Muscle fatigue: what, why and how it influences muscle function

Summary & key facts

This review explains that muscle fatigue can come from many different causes and that there is no single global cause. Fatigue is usually measured as a drop in the maximum force or power a muscle can make, but people can still do less-demanding tasks after that drop begins. Which processes fail depends on the exact task and the person — for example, older adults lost more peak torque during repeated maximal ankle contractions, yet older adults lasted longer than young adults during a low‑effort sustained elbow task. Overall, the authors say we still lack simple general rules that explain fatigue in all situations.

Key facts:

• The review defines muscle fatigue as an exercise-induced reduction in the ability of muscle to produce force or power.
• There is no single cause of muscle fatigue; the dominant mechanism depends on the specific task (this is called task dependency).
• Fatigue is commonly quantified as the decline in maximal force or power, often measured by interrupting a task with brief maximal contractions to estimate the drop in capacity.
• In Baudry et al. (2007), young adults (mean age 30.5 ± 2.5 years) had a peak dorsiflexor torque of 38.3 ± 3.1 Nm, while old adults (77.2 ± 1.4 years) had 28.6 ± 1.3 Nm.
• In the same Baudry study, final peak torque during maximal lengthening contractions declined by 27.1% for young adults and by 42.1% for old adults; after 150 maximal shortening contractions the decline was 40.9% for young and 50.2% for old.
• Baudry et al. reported that fatigue in both age groups was linked to changes in Ca2+-controlled excitation–contraction coupling, and that old adults also showed impairment of neuromuscular propagation.
• In Hunter et al. (2005), eight pairs of men matched for strength (young MVC 65.9 ± 8.0 Nm; old MVC 65.4 ± 8.7 Nm) sustained a 20% MVC elbow isometric task. Old men (71.3 ± 2.9 years) lasted 22.6 ± 7.4 min and young men (21.5 ± 4.4 years) la
• At the end of the Hunter et al. task, the decrease in MVC torque (amount of fatigue) was similar for young and old men: −31.4 ± 10.6%.
• The authors note that despite a large research literature, few general principles have emerged and we are often unable to state with certainty why an individual becomes fatigued under different conditions.

Abstract

Much is known about the physiological impairments that can cause muscle fatigue. It is known that fatigue can be caused by many different mechanisms, ranging from the accumulation of metabolites within muscle fibres to the generation of an inadequate motor command in the motor cortex, and that there is no global mechanism responsible for muscle fatigue. Rather, the mechanisms that cause fatigue are specific to the task being performed. The development of muscle fatigue is typically quantified as a decline in the maximal force or power capacity of muscle, which means that submaximal contractions can be sustained after the onset of muscle fatigue. There is even evidence that the duration of some sustained tasks is not limited by fatigue of the principal muscles. Here we review experimental approaches that focus on identifying the mechanisms that limit task failure rather than those that cause muscle fatigue. Selected comparisons of tasks, groups of individuals and interventions with the task-failure approach can provide insight into the rate-limiting adjustments that constrain muscle function during fatiguing contractions.

Topics

Motor Control and Adaptation Muscle activation and electromyography studies Sports Performance and Training

Categories

Biomedical Engineering Engineering Physical Sciences

Tags

Electromyography Engineering Epistemology Internal medicine Limiting Mechanical engineering Mechanism (biology) Medicine Muscle contraction Muscle fatigue Philosophy Physical medicine and rehabilitation Systems engineering Task (project management)