Motor Unit Recruitment & Muscle Fatigue

This simulator renders a muscle cross-section as 20 motor units built from hundreds of individually instanced fiber cylinders, and drives them with a real motor-control model: Henneman's size principle for recruitment order, rate coding for force within a unit, and a differential fatigue/recovery model that fatigues fast, high-force units much faster than slow, fatigue-resistant ones. Set a target force and watch small Type I units fire first, larger Type IIa/IIx units join only as effort climbs toward maximum, and — under a sustained hold — watch fatigued early units lose capacity and force the recruitment of additional units just to keep the same target force, the mechanism behind fatigue and failure in a real sustained voluntary contraction.