Modeling the stiffness characteristics of the human body while running with various stride lengths

A modified mass-spring-damper model was used to simulate the vertical groun d reaction forces of a human runner as stride length was altered. Spring st iffness values were selected by an optimizing routine that altered model pa rameters to match the model ground reaction force curve to a runner's actua l ground reaction force curve. A mass in series with a spring was used to s imulate the behavior of body structures that produce the active portion of the ground reaction force. A second mass in series with a spring-damper sys tem was used to simulate the behavior of those components that cause the im pact portion of the ground reaction force. The stiffness of the active spri ng showed a 51% decrease as subjects increased their stride length. The sti ffness value of the impact spring showed a trend opposite that of the activ e spring, increasing by 20% as strides lengthened. It appears that the impa ct stiffness plays a role in preventing the support leg from collapsing in response to the increased contact velocities seen in the longer strides.