Above is a slow motion clip of Usain Bolt – the fastest man on earth – running the 100m (9.58 secs).

Look at his:

1. Vertical head oscillation is minimal (energy efficient)
2. Arm anteflexion in sync with ipsilateral leg propulsion phase
3. Heel to butt action after terminal stance – this reduces force needed for hip flexion
4. Hip extension for prolonged propulsion
5. Fore foot striking
6. Mild inversion on initial touch
7. Loading phase occurs right underneath torso
8. Stance : Swing phase ratio

…amazing to watch!

A Biomechanical Analysis:
(NB: only transverse plane forces are discussed, sagittal plane forces also play a large part in these movements)

‘Arm ante flexion in sync with ipsilateral leg propulsion phase’ – ask yourself why this is beneficial? From newtons 3rd law of force, we know that every action has an equal and opposite reaction… so lets apply that here.

Looking at the arm being accelerated into anteflexion (F=ma), this would produce an equal and opposite force travelling inferior and posterior, acting at its connecting joint – the shoulder.

The contralateral arm travels in the opposite way… creating an anterior and superior transfer of forces at the shoulder.

With these combined we have a rotation effect in the transverse plane – anticlockwise!

Here is where this is important:

The lower extremity is also exerting rotation forces. The leg in the propulsive phase receives the reaction force from the ground… which travels through the leg and acts at the hip producing a superior and anterior force. The contralateral leg is acting like the arm in anteflexion… producing a down and posterior force acting at the hip. This creates a clockwise force!

So we have an upper body anti-clockwise force, conflicting with a lower body clockwise force… which is exactly what we need to prevent rotation. When two forces act against each other, they also successfully transfer energy (think how this couldn’t happen if they were acting in the same direction) through the body for movement. These forces meet and match which allows forces from the legs to effectively propel the body forward. The torso expresses ‘rigidity’ with this anti-rotational response, allowing transfer of force between body segments (mostly from the bottom up). If rigidity did not occur then there would only be absorption of force (think of how our bones are permanently rigid for this purpose). This transfer of force through the body allows for forward propulsion to occur. We cannot see any rotatory motion in the trunk as muscles counter act these instantly. In sprinters, ideally you shouldnt see any movement in the torso, only the legs and arms. This contributes to efficiency, and efficiency makes a sprinter faster.

So watch the clip again and consider how all the static parts of Usain Bolts body are from reactive isometric contractions which produce the most efficient transfers of force… effectively making him the fastest man on earth!

EXTRA:
The motions in the sagittal plane which produce vertical forces also contribute to efficient force transfer in the body. Think of those yourself, and how the contralateral side of the torso may have to react. Remember the body needs to produce rigidity to allow an efficient transfer of force through the body.

Fascinated by the first video? Finish off with this clip: