Are maximum ground forces and leg compression in phase? A test of the classical spring mass model of running gaits

The mechanical understanding of human running has classically been described as a spring-mass system, with subsequent models predicting the movements of the body’s center of mass and the forces applied by the leg against the ground. A central requirement of any spring system is the phasic relationship between the length of the elastic elements and the forces applied to these structures. Specifically, elastic elements compress under load and extend as the load is released. We tested whether this model applies to individuals with specialization for extreme performance in human gait. Recent work from elite level sprint runners suggest that their patterns of force application differ from those used during slow speed running, and similarly differ between individuals capable of high speed running and those that are not. We measured force application and center of mass movements in collegiate sprinters (n=7; top speed 10.1 ± 0.7 m s-1) and recreational runners (n=9; top speed 8.4 ± 0.1m s-1) as they ran on an instrumented force treadmill at speeds spanning each individual’s range. Between these groups we found sprinters applied greater stance average forces at common speeds (mean difference = 11 ± 0.2%) and used an asymmetrical pattern of force application to do so when running at speeds great than 7.0 m s-1. Further at speeds greater than this threshold peak force application preceded minimum center of mass height by 13±1% when expressed relative to the duration of foot-ground contact. This result produced force-length relationships, a method to describe the elastic properties of the leg, that were unique among terrestrial species displaying increased compression of the leg despite lesser levels of force application. We conclude sprint runners use novel gait mechanics to obtain increased whole-body performance rather than a reliance on the storage and release of elastic energy, classically documented at low speeds and for recreational runners