Running, at any speed, requires the contraction of the skeletal muscles. But it’s the varying concentrations of the muscular fibre types in the skeletal muscles that will dictate the force of contraction, how fast we go and for how long.
There are two main types: slow (Type 1) twitch. Active during steady tempo runs as they utilise oxygen for fuel, they fatigue slowly and can contract for extended periods. And fast (Type 2) twitch, which are mainly used when we sprint. They fatigue quickly, provide short bursts of speed and use anaerobic metabolism for fuel.
When training for tri, our muscles are primarily made up of Type 1 fibres, so engaging the muscles to contract faster can be quite difficult. If we’re able to increase the concentration of fast-twitch fibres, we’ll be able to sprint more effectively and improve our race performance. This can be achieved by doing high-intensity intervals sessions and finishing a long swim, bike or run with some 10-15sec max sprints.
The overall effect can lead to a greater ability to change pace, passing other competitors confidently and being more competitive in a sprint finish.