Age-differences in the free vertical moment during step descent.

Abstract

This study utilises a rarely examined biomechanical parameter--the free vertical moment--to determine age-related differences in rotational kinetics of the body about the vertical-axis when stepping down from a stationary position. Ten older and 10 young adults completed step-downs from three heights. Free vertical moment impulse and peak during step-initiation double-support and the subsequent step-execution phase, and vertical-axis pelvis angular displacement and velocity at instant of landing were compared. The free vertical moment during double-support was directed away from the intended lead-limb side, producing a change in vertical-axis rotational momentum that moved the lead-limb in a forwards-medial direction about the stationary support/trailing limb during the subsequent step-execution phase. The free vertical moment during step-execution was directed towards the lead-limb side and acted to slow/halt the body's vertical-axis rotation away from lead-limb side. Free vertical moment impulse and peak during double-support were similar between groups (P>0.05), but during step-execution were significantly reduced in older adults (P=0.002). As a result older adults had greater vertical-axis pelvis angular displacement and velocity at instant of landing (directed away from lead-limb side), with significant (P<0.001) group-by-step height interactions indicating that differences between groups became more pronounced with increasing step-height. These findings highlight that older adults were unable to exert the same vertical-axis control during single-support as young subjects did. Findings also highlight that the analysis of free vertical moment data can be a useful biomechanical tool to highlight age-related differences in how steps/stairs are negotiated.