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When we break down sprinting, the muscle demands aren’t static…they shift depending on whether you’re accelerating or already at max velocity. In accel, the gastroc and soleus were the major accelerators, contributing the largest share of propulsive impulse across 19 steps.
The hamstrings and glute medius also assisted, while the quads (vasti, rectus femoris) acted more as brakes…slowing the body down in early stance even though they’re producing high forces. Soleus and gastroc were critical, but with different roles, gastroc consistently
producing propulsion, soleus acting more as a supporter and shifting into braking as speed increased. At max v, the story changes. Hamstring forces spike to ~9× bodyweight, the iliopsoas (hip flexors) reach ~9× as well, and the soleus remains massive (~7× BW). In other words,
the loading shifts toward hamstrings + hip flexors + plantarflexors as you transition to top-end speed. Do these numbers mean your body literally experiences these exact loads in every sprint? Probably not. But they give us a framework - To think about which muscles are most
stressed in different phases - To conceptualize programming for speed - To help with return-to-sport or diagnosing issues in sprint mechanics This is exactly the type of detail I dive into in Speed Kills, my most complete resource yet. Over 80 pages of education on what
drives speed, how to train each phase of the sprint, and an 8-week program that shows you how to combine sprint work, weight training, plyometrics, and conditioning into a system that actually produces progress.
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