Effects of medial-lateral stability on the postural adjustments associated with reaching movements

• Main Author: Simpkins, Susan Diane
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-1999
• Subjects: Neurology; Neurosciences; Physical therapy; Rehabilitation; Therapy
• ISBN: 9780599408913; 059940891X

The aim of the present study was to investigate the effects of medial-lateral (M-L) stability on the postural adjustments that precede reaching movements. Medial-lateral stability was varied by changing the size of the base of support (BoS) and by having subjects perform bilateral and unilateral reaching movements. Eight subjects stood on a force plate to reach and grasp a bottle with one or both arms, from a wide and narrow base of support (BoS), under simple reaction time (RT) conditions. Anticipatory postural adjustments were measured as changes in M-L and anterior-posterior (A-P) center of pressure (CoP) displacements and center of mass (CoM) accelerations. The organization of M-L APAs depended on stance width and the type of reaching movement performed, while A-P APAs showed a similar organization across movement conditions. The occurrence and amplitude of M-L APAs increased when reaching movements were unilateral and associated with a strong potential for M-L destabilization. Although stance width had no effect on the occurrence of APAs, the amplitude of M-L CoP displacements and the rate of change of the M-L CoP increased as stance width increased. Anterior-posterior and M-L CoM accelerations were usually in the direction that opposed the destabilizing forces of the arm movement. When subjects stood with a narrow BoS the direction of M-L CoM accelerations and the M-L CoP position at movement onset was variable. Although RT was not affected by task demands a correlation was found between the onset latencies of reaching movements and anticipatory CoP displacements. The results suggest that the organization of M-L APAs was not always specific to the postural requirements of the task. When subjects stood with a wide BoS an increase in the mechanical coupling of the hip and ankle allowed subjects to generate strong stabilizing forces in the M-L direction, independent of the destabilization associated with the arm movement. When stance posture was unstable the direction of M-L APAs was variable. Thus, the organization of M-L APA depended not only on the dynamics of the reaching movement but, also on the initial stance conditions.