Biomechanics of Acceleration

The physics of horizontal thrust

The acceleration phase is the critical component that defines the start of any sprint race, from 60m to 400m.

The primary biomechanical objective in this stage is to move the body from static inertia (zero velocity) to the maximum possible velocity in the shortest possible time.

To accomplish this, the athlete must apply massive forces against the ground. However, it is the direction of these forces that determines the efficiency.

Unlike sprinting, where the forces are vertical, in acceleration we need to generate extreme horizontal forces.

For this reason, the sprinter adopts a pronounced forward lean of the trunk (approximately 45 degrees). This angulation is not aesthetic; it is functional.

It allows the center of mass to be aligned in such a way that, when pushing backwards against the track, the physical reaction projects the athlete forwards.

The Triple Extension and the line of force

The central technical concept that the coach must observe is the "Triple Extension".

This refers to the full and simultaneous extension of three key joints: the hip, knee and ankle of the pushing leg.

At the moment the foot leaves the ground or starting block, there must be a perfect straight line connecting the shoulder, hip, knee and ankle.

If this line is broken (e.g., if the hip remains flexed or the knee does not extend), the force is dissipated and is not effectively transmitted forward.

The athlete should feel as if he is pushing the ground "backwards", as if he wants to move the world under his feet, projecting his body through this 45 degree line of force.

Piston and counterbalance action

While one leg performs this triple extension, the free leg plays an equally vital role. It must be propelled forward with a high, aggressive knee action.

The key is timing: at the exact instant the back leg reaches its maximum extension, the free knee must be at its highest point in front. This sets up the next impact against the ground.

The foot contact on acceleration should always occur below the hip or slightly behind it ("piston action").

If the foot lands in front of the center of gravity, it acts as a biomechanical brake.

To balance these violent leg forces, the arm