Intensity of activation and timing of deactivation modulate elastic energy storage and release in a pennate muscle and account for gait-specific initiation of limb protraction in the horse.

The equine biceps brachii (biceps) initiates rapid limb protraction through a catapult mechanism. Elastic strain energy is slowly stored in an internal tendon and is then rapidly released to protract the forelimb. The muscle fibres are short, have little scope for length change and can therefore only shorten slowly compared with the speed at which the whole muscle must shorten, which makes them poor candidates for driving rapid limb protraction.