Which muscles power our running? And do they function differently the faster we run?
Muscular strategy shift in human running: dependence of running speed on hip and ankle muscle performance
Tim W. Dorn, Anthony G. Schache, Marcus G. Pandy
Journal of Experimental Biology 2012 215: 1944-1956; doi: 10.1242/jeb.064527
There are two ways to increase your speed while running. You can either take longer strides (stride length) or swing your legs faster (stride rate). However, it can be difficult in practice to manipulate only one of these variables because they are inversely proportional, and so running speed can be increased only when an increase in stride length is not accompanied by a similar decrease in stride frequency and vice versa.
So how do humans run faster? The answer may lie within what muscles are working at different speeds. At slower speeds, running speed is increased by exerting larger support forces during ground contact, in effect increasing stride length (more time spent in the air).
In contrast, at faster speeds, ground contact time is reduced so the body does not have the same amount of time to generate the forces needed to increase stride length. Hence, we generate more speed by increasing leg speed by increasing peak force generation through both the swing and stance phase of gait. (though the study is only talking about swing phase mainly)
A study by Dorn et al, 2012 looked at peak muscle forces at increasingly faster speeds. The results were very interesting.
What does the data shows:
Very little glute activity especially at slower speeds (<7m/sec or 25km/hr)
Very little quad activity, except for rectus femoris at increasingly faster speeds (> 7m/sec or 25km/hr)
Soleus is very active at all running speeds with gastocnemius doing much less work
Hamstring and iliopsoas activity increases as speed increases
Clinical Implications:
The findings of this study show how the soleus is probably one of the most important muscles when it comes to running mechanics and that the glutes have much less of a role than we often think. The hamstrings and iliopsoas muscles become increasingly important, the faster we run. This may have some implications on strengthening programs directed towards runners. The evidence would indicate that there be more emphasis on strengthening of the soleus in all runners and hamstrings/iliopsoas in sprinters.