Many times, we hear about fatigue but we don’t really know what the effect of it is on hypertrophy and recovery.
Weightlifters often speak of fatigue that interferes with their ability to train or progress. Additionally, strength training programs are often designed or periodized with the specific goal of managing fatigue.
What Is Fatigue?
Although most people think of fatigue as a subjective feeling, it is actually an objective measure.
It is a temporary and reversible reduction in our ability to produce voluntary force with a muscle, as a result of previous exercise.
We are experiencing fatigue in a muscle if it can currently produce less force than it was before exercising (whether we feel tired or not is largely irrelevant).
Fatigue usually occurs during bouts of strength or very intense training within each individual workout.
When fatigue reaches the point where we can no longer produce enough force to lift the weight over the bar in a series of “X” reps, we say we have reached muscle failure.
However, fatigue also occurs after strength training. 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 training itself.
The various underlying mechanisms that cause these two phenomena are similar but not identical. Understanding them requires that we understand what really causes fatigue in the first place.
What Causes Fatigue?
Fatigue occurs due to mechanisms within the central nervous system (central fatigue) and within the muscle (peripheral fatigue). You can see this specific process in: CNS fatigue and recovery: what happens to us after training?
Peripheral fatigue is further divided into two separate groups of mechanisms:
- Those that are very transient and only occur during and immediately after exercise(these are also confusingly referred to as “peripheral fatigue”)
- Those that are longer lasting and tend to occur in the hours and days after exercise (called “muscle damage”).
In practice, therefore, we tend to refer to central fatigue, peripheral fatigue, and muscle damage as separate mechanisms.
Central Fatigue
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 due to a reduction in the size of the original signal sent from the brain or spinal cord, or due to an increase in afferent feedback that subsequently reduces the excitability of the motor neuron.
Central fatigue can affect our ability to produce force both within a workout and after a workout.
Although some strength training experts and researchers believe that central fatigue occurs after a workout, it occurs as a result of reaching high levels of motor unit recruitment within training, this is actually highly unlikely.
Central fatigue is more likely to occur secondary to peripheral factors, including afferent feedback, aerobic demand, and aspects of muscle damage.
Peripheral Fatigue
Peripheral fatigue refers to reductions in the muscle’s ability to produce force, regardless of the signal from the central nervous system.
It occurs 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 appear to occur due to (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 involve alterations in the actin-myosin cross-bridge function, probably due to the generation of some metabolic by-products (adenosine phosphate and diphosphate ions). In contrast, the accumulation of lactate (and also the associated accumulation of hydrogen ions, when taken in isolation) is not critical to the fatigue process.
Importantly, peripheral fatigue only affects our ability to produce force within a workout, and it does not affect our ability to produce force for a long time after a workout.
Muscle Damage
Muscle damage occurs when the internal structures of a muscle fiber, or its outer sheath layers, are broken.
These interruptions cause a reduction in our ability to exert force with the fiber (although an immediate repair process is activated when this happens, preventing the complete loss of function even at the level of the muscle fiber).
Muscle damage is probably not caused solely by forceful stretching of muscle fibers, although eccentric contractions cause muscle damage more easily than other types of loading.
In fact, it is very likely that muscle damage also occurs in response to the accumulation of intracellular calcium and inflammatory neutrophils during any type of fatigue contractions, since these degrade the interior of the muscle fiber.
Importantly, muscle fibers can be damaged to varying degrees. Myofibrils and the cytoskeleton that supports them are more easily damaged.
This can be seen as changes in the position of the Z disk, which is an easily identifiable feature of the sarcomere (see information above). The outer sheath layers of the muscle fiber are also easily damaged, making them more permeable.
When they become permeable, this causes some of the content of the muscle fiber to leak into the spaces between the muscle fibers and subsequently into the bloodstream, which is seen as a rise 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, the fiber is repaired.
Existing structures are retained, but the broken parts are removed and replaced with new proteins. If the fiber is too damaged to repair, such as when it breaks in half, it becomes necrotic and dies. When this happens, the fiber breaks down within its cell membrane, and a new replacement fiber grows within 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 causes fatigue within the workout itself.
How Do Various Types of Fatigue Affect the Strength Training Stimulus?
When present (either because they are stimulated to occur during the workout itself, or because they are still present in a previous workout), central fatigue, peripheral fatigue, and muscle damage have slightly different effects on the outcome of a workout.
Central fatigue probably prevents full motor unit recruitment from being achieved, so training for muscle failure with very light loads (20% of 1RM) does not cause as much muscle growth as training for muscle failure with little load (40% of 1RM).
Although all series with all loads involve central and peripheral fatigue, series with very light loads achieve failure through a higher proportion of central fatigue, due to the higher aerobic demand. Therefore, the presence of central fatigue (either caused by previous sets in the training itself or by previous training) has a negative impact on the stimulating effects of a strength training.
When considered in isolation and its effects on central fatigue are ignored, peripheral fatigue appears to increase levels of motor unit recruitment during strength training.
This makes perfect sense, because when working muscle fibers are fatigued, other muscle fibers must be activated to maintain desired strength levels. Therefore, peripheral fatigue can be beneficial during a workout, as it allows us to increase the recruitment of motor units and thus train more muscle fibers.
Therefore, the presence of muscle damage from a previous workout may have a negative impact on the stimulating effects of a strength training, although these effects are probably more short-lived than the side effects on central fatigue.
How Long Do the Various Types of Fatigue Last?
Peripheral fatigue dissipates very quickly, and its effects only have some effect on us within training.
Muscle damage is the opposite, insofar as it does not normally affect our ability to produce force during a workout, but it does affect our ability to produce force after training. However, the duration of this effect can vary greatly, depending on the extent of the damage.
Minor muscle damage is repaired in a few days, and severe cases take up to a week. Severe muscle damage that requires regeneration of muscle fibers can take a month or even longer. While muscle fibers are generally only repaired (and not regenerated) after conventional strength training, signs of necrotic and regenerative fibers have been observed in high-level strength athletes.
Central fatigue is more complex, because it can occur both within training and after training.
- Central fatigue within a workout appears to occur secondary to (some aspects of) the peripheral fatigue that occurs during strength training, gradually accumulating over multiple sets, perhaps due to afferent feedback. The presence of central fatigue in a workout can help explain why hypertrophy is affected by the order of exercise and the length of the rest period, and could also help explain the diminishing effects of increasing training volumes above a certain threshold. The exercises we do at the beginning of training are less susceptible to central fatigue, because we are still without fatigue. Exercises we do later in training are affected by core fatigue caused by earlier exercises. Similarly, rushing through the next set by taking short rest periods may not be ideal, because it takes time for the core fatigue to dissipate from the previous set.
- Central fatigue after a workout appears to be secondary to muscle damage. In fact, central fatigue is greatest when measured in the days after workouts that cause the most muscle damage (whether the damage is from high training volumes or eccentric contractions). This central fatigue may be caused by a post-workout inflammatory response involving certain cytokines entering the brain, which are known to influence fatigue. The presence of central fatigue as a result of muscle damage is probably the reason why high training frequencies are not always more effective than less frequent workouts, and may also partly explain the interference effect caused by concurrent aerobic exercise, as this also causes muscle damage.
What Else Can Affect the Amount of Core Fatigue We Experience in A Workout?
The amount of muscle mass used in an exercise also affects the amount of core fatigue.
Research has shown that when we exercise that involves less muscle mass, we can tolerate greater peripheral muscle fatigue than when we perform exercise that involves more muscle mass. On the contrary, when we perform an exercise that involves a greater amount of muscle mass, we tend to experience greater central fatigue and therefore stop before reaching such a high level of peripheral muscle fatigue.
This fascinating phenomenon has been observed when comparing single or multi-joint exercises, single or double leg exercises, and larger leg muscles with smaller arm muscles.
Therefore, we can reduce the amount of core fatigue we experience by using exercises with small amounts of muscle mass, such as those that involve only individual joints or individual limbs. Such exercises can be particularly beneficial at the end of a workout, when core fatigue is naturally high.
Still, exercising with high levels of peripheral fatigue causes more muscle damage after training, because muscle fibers are exposed to high levels of intracellular calcium over a longer period of time. Consequently, using exercises that * decrease * core fatigue within a workout actually * increases * post-workout core fatigue, because muscle damage is the main determinant of post-workout core fatigue. Therefore, these exercises may be more appropriate in part-body 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 keep core fatigue as low as possible during strength training workouts, otherwise our sets will reach failure before all motor units within the muscle have been recruited. If we are experiencing core fatigue, we will think we are training hard, because we are still reaching muscle failure, but the muscle fibers of the high-threshold motor units will not be working.
We need to manage core fatigue both within each workout as well as from workout to workout.
Within each workout, core fatigue appears to be increased by training methods that involve strong negative sensations, such as high repetitions and short rest periods, as well as through the use of exercises with a large muscle mass, perhaps due to increased demand aerobic that this causes.
This may be the reason why lower levels of motor unit recruitment are recorded during isometric contractions performed to muscle failure. Therefore, the use of heavy or moderate loads (1–15RM) may be better than light loads (> 15RM) for long-term muscle growth.
In practice, it is difficult to accumulate enough stimulating reps with heavy loads (1–5RM), and the upper 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 bodybuilders use the most (contrary to what you can read elsewhere).
After each workout, core fatigue can increase after workouts that involve large 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 hope. In practice, this means that training too frequently will not produce optimal results, as will using excessive training volumes or advanced techniques that cause a lot of muscle damage as a result of prolonged periods of peripheral fatigue.
