Age-related Changes in Posture-Leg Movement Coordination during Rhythmic Unipedal Stepping

INTRODUCTION: During rhythmic stepping the body center of mass (CM) must reverse direction prior to limb liftoff. This postural adjustment serves to unweight the upcoming stepping leg. Given that aging affects both posture and movement, it is surprising that relatively little is known about the age-related changes in the coordination between posture and locomotion. Such information could be important for understanding the problem of falls in the elderly. Lateral rhythmic unipedal stepping performed across a range of frequencies is examined here to identify the effect aging has on the coordination between posture and stepping. METHOD: 7 young adult females (mean, 24 years) were height matched with 7 older adult females (mean, 71 years). Subjects were instructed to rhythmically step a comfortable distance to the side and return to the beat of a metronome using their dominant (right) leg. Pacing began at 1Hz, progressed to 2.8Hz and returned to 1Hz in 0.6Hz intervals. 6 Qualisys motion capture units (Gothenburg, Sweden) were used (sampling 100Hz) to determine lateral step timing using a toe marker. Body CM was calculated using an 8-segment model (Visual3D, C-Motion, Inc., Rockville, MD) to establish time when the CM reversed direction representing the necessary postural prerequisite for stepping. One-way repeated measures ANOVA was performed to test the effects of age and frequency on posture-stepping coordination (CM reversal time ? step onset time / step period). RESULTS: There was a significant effect of age on coordination (F(1,12)=5.51, p=.037). On average older adults shifted their CM back towards the stance limb earlier relative to step onset given a particular step period (which is dictated by frequency) across frequencies and most notably the faster pacing frequencies. A non-significant frequency effect and age x frequency interaction demonstrates subjects maintained a relatively constant phase relationship across frequencies. CONCLUSION: The results are consistent with the notion that postural adjustments during locomotion are under the influence of the stepping limb, particularly its current state (Orlovsky, Deliagina, & Grillner, Neuronal Control of Locomotion, 1999). Age-related changes to postural (CM) and stepping motions separately (i.e., less postural motion, maintained weight bearing on the support limb, and smaller lateral steps - to be submitted for publication), likely serve to maintain a posture-stepping coordination conducive to task completion and perhaps stability / safety maintenance during paced rhythmic stepping. It appears the typically-aging movement control system exhibits a functional adaptive capacity to meet challenges associated with time-critical stepping.