As everyone is already aware, in endurance sports, training must be sufficiently demanding both quantitatively and qualitatively for progressive development to occur. However, with the accumulation of high training volumes and overall stress from non-sporting activities, it is essential to be careful so that the body has enough time for rest and recovery.

Recovery often receives insufficient attention during intense training periods. Ironically, a tired body cannot perform training effectively, and optimal development becomes unattainable. Inadequate recovery manifests as fatigue and increases the risk of overuse injuries. So, how can one promote recovery effectively—and how can you tell if your body is ready for training or needs more rest?

These challenges are discussed in the athlete recovery series, which aims to present a wide range of recovery and recovery monitoring methods. From the second part onwards, we will introduce pressure-based recovery techniques, including compression, foam rolling, and massage. Subsequent sections will also explore temperature-based recovery and monitoring methods.

However, before examining specific recovery methods, it is essential to understand why recovery is crucial and what its objectives are. What exactly is fatigue, what causes it, and is it good or bad? Fatigue as a physiological phenomenon encompasses several mechanisms, which are explored below.

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The feared, desired fatigue

Following a single high-intensity or long-duration exercise session, as well as after several consecutive sessions, the body is often fatigued, making recovery crucial for the body to perform efficiently in the next day’s (or shorter-term) training session. Fatigue is the factor against which recovery methods are aimed.

The fatigue induced by exercise is beneficial in the short term because development cannot be expected without disrupting the body’s homeostasis, or balance. However, prolonged and/or high-intensity training depletes the body’s resources. Without adequate replenishment, the athlete soon finds themselves in an unfavorable situation where the imbalance between stress and rest escalates into more significant issues such as illness, injury, or overtraining.

Read More: How to avoid overtraining?

The physiology of fatigue

As mentioned earlier, endurance training, as well as strength training (which will not be further explored here), stresses the body in various ways. The musculoskeletal system, energy metabolism systems, and the central nervous system are all affected. To understand the causes of fatigue, it is crucial to grasp the complexity of this phenomenon.

Muscle soreness 

Let’s start with the musculoskeletal system, which includes muscles, bones, and tendons. Exercise causes small micro-injuries, or microtears, in muscles and tendons, the repair of which is essential during the recovery phase. These injuries cause local swelling in the muscles due to an inflammatory reaction, and the muscles often feel sore or tender, as seen in the case of delayed onset muscle soreness (DOMS).

Muscle strength

Muscles contain contractile elements, actin and myosin filaments, whose efficient operation is essential for muscle contraction. The effectiveness of these contractile elements diminishes due to fatigue during prolonged exercise or high intensity. When muscle contraction strength decreases, movement becomes less fluid, and the pace must be slowed down to complete a run.

Muscle stiffness

Muscle flexibility decreases due to fatigue, resulting in the muscle-tendon unit being less effective as a spring-like structure than before fatigue. A fatigued muscle stretches poorly, so the load shifts increasingly onto the tendons. These, in turn, are forced to stretch beyond their comfort zone, which can cause micro-injuries and possibly inflammation. This happens, for example, when an athlete goes for a run while tired, and the calf muscles cannot stretch and perform their function properly; the load shifts to the Achilles tendon, potentially causing significant damage as the run continues. Similarly, as muscles and tendons fatigue, the bones experience increased stress, and without time to recover, stress fractures can occur.

Energy stores

Regarding energy production systems, fatigue in endurance training often relates to the depletion of muscle and liver glycogen stores. When carbohydrates are no longer available for energy production, the body must rely solely on fats and even proteins to ensure energy production and maintain normal brain function, among other functions. Because the conversion of fats (and proteins) into energy is slower, performance declines, and the athlete experiences fatigue.

Acidosis 

Similarly, during high-intensity exercise, when lactate and hydrogen ions are transported from muscles into the bloodstream, fatigue occurs. In this case, fatigue is related to acidosis due to hydrogen ions, and acidity, in turn, affects muscle function by making the operation of contractile elements more difficult. Nutrition as part of recovery is crucial, but it will not be covered in this series as it is likely familiar to many readers.

Dehydration 

Fatigue can also result from dehydration, especially in hot and humid conditions. Dehydration typically reduces performance because it decreases plasma volume, resulting in a lower blood volume than in normal conditions. The decrease in circulating blood volume also facilitates the transport of oxygen and waste products to and from cells, affecting performance. Additionally, dehydration affects thermoregulation, raising body temperature and impacting the central nervous system, which can lead to feelings of fatigue.

Central Nervous System 

The cases mentioned above relate to peripheral fatigue, or fatigue in the peripheral parts of the body. However, in recent years, central fatigue, associated with the central nervous system, has also been discussed. Since the brain is the source of most essential commands for movement, its function is likely to affect not only performance during exercise but also post-exercise fatigue and recovery. Especially, the functioning of the nervous system and muscle command is dependent on the state of the central nervous system. Several factors, including environmental conditions, energy intake, and psychological factors, impact brain function. The amount and quality of sleep also greatly affect the central nervous system and emotional states, although they will not be addressed in this series.

Now that readers have reviewed the causes and complexity of fatigue, the following articles in this series will explore how to counter it effectively, thereby maximizing performance and progress in endurance sports.

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*This article was originally published on Maastohiihto.com and updated for ProXCskiing.com in October 2025.

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