Unpack the biological feedback loop between progressive tension, mechanotransduction, and sustained myofibrillar protein synthesis.
Why does doing the same workout with the same weight stop producing results after a few weeks? To grow, muscle fibers must encounter tension that exceeds their established tolerance. Here is the exact biology linking progressive overload to myofibrillar remodeling.
Henneman's Size Principle & Motor Recruitment
Under Henneman's Size Principle, the central nervous system recruits motor units in order of increasing size: low-threshold motor units (small slow-twitch Type I fibers) fire first, followed by high-threshold motor units (large, fast-twitch Type IIa and IIx fibers) as mechanical demand rises.
High-threshold motor units control the fibers with the greatest capacity for growth. By progressively adding load or performing sets close to failure, you ensure that these critical high-threshold units are fully engaged.
The 'Stimulating Reps' Model
Research suggests that not every repetition in a set provides an equal hypertrophy stimulus. In a set of 10 repetitions taken to failure, the initial 5 reps feel relatively effortless and primarily recruit low-threshold fibers. The final 3 to 5 repetitions—where contraction velocity visibly slows—force the recruitment of high-threshold fibers under high mechanical strain.
To maximize stimulating repetitions without generating excessive systemic fatigue, terminate the majority of your working sets 1 to 3 repetitions shy of absolute failure (1–3 RIR).
References
- Henneman, E., Somjen, G., & Carpenter, D. O. (1965). Functional significance of cell size in spinal motoneurons. Journal of Neurophysiology, 28(3), 560-580.
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