How do different types of fatigue affect hypertrophy and recovery?

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Lifters often talk about fatigue interfering with their ability to train, or to make progress. Moreover, strength training programs are often designed or periodized with the specific goal of managing fatigue. But what is fatigue anyway, and how does it cause negative effects on the results we get in the gym?

What is fatigue?

Although most people think of fatigue as being a subjective sensation, it is actually an objective measurement. It is a temporary and reversible reduction in our capacity to produce voluntary force with a muscle, as a result of a previous bout of exercise.

We are experiencing fatigue in a muscle if it can currently produce less force than it could previously, before we exercised (whether we are feeling tired or not is largely irrelevant).

Fatigue happens during sets of strength training, within each individual workout. When fatigue reaches the point such that we can no longer produce enough force to lift the weight on the bar in a set, we say that we have reached muscular failure.

However, fatigue also happens after a strength training workout. For several hours (and sometimes even several days) afterwards, we still cannot produce as much force with the worked muscle as we did during the workout itself.

The various underlying mechanisms that cause these two phenomena are similar but not identical. To understand them requires us to understand what actually causes fatigue in the first place.

What causes fatigue?

Fatigue occurs because of mechanisms inside the central nervous system (central fatigue) and inside the muscle (peripheral fatigue).

Peripheral fatigue is further divided into two separate groups of mechanisms: (1) those that are very transitory and only occur during and immediately after exercise (these are confusingly also referred to as “peripheral fatigue”) and (2) those that are longer lasting and tend to occur in the hours and days after exercise (these are termed “muscle damage”).

In practice, we therefore tend to refer to central fatigue, peripheral fatigue, and muscle damage as separate mechanisms.

Central fatigue refers to reductions in our ability to produce voluntary force that result from actions within the central nervous system.

Central fatigue can occur either because of a reduction in the size of the original signal sent from the brain or the spinal cord, or because of an increase in afferent feedback that subsequently reduces motor neuron excitability.

Central fatigue can affect our ability to produce force both within a workout and also after a workout.

Although some strength training experts and researchers believe that the central fatigue that occurs after a workout happens as a result of reaching high levels of motor unit recruitment within the workout, this is actually highly unlikely. It is more likely that central fatigue occurs secondary to peripheral factors, including afferent feedback, aerobic demand, and aspects of muscle damage.

Peripheral fatigue refers to reductions in the ability of the muscle itself to produce force, regardless of the signal from the central nervous system. It happens through two separate mechanisms: (1) reductions in the activation of individual muscle fibers, and (2) reductions in the ability of individual muscle fibers to produce force.

  • Reductions in the activation of individual muscle fibers seem to occur due to either (1) a decrease in the sensitivity of actin-myosin myofilaments to calcium ions (perhaps due to the actions of reactive oxygen species), or (2) a reduction in the release of calcium ions from the sarcoplasmic reticulum, (perhaps due to an accumulation of other ions, such as potassium).
  • Reductions in the ability of individual muscle fibers to produce force involves impairments actin-myosin crossbridge function, likely due to the generation of some metabolic byproducts (phosphate ions and adenosine diphosphate). In contrast, lactate accumulation (and also the associated accumulation of hydrogen ions, when taken in isolation) is not central to the process of fatigue.

Importantly, peripheral fatigue only affects our ability to produce force within a workout, and does not affect our ability to produce force for very long after a workout.

Muscle damage occurs when the internal structures of a muscle fiber, or its outer wrapping layers, are disrupted. These disruptions cause a reduction in our ability to exert force with the fiber (although an immediate repair process is triggered when this happens, which prevents complete loss of function even at the muscle fiber level).

Muscle damage is probably not caused solely by stretching muscle fibers forcibly, although eccentric contractions do produce muscle damage more easily than other types of loading. In fact, muscle damage very likely also occurs in response to the build-up of intracellular calcium and inflammatory neutrophils during any kind of fatiguing contractions, because these degrade the inside of the muscle fiber.

Importantly, muscle fibers can be damaged to varying degrees. The myofibrils and the cytoskeleton that supports them are most easily damaged. This can be observed as shifts in the position of the Z disk, which is an easily identifiable feature of the sarcomere. The outer wrapping layers of the muscle fiber are also easily damaged, which makes them more permeable. When they become permeable, this causes some of the contents of the muscle fiber to leak out into the spaces between muscle fibers, and subsequently into the bloodstream, which is observed as an elevation in creatine kinase levels.

After being damaged, muscle fibers undergo one of two processes, depending on the extent of the damage. When the damage is minor, then the fiber is repaired. The existing structures are retained, but any broken parts are removed and replaced with new proteins. If the fiber is too badly damaged to be repaired, such as when it is torn completely in half, it becomes necrotic and dies. When this happens, the fiber is degraded inside of its cell membrane, and a new, replacement fiber is grown inside it. This is called regeneration.

Muscle damage probably only affects our ability to produce force after a workout, and is not a major factor that leads to fatigue within the workout itself.

How do the various types of fatigue affect the strength training stimulus?

When they are present (either because they are stimulated to occur during the workout itself, or because they are still present from a previous workout), central fatigue, peripheral fatigue, and muscle damage each have slightly different effects on the result of a workout.

Central fatigue most likely prevents full motor unit recruitment to be reached, which is why training to muscular failure with very light (20% of 1RM) loads does not cause as much muscle growth as training to muscular failure with light (40% of 1RM) loads. Although all sets with all loads involve both central and peripheral fatigue, the sets with very light loads reach failure through a greater proportion of central fatigue, due to the greater aerobic demand. Therefore, the presence of central fatigue (whether caused by previous sets in the workout itself, or when caused by a previous workout) has a negative impact on the stimulating effects of a strength training workout.

When considered in isolation, and ignoring its effects on central fatigue, peripheral fatigue seems to increase motor unit recruitment levels during strength training. This makes a lot of sense, because when the working muscle fibers are fatigued, other muscle fibers must be activated in order to maintain the desired levels of force. Therefore, peripheral fatigue can be beneficial during a workout, because it enables us to increase motor unit recruitment, and thereby train more muscle fibers.

When considered in isolation, and ignoring its effects on central fatigue, muscle damage seems to interfere with the stimulating effects of a workout in at least two ways. Firstly, the associated oxidative stress seems to inhibit elevations in post-workout muscle protein synthesis rates directly. Secondly, the need to use some of the muscle protein synthesis to repair muscle damage seems to reduce the proportion that can be directed towards increasing the size of the muscle fibers. Therefore, the presence of muscle damage from a previous workout can have a negative impact on the stimulating effects of a strength training workout, although these effects are probably more short-lived than the secondary effects on central fatigue.

How long do the various types of fatigue last?

Peripheral fatigue dissipates very quickly, and its effects only ever have any effect on us within the workout itself.

Muscle damage is the opposite, insofar as it normally does not affect our ability to produce force during a workout, but does affect our ability to produce force after the workout. However, the duration of this effect can vary a great deal, depending on the extent of the damage. Minor muscle damage is repaired over a few days, with severe cases taking up to a week. Severe muscle damage requiring the regeneration of muscle fibers can take a month or even longer. While muscle fibers are usually only repaired (and not regenerated) after conventional strength training, signs of necrotic and regenerating fibers have been observed in high-level strength athletes.

Central fatigue is more complex, because it can occur both within the workout and also after the workout.

What else can affect the amount of central fatigue we experience in a workout?

The amount of muscle mass that is used in an exercise also affects the amount of central fatigue.

Research has shown that when we perform exercise that involves a smaller amount of muscle mass, we can tolerate greater peripheral muscular fatigue than when we perform exercise that involves a larger amount of muscle mass. In contrast, when we perform an exercise that involves a larger amount of muscle mass, we tend to experience greater central fatigue, and therefore we stop before reaching as high a level of peripheral muscular fatigue.

This fascinating phenomenon has been observed when comparing single-joint and multi-joint exercises, single-leg and two-leg exercises, and larger leg muscles with smaller arm muscles.

We can therefore reduce the amount of central fatigue that we experience by using exercises with smaller amounts of muscle mass, such as those that involve only single joints, or single limbs. Such exercises may be particularly beneficial at the end of a workout, when central fatigue is naturally high.

Even so, performing exercises with high levels of peripheral fatigue causes more post-workout muscle damage, because the muscle fibers are exposed to high levels of intracellular calcium for a longer period of time. Consequently, using exercises that *decrease* central fatigue within a workout actually *increases* central fatigue after the workout, because muscle damage is the main determinant of post-workout central fatigue. Such exercises may therefore be most appropriate in body part split routines, where high volumes are performed for individual muscles in less frequent workouts.

What does this mean in practice?

In practice, we want to maintain central fatigue as low as possible during strength training workouts, otherwise our sets will reach task failure before all of the motor units within the muscle have been recruited. If we are experiencing central fatigue, then we will think that we are training hard, because we are still reaching muscular failure, but the muscle fibers of the high-threshold motor units will not be working.

We need to manage central fatigue both within each workout, and also from one workout to the next.

Within each workout, central fatigue seems to be increased by training methods that involve strong negative sensations, such as high reps and short rest periods, as well as by using exercises with a large muscle mass, perhaps due to the greater aerobic demand that this causes. This may be why lower levels of motor unit recruitment are recorded during isometric contractions performed to task failure. Therefore, using heavy or moderate loads (1–15RM) may be better than light loads (>15RM) for long-term muscle growth. In practice, it is hard to accumulate enough stimulating reps using heavy (1–5RM) loads, and the higher end of moderate loads can still involve a lot of afferent feedback. So the low end of moderate loads (6–8RM) seems optimal. This rep range is actually what most competitive bodybuilders use (contrary to what you may read elsewhere).

After each workout, central fatigue can be heightened after workouts that involve (high volumes of) muscle-damaging contractions. This may explain why short periods of high volume eccentric training fail to increase strength as effectively as we might expect. In practice, this means that training too frequently will fail to produce optimal results, as will using excessive training volumes or advanced techniques that produce a lot of muscle damage as a result of extended periods of peripheral fatigue.

What is the takeaway?

Fatigue is a temporary and reversible reduction in our capacity to produce voluntary force with a muscle, as a result of a previous bout of exercise. It occurs through central and peripheral mechanisms. Some of these fatiguing mechanisms lead to beneficial effects, such as increased motor unit recruitment during a set of an exercise, while others produce negative effects, such as decreased motor unit recruitment or impaired post-workout rates of muscle protein synthesis. Knowing how these mechanisms occur is critical for an understanding of how fatigue affects the adaptations to strength training.

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