Organization Of Postural Adjustments Research Articles

The function and organization of postural adjustments during voluntary pulls made by standing humans

Summary form only given. Postural adjustments preceding voluntary arm movements generally are assumed to stabilize standing balance relative to the base of support, body joint configuration, or orientation of one or more segments to the inertial reference frame. While this assumption is reasonable and consistent with much biomechanical data, postural adjustments also can contribute directly to the primary voluntary act. This role of postural adjustments has been demonstrated analytically for abrupt pulls made in the horizontal direction of the sagittal plane by freely standing humans. Under the constraints of the experiment, the biomechanical system has two kinematic degrees of freedom, which can be represented by the vertical and anterior-posterior coordinates of the body's center of mass (CMV and CMAP). A model which treats the CMAP as a point mass at the end of an inverted pendulum that rotates about a fixed axis in a gravitational field is presented. The model has three parameters: stiffness of an elastic cord between the CMAP and the external world; the slack that has to be taken out before the cord exerts force on the CMAP; and the diameter of a weighted pulley centered at the axis of rotation, which generates torque that initiates pendulum motion. >

Effects of varying load conditions on the organization of postural adjustments during voluntary arm flexion.

One purpose of the experiments reported here was to further clarify the effect of varying loads on postural adjustments. Another was to reevaluate whether or not the timing of electromyographic (EMG) activity in the postural muscle is preprogrammed. To accomplish these goals, we compared the effect of the presence or absence of prior knowledge of a load on the timing of EMG activity in the postural muscle (biceps femoris [BF]) with that in the focal muscle (anterior deltoid [AD]). Although the sequence of EMG activation was similar under conditions with and without a load, the timing of postural EMG activities (BFi, ipsilateral BF; BFc, contralateral BF) in associated postural adjustments was dependent on the force of arm movement, and the latencies of postural EMG activities (BFi-BFc) were dependent on the speed of arm movement. This indicates that EMG changes in the upper (focal muscle) and lower limbs (postural muscle) were triggered by different motor programs. Moreover, similar EMG activities were observed in postural muscles when the subject had advance knowledge of the presence or the absence of a load. Thus, this suggests that BFi may be centrally preprogrammed (anticipatory regulation) and BFc may be feedback regulated. Furthermore, environmental information may be a critical source of influence on those postural responses.

Effects of arm acceleration and behavioral conditions on the organization of postural adjustments during arm flexion.

Nine standing subjects performed unilateral arm flexion movements over an eight-fold range of speeds, under two behavioral conditions. In the visually-guided condition, a visual target informed subjects about the correct movement speed. Seven subjects also made movements of different speeds during a self-paced condition, without a visual target. Angular displacement and acceleration of the arm, and EMG activity from the hamstrings (HM), erector spinae (ES) and the anterior deltoid (AD) muscles were measured. The following results were observed. Mean rectified amplitudes of EMG activity in HM and ES were typically correlated with the average arm acceleration and presumably the disturbance to posture and/or balance. HM and ES amplitudes were correlated for only six subjects. Functions relating the ratios of HM/ES EMG amplitudes to acceleration varied between subjects. HM onset latencies were highly variable for slow movements and usually lagged movement. For movements above a threshold-like point in acceleration, HM latencies were correlated with arm acceleration and recruited before movement. ES latencies were constant for fast movements, and negatively correlated with acceleration for slower movements. The recruitment order of HM and AD was influenced by the behavioral condition but not by arm acceleration for fast movements. HM and AD were recruited coincidentally for visually-guided movements, while for self-paced movements, HM was recruited before AD. We conclude that for the arm flexion task: HM and ES are not tightly coupled; both behavioral and mechanical conditions affect the recruitment of postural muscles; and postural and focal components of the movement are probably organized by parallel processes.