Over the last few years, researchers have discovered that it is possible to achieve meaningful muscle growth when lifting light weights, so long as sets are performed to muscular failure.
Additionally, some studies indicate that training to failure may lead to more hypertrophy than avoiding failure.
Muscular failure during strength training is simply the point at which fatigue is high enough to prevent a muscle from exerting the amount of force necessary to complete the current repetition, with a given load. Yet, if we lower the weight after reaching failure (as when using drop sets), we can immediately continue exercising.
So why does achieving this level of fatigue during strength training help increase the amount of muscle growth that occurs? And does it always apply, or are there some situations when it is not necessary?
How does muscle growth happen?
Muscle growth is primarily determined by mechanical loading on individual muscle fibers. Once stimulated by this mechanical loading, the individual muscle fibers then adapt, and increase in diameter.
In general, for an individual muscle fiber to experience this mechanical loading during a strength training exercise, it needs two things.
- It needs to be activated.
- It needs to contract at a speed that is slow enough to allow sufficient actin-myosin bindings to form simultaneously, and thereby produce a high level of mechanical loading on the individual muscle fiber. Fast fiber contraction velocities do not allow this to occur, because the higher contraction speed causes actin-myosin bindings to detach too quickly, and this keeps the mechanical loading on the individual muscle fiber low.
(Note: muscles can grow when they are subjected to prolonged passive stretching without activating any of the muscle fibers, but this does not typically occur during strength training, as a long period of time in a lengthened position seems to be necessary).
If we look at how each of these two conditions are brought about when lifting heavy and light weights, we can see how they produce hypertrophy.
What happens when we lift heavy weights?
When we lift a heavy weight (>85% of our maximum strength), the central nervous system recruits all motor units regardless of the amount of fatigue, because it needs all of the available muscle fibers to contract in order to produce the necessary level of force. Thus, all muscle fibers are activated from the beginning of a set.
As we do additional repetitions and experience the onset of fatigue, this causes an increase in the rate of signals from the central nervous system (which is called motor unit firing frequency), but it does not produce an increase in the amount of motor unit recruitment.
When lifting a heavy weight, the muscle fibers can only contract slowly, because of the force-velocity relationship. Therefore, the fibers that are activated are also subjected to a high level of mechanical loading.
Consequently, all muscle fibers are both (1) activated, and (2) contracting slowly. This enables heavy strength training to produce hypertrophy, (largely) irrespective of the degree of fatigue that is present. And this is why cluster training with heavy loads very often produces quite similar amounts of muscle growth to straight sets.
What happens when we lift light weights?
When we lift a light weight without fatigue, the amount of motor unit recruitment depends upon the velocity.
If we use a maximum velocity, the central nervous system recruits all motor units, and activates all muscle fibers. This happens because it needs all of the muscle fibers to contract in order to exert the required level of force, because the force-velocity relationship means that the force produced by each individual muscle fiber is low.
When using a maximum velocity, although all of the muscle fibers are active, the contraction velocity is high, so the individual muscle fibers are not stimulated by a high level of mechanical loading, and therefore do not grow. This is why high-velocity exercises like plyometrics and ballistic strength training produce little muscle growth.
In contrast, when we lift a light weight at a deliberately slow tempo without fatigue, the central nervous system recruits very few motor units. So even though the activated muscle fibers are contracting slowly and are exposed to a high level of mechanical loading, the muscle fibers of the high-threshold motor units are not activated, and these are the ones that are most able to grow. This is why deliberately slowing down the lifting (concentric) phase of an exercise has no beneficial effects for bodybuilders.
What happens when we lift light weights to failure?
When lifting light weights for a high number of repetitions in a single set, fatigue arises. This fatigue occurs primarily through the accumulation of metabolites inside the muscle.
The accumulation of metabolites has bee proposed to interfere with the actin-myosin bindings, and in this way reduces the ability of each individual muscle fiber to produce maximum force. The onset of fatigue also reduces their maximum contraction velocity.
Therefore, in order to keep force at the required level as we approach muscular failure, the central nervous system increases the number of motor units that are recruited. The greater number of fibers that are activated compensates for the lower force being exerted by each individual muscle fiber.
As muscular failure approaches, the central nervous system recruits all of the available motor units to help. The full motor unit recruitment at a slow contraction velocity then provides sufficient mechanical loading on the muscle fibers linked to the high-threshold motor units such that they are stimulated to grow.
This is how lifting light weights to failure produces hypertrophy.
What is the difference between heavy and light loads?
When we lift a heavy load (<85–90% of maximum strength), we achieve maximal motor unit recruitment from the first repetition of a set, and there is no need to increase fatigue further in order to recruit more motor units. In addition, the bar speed is slow enough because of the force-velocity relationship such that sufficient mechanical loading is experienced by the working muscle fibers.
In contrast, when we lift light or only moderately-heavy loads, we do not achieve maximal motor unit recruitment from the first repetition of a set, and therefore fatigue is essential if we want to recruit the high-threshold motor units that produce the necessary stimulus for muscle growth.
Therefore, training to failure is likely only helpful for enhancing muscle growth when lifting loads that are lighter than approximately 85–90% of maximum strength, and not when using heavy loads.
What is the takeaway?
Training to failure can be used to increase motor unit recruitment when lifting light or moderately-heavy loads, which in turn increases the likelihood that the muscle fibers attached to high-threshold motor units will grow.
However, since heavy loads already achieve full motor unit recruitment even without the onset of fatigue, it seems unlikely that training to failure will have any incremental benefit under those circumstances.