Runners appear to use two different strategies to increase their speed according to this article we are appreciating by Dorn, Schache and Pandy
In specific observance of the lower limb muscles,
"Humans run faster by increasing a combination of stride length and stride frequency. In slow and medium-paced running, stride length is increased by exerting larger support forces during ground contact, whereas in fast running and sprinting, stride frequency is increased by swinging the legs more rapidly through the air". . . . .
"For speeds up to 7ms–1, the ankle plantarflexors, soleus and gastrocnemius, contributed most significantly to vertical support forces and hence increases in stride length. At speeds greater than 7ms–1, these muscles shortened at relatively high velocities and had less time to generate the forces needed for support. Thus, above 7ms–1, the strategy used to increase running speed shifted to the goal of increasing stride frequency. The hip muscles, primarily the iliopsoas, gluteus maximus and hamstrings, achieved this goal by accelerating the hip and knee joints more vigorously during swing."
Muscular strategy shift in human running: dependence of running speed on hip and ankle muscle performance
Tim W. Dorn, Anthony G. Schache and Marcus G. Pandy*
Department of Mechanical Engineering, University of Melbourne, Victoria 3010, Australia. The Journal of Experimental Biology 215, 1944-1956
© 2012. Published by The Company of Biologists Ltd