What is fatigue and their types?

Fatigue

Fatigue is a complex phenomenon that affects muscle performance and must be considered in a resistance exercise program. There are several types of fatigue, each with its own definition.

Muscle (Local) Fatigue

Most relevant to resistance exercise is the phenomenon of skeletal muscle fatigue. Muscle fatigue, the diminished response of a muscle to a repeated stimulus, is reflected in a progressive decrement in the amplitude of motor unit potentials. This occurs during exercise when a muscle repeatedly contracts statically or dynamically against an imposed load.

Muscle fatigue is an acute physiological response to exercise that is normal and reversible. It is characterized by a gradual decline in the force-producing capacity of the neuromuscular system. This is a temporary decline that leads to a decrease in muscle strength.

The diminished contractile response of the muscle is caused by a complex combination of factors, which include disturbances in the contractile mechanism of the muscle itself (associated with a decrease in energy stores, insufficient oxygen, reduced sensitivity and availability of intracellular calcium, and a build-up of H+) and perhaps reduced excitability at the neuromuscular junction or inhibitory (protective) influences from the central nervous system (CNS).

Fiber-Type Distribution and Fatigue Resistance

The fiber-type distribution of a muscle, which can be divided into two broad categories (type I and type II), affects its fatigue resistance. Type II (phasic, fast-twitch) muscle fibers are further divided into two additional classifications (types IIA and IIB) based on contractile and fatigue characteristics.

• Type II fibers generate a great amount of tension within a short period of time, with type IIB being geared toward anaerobic metabolic activity and having a tendency to fatigue more quickly than type IIA fibers.

• Type I (tonic, slow-twitch) muscle fibers generate a low level of muscle tension but can sustain the contraction for a long time. These fibers are geared toward aerobic metabolism, as are type IIA fibers. However, type I fibers are more resistant to fatigue than type IIA.

Because muscles are composed of varying proportions of tonic and phasic fibers, their function becomes specialized. For example:

• A heavy distribution of type I (slow-twitch, tonic) fibers is found in postural muscles, allowing them to sustain a low level of tension for extended periods of time. Functionally, postural muscles are suited to hold the body erect against gravity or stabilize against repetitive loads.

• On the other end of the fatigue spectrum, muscles with a large distribution of type IIB (fast-twitch, phasic) fibers can produce high increases of tension over a relatively short time. These muscles function to move heavy loads but are susceptible to fatigue.

Clinical Signs of Muscular Fatigue

Clinical signs of muscular fatigue during exercise are summarized in Box 6.2,184,202. When these signs and symptoms develop during resistance exercise, the therapist should decrease the load on the exercising muscle or stop the exercise and shift to another muscle group to allow time for the fatigued muscle to rest and recover. When resistance exercises are part of a home program, the therapist should teach the patient to recognize signs of fatigue and the strategies to minimize its effects.

Cardiopulmonary (General) Fatigue

This type of fatigue is the systemic diminished response of an individual to a stimulus as the result of prolonged physical activity such as walking, jogging, cycling, or repetitive work. Cardiopulmonary fatigue is related to the body’s ability to use oxygen efficiently.

Cardiopulmonary fatigue associated with endurance training is probably caused by a combination of the following factors:

• Decreased blood sugar (glucose) levels

• Decreased glycogen stores in muscle and liver

• Depletion of potassium, especially in the elderly patient

Threshold for Fatigue

The threshold for fatigue is the level of exercise that cannot be sustained indefinitely. A patient’s threshold for fatigue could be noted as the length of time a contraction is maintained or the number of repetitions of an exercise that initially can be performed. This sets a baseline from which adaptive changes in physical performance can be measured.

Factors That Influence Fatigue

A patient’s health status, diet, or lifestyle (sedentary or active) all influence fatigue thresholds. In patients with neuromuscular, cardiopulmonary, inflammatory, cancer-related, or psychological disorders, the onset of fatigue is often irregular. For instance, it may occur abruptly, more rapidly, or at predictable intervals.

It is advisable for a therapist to become familiar with the patterns of fatigue associated with different diseases and medications. For example:

• In multiple sclerosis, the patient usually awakens rested and functions well during the early morning. By mid-afternoon, however, the patient reaches a fatigue threshold and becomes notably weak. By early evening, fatigue often diminishes and strength returns.

• Patients with cardiac, peripheral vascular, and pulmonary diseases, as well as patients with cancer undergoing chemotherapy or radiation therapy, all have deficits that compromise the oxygen transport system. Therefore, these patients fatigue more readily and require a longer recovery period from exercise.

Environmental factors, such as outside or room temperature, air quality, and altitude, also influence how quickly the onset of fatigue occurs and how much time is required for recovery from exercise.

Recovery From Exercise

Adequate time for recovery from fatiguing exercise must be built into every resistance exercise program. This applies to both intrasession and intersession recovery. After vigorous exercise, the body must be given time to restore itself to a state that existed prior to the exercise.

Recovery from acute exercise, in which the force-producing capacity of muscle returns to 90% to 95% of the pre-exercise capacity, usually takes 3 to 4 minutes, with the greatest proportion of recovery occurring in the first minute.

• Oxygen and energy stores replenish quickly in muscle recovery.

• Lactic acid is removed from skeletal muscle and blood within approximately 1 hour after exercise.

• Glycogen stores are replaced over several days.

Clinical Tip

Muscle performance (strength, power, or endurance) only improves over time if a patient is allowed adequate time to recover from fatigue after each exercise session. With insufficient rest intervals during a resistance exercise program, a patient's performance plateaus or deteriorates. Evidence of overtraining discussion in the Overtraining and Overwork section of this or overwork weakness may become apparent. It has also been shown that fatigued muscles are more susceptible to acute strains.