Energy Expenditure and Fatigue

Measuring Energy Expenditure：The energy used by contracting muscle fibers during exercise cannot be directly measured. But numerous laboratory methods can be used to calculate whole-body energy expenditure at rest and during exercise. Several of these methods have been in use since the early 1900s. Others are new and have only recently been used in exercise physiology.

Direct calorimetry involves a large sophisticated chamber that directly measures heat produced by the body; while it can provide very accurate measures of resting metabolism, it is not a commonly used tool for exercise physiologists.

Indirect calorimetry involves measuring whole-body O2 consumption and CO2, production from expired gases. Since we know the fraction of O, and CO, in the inspired air, three additional measurements are needed: the volume of air inspired (Vl) or expired (VE), the fraction of oxygen in the expired air ( FEO2), and the fraction of CO2 in the expired air (FECO2)·

By calculating the RER (the ratio of CO2 production to O2 consumption) and determining the metabolic substrates being oxidized, we can convert VO2 into energy expenditure in kilocalories.

The RER value at rest is usually 0.78 to 0.80. The RER value for the oxidation of fat is 0.70 and is 1.00 for carbohydrates.Isotopes can be used to determine metabolic rate over longer periods of time. They are injected or ingested into the body. The rates at which they are cleared can be used to calculate CO2 production and then caloric expenditure.

Energy Expenditure at Rest and During Exercise

The basal metabolic rate (BMR) is the minimum amount of energy required by the body to sustain basic cellular functions and is related to fat -free body mass and, to a lesser extent, body surface area. It typically ranges from 1,100 to 2,500 kcal/day; but when daily activity is added, typical daily caloric expenditure is 1,700 to 3,100 kcal/day.

VO2 increases linearly with increased exercise intensity, but eventually reaches a plateau. Its maximal value is called the ѶO2max, When volitional fatigue limits exercise before a true maximum is reached, the term ѶO2veak is used.

Successful aerobic performance is linked to a high VO2max, to the ability to perform for long periods at a high percentage of ѶO2max; to the running velocity at lactate threshold, and to a good economy of movement.

The EPOC is the elevated metabolic rate above resting levels that occurs during the recovery period immediately after exercise has ceased

Lactate threshold is that point at which blood lactate production begins to exceed the body's ability to clear lactate, resulting in a rapid increase in blood lactate concentration during exercise of increasing intensity. Generally, individuals with higher lactate thresholds, expressed as a percentage of their VO2max, are capable of better endurance performances. Lactate threshold is a strong determinant of an athlete's optimal pace in endurance events such as distance running and cycling

Fatigue and Its Causes

Depending on the circumstances, fatigue may result from depletion of PCr or glycogen; both situations impair ATP production. Glycogen depletion may occur in select fiber types or specific muscle groups depending on the exercise.

Increased metabolic by-products like phosphate ions and heat may contribute to fatigue Lactic acid often has been blamed for fatigue, but it is generally not directly related to fatigue during prolonged endurance exercise, and may serve as a fuel source (see chapter 2).

In short-duration exercise, like sprinting, it is actually the H+ generated by dissociation of lactic acid that often contributes to fatigue. The accumulation of H+ decreases muscle pH, which impairs the cellular processes that produce energy and muscle contraction.

Failure of neural transmission may be a cause of some types of fatigue. Many mechanisms can lead to such failure, and further research is needed.

The CNS plays a role in most types of fatigue, perhaps limiting exercise performance as a protective mechanism. Perceived fatigue usually precedes physiological fatigue, and athletes who feel exhausted can often be encouraged to continue by various cues that stimulate the CNS, such as listening to music.

Muscle Soreness and Muscle Cramps :

   Muscle soreness generally results from exercise that is exhaustive or of very high intensity. This is particularly true when people perform a specific exercise for the first time.While muscle soreness can be felt at any time, there is generally a period of mild muscle soreness that can be felt during and immediately after exercise and then a more intense soreness felt a day or two later.

Acute Muscle Soreness

Pain felt during and immediately after exercise is classified as a muscle strain and is perceived as muscle stiffness, aching, or tenderness. It can result from accumulation of the end products of exercise, such as H+, and from tissue edema that is caused by fluid shifting from the blood plasma into the tissues. Edema is the cause of the acute muscle swelling that people feel after heavy endurance or strength training. The pain and soreness usually disappear within a few minutes to several hours after the exercise. Thus, this soreness is often referred to as acute muscle soreness.

Delayed-Onset Muscle Soreness

The precise causes of muscle soreness felt a day or two after a heavy bout of exercise are not totally understood. Because this pain does not occur immediately, it is referred to as delayed -onset muscle soreness (DOMS). Delayed -onset muscle soreness can vary from slight muscle stiffness to severe, debilitating pain that restricts movement. In the following sections, we discuss some theories that attempt to explain this form of muscle soreness.

Almost all current theories acknowledge that eccentric muscle action is the primary initiator of DOMS. This has been clearly demonstrated in a number of studies examining the relationship of muscle soreness to eccentric, concentric, and static actions. Individuals who train solely with eccentric actions experience extreme muscle soreness, whereas those who train using only static and concentric actions experience little soreness. This idea has been further explored in studies in which subjects ran on a treadmill for 45 min on two separate days, one day on a level grade and the other day on a 10% downhill grade.17,18 No muscle soreness was associated with the level running. But the downhill running, which required extensive eccentric action, resulted in considerable soreness within 24 to 48 h, even though blood lactate concentrations, previously thought to cause muscle soreness, were much higher with level running.

In the following sections we examine some of the proposed explanations for exercise-induced DOMS. In general, the pathway for developing DOMS begins with structural damage to muscle fibers (microtrauma) and to the surrounding connective tissues. Thisdamage is followed by an inflammatory process that leads to edema as fluid and electrolytes shift into the area. To make matters worse, muscle spasms can occur, prolonging the condition and making the soreness worse.Acute muscle soreness occurs during or immediately after an exercise bout.

Delayed- onset muscle soreness usually peaks a day or two after the exercise bout.Eccentric muscle action seems to be the primary initiator of this type of soreness.

Proposed causes of DOMS include structural damage to muscle cells and inflammatory reactions within the muscles. The proposed sequence of events includes structural damage, impaired calcium homeostasis, inflammatory response, increasedmacrophage activity, and edema.

Reduced muscle strength with DOMS is likely the result of physical disruption of the muscle, failure of the excitation- -contraction process, and loss of contractile protein.

Muscle soreness can be minimized through the use of lower intensity and fewer eccentric contractions early in training. However, muscle soreness may ultimately be an important part of maximizing the resistance training response.

Muscle fatigue- -ssociated cramps are related to sustained a-motor neuron activity, with increased muscle spindle activity and decreased Golgi tendon organ activity.

Exercise- associated muscle cramps may be caused by altered neuromuscular control, fluid or electrolyte imbalances, or both. Heat-associated cramps, which typically occur in athletes who have been sweating excessively, involve a shift in fluid from the interstitial space to the intravascular space, resulting in a hyperexcitable neuromuscular junction.

Rest, passive stretching, holding the muscle in the stretched position, and fluid and electrolyte restoration can be effective in treating EAMCs. Proper conditioning, stretching, and nutrition are also possible prevention strategies.

Exercise-Induced Muscle Cramps

   Skeletal muscle cramps are a frustrating problem in sport and physical activity and commonly occur even in highly fit athletes. Skeletal muscle cramps can come during the height of competition, immediately after competition, or at night during deep sleep. Muscle cramps are equally frustrating to scientists, because there are multiple and unknown causes of muscle cramping, and little is known about the best treatment and prevention strategies. Nocturnal muscle cramps, especially in the calf muscle, have been experienced by 60% of adults. This type of cramp is probably caused by muscle fatigue and nerve dysfunction and may or may not be associated with exercise. Electrolyte imbalances and hydration do not seem to play an important role.

In Closing

In previous chapters, we discussed how muscles and the nervous system function together to produce movement. In this chapter we focused on energy expenditure during exercise and fatigue. We considered the energy needed by the body at rest and during movement. We explored how energy production and availability can limit performance and learned that metabolic needs vary considerably. We discussed the many potential factors involved in fatigue, both those resulting from cellular metabolism and those associated with the nervous system, and examined delayed-onset muscle soreness and muscle cramps as additional limiting factors in exercise. In the next chapter, we turn our attention to the cardiovascular system and its control.

Study Questions

1. Define direct calorimetry and indirect calorimetry and describe how they are used to measure energy expenditure.

2. What is the respiratory exchange ratio (RER)? Explain how it is used to determine the oxidation of carbohydrate and fat.

3. What are basal metabolic rate and resting metabolic rate, and how do they differ?

4. What is maximal oxygen uptake? How is it measured? What is its relationship to sport performance?

5. Describe two possible markers of anaerobic capacity.

6. What is the lactate threshold? How is it measured? What is its relationship to sport performance?

7. What is economy of effort? How is it measured? What is its relationship to sport performance?

8. What is the relationship between oxygen consumption and energy production?

9. Why do athletes with high VOzmax values perform better in endurance events than those with lower values?

10. Why is oxygen consumption often expressed as milliliters of oxygen per kilogram ofbody weight per minute (ml· /kg· min)?

11. Describe the possible causes of fatigue during exercise bouts lasting 15 to 30 s and 2 to 4 h.

12. Discuss three mechanisms through which lactate can be used as an energy source.

13. What is the physiological basis for delayed-onset muscle soreness?

14. What is the physiological basis for exercise· associated muscle cramps?