the focal movement. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Anticipatory postural adjustment; Balance; Motor synergy
It is well established that anticipatory postural adjustments (APAs), which are dynamic phenomena, depend on type of movement and postural parameters. For example, the APA durations or magnitudes, associated with upper or lower limb movements, increase with velocity of the forthcoming voluntary movement [3,12,18] or when the support base perimeter is reduced [19] or unstable [8,13]. In lower limb movements, APAs depend on the initial [15] and ®nal postural [10] balance, and also on experience [14]. To explain these results, it has been suggested that the dynamics of the intentional movement is a perturbation to postural equilibrium, and that a counterperturbation must be developed to allow ef®cient movement. Thus, it has been hypothesized that APAs serve to minimize the subsequent postural destabilization [4]. However, it was recently established that APAs lacked [15] when lower limb ¯exion is performed from an unipedal stance. To interpret this result, it has been suggested that APAs could perturb postural equilibrium because they are dynamic phenomena [5]. Thus, their absence could represent an adaptive adjustment by * Corresponding author. E-mail address: [email protected] (M.C. Do)
the central nervous system in movements performed in postural instability as on a narrow support area [1], in Parkinson's disease [2], or in unipedal stance [17]. In these studies the APAs were characterized by global biomechanical variables, i.e. dynamics of centre of feet pressure and/or centre of mass. Therefore, it could not be excluded that APAs could occur at some local level [15]. However, if there are no local anticipatory postural phenomena then to preserve equilibrium an other segmental postural organization might be supposed. This assumption is supported by postural synergies evidenced resulting from perturbation of upright standing on normal or altered ground [13], or in studies where a transient modi®cation of the postural base occurs [3,14]. Segmental motor strategies were reported to change, at the hip and trunk levels. However, in these studies APAs were still observed at the trunk level. Now, when lower limb is involved in the intentional movement such as in lower limb ¯exion or gait initiation, the perturbation to body balance is ®rst applied to the postural hip through the pelvis [7,9], i.e. to the lower part of the trunk. As trunk is an important element for global body equilibrium due to its inertia, its control yields a major factor in the posturo-kinetic organization. It was why it seemed
0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 9 9) 00 94 7- 7
78
P. Nouillot et al. / Neuroscience Letters 279 (2000) 77±80
worthwhile to consider conjointly trunk and postural hip movements. The aim of this research was to determine whether an unstable body balance, i.e. body balance in an unipedal stance, entails a different postural organisation when lower limb ¯exion is performed. In other words, ®rst: (1) are APAs present at the trunk level even when global APAs are lacking, and is a different local postural organization associated to lower limb ¯exion when the initial support base is poor? and second; (2) as the APAs serve to minimize the subsequent destabilization, the magnitude of postural kinematic must be increased if they are absent. To this end, the posturo-kinetic organization of trunk and hip was assessed by the accelerations. Six subjects performed the experiment under their informed consent. The subject, in upright posture and with upper limbs along the body, performed two series (2 £ 10 trials) of right lower limb ¯exions (focal movement) at maximal velocity following an auditory signal. The lower limb ¯exion was to elevate the thigh to a horizontal position and to hold it for 2±3 s. In the ®rst series, the initial posture was bipedal and the ®nal posture was unipedal (Fbu). In the second one, the initial and ®nal postures were unipedal (Fuu). In Fuu, the right lower leg was initially slightly ¯exed so that the foot was just off the ground. The local kinematics was recorded by three accelerometers, two tri-axial and one mono-axial (ENTRAN, ECG D, ^5 g). One tri-axial accelerometers was ®xed accurately to the trunk at the xyphoõÈd point, and the other at the contralateral stance hip, accurately at the coxo-femoral level. The mono-axial accelerometer was ®xed on the right thigh, above the knee, and was used to date the onset of the lower limb movement. All the accelerometers were mounted on appropriately shaped splints. The biomechanical variables were digitized at a sampling rate of 1 kHz and stored on a PC hard disk for subsequent analysis. The statistic methods used were paired t-test. The signi®cant differences were considered when P , 0:05. When lower limb ¯exion was performed from a bipedal stance (Fbu), local anticipatory postural accelerations occurred prior to the onset of the lower limb ¯exion, t0 (Fig. 1), as had been shown previously for global dynamics. The earliest APAs occurred at trunk level along the lateral axis (Atry) (Table 1), whose aim was to project the body mass towards the supporting foot and to release the moving lower limb from its postural function [15,16]. In contrast, in the Fuu stance there were no more anticipatory local phenomena. The onset of the trunk and hip patterns of accelerations started simultaneously with the onset of thigh acceleration (Table 1). Regardless of the presence or lack of APAs, the patterns of trunk's accelerations are similar in the three axes for the two conditions, whereas it is not the case for the stance hip. It was also observed that the signs of anticipatory (Fbu) and early (Fuu) trunk' accelerations according to the three reference axes are similar to the corresponding features of the
Fig. 1. Accelerometric traces of the trunk and the contralateral hip. Left. Fbu: ¯exion with initial bipedal stance and ®nal unipedal stance. Right. Fuu: ¯exion with initial and ®nal unipedal stance. Ati: ipsilateral thigh acceleration (dating to onset of focal movement). Atrx, Atry and Atrz: anteroposterior, lateral and vertical trunk accelerations. Ahcx, Ahcy and Ahcz: anteroposterior, lateral and vertical contralateral hip accelerations. t0: onset of thigh acceleration, i.e. onset of voluntary movement. t1: end of thigh acceleration, i.e. end of voluntary movement. f, b, l, r, u and d: forward, backward, left, right (moving limb), upward and downward, direction of trace variations. (average of seven trials for one subject).
centre of mass accelerations, as it has been previously reported [15]. Such results cannot be considered surprising, owing to the important mass of the trunk which amounts (head and upper limbs included) to more than 65% of the whole body mass. For the stance hip, on the lateral axis, the direction of accelerations in the beginning were in the opposite direction between the two tasks. It was leftward accelerated, i.e. towards the forthcoming stance foot, for Fbu, whereas it was systematicaly rightward oriented for Fuu, i.e. towards the moving foot. At last, on the antero-posterior and vertical axes, the patterns of stance hip accelerations were variable between conditions and between subjects. On the vertical
P. Nouillot et al. / Neuroscience Letters 279 (2000) 77±80
79
Table 1 Kinematic APAs durations a
Atrx Atry Atrz Ahcx Ahcy Ahcz
Fbu (mean ^ SD) (ms)
Fuu (mean ^ SD) (ms)
t-test (n 6)
271 ^ 34 2127 ^ 35 2137 ^ 5 2129 ^ 28 246 ^ 68 2105 ^ 43
15 ^ 13 17 ^ 26 17 ^ 20 21 ^ 9 114 ^ 19 116 ^ 7
P , 0.01 P , 0.01 P , 0.001 P , 0.001 NS P , 0.01
a The values correspond to the durations between the local acceleration onsets and the voluntary movement onset (t0). The negative sign signi®es that the local acceleration precedes t0. Atr, Ahc: respectively, accelerations of the trunk and contra-lateral hip; x, y, z: respectively antero-posterior, lateral and vertical directions. Fbu: ¯exion with initial bipedal stance and ®nal unipedal stance; Fuu: ¯exion with initial and ®nal unipedal stance. The comparisons between the two conditions are obtained with the paired t-test. Mean (of subjects) and standard deviations (SD) are expressed in milliseconds.
one, in the early part of the acceleration, after a very weak magnitude displayed a large peak which was oriented upwards, i.e. in the direction of focal movement. During the voluntary movement, with the aim of accounting for representative parameters of imbalance, the peak to peak magnitudes of segmental accelerations were measured. A large difference between subjects was observed. Consequently, the comparison between the two conditions was made on the difference of magnitude, for each of the three perpendicular axes. The only signi®cant differences between the two conditions were observed on the anteroposterior axis for the contralateral hip (*Ahcx) and the vertical axis for the hip and trunk (*Ahcz and *Atrz) (Fig. 1; Table 2). In other words, the accelerations in Fuu were larger than in Fbu on the anteroposterior axis for the contra lateral hip, contrary to the vertical axis on which they were smaller. The ®rst main result is the absence of local APAs when the lower limb ¯exion was performed from an initial unipedal posture (Fuu), whereas they were present when it was performed from a bipedal posture (Fbu). Thus, local kinematics data showed similar results to those of global kinematics previously reported [15]. Therefore, it is established that there are really no local APAs and the hypothesis according to which there are local APAs compensating each others can be discarded. It had been previously argumented [10] that APAs in Fbu are necessarily programmed, because they varied according to the ®nal balance. In Fuu, in so far as the onset of global and local postural kinematics were simultaneous of intentional ¯exion movement, it could be supposed that these onsets of postural adjustments were also programmed, at least for the ®rst 20±25 ms [2]. The second main result concerns the magnitude of postural accelerations accompanying the voluntary movement. We had hypothesized that, if the APAs were absent, the magnitude of postural kinematics would be increased when the stance was unipedal (Fuu), in comparison to bipedal (Fbu). The results showed that it was partly the case. Indeed, the amplitude of the local postural accelerations were equivalent in the two postural conditions or greater in Fuu on the lateral and antero-posterior axes, i.e. in the
horizontal plane where balance is more precarious, whereas the voluntary movement being performed in a para sagittal plane the perturbing forces had to be oriented along the three axes. The signi®cant difference is located at the stance coxo-femoral joint on the antero-posterior axis. The results concerning the equivalence of magnitude between the two postural conditions can be explained from the performance of the voluntary movement. The velocity of the voluntary movement is signi®cantly lower by 40% in Fuu comparatively with Fbu [15]. As APA durations or magnitudes vary with velocity [3,12,18], one can think that the reduction of the velocity can have as effect the reduction of the postural perturbation, because of inertial forces which are likely much less. Thus, we can hypothesize that the lack of APAs in Fuu excludes the possibility to minimize in advance the postural perturbation. In other words, the lack of APAs do not allow the limitation of the local postural accelerations on the plane of imbalance. However, on the vertical axis, on the contrary to the other axes, the magnitudes of the accelerations are greater in Fbu. We can suppose that on this axis no balance requirement needs to be preserved, because, for example, the displacements of the centre of mass remain on the support area. In an experiment presented by [11], concerning a movement of an elbow ¯exion, the results strengthen this hypothesis. In this Table 2 Difference of local acceleration magnitudes between Fbu and Fuu a
Atrx *Ahcx Atry Ahcy *Atrz *Ahcz
Mean ^ SD (ms 22)
t-test
0.27 ^ 2.59 4.32 ^ 3.20 0.97 ^ 3.46 0.28 ^ 1.54 8.64 ^ 7.96 7.23 ^ 3.37
NS P , 0.05 NS NS P , 0.05 P , 0.01
Fbu , Fuu Fbu . Fuu Fbu . Fuu
a Atr: accelerations of the trunk; Ahc: accelerations of the contra-lateral hip; x, y, z: respectively, antero-posterior, lateral and vertical directions. The comparisons between the two conditions are obtained with the paired t-test. Mean (of subjects, n 6) and standard deviations (SD) are expressed in ms 22.
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P. Nouillot et al. / Neuroscience Letters 279 (2000) 77±80
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