Traditional strength training guidelines recommend loads ≥70% of 1RM to maximize hypertrophy.
However, low-load resistance exercise training (RET) to failure (≤60% 1RM) can yield similar muscle growth.
This review explores whether muscle fatigue itself is a stimulus, directly and indirectly, for hypertrophy in low-load RET.
Does fatigue itself drive skeletal muscle hypertrophy in low-load resistance training?
What did the researchers do?
The authors explored indirect and direct mechanisms by which fatigue could stimulate hypertrophy.
Covered concepts include:
- Fiber recruitment
- Mechanical tension
- Muscle damage
- Hormonal responses
- Metabolic stress, inflammation, and oxidative signaling
Proposed mechanisms contributing to low-load fatigue-induced hypertrophy:
What were the results?
Indirect Mechanisms
- Increased motor unit recruitment (especially type II fibers) due to fatigue.
- Mechanical tension shifts to newly recruited fibers as early fibers fatigue.
- Exercise-induced muscle damage (EIMD) triggers repair signaling but may not directly drive hypertrophy.
- Hormonal changes (testosterone, GH, IGF-1) rise with fatigue but may not be causally linked to growth.
- Myostatin suppression may be enhanced with fatiguing work, especially with BFR.
Direct Mechanisms
- Lactate & metabolic by-products accumulate more in low-load sets to failure but likely have limited hypertrophic impact in humans.
- Oxidative stress & inflammation may activate anabolic pathways, but there's a “Goldilocks” zone: too much or too little may blunt gains.
- Cell swelling is proposed to signal growth via mTOR and MAPK activation.
Load-Reps Recruitment
Proposed effects of load on muscle recruitment, damage, and fatigue in single muscle fibers between the first and last repetition of acute resistance exercise.
What does this mean?
Fatigue is a critical requirement in low-load RET for maximizing hypertrophy, largely through indirect mechanisms (fiber recruitment and tension).
- Training to failure is more important with low loads than with high loads.
- Direct metabolic signals (e.g., lactate, ROS) likely play a supporting rather than a leading role.
- Fatigue-induced recruitment is the central mechanism, particularly activating high-threshold motor units that wouldn’t be recruited at low loads without fatigue.
Coach's Takeaway
- Training with lighter loads requires sets to be taken close to failure to stimulate muscle growth.
- Don’t obsess over lactate or hormones; focus on execution and proximity to failure.
- Fatigue and proximity to failure drive fiber recruitment.
