Why do very light loads not produce as much muscle growth as light loads?

Recently, a study was published comparing the long-term effects of strength training with very light (20% of 1RM), light (40% of 1RM), moderate (60% of 1RM), and heavy (80% of 1RM) loads. All sets were performed to failure, and volume load (sets x reps x load) was matched between conditions.

Muscle growth after 12 weeks was similar in the conditions that involved light (40% of 1RM), moderate (60% of 1RM), and heavy (80% of 1RM) loads.

Yet, training with very light (20% of 1RM) loads produced about half the amount of muscle growth as the other conditions (both in absolute terms and also by reference to effect sizes).

Why did this happen?

How does strength training produce muscle growth?

To understand exactly how the volume-matched, proximity-to-failure-matched very light load (20% of 1RM) strength training program was unable to produce the same amount of muscle growth as the heavier load programs, we need to start right at the very beginning.

How does strength training cause muscle growth?

There are three proposed mechanisms by which muscle fibers increase in size (mechanical loading, metabolic stress, and muscle damage), but only mechanical loading has a strong weight of evidence behind it. Conceivably, the other mechanisms may be effective simply because they also stimulate mechanical loading on individual fibers (but we do not know for sure yet).

For an individual muscle fiber to experience mechanical loading during a strength training exercise, it needs both:

1. to be activated; and

2. to contract at a speed that is slow enough to allow enough actin-myosin bindings to form simultaneously (the number of simultaneous actin-myosin bindings affects the force produced by a fiber, and this is determined by the contraction velocity).

When lifting heavy loads — most motor units are quickly recruited, and the weight is heavy enough that a fast bar speed is impossible. Thus, heavy loads automatically cause (1) activation of fibers attached high-threshold motor units, and (2) enough actin-myosin bindings to form simultaneously.

When lifting light loads to failure — few motor units are initially activated, but as metabolites accumulate and cause fatigue, high-threshold motor units are recruited in order to compensate for the reduced capacity for force production in the working muscle fibers. Similarly, bar speed is initially quick, but as metabolites accumulate and cause fatigue, bar speed involuntarily slows down. By the end of a set to failure, light loads also cause (1) activation of fibers attached high-threshold motor units, and (2) enough actin-myosin bindings to form simultaneously.

Strength training, either with heavy loads, or with light loads to failure, therefore imposes mechanical loading on individual muscle fibers, and therefore brings about hypertrophy.

(N.B. deliberately slowing down bar speed with light loads does not work, because it reduces motor unit recruitment even though it increases the number of actin-myosin bindings that form simultaneously).

Why do very light loads work less effectively?

When we lift light loads to failure, the mechanical loading produced on the muscle fibers only occurs if the fatigue we experience leads to increased motor unit recruitment.

Fatigue is a very complex phenomenon that arises from multiple causes, and its nature is hotly debated by researchers. Even so, it is generally agreed that the nature of fatigue changes as we move away from short-duration, anaerobic activities and towards longer-duration, aerobic activities.

Fatigue in very intense, short-duration efforts is primarily peripheral in nature, and seems to be linked mainly to the accumulation of metabolites, while fatigue in longer-duration efforts involves a greater central component.

When training with light (40% of 1RM) or moderate (60% of 1RM) loads, it is likely that fatigue is caused primarily by the accumulation of metabolites. This accumulation of metabolites interferes with actin-myosin binding capability, and reduces individual muscle fiber force. This reduction in individual muscle fiber force is then compensated for by the recruitment of high-threshold motor units.

When using very light (20% of 1RM) loads, fatigue may be caused by both accumulated metabolites as well as other mechanisms, and this may mean that a smaller amount of motor unit recruitment occurs. For example, if there was a larger amount of central fatigue, this would reduce voluntary force by decreasing neural drive to the muscle. This would lead to a set of reps being stopped even before high-threshhold motor units have been recruited.

What does this mean in practice?

There is probably a threshold repetition-maximum (RM) that represents a shift from one type of fatigue to another.
In this study, the light load (40% of 1RM) condition involved approximately 30 reps to failure, while the very light load (20% of 1RM) condition involved approximately 65 reps to failure.

Somewhere between these two repetition maximums (RMs) is the point at which fatigue shifts from being primarily peripheral (and caused by the accumulation of metabolites), to being affected by other factors, including those that may be central in nature.

For bodybuilders, staying under a 30RM is probably a good plan in practice.

N.B. For researchers, it bears pointing out that if the nature of fatigue is the factor that determines the smaller hypertrophic response with very light loads, then categorizing each type of relative load based on repetition maximums makes a lot more sense than using percentages of 1RM, because it is exercise duration that seems to alter the nature of fatigue, and not the size of the load in relation to the maximum force that can be exerted.

What is the takeaway?

Strength training to muscular failure means training to the point where fatigue prevents you from doing another repetition. Depending on the size of the relative load (percentage of 1RM) you are lifting, the nature of the fatigue will probably be different. It seems likely that only (peripheral) fatigue that involves (the accumulation of metabolites and therefore) increased motor unit recruitment will stimulate muscle growth, and this type of fatigue may not be present at loads lighter than approximately 30RM.