Recruitment and tension

A fibre needs two things at once. Miss either and the rep is a fatigue tax.

#Condition one — recruitment

Motor units fire in strict ascending order of size. This is Henneman's size principle, and it is about as settled as anything in motor control.

Small, low-threshold units driving fatigue-resistant type I fibres go first. Large, high-threshold units — driving the type II fibres with the most growth potential — go last.

The important part: recruitment tracks effort, not load. Full recruitment arrives around 90% of maximal unfatigued force, roughly a 5RM. But you reach the same recruitment with a lighter weight by taking the set close enough to failure that accumulating fatigue forces the nervous system to call up the remaining units.

That is why a set of 25 to failure and a set of 6 to failure produce similar growth per set. The load is not the stimulus.

#Condition two — mechanical tension per fibre

An activated fibre that is not loaded hard does not grow.

Force in a sarcomere is a function of how many myosin heads are bound to actin at any instant. During fast shortening, heads detach before completing their power stroke and have to rebind — few crossbridges engaged, low per-fibre force. During slow shortening, they attach, stay attached longer, and accumulate — high per-fibre force.

That is the force-velocity relationship, and it is why a grinding rep at the end of a set transmits far more tension per fibre than a fast one early on, despite feeling like less total effort.

#Where both are satisfied

Only in the last handful of reps before failure — call it five. Early reps are fast (low tension) and incompletely recruited. Late reps are slow (high tension) and fully recruited.

Everything before that is the price of admission.

#What this immediately explains

• Why load barely matters between 5RM and 30RM at matched proximity to failure
• Why "feeling it" is not a stimulus measure
• Why junk volume is worse than no volume — it costs recovery and delivers nothing
• Why proximity to failure is the single most important variable in the program

Source tier: Henneman's principle is primary literature and long-settled. The stimulating-rep synthesis is Chris Beardsley's, argued from primary literature — read it as a well-supported model rather than consensus. See source tiers.