How Light Is Too Light for Maximizing Muscle Growth?

Описание: What loading may be too light to maximzie muscle hypertrophy? This video, we'll assess some theoretical & experimental evidence, with the aim of answering this.

2 meta-analyses indicate light loads can produce similar growth to heavier loads, when reps are performed to or close to failure. In the 2 meta-analyses, most studies used loads from 30%-85% of a person's one-rep max (1RM).

For instance, Mitchell et al. had subjects perform 3 sets to failure on the leg extension with a 30%1RM or 80% 1RM, 3x per week for 10 weeks. Increases in quadriceps volume were similar when using a 30% or 80% 1RM load.

A strong reason behind why loads as light as this can produce similar growth to heavier loads relates to mechanical tension, the best-understood mechanism of hypertrophy at this time.

A key component of mechanical tension is active tension. This is equal to the force produced by a muscle. High levels of muscle fiber recruitment & high levels of force generated by the recruited muscle fibers means high levels of active tension.

Heavier loads readily demand high levels of muscle fiber recruitment & high levels of force produced by the muscle fibers. As you near failure, these things would likely increase further.

But, evidence indicates light loads can achieve this too. Taking a light load to or very close to failure will require increased muscle fiber recruitment & force produced by the muscle fibers. Resulting in a similar stimulus to heavier loads.

If we were to take these ideas alone, an interesting question arises. Why would a load be too light for muscle growth? So long as we take any load to or very close to failure, surely high levels of muscle fiber recruitment, & high levels of force produced by those fibers, are present?

1000 reps, so long as the last rep was a failure, should be as good as 10 reps. Instinctively, this does not sound right.

One potential hypothesis that can help us here relates to central fatigue.

Remember, the reason muscles produce force is because they receive electrical signals originating from the central nervous system. The more electrical signals a muscle receives, the more muscle fibers are recruited & the more force produced by the recruited muscle fibers.

Central fatigue refers to a decrease in the electrical signals sent down to the muscle by the nervous system, resulting in a reduction in muscle fiber recruitment & less force produced by recruited muscle fibers.

Low load longer duration sets appear to produce more central fatigue than higher load shorter duration sets. Yoon et al. had 9 men and 9 women, on separate days, perform a contraction of their biceps at either 20% of their maximum voluntary contraction force or 80% of their maximum voluntary contraction force. On average, subjects held the 20% contraction for around 14 minutes before they could no longer produce the required force, whereas the 80% contraction was only held for around 25 seconds. But, the 20% contraction produced greater central fatigue than the 80% contraction.

During sets on typical exercises, some degree of central fatigue will probably be present. Lighter loads, when taken to or close to failure, would produce more central fatigue than heavier loads.

Given evidence indicates loads as light as 30%1RM can produce similar hypertrophy to heavier loads, we can safely say any central fatigue produced with this load is not an issue.

But it possible that some load below 30% of your one-rep max, whereby if taken to failure, may fail to achieve sufficiently high levels of muscle fiber recruitment & force produced by recruited muscles, due to the interference of central fatigue.

Let us now look at the actual research.

A few papers have assessed the impact of using loads below 30%1RM for growth.

Jessee et al. found that increases in anterior and lateral thigh thickness were similar when using a 15% of 70%1RM load on the leg extension, indicating a 15% 1RM load may still be able to maximize hypertrophy.

However, Buckner et al. found that increases in elbow flexor thickness was greater for a group using a 70%1RM load versus a 15%1RM load on the dumbbell biceps curl. Suggesting a 15%1RM load may be too light to maximize muscle growth.

Looking at other research, Lasevicius et al. found increases in elbow flexor and vastus lateralis cross-sectional area were similar when using a 40, 60, or 80% 1RM load on the dumbbell curl and leg press and all greater than when using a 20% 1RM load. Indicating a 20%1RM load is not able to maximize muscle growth.

A study by Roie et al. further confuses things. They found that in older subjects, using a 20%1RM load on the leg extension produced similar increases in upper leg volume to using an 80%1RM load.

Due to the conflicting evidence, it's probably sensible to ensure that if your goal is hypertrophy, most of your training should be done within the 30%-85%1RM zone

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