Abstract
It has been shown that increased muscular activity along the trunk is likely to impair body balance, but there is little knowledge about its consequences on more dynamic tasks. The purpose of this study was to determine the effect of unilateral and bilateral increases of muscular tension along the trunk on the sit-to-stand task (STS) performance and associated anticipatory postural adjustments (APAs). Twelve healthy females (23 ± 3 years, 163 ± 0.06 cm, 56 ± 9 kg), free of any neurological or musculoskeletal disorders, performed six trials of the STS at maximum speed, in seven experimental conditions varying the muscular tension along each side of the trunk, using a specific bimanual compressive load paradigm. A six-channel force plate was used to calculate the coordinates of the center of pressure (CP) along the anterior-posterior and medial-lateral axes, and the kinematics of the head, spine and pelvis, were estimated using three pairs of uni-axial accelerometers. The postural and focal components of the task were assessed using three biomechanical parameters calculated from CP signals: the duration and magnitude of APAs, and the duration of focal movement (dFM). Results showed that beyond a given level, higher muscular tension along the trunk results in longer APAs, but with a stable duration of the focal movement. In addition, no significant variation of APAs and FM parameters was found between bilateral and unilateral increases of muscular tension. It was suggested that restricted mobility due to higher muscular tension along the trunk requires an adaptation of the programming of APAs to keep the same level of performance in the STS task. These findings may have implications in treatment strategies aimed at preserving functional autonomy in pathologies including a rise of muscular tension.
Highlights
According to the seminal work of Bouisset and Zattara (1981), voluntary movement induces a perturbation to balance, and a counter-perturbation must be developed for the movement to be performed efficiently
The center of pressure (CP) time course along the anterior-posterior axis revealed a typical shape which always starts with an almost linear backward displacement, followed by a forward displacement with a steeper slope, and which ends with a less regular stabilization phase preceding the plateau. This pattern, which is consistent with data displayed in previous experiments (e.g., Schenkman et al, 1990; Diakhaté et al, 2013) was reproducible across subjects and conditions. No such typical trace was found along the medial-lateral axis, with a high variability of CP displacements during anticipatory postural adjustments (APAs) and focal movement (FM)
The analysis of variance (ANOVA) revealed that increased compressive load level was associated with a longer duration of the APAs, with a significant main effect (p < 0.05)
Summary
According to the seminal work of Bouisset and Zattara (1981), voluntary movement induces a perturbation to balance, and a counter-perturbation must be developed for the movement to be performed efficiently This counter-perturbation starts prior to the onset of the voluntary movement, and serves to create in the rest of the body a movement whose forces of inertia would, when the times comes, balance the inertia forces due to the voluntary movement. According to the hypotheses developed by Gelfand et al (1966), the voluntary movement may be broken up into two components, one focal and one postural The former refers to the voluntary movement itself, and the latter to the associated stabilizing activity of the body (Bouisset and Do, 2008). Restricted mobility of the bony chain may impair motor performance, even though its source is located in a part remote from the focal movement (FM)
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