The first four steps out of the blocks: the shape a sprinter is trying to reach

Why the body is angled at all

Force applied to the ground has a direction, and a runner leaning forward directs more of that force backwards, which is what produces forward acceleration. A runner standing upright can only push downwards, which supports the body against gravity and contributes very little to changing its speed. The blocks exist to establish the angle, holding the body in a position it could not maintain unsupported at the moment of the start.

The first steps therefore preserve that angle for as long as the runner can continue to generate force along it. The transition to upright running is inevitable, and the skill lies in delaying it until acceleration has done most of its work.

The first step and where the foot lands

The first foot lands beneath or slightly behind the runner's centre of mass rather than in front of it, which is the opposite of ordinary running. A foot landing in front produces a braking force, since the ground pushes back along the line from the contact point to the body. That braking is the single most common fault in a sprint start and it is usually caused by the runner trying to take a long first stride.

Short, powerful early strides therefore produce more speed than long ones, which is counter-intuitive and is why the pattern has to be trained. Stride length increases naturally as speed rises, so a runner who forces it early is spending effort on something that would have happened anyway.

Rising too soon

The commonest error after the first step is lifting the head and chest, which raises the trunk before the runner has generated the speed to justify it. Once upright, the runner is applying force downwards and the remaining acceleration must come from a much less efficient mechanism. The head position is the usual culprit, because lifting the eyes to look down the track pulls the shoulders and the spine with them.

Coaches address it by controlling where the runner looks rather than by instructing the trunk, since the trunk follows the head automatically. The correct rise is gradual and occurs across several steps rather than at a single moment, which is why it is difficult to identify in real time.

The arms and what they are doing

The arm action in the first steps is longer and more forceful than in the upright phase, and it is doing more than balancing the legs. A vigorous backward drive of the arm assists the opposite leg by managing the rotation the leg drive would otherwise impose on the trunk. A runner whose arms cross the body introduces rotation around the vertical axis, which the legs must then correct at a cost in forward force.

The elbow angle changes through the start as the stride shortens and lengthens, so a fixed arm position is a sign of a rehearsed rather than a functioning action. These are small effects individually and they matter because the early steps are where the largest changes in speed occur.

Why the reaction time matters less than it seems

The interval between the signal and the first movement is a small fraction of the time spent in the acceleration phase, and it varies by very little between athletes. What follows the reaction accounts for far more, since a runner who leaves the blocks quickly and rises too early has spent the advantage immediately. A runner who is slightly late but holds the correct angle can recover the deficit within the first few steps, which is why starts are judged over distance rather than at the gun.

Anticipating the signal is also constrained by the rules, which penalise movement before a defined interval and remove the option of guessing. The useful focus is therefore the position rather than the reaction, which is the part a runner can actually influence through training.