Fatigue is one of those things that’s easy to recognize, yet difficult to quantify, and for which it is nearly impossible to determine causality. Fatigue arises from multiple factors ranging from poor quality sleep to a challenging, high-intensity interval session. Yet, fatigue might also be considered a necessary evil, as it’s required to elicit favorable training adaptations.

So, how do you know if the fatigue your clients are accumulating is the kind of fatigue you want them to have? How do you mitigate the different types of fatigue they may encounter? In this article, we answer these questions by examining the different types of exercise fatigue and exploring how you can help your clients manage them.

Defining Fatigue

First, let’s clarify what “fatigue” means. Fatigue refers to a reduction in performance due to stressors imposed on the body. These stressors can come from non-training sources, such as poor sleep, inadequate nutrition, work deadlines or relationship stress, for example. For this article, however, we focus primarily on training-induced stress. Within a training cycle, there are three categories of fatigue that commonly occur—local fatigue, axial fatigue and systemic fatigue.

Local Fatigue

The focal point of local fatigue is the working muscle cell itself, but it also comprises the peripheral nervous system (PNS) and the nearby joint and connective tissue. When we train, we reduce the amount of available cellular energy in the form of adenosine triphosphate (ATP), glycogen and creatine phosphate (CP). This lack of energy results in an immediate drop in performance until energy stores are refilled. The treatment and time needed for the body to resupply can vary greatly. In the case of glycogen, hours may be needed for carbohydrate digestion. For CP replenishment, mere minutes of rest may be all that is required. Exercise also results in structural damage to muscle and joint tissue and a build-up of metabolites such as lactate and phosphate. These metabolites limit the ability of the PNS to signal cell contraction in a process known as excitation-contraction decoupling. The nerve is trying to signal for muscular contraction but is unable to transmit the signal effectively, which results in either no contraction or poor timing.

Local fatigue is an unavoidable part of the training process. Fortunately, a large portion of local fatigue is the driving force of the favorable training adaptations you want to see in your clients. For instance, metabolic stress and muscle damage both contribute to muscle hypertrophy. Additionally, cellular energy depletion leads to increases in glycogen and CP stores, angiogenesis to improve oxygen and blood delivery, and an increase in mitochondrial density. However, not all local fatigue is good. Accumulative glycogen depletion and connective tissue damage throughout a training cycle are a major reason it’s not always possible to train hard.

Systemic Fatigue

The second type of fatigue is systemic fatigue. With this fatigue, the stress and symptoms are no longer confined to the areas that are being exercised but to the whole body. Symptoms of systemic stress can be felt in the central nervous system (CNS), the immune system and the cardiovascular system. The CNS experiences problems that are similar to what the PNS experiences with local fatigue. Physically, the nerves must reestablish chemical gradients and, until they do so, force production is reduced across the entire body. However, the problems in the CNS aren’t just physical. Hard training is extremely demanding from a psychological perspective. If you client spends weeks pushing themselves to near failure, they are likely going to feel beat down on a mental and emotional level. This becomes the limiting factor in their performance, which means they can no longer effectively train the muscles and/or energy systems they want to target.

Systemic fatigue also includes the immune system. With large exercise demands, working cells release cytokines that alert the immune system that the cells are damaged. This immune response greatly reduces performance. If you’ve ever felt ill shortly after a really hard workout, it’s possible that this response contributed, at least in part, to your illness.

Axial Fatigue

The final form of fatigue may not occur with all clients. However, it is worth considering for those clients that are performing a large amount of heavy lifting. Axial fatigue is caused by the vertical loading of the spine and limits force production throughout the body to protect the spine from damage. For example, it will limit the force output of the legs during a squat or the arms during an overhead press. This type of fatigue is poorly understood but can be observed in training regimes with a high volume of exercises that load the axial skeleton. Axial fatigue may be caused, at least in part, by the spinal erector muscles being particularly sensitive to changes in length and force detected by the Golgi tendon organs and muscle spindles. This leads to an inhibitory feedback mechanism within the spinal cord that limits force production throughout the body.

Detecting Fatigue

Now that we’ve defined the different types of fatigue you may encounter in your clients, how do you detect fatigue and find its source? Fundamentally, fatigue is detected through a decline in exercise performance. When and where an individual client’s performance is down will tell you a lot about the type of fatigue they are experiencing, and it may impact how you help them manage it.

With local fatigue, an individual experiences a reduction in performance of the local muscle. For example, if your client can bench 185 pounds (84 kg) for a set 10 repetitions, but a minute later they can only do a set of six, they are fatigued. If two days later they can only do a set of nine, they are still experiencing some local fatigue. For days after their training session, clients may also report some painful or burning sensations in the muscle during exercise, as well as soreness in the muscle and/or connective tissue. Importantly, with local fatigue, performance will only be diminished on the exercises that use that muscle. Continuing the bench press example from above, the client may have reduced performance in the bench press or chest fly, but if you have them squat or row and they are still able to perform at their best, you can rightly assume that the fatigue is limited to the pectoral muscles.

With systemic fatigue, you will a see a degradation in performance across all exercises, no matter what muscles are being used. For example, at the end of a hard leg workout, it may be physically and/or psychologically impossible to perform well on any additional exercise, regardless of the muscle being worked. In chronic levels of fatigue, an individual will likely experience poor performance in all exercise for days, perhaps even weeks. When a high level of systemic fatigue is present, clients start to develop symptoms. Their resting heart rates can be elevated, and they may experience disturbances in their sleep. They also tend to get sick more frequently or experience consistent, light flu-like symptoms due to the immune response from cytokines.

With axial fatigue, you will see a drop in performance only during exercises that feature large amounts of axial loading. Say you have a client who is struggling to perform squats. An easy way to test a client to determine if axial fatigue is the issue is to check their performance on the leg press. If fatigue is isolated to the quads, they will perform poorly on both. If the fatigue is arising due to stress on the spinal column, they will be able to still perform well on the leg press. To confirm a suspicion of axial fatigue, you should see a reduction in performance on most exercises that load the axial spine.

Managing Fatigue

When it comes to managing fatigue, always keep in mind that it is a necessary and unavoidable part of the training process. Local fatigue, in particular, drives the favorable adaptations your clients want to achieve with training. Trying to avoid or suppress this fatigue too much can hinder the training results for your clients. Previous research has shown that muscle hypertrophy is reduced due to the suppression of post-exercise inflammation, either through the use of anti-inflammatories or interventions like ice baths. You want local fatigue to be the limiting factor in your clients’ training performance to encourage the production of adaptations. Therefore, you must aim to limit systemic and axial fatigue, where possible.

Quality fatigue management comes primarily from well-designed programs and good exercise selection. For example, choose exercises that lead to a high amount of hypertrophy and strength improvements but have lower overall fatigue demands. Typically, this means using exercises that have a large range of motion and lower external loads. These exercises still result in the strength and hypertrophy adaptations you want to produce in your clients, because those adaptations are driven by intracellular muscle tension and not external load.

With proper programming, you can start off a client’s training cycle at a relatively low-intensity level to facilitate a longer progression without risk of building up too much fatigue. A review by Grgic and colleagues showed that, compared to training to failure, training a few repetitions shy of failure results in similar degrees of improvement in both strength and hypertrophy.

Therefore, when designing a training cycle, you can start a few repetitions shy of failure and progress closer and closer as you approach a deload. This approach also helps from a psychological perspective. Additionally, verbal encouragement and psychological stimulation, such as music, can have a profound effect on athletic performance. However, this results in a large amount of psychological stress, which also accumulates week to week like physical stress. By starting your clients’ training cycles farther from failure, you can reserve these big "psyche up" moments for the period just before a deload.

The final consideration for managing fatigue through exercise programing is designing proper deloads and rest/recovery sessions. All three types of fatigue will accumulate throughout a training cycle. When a client’s performance is down, you need to reduce their overall volume of training to promote recovery.

Final Thoughts

Careful exercise programming is critical to the management of fatigue, but it’s not the only factor to consider. Adequate sleep, proper nutrition and reducing psychological stress are all research-substantiated recovery techniques that have a critical role in preventing undue fatigue. For more information on post-exercise recovery strategies, please read our previous article here.