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Sports Conditioning and Fatigue
Lance C. Dalleck, M.S. and Kravitz, Ph.D.

Wouldn’t it be great if you could perform your favorite sporting activity or exercise program as long as you wanted? Woefully, those of you who push the intensity threshold of your recreational pursuits often experience a multi-faceted phenomenon known as fatigue. Simply defined, yet physiological quite complex, fatigue, refers to the inability to continue exercise at a given intensity (Roberts & Robergs, 1997). In all sports and exercise training, the onset of fatigue will vary depending on a person’s fitness level, the exercise intensity, and environmental conditions (e.g. heat, humidity and altitude) (Fitts, 1994). This article will provide a brief overview of the various types of fatigue and examine the role gender plays in fatigue.

Fatigue Types in Sports and Exercise
The two dominant sport conditioning scenarios leading to fatigue are short-term intense exercise and extended, submaximal training events. (Roberts & Robergs, 1997). Though the physiological mechanisms of these training conditions is distinctly different, their reduction in muscle performance capacity is similar.

Short-term, Intense Sport and Exercise
During vigorous exercise bouts such as sprinting, short burst interval training, and high intensity resistance exercise, continued muscle contraction is dependent on the formation of adenosine triphosphate (ATP) for the demanding energy needs. Under these exercise conditions, creatine phosphate, which resynthesizes ATP (the universal energy molecule), and glucose breakdown (called glycolysis) are primarily responsible for maintaining ATP levels. It has been found that during intense muscle contraction, creating phosphate becomes depleted rapidly, resulting in an incomplete supply of ATP (Fitts, 1994). To make-up for this ATP deficiency, glyolysis increases. However, the increased output of glycolysis results in the accumulation of by-products, including lactate and protons (hydrogen ions also shown regularly as H+), which have been identified as potential contributors to fatigue. Lactate production is thought to disturb electrochemical events in muscle cells (Robergs & Roberts, 1997). Historically, researchers have associated lactate (or lactic acid) production during increased rates of glycolysis with the development of cellular acidosis (decreased pH we commonly refer to as ‘the burn’) and fatigue. Although lactate production does play an independent role in fatigue as described above, it has not been shown to be the primary cause (Robergs, 2001). It is the proton accumulation which results in decreased cellular pH (acidosis), which impairs muscle contraction through a number of mechanisms. For instance, enzymes involved in the cellular regulation of sodium and potassium during muscle contraction become impaired by this increasing acidity from proton accumulation (Robergs & Roberts, 1997).

A neural fatigue also exists during short-term, intensive exercise and sports bouts. Each single motor nerve (meaning under your voluntary control) activates a group of muscle fibers (often in the hundreds), and is collectively referred to as a motor unit. The chemical messengers, or neurotransmitters, that carry the nerve’s excitation message to the muscle at the neuromuscular junction, also become impaired with intense exercise. This inhibition results in a decreased efficiency of the muscle fibers ability to contract (Gardiner, 2001). In addition, the protons produced from the increased glycolysis often interfere with calcium ions reacting with proteins at the myofilament site in muscle where contraction is occurring, further inhibiting the muscle’s ability to contract.

Extended, Submaximal Sports and Exercise
During prolonged exercises such as cycling, cross-country skiing, and distance running, muscle contraction is also dependant on the ability of metabolic (breakdown of a fuel to release energy) pathways to continuously regenerate ATP. Mitochondrial respiration (aerobic metabolism in the mitochondrion of the cell) becomes the primary supplier of ATP. Many fuels, or substrates from fat, carbohydrates and proteins, are available for mitochondrial respiration, however, the two most important with regards to fatigue are blood glucose and muscle glycogen. Fats in the form of triglycerides are also readily available for ATP production, but their breakdown is much slower than glucose and glycogen. Decreased levels of blood glucose and low levels of muscle glycogen have been more associated with the onset of fatigue in sustained exercise events (Fitts, 1994).
Some sporting activities such as hiking, cross-training events, and endurance races may lead to dehydration. Insufficient fluid intake and replacement, as well as fluid loss, will impair bodily temperature control systems and cardiovascular function. The repeated muscle contraction during these sustained events results in a continual release of bodily heat, which can lead to high body temperatures, referred to as hyperthermia. Research has suggested that rising body core temperatures may cause fatigue in both the contracting muscles and central nervous system (Fitts, 1994). Failure to maintain fluid balance throughout prolonged sports events and exercise sessions may eventually result in decreased availability of blood flow to both exercising muscles and the skin for dissipation of heat. This dehydration may lead to higher heart rates and stress on the cardiovascular system (due to the lack of blood volume since blood is approximately 50% water) as the event continues. The importance of fluid intake for total body health, sport performance and reduction of fatigue cannot be under emphasized.

Article Reviewed
Hicks, A.L., Kent Braun, J., and Ditor, D.S. (2001). Sex differences in Human Skeletal Muscle Fatigue. Exercise and Sport Sciences Reviews, 29, 109-112.
Fatigue and Gender
Researchers have identified several gender-specific differences in muscular fatigability related to muscle mass, substrate utilization and neuromuscular activation (Hicks, Kent-Braun, & Ditor, 2001). These differences may have important implications for the future direction of training methodology in exercise and sport.
When performing the same relative muscular work, females tend to produce lower absolute muscle forces compared to males. This results in less oxygen demand in the exercising muscles (Hicks, Kent-Braun, & Ditor, 2001). During submaximal exercise, these conditions enhance both oxygen delivery and metabolic by-product (primarily carbon dioxide) removal, resulting in the delayed onset of fatigue. This apparently describes a female ‘fatigue resistance’ advantage in submaximal sporting and exercise events.
Past research has also observed gender differences in substrate utilization, with males displaying greater glycolytic capacities and females possessing greater capacities for fat breakdown, referred to as fat oxidation (Hicks, Kent-Braun, & Ditor, 2001). These gender differences tend to predispose females to success in extended submaximal activities such as long-distance swimming and ultra-marathon running.
Following both maximal and submaximal exercise, studies have reported that males tend to become more fatigued than females with regards to neuromuscular activation (Hicks, Kent-Braun, & Ditor, 2001). This suggestion is based on findings that electromyography (EMG) activity, which measures motor unit recruitment patterns, is proportionately lower following fatiguing muscular contraction in males compared to females.

Fatigue: Training Implications due to Gender Differences
Interpretation of the reported gender differences in fatigability advocate some interesting training considerations for personal trainers with their clients. If females become less fatigued following the same relative submaximal workload, this infers males and females respond differently to similar training stimuli. Therefore, females may require greater volume workouts, including more sets and repetitions at the same submaximal intensity, during training in order to attain similar physiological benefits. In addition, males may require additional recovery time, both between sets and days of training in order to fully recover. Conversely, females may need less rest between sets and possibly fewer recovery days between intense training sessions. These adaptations to training methodology, based on the research of gender differences in fatigability, may improve sport performance and exercise conditioning in both the competitive athletes and recreational enthusiasts you train.

Additional References:
Fits, R. H. (1994). Cellular mechanisms of muscular fatigue. Physiological Reviews, 74 (1), 49-94.
Gardiner, P. F. (2000). Neuromuscular aspects of physical activity. Human Kinetics.
Robergs, R. A. (2001). Exercise-induced metabolic acidosis: Where do the protons come from? Sportscience 5(2),
Robergs, R. A. and Roberts. S. (1997). Exercise physiology: Exercise, performance, and clinical applications. Mosby.

Mechanisms of Skeletal Muscle Fatigue

Short-Term Intense Exercise
Creatine phosphate depletion
Decrease in ATP
Lactate accumulation
Proton accumulation
Decrease in neural muscular activity

Prolonged Submaximal Exercise:
Decreased blood glucose
Decrease muscle glycogen
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