How to Build Muscle With Light Weights
It can be done, but there are some rules.
This 2025 narrative review by Flewwelling et al. examines the potential mechanisms by which muscle fatigue during low-load resistance exercise training (RET) might stimulate skeletal muscle hypertrophy. The review synthesises current evidence and proposes both direct and indirect pathways linking fatigue to muscle growth, with a focus on practical implications for lifters.
PMID: 39726254
Aim
The primary aim is to explore whether muscle fatigue acts as a direct or indirect mechanism contributing to skeletal muscle hypertrophy during low-load RET, and to summarise the current literature on this topic.
Methods
An evidence-based narrative review of existing studies on low-load RET and muscle hypertrophy.
Analysis of both physiological and molecular mechanisms, comparing high-load and low-load RET performed to or near task failure.
Key Points
1. Low-Load RET Can Stimulate Similar Hypertrophy as High-Load RET
When performed to or near task failure, low-load RET (≤60% 1RM) can produce muscle growth comparable to traditional high-load RET (>60% 1RM).
This is especially relevant for those seeking alternatives to heavy lifting due to joint concerns or preferences.
2. Fatigue as a Key Stimulus
Proximity to task failure is crucial for maximising hypertrophy with low-load RET.
Fatigue during low-load RET may be essential for recruiting all available muscle fibers, including high-threshold motor units typically activated by heavier loads.
3. Indirect Mechanisms of Fatigue-Induced Hypertrophy
Increased Fiber Recruitment: As fatigue sets in, more muscle fibers (including type II fibers with high growth potential) are recruited to maintain force output.
Mechanical Tension: Sustained contractions under fatigue maintain mechanical tension on muscle fibers, a known driver of hypertrophy.
Muscle Damage: Fatiguing contractions may cause minor muscle damage, stimulating muscle protein synthesis (MPS) for repair and growth.
Hormonal Responses: Fatigue may indirectly increase anabolic hormones (testosterone, GH, IGF-1), though the role of acute hormone spikes in muscle growth is debated.
Gene and Protein Expression: Fatigue may alter the expression of proteins like myostatin, which regulates muscle mass.
4. Direct Mechanisms of Fatigue-Induced Hypertrophy
Metabolite Accumulation: The build-up of by-products like lactate during fatiguing exercise may directly signal muscle growth pathways.
Inflammation and Oxidative Stress: Localised inflammation and oxidative stress from fatigue may also promote hypertrophic signalling.
5. Not All Fatigue-Inducing Exercise Leads to Hypertrophy
Only RET with sufficient load and performed to or near failure reliably stimulates muscle growth.
Fatigue from endurance exercise does not produce the same hypertrophic response and may even interfere with muscle growth from RET.
