Motor Unit Recruitment: Spike-Train Twitch Summation (2D)

This 2D simulator reimplements Henneman's size principle from first principles: each of 20 motor units fires an actual discrete spike train once the shared excitatory drive crosses its own recruitment threshold, and every spike kicks a critically-damped twitch-mechanics oscillator whose impulse response is the textbook motor-unit twitch shape. Unlike the 3D version's smoothed activation proxy, force here emerges from genuine summation of individual twitches — visible directly on the live spike raster as separated twitches at low firing rates and fused tetanus at high rates — while the same differential fatigue/recovery model depletes fast, high-force units several times faster than slow, fatigue-resistant ones. Set a target force, hold it under sustained effort, and watch fatigue in the early-recruited units force additional, larger units to join just to keep the same output.