Frame of reference

Take a acceleration phase in sprinting for an example. Ground reaction force passes through body’s center of mass (or slightly outside of it causing certain torque around body’s sagital axis) and certain component of it provides the propulsion force. The bigger the propulsion force, the greater the acceleration. The body inclination can affect this, but to get into more incline position some pre-requests needs to be in place first.

Anyway, recently there are some opinions regarding how to improve this propulsive force which is horizontal to the ground. Most (if not all) of the strength training force vectors are vertical (since the gravity is vertical), thus is is questionable whether strength training improves the ability of the athlete to provide propulsive force extremely important in acceleration phase.

This is why there is a recent trend toward sled pushes, car pushes and some new exercises (or old, but brought to life one more time) that supposedly work the muscles that are responsible for propulsive force generation.

Did you noticed that we utilized ground as a reference system? What happens if we take the body as a reference system? Just rotate the picture

What happened now? What does the posture of the athlete look like, and where is the ground reaction force passing through in relation to the athlete’s body? Does it looks something like:

Another paralysis by analysis example? Since the body is build to live in a gravity world, the best force capabilities are vertical (to fight gravity) or should I say axial. Let’s do a simple experiment. Go to a parking lot, put your car in neutral. First try to maintain erect position of the body and try to push (accelerate) the car. Then incline as much as you can and try to push. Where did you generated the most propulsive force to accelerate the car?

This is exactly why the body goes for a inclined position in acceleration phase. The distinction of vertical and propulsive forces are the results of the analysis (which is constrained by frame of reference we use) and NOT the „real“ forces that the body can differentiate per se and use them accordingly. That’s why you incline during the acceleration phases so that the force producing capabilities of the body are oriented toward the task goal of increasing the horizontal acceleration and speed.

The point being taken here is that you cannot ‘train’ the propulsive force since the propulsive force is the result of the analysis. So stop fantasizing about special exercises that target propulsive forces and start doing squats.

Other ‘internal’ factors might explain why some athletes are good jumpers and poor sprinters (to what distance we are talking about?) and vice versa. I can be wrong, but I can bet that the athlete with the greatest relative squat, squat jump will alos have the best speed in first 5 meters. If we talk about other types of jumps (reactive, single leg, running, etc) and sprinting distances (5m, 10m, 20m, 40m, 60m, etc) the correlation mmi might be different.