Full mechanism, in text

Each contraction sends three signals

1 · Motor unit

The basic unit of contraction is the motor unit, one α-motor neuron with all the fibres it innervates, and it either fires fully or not at all.The basic unit of muscle contraction isn't "a single fibre" but one α-motor neuron plus all the fibres it innervates, called a motor unit (MU):

· Small MU: one neuron controls a few Type I (slow, oxidative) fibres; fatigue-resistant, suited to sustained light effort
· Medium MU: mostly Type IIa fibres, the workhorses of daily activity
· Large MU: one neuron controls many Type IIx (fast, glycolytic) fibres, the source of explosive force

How many fibres one motor unit controls ranges from a handful to over a thousand, depending on the muscle: muscles for fine movements have small units, the big leg muscles have large ones.

Key: a motor unit either fires fully or not at all (all-or-none) — force is graded by how many MUs you recruit, not by "firing one MU lighter".

So the physical implementation of "how much force do I want?" in the brain is "which MUs to fire".

2 · Low force · small MUs only

Low-force tasks such as walking or writing recruit only small Type I MUs, because motor neurons are recruited by cell-body size, small first and large last.Henneman size principle (1957): motor neurons are recruited in order of cell-body size — small first, large last.

A small cell body = high input impedance = the same synaptic current pushes it past threshold more easily.

Walking / writing / standing still:
· Only recruits small Type I MUs
· Force demand is far below maximum voluntary contraction (MVC)
· These fibres have lots of mitochondria + are fatigue-resistant — walking an hour is barely tiring.

Key implication: an easy walk rarely calls on IIa and almost never on IIx — they are there when you need them, but if they are not woken up for a long time, they are hard to maintain.

3 · High force · IIx recruited

As force demand rises, IIa MUs join in, and IIx is recruited only near maximal effort or when a set is taken close to failure.As force demand rises, medium-sized IIa MUs join in, and IIx only fires near maximal effort (the exact threshold differs between muscles):

· Smaller forces: I + some IIa — jogging, light-load resistance work
· Moderate forces: I + IIa — moderate-intensity training, climbing steep hills
· Near-maximal forces: + IIx — heavy squats, sprints, jumps

Recruitment under fatigue: at a given load, as the muscle nears exhaustion, small MUs lose output and the nervous system is forced to recruit larger MUs as relay — so any intensity, taken to failure, eventually recruits IIx. This also explains why light weights taken close to failure still build muscle (Schoenfeld 2017).

Clinical implications:
· Sarcopenia in older adults is mainly Type II fibre atrophy — walking alone struggles to reverse it; strength training that is heavy enough or taken close to failure is needed
· Resistance training grows Type II fibres more clearly than endurance training does

4 · Training implications

The size principle explains several training facts: light weights to failure and heavy weights both recruit IIx, while long easy cardio and walking barely wake it.Once you understand Henneman, several apparently contradictory training facts make sense:

"Light weight to failure" ≈ "heavy weight low reps" (roughly):
· 30% × 30 reps to failure — IIx is recruited in the final reps
· 85% 1RM × 5 reps — IIx is recruited directly
· The two produce similar hypertrophy (Schoenfeld 2017 ), but the light-weight route takes longer and feels worse, so most people find moderate-to-heavy weights easier.

"Long-duration light cardio doesn't train IIx":
· A 5 km easy run barely recruits IIx — long-distance runners actually have a lower IIx proportion
· Likewise, long low-intensity rowing in the gym is no substitute for barbell training.

"Walking isn't enough for older adults":
· Walking maintains Type I fibres + cardiopulmonary fitness, but IIx keeps atrophying
· Fall prevention and fast force production → need strength training that is hard enough
· The US Physical Activity Guidelines (HHS 2018) also advise older adults to do muscle-strengthening work on at least 2 days a week, plus balance training.

"Neural recruitment efficiency vs muscle mass":
· In the first weeks of resistance training, most of the strength gain comes from improved neural recruitment (calling on more MUs at once, firing them in better sync), not from real growth
· This is why beginners gain strength quickly without obvious size change
· After that, the share from muscle growth gradually rises (see neural-vs-hypertrophy).

Full mechanism, in text

1 · Motor unit

2 · Low force · small MUs only

3 · High force · IIx recruited

4 · Training implications

Size principle · motor unit recruitment

Full mechanism, in text

1 · Motor unit

2 · Low force · small MUs only

3 · High force · IIx recruited

4 · Training implications

Size principle · motor unit recruitment