What really limits sprint speed: ground force, force direction and propulsive impulse — and how to train exactly that via your force-velocity profile.
Straight talk: your top speed comes down to the ground force you drive into the track per contact — not how fast you move your legs through the air. Top runners press more than 2.5 times their body weight into the ground per contact, while flight and leg-repositioning times stay almost identical across every performance level. So you don't get faster by "making the legs faster", but by learning to put more force into the ground in less time and in the right direction.
That's the uncomfortable truth behind the eternal "stride length vs. stride frequency" debate: both are consequences, not causes. The cause is the force-impulse profile of your ground contacts — how much force you deliver, in which direction, and in what time. In this guide we break your sprint mechanics down into exactly those levers — ground force, force direction (ratio of force), propulsive impulse, and your force-velocity profile — and derive what you actually train.
Weyand and colleagues showed that fast and slow runners don't differ in the time needed to reposition the legs — flight time is around 0.12 s for everyone. The only decisive difference is the magnitude of ground force: the fastest push more than 2.5 times their body weight into the ground. Your training consequence: it's about force per contact at an extremely short contact time (high rate of force development), not "faster leg spinning".
Rabita and colleagues compared elite and sub-elite sprinters: the elite produce more forward force at any speed — and the force-velocity relationship is nearly a straight line (R² ≈ 0.89). The key is the "ratio of force": how much of your ground force points forward rather than just down. In acceleration, on average around 0.35 body weights goes forward, and even 0.42 in the first steps. So what separates elite from the rest isn't raw leg strength, but the ability to aim it horizontally.
Force alone isn't enough — what counts is impulse (force × time) and where it points. Your relative propulsive impulse explains around 57% of the differences in sprint speed, while vertical impulse barely matters. Every foot strike ahead of your centre of mass creates a braking impulse that costs you speed. Remember the principle: enough vertical impulse for flight time and leg turnover, but brake out and drive in — "push more, brake less".
Horizontal force is generated where the hamstrings are maximally activated just before ground contact: those who produce the most horizontal force show high eccentric knee-flexor torque and strong biceps femoris activity in the late swing phase. Pure "pawing" kinematics (foot speed, knee position), by contrast, don't correlate with propulsive force. So train eccentric hamstring strength and horizontal force output, not cosmetic movement details.
Build an individual force-velocity profile (F0 = theoretical maximal force, V0 = theoretical maximal velocity, Pmax = maximal power) from a sprint with splits and an app/radar. A force-deficient profile is trained with heavy resistance (sled runs, hills), a velocity-deficient one with assisted or maximal-speed runs. That way you aim the stimulus precisely at your limiting factor instead of just "sprinting more".
The stimuli described here (maximal ground forces, high RFD, heavy resistance) place extreme load on tendons and the posterior chain. Never increase resistance and sprint volume at the same time, schedule adequate recovery between maximal sessions, and monitor hamstring load tolerance — it's the number-one injury even at pro level. Use video analysis to catch overstriding early, before it leads to a strain.
Repeat your force-velocity profile every four to six weeks under identical conditions. The control signal isn't your best time but the shift in F0, V0 and Pmax: if your deficit moves toward balance, your programme is correctly targeting the limiting factor. If the profile stays unchanged despite training, change the stimulus — not the volume.
Ground force per contact, not leg frequency. Fast and slow runners take almost the same time to reposition their legs (flight time around 0.12 s); the difference is that the fastest push more than 2.5 times their body weight into the ground. Top speed is therefore primarily a force and force-direction problem at an extremely short contact time.
Neither, as a cause — both are outcomes of your force-impulse profile. Trying to raise frequency directly usually fails because leg-repositioning time is barely changeable. Train ground force and its horizontal orientation (ratio of force), and length and frequency improve on their own as a consequence.
Not more absolute force, but better-directed force: elite produce more horizontal ground force at any speed, visible in a higher ratio of force. Add a larger propulsive impulse with minimised braking impulse and a posterior chain that can be activated very strongly eccentrically. It's force-technique, not just force.
If your foot lands ahead of your centre of mass, yes. Every strike ahead of the centre of mass creates a braking impulse that costs propulsion. Since relative propulsive impulse explains around 57% of sprint-speed variance, reducing the braking share is one of the most effective levers. Check via video analysis whether your foot strike sits close under the hip.
Leichtathletik: Recovery and Longevity