Interaction of leg stiffness and surface stiffness during human
hopping.
Ferris, Daniel P., and Claire T. Farley.
Department of Human Biodynamics, University of California,
Berkeley, CA 94720-4480
APStracts 3:0422A, 1996.
When mammals run, the overall musculoskeletal system behaves as a
single linear "leg spring". We used force platform and
kinematic measurements to determine if leg spring stiffness is
adjusted to accommodate changes in surface stiffness when humans hop
in place, a good experimental model for examining adjustments to leg
spring stiffness in bouncing gaits. We found that the leg spring
stiffness was greatly increased to accommodate surfaces of lower
stiffness. The series combination of the leg spring stiffness and
surface stiffness (total stiffness) was independent of surface
stiffness at a given hopping frequency. For example, when humans
hopped at a frequency of 2 Hz, they tripled their leg spring
stiffness (kleg) on the least stiff surface (ksurf = 26.1 kN/m, kleg
= 53.3 kN/m) compared to the most stiff surface (ksurf = 35,000 kN/m;
kleg = 17.8 kN/m). The total stiffness was not significantly
different on the least stiff surface (ktot = 16.7 kN/m) and the most
stiff surface (ktot = 17.8 kN/m). Because of the leg spring stiffness
adjustment, many aspects of the hopping mechanics (e.g., ground
contact time and center of mass displacement) remained remarkably
similar in spite of a more than 1000-fold change in surface
stiffness. This study provides insight into how leg stiffness
adjustments can allow similar locomotion mechanics on the variety of
terrain encountered by runners in the natural world.
Received 29 April 1996; accepted in final form 20 August 1996.
APS Manuscript Number A413-6.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 19 September 1996