Let’s explain muscle fibre recruitment

For decades the fitness industry told the same story when it came to how we build strength which is lift heavy weights for 6-12 reps. But more research is challenging that idea. Interestingly, low-load resistance training can build similar muscle mass to heavy training only if the sets are taken close to failure.

When you lift heavy weights, your body recruits a large number of muscle fibres immediately. These include high-threshold motor units (also known as fast-glycolytic), which control the powerful Type II muscle fibres responsible for strength and growth. With lighter weights, the process looks different. At first, your body recruits mostly lower-threshold fibres. But as fatigue builds and those fibres can no longer maintain force… Your nervous system begins recruiting additional fibres to keep the movement going.

Eventually, if the set continues long enough, nearly all available muscle fibres become involved. This is why taking lighter loads close to failure can stimulate similar hypertrophy.

One of the most interesting insights from the research is the role of fatigue. Fatigue appears to stimulate muscle growth both indirectly and directly.

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Indirect mechanisms include:

• Increased muscle fibre recruitment

• Mechanical tension on newly recruited fibres

• Cellular signalling pathways related to growth

Direct mechanisms may include:

• Metabolic stress (build-up of metabolites like lactate)

• Cell swelling

• Local inflammation and oxidative stress

As the muscle fatigues, more biological signals for growth are triggered.⁠

They tend to produce:

• Greater strength adaptations

• Faster recruitment of high-threshold motor units

• Less time required per set

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Takeaway

Muscle growth is about how much of the muscle you challenge. Heavy loads do that quickly. Lighter loads can do it too only if the effort is high enough. Muscle adapts to challenge, not just weight.