76 - Does angular momentum conservation explain small pelvic rotations in human bipedalism?

Poster Board Number: 76
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Co-authors:

Erica Vazquez - PT Student, NYIT School of Health Professions; Nathan Thompson, Ph.D. - Associate Professor, Department of Anatomy, NYIT College of Osteopathic Medicine

Abstract Body : Introduction: Compared with facultatively bipedal primates, human bipedalism utilizes uniquely small transverse plane pelvic rotations. Bipedal primates use large pelvic rotations, in part, to maximize stride length (the ‘pelvic step’). Why humans have abandoned this strategy is unknown. One hypothesis is that small pelvic rotations are needed to conserve angular momentum during human locomotion. In humans, the thorax and pelvis rotate out of phase, a characteristic not seen in primates. This helps to drive energetically efficient arm swing and lower whole-body angular momentum. Small pelvic rotations may be needed to maintain this balance. If so, humans may have abandoned the ‘pelvic step’ and longer strides in favor maintaining small pelvic rotation and lower whole-body angular momentum. Here, we investigated this hypothesis by experimentally reducing thorax and pelvic counterrotations during human walking and running using a trunk brace. If the ability to balance angular momentum is interrupted, participants should adopt larger pelvic rotations, longer strides, a larger ‘pelvic step’, and larger fluctuations of whole-body angular momentum.

Methods: Three-dimensional data were collected from 5 recreationally fit humans (mass = 65.2±6.4 kgs) while walking (~1.0 m/s and ~1.6 m/s) and running (~2.6 m/s) on a force-instrumented treadmill. Data was collected during normal locomotion and while subjects wore a Donjoy trunk brace to restrict rotations between the thorax and pelvis. A 12-camera Vicon system was used to track 51 infrared markers. Kinematics were calculated using a 15 segment skeletal model in Visual3d and custom code in Matlab.

Results: Use of a brace reduced relative motion between the thorax and pelvis by 70±15% compared with normal locomotion. While wearing a brace pelvic rotation range of motion significantly decreased during walking (walking: 26%; fast walking 54%; p< 0.01) but increased during 59% running (p< 0.01). Stride lengths did not change significantly between walking conditions but were significantly shorter for braced running compared to normal (normal run: 1.68±0.12 m; braced run: 1.62±0.08 m). The ‘pelvic step’ was significantly smaller for all braced conditions (p< 0.01). Root-mean-squared whole-body angular momentum increased somewhat during braced walking conditions (walking: 25%, p< 0.05; fast-walking: 21%, n.s.) but decreased significantly during braced running (14%, p< 0.05).

Conclusion: Interrupting counterrotations between the pelvis and thorax did not appear to induce participants into using larger pelvic rotations, longer stride lengths, and a longer pelvic step, as in bipedal primates. Whole body angular momentum was affected, but only negligibly for walking, and in the opposite direction of expectations for running.