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Getting the Facts on Fatigue
By Len Kravitz, PhD

To the exerciser, the word 'fatigue' denotes many different concepts, such as not completing a set or being too exhausted to run a few more steps on a long distance run. Indeed, the physiology of fatigue has been of great interest to researchers for many decades and was recently a special section of articles in the journal, Medicine & Science in Sports & Exercise. In order to address some of the diverse topics on fatigue, this column will approach these issues with a 'Question and Answer' format to specifically address relevant issues of exercise fatigue.

What is Fatigue?
World-renowned muscle fatigue researchers, Roger Enoka and Doug Stuart (1992), describe fatigue as an over-all concept intended to symbolize an acute impairment of performance that includes both an increase in perceived effort necessary to exert a desired force and an eventual inability to produce this force. Citing early work by Mosso (which was further developed by Kluger and colleagues), Enoka and Duchateau (2016) further elucidate that performance fatigability has some type of measure (e.g., contractile function or muscle activation) of a decline in exercise performance over a distinct period of time. Physiological processes that contribute to fatigue are grouped into two categories: 1) Central, which is muscle activation signaling from the brain, and 2) Peripheral, which are the physiological mechanisms occurring at the neuromuscular junction and within the contracting muscle. The researchers describe perceived fatigability as the measurement (e.g., psychological state, homeostasis) of sensations that regulate the integrity of the performer.

What is Causing the Fatigue in Muscle?
Reid (2016) summarizes and concludes that 30 years of research by muscle biologists has developed a clear picture that reactive oxygen species (ROS), reactive molecules and free radicals derived from molecular oxygen that can impair or damage muscle proteins (and other molecules), are generated during strenuous exercise. Reid continues that the large body of research now conclusively agrees that these generated ROS clearly impact the loss of muscle function occurring during fatigue. The evidence of ROS having a causal influence to fatigue has been shown in small muscles, at the neuromuscular junction and in whole-body exercise (like running) by elite athletes (Reid). Reid highlights that future research efforts by scientists will be to directly try to find ways to enhance the body's acute ability to make antioxidants, those specialized molecules that neutralize the ROS in muscle. If and when this occurs, it is likely that we will see elite athletes setting new cardiovascular and anaerobic world records!

What are the Sex Differences in Fatigability?
Dr. Sandra Hunter (2016) explains that every human cell has a sex (defined by the chromosome complement of XX in men and XY in women) that influences fatigue in women and men differently. Hunter recaps research indicating that women are less fatigable then men for many isometric tasks and some dynamic tasks, when the women and men are performing similar intensity contractions. It should be noted that much of the research comparing the fatigue in sexes uses isometric contractions, due to the ability to quantify a decline in force production so accurately. Hunter summarizes that women are less fatigable then men during isometric fatiguing tasks for several muscle groups. It is interesting to note that the sex fatigue differences actually vary between muscle groups. For instance, the sex differences in fatigability are less for the ankle dorsiflexor muscles as compared to the elbow flexor muscles (Hunter). In addition, Hunter notes that sex fatigue differences are less for high-intensity isometric contractions as compared to low-intensity isometric muscle contractions.

In regards to dynamic muscle functions, the emerging evidence indicates that sex differences in fatigue are task specific (Hunter). For instance, Hunter elucidates evidence denoting that women are less fatigable then men for a dynamic task with the elbow flexor muscle at a slow contraction speed, but not at a fast speed of contractions.
Interestingly, Hunter cites studies that show after long-duration running and cycling, reductions in lower body maximal strength are less for women as compared to men. More research in this area will eventually elucidate new and difference recovery strategies for men and women after fatiguing endurance exercise (Hunter).

What are the Mechanisms for Sex Differences in Fatigue?
The main area of muscle fatigue difference in men and women is in the contractile mechanisms of muscle (Hunter, 2016). In addition, there are differences in the delivery of blood to a capillary bed in muscle (called muscle perfusion) between men and women. As well, the voluntary activation of muscle groups has been shown to vary between the sexes (Hunter).

Hunter (2016) spotlights evidence indicating sex differences in whole-body foodstuff use during endurance exercise. Women break down less carbohydrate and more fat then men during similar intensity endurance exercise. These differences in muscle energy metabolism are largely attributable to differences in the muscle fiber PROPORTIONS between the sexes. To briefly review fiber types, type I muscle fibers are referred to as slow twitch muscle fiber types. They are highly involved in activities of sustained muscle contractions, such as endurance events. The type II muscle fibers are referred to as fast twitch muscles fibers. They are engaged mainly in shorter length activities of greater force production, such as sprinting and burst-type exercise. Hunter explains that the relative NUMBER of muscle fibers may not differ between men and women, however women have smaller type II fibers than men and a greater relative area of type I fibers. Hunter explains that this fiber-type proportional area difference leads to a more fatigue-resistant muscle. Hunter suggests that the mechanism for recovery after a strength event may differ between the sexes, but more research is needed. This may eventually suggest different recovery strategies for men and women after resistance training.

How Does the Brain Influence Fatigue?
As noted earlier in the column, central fatigue (the activation signal originating from the brain) is a contributing factor to fatigue. Taylor et al. (2016) explain that a chain of processes of the nervous system preceding the muscle contraction may also present impairment to the force generated by the muscle. The brain's signal to muscle referred to as neural drive determines when and to what degree the muscle fibers are activated (Taylor et al). However, feedback mechanisms such as pain, discomfort, and perception of effort may directly affect neural drive. In addition, Taylor and colleagues explain that the disturbance of homeostasis with fatiguing exercise leads to multiple neural alternations contributing to the central fatigue. However, the brain tries to compensate by recruiting other motor units (a motor unit is the single nerve and the many muscle fibers innervated by the nerve). Thus, during fatiguing exercise the brain is evaluating all of the sensory input and determining whether to reduce the signals to muscle or compensate to attempt to continue the specific muscle task performance.

What is the Future of Fatigue Research?
Although this column focused a great deal on the current evidence involving muscle performance fatigue issues, fatigue is also implicated in many diseases such neuromuscular disease, cancer, chronic inflammatory disease and acute critical illness (Powers et al, 2016). Powers and colleagues underscore that muscle wasting and fatigue is a clinical concern, as it is associated with a decreased quality of life and increased morbidity and mortality. Therefore, it is hoped that future research on fatigue may not only help improve exercise performance, but introduce health-promoting strategies to combat many diseases.

Len Kravitz, PhD, CSCS is the program coordinator of exercise science and a researcher at the University of New Mexico, where he received the Outstanding Teacher of the Year award. In addition to being a 2016 inductee into the National Fitness Hall of Fame, Len was awarded the 2016 CanFitPro Specialty Presenter Award.

Enoka, R.M. and Stuart, D.G. (1992). Neurobiology of fatigue. Journal of Applied Physiology, 72(5), 1631-1648.

Enoka, R.M. and Duchateau, J. (2016). Translating fatigue to human performance. Medicine & Science in Sports & Exercise, 48(11), 2228-2238.

Hunter, S.R. (2016). The relevance of sex differences in performance fatigability. Medicine & Science in Sports & Exercise, 48(11), 2247-2256.

Powers, S.K., Lynch, G.S., Murphy, K.T. et al. (2016). Disease-induced skeletal muscle atrophy and fatigue. Medicine & Science in Sports & Exercise, 48(11), 2307-2319.

Reid, M.B. (2016). Reactive oxygen species as agents of fatigue. Medicine & Science in Sports & Exercise, 48(11), 2239-2246.

Taylor, J.L., Amann, M., Duchateau, J. et al. (2016). Neural contributions to muscle fatigue: From the brain to the muscle and back again. Medicine & Science in Sports & Exercise, 48(11), 2294-2306.