Adaptive postural control for joint immobilization during multitask performance

Abstract

Motor abundance is an essential feature of adaptive control. The range of joint combinations enabled by motor abundance provides the body with the necessary freedom to adopt different positions, configurations, and movements that allow for exploratory postural behavior. This study investigated the adaptation of postural control to joint immobilization during multi-task performance. Twelve healthy volunteers (6 males and 6 females; 21-29 yr) without any known neurological deficits, musculoskeletal conditions, or balance disorders participated in this study. The participants executed a targeting task, alone or combined with a ball-balancing task, while standing with free or restricted joint motions. The effects of joint configuration variability on center of mass (COM) stability were examined using uncontrolled manifold (UCM) analysis. The UCM method separates joint variability into two components: the first is consistent with the use of motor abundance, which does not affect COM position (VUCM); the second leads to COM position variability (VORT). The analysis showed that joints were coordinated such that their variability had a minimal effect on COM position. However, the component of joint variability that reflects the use of motor abundance to stabilize COM (VUCM) was significant decreased when the participants performed the combined task with immobilized joints. The component of joint variability that leads to COM variability (VORT) tended to increase with a reduction in joint degrees of freedom. The results suggested that joint immobilization increases the difficulty of stabilizing COM when multiple tasks are performed simultaneously. These findings are important for developing rehabilitation approaches for patients with limited joint movements.

Figure 1. Experimental setup.

Schematic illustration of the angles studied in the sagittal plane: ankle, knee, hip, lumbarsacral spine (LSJ), scapular, shoulder, elbow, wrist, and cervical-thoracic junction (CTJ). Note that the knee and LSJ were immobilized in the immobilized conditions.

Figure 2. Mean trajectory of joints, center of mass (COM), and hand mean trajectories with respect to the coordinate system origin for a representative participant when targeting a small target and balancing a ball in the unconstrained condition (TWB9).

Dashed lines represent ±1 SD from the mean. LSJ: lumbo–sacral junction. CTJ: cervical-thoracic junction.

Figure 3. Mean trajectory of joints, center of mass (COM), and hand with respect to origin of the coordinate system for a representative participant in the targeting to a small target and combining with the ball-balancing in the immobilized condition (TWB7).

Dashed lines represent ±1 SD from the mean position. LSJ: lumbo–sacral junction. CTJ: cervical-thoracic junction.

Figure 4. Mean (+SEM) of the sum of all joints variability across values of all participants.

The bars represent: the static standing task in an unconstrained condition (STAT9), static standing task in an immobilized condition (STAT7), targeting task with no ball balancing in an unconstrained condition (TNB9), targeting task with no ball-balancing task in an immobilized condition (TNB7), targeting task combined with the ball-balancing task in an unconstrained condition (TWB9), and targeting task combined with the ball-balancing task in an immobilized condition (TWB7).

Figure 5. Mean (+SEM) of task-related variable variability in the anterior-posterior movement direction for center of mass (COM) position.

Condition acronyms are the same as in Figure 4.

Figure 6. V_UCM and V_ORT with respect to the COM control for a representative participant.

A: Targeting combined with ball-balancing task in an unconstrained condition (TWB9). B: The same task with immobilized joints (TWB7). Solid lines represent the mean V_UCMof all cycles and dashed lines represent the mean V_ORT of all cycles.

Figure 7. Mean (+SEM) of joint configuration variability for COM position control.

Adjacent pairs of bars represent V_UCM (left, diagonal filled bars) and V_ORT (right, spotted bars). Condition acronyms are the same as in Figure 4. The scale of V_UCM is larger than the scale of V_ORT, however, the relationship between V_UCM and V_ORT did not change, that is, V_UCM>V_ORT. Asterisks indicate significant differences between bars (*p<0.05).