Motor Unit Recruitment & Muscle Fatigue

Voluntary Effort

Target force 30% MVC

Fast-twitch (Type II) fraction 45%

Live Readouts

Output: 0% MVCTarget

Units recruited

0 / 20

Elapsed time

0.0 s

Average fatigue (capacity remaining)

100%

Motor Unit Types

Type I — slow, small, fatigue-resistant (recruited first)

Type IIa — fast, moderate fatigability

Type IIx — fast, large, fatigues quickly (recruited last)

How it works

Henneman's size principle: motor neurons are recruited in strict order of increasing size as the net excitatory drive e (0–1, set by the target-force slider) rises. Each of the 20 motor units i has a recruitment threshold θ_i spaced on an exponential scale, so small, low-force units fire first and large, high-force units join only near maximal effort:

θ_i = θ_min · (θ_max/θ_min)^(i/(N-1))     unit i active  ⇔  e ≥ θ_i
rate_i(e) = clamp(rate_min + gain·(e − θ_i), 0, rate_max)

Above threshold, firing rate codes the force each unit contributes (rate coding), summed with the recruited unit's own peak-force capacity F_max,i (bigger, later-recruited units carry more force per fiber — the "size" in size principle):

contribution_i = F_max,i · σ(rate_i)         (σ = saturating twitch-summation curve)
Total force(t) = Σ contribution_i(t)

Fatigue accumulates only in units that are currently active, and recovers only in units that are resting — fast (Type II) units fatigue several times faster than slow (Type I) units, mirroring their glycolytic vs. oxidative metabolism:

active:   dF_max,i/dt = −F_max,i / τ_fatigue,i     (τ_fatigue,i small for fast units)
resting:  dF_max,i/dt = (F0_i − F_max,i) / τ_recover,i   (recovery is always slower than fatigue)

Because total output is target-force minus what's already fatigued, holding a constant target under sustained effort forces the size principle forward in real time: as early units fatigue and lose capacity, additional (larger, faster-fatiguing) units must recruit to make up the shortfall — the same mechanism behind the burning, shaking sensation near failure in a sustained muscular contraction.

Type I fiber Type IIa fiber Type IIx fiber

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