Moment Arms & the Achilles Complex in Sprinting

Muscles produce movement by applying forces to bones. A muscle attaches to the bone at a distance from the joint and use leverage to move the limbs. This leverage or mechanical advantage is the moment arm. The moment arm is increased when there is a larger distance between the muscle attachment site on the bone and the joint’s axis of rotation

Think about a door 🚪

If the door handle is further from the hinge, the moment arm is longer and it’s easier to open that door. This means a muscle with a smaller moment arm produces a greater amount of force to produce the same joint moment as a muscle with a larger moment arm and greater mechanical advantage

(Joint Moment is also known as Torque- the rotational equivalent of linear force)

However, muscles with a large moment arm operate over a larger range of the force length curve & a muscle with a larger moment arms will also experience higher shortening velocities than one with a shorter moment arm. These variables have interesting implications on force length relationships & generating force in sprinting. These effects mean that it’s most likely beneficial for muscles that operate joints that undergo high angular velos to have smaller moment arms and longer fibers

For example, Lee & Piazza (2009) found that elite sprinters actually have Achilles tendon moment arms that are 25% smaller & ankle plantarflexor muscle fibers (lateral gastrocs) 11% longer than what was seen in non-sprinters. The longer muscle fibers and smaller moment arms allow the sprinters to maintain plantarflexor fiber lengths closer to optimal lengths & to reduce fiber shortening velo as the ankle plantarflexes during the push-off phase. These effects increase the force-generating capacity of the plantarflexor muscles. This allows them to generate larger forces during high-speed sprinting. This is what you see with a lot of explosive athletes having a “long achilles”

A lot of these features are most likely genetic, but with a long term athletic development program, these features can be developed to an athletes full genetic potential through elastic movements like sprinting, plyometrics and playing explosive sports