Abstract
Controllability of posture in the medial-lateral direction is critical for balance maintenance, particularly in step initiation. The objective of the current study was to examine the effects of external perturbation and landing orientation on medial-lateral control stability in step initiation. Eleven young healthy participants stood on the force platform and waited for the instruction of taking a step while experiencing a pendulum perturbation applied at the lateral side of the right shoulder. Eight experimental conditions were conducted by two levels of step side (right or left), two levels of perturbation (with or without), and two levels of landing orientation (forward or diagonal). The center of pressure (COP), pelvic movements, and muscle activities were recorded and analyzed as the onset of COP and pelvic movement, the COP displacement, and cocontraction and reciprocal muscle activation pattern. The temporal events of COP and pelvic movement were not significantly different in all experimental conditions. However, COP and pelvic movement were significantly later in the diagonal condition. Most of the segments showed reciprocal muscle activation patterns in relation to the perturbation released time. Subsequently, all segments showed cocontraction muscle activation patterns, which was significantly affected by step side, perturbation, and orientation. The results suggest that how the CNS initiated a step was identical with the COP then pelvic movement. The outcome highlights the importance of external perturbation and foot landing orientation effects on postural adjustments, which may provide a different approach to help step initiation.
Highlights
Controllability of posture in the medial-lateral direction is critical for balance maintenance, in step initiation
Prior to executing a swing in step initiation, the body’s weight has to be shifted from even distribution of its weight to a single loading leg. This sequential pattern was inferred from measurements of the center of pressure (COP) displacements and is usually divided into three components: COP moves to the initial swing leg and backwards, COP moves to the initial stance leg, and COP moves forward until toe-off of the swing leg[2,3]
The external pulling system at the waist level could potentially improve step initiation in older adults[21] and Parkinson’s patients[22]. This implies that the lateral perturbation might either assist or hinder movement depending on the impact of the swing or stance leg during step initiation
Summary
Controllability of posture in the medial-lateral direction is critical for balance maintenance, in step initiation. The forward/backward COP displacement is caused by reciprocal activation between ventral and dorsal muscles of the shank segment It is an anticipatory postural adjustment (APA) handling the perturbation in the anterior-posterior direction during step initiation from COP shifting backward on the stance leg until heel off of the swing leg[3,9]. When handling two perturbations simultaneously, instead of compromising the acceleration of arm flexion movement, the CNS recruits additional postural muscle activity to stabilize posture and reduce the COP displacement for the translational perturbation from the lower limb[16,17] This indicates that strategies are adjusted based on the level of challenge to posture and balance, i.e. based on the context. The objective of the current study was to investigate how medial-lateral perturbation (a pendulum would impact at the shoulder level on the right side of the body) affect postural muscles activity and COP in step initiation. Changes in foot landing orientation might affect the COP displacement and postural muscle activity of shank, thigh, and trunk segments
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