Heat Dispersion and Muscle Fatigue During Exercise

Water Loss A patient's ability, while exercising, to dissipate meta-bolically-produccd heat de­pends largely on his or her ability to form and vaporize sweat. The magnitude of sweat production increases significantly with increases in the intensity of exercise.1 A decrease in body weight of more than 2% by cxer-cise-induced sweating places severe demands on both the car­diovascular and thermal regulatory sys­tems. The concentration of ions in sweat is influenced by a number of factors, such as sweat rate. A marked flux of water out of plasma into the active muscles occurs at the onset of exercise. Body water lost during prolonged exercise appears to be derived from the interstitial or intracellu-lar spaces or both. Progressive exercise-dehydration seems to have little effect on the plasma potassium content. The plasma magnesium ion declines during exercise and muscle magnesium content increases. Muscle water declines steadily with progressive dehydration. No direct evi­dence of tissue hypokalemia with heavy exercise and profuse sweating has been obtained. Plasma volume changes dur­ing days of repeated dehydration. So­dium is conserved, apparently, through an increase in mineral corticoid function. Low potassium intake appears to reduce sodium loss and urine output during re­peated days of heavy exercise. There is no evidence that heavy exercise on re­peated days threatens muscle and plasma potassium stores, even when the dietary potassium intake is very low. Costill2 indicates the body minimizes electrolyte losses during acute and re­peated bouts of exercise and dehydration. In general, large sweat losses during train­ing are adequately tolerated. Despite siz­able excretion of ions in sweat, caloric intake and renal conservation of sodium minimize the threat of chronic dehydra­tion or electrolyte deficiencies or both. Body water lost as a con­sequence of heat expendi­ture during exercise is ac­complished at the expense of larger water losses from extracellular, rather than from intracellular, compartments. Furthermore, the losses in sweat and urine have little effect on potassium content of either plasma or muscle. With repeated days of de­hydration and heavy exer- cise, plasma volume does increase, and this relative increase of plasma water may result in a reading of fictitious anemia as a phenomenon of hemodilution. This may also produce an apparent hypokalemia that is also fictitious. Blood Flow Skin blood flow is the chief mechanism for transferring heat from the core to skin during heat stress and exercise. The cir­culatory system also must supply the in­creased metabolic requirements of exer­cising muscles. Roberts, et a/.,3 exam­ined the influence of body temperature on skin blood flow in exercise. Tempera­tures were measured in the esophagus and at eight skin sites in subjects who exercised on a cycle ergometcr in three different positions. Cardiac output was measured by a CO2 re-breathing tech­nique, and chest sweating was measured by resistance hygrometry. Exercise increased internal body tem­perature and cutaneous blood flow at any given environmental and mean skin tem­perature. At high environmental tempera­tures, with skin temperature elevated, skin blood flow at any given internal tempera­ture was higher than at cooler skin tem­peratures. At high levels of skin blood flow, peripheral vascular pooling and fluid loss through filtration led to reduced cen­tral venous pressure, which lowered car­diac stroke volume and required a higher heart rate to maintain a given cardiac out­put. Mechanisms for alleviating some of the cardiovascular strain include reflexes arising from receptors in working muscles that produce vasoconstriction in central and peripheral vascular beds. Reflexes arising from cardiac baroreceptors pro­duce additional peripheral vasoconstric-tion when cardiac filling is impaired. In the long term, physical conditioning and heat acclimatization lead to increased sweat output during thermal stress, cooler skin and core temperatures on ex­ercise, and a decrease in the level of skin blood flow needed for regulation of body temperature. An environment that inhib­its evaporation will preclude large reduc­tions in skin temperature. The demand for skin blood flow is an important limiting factor in sustained ex­ercise, particularly if the environment is warm or hot. With maximal cutaneous vasodilatation, flow can reach 6-1OL per minute, 25% or more of the total cardiac output. Muscle Fatigue Local muscle fatigue causes a shift in the frequency spectrum of the surface electromyogram (EMG) toward lower fre­quencies. Sato and Hayami4 examined the functional differentiation of main human limb muscles with respect to fatigability for sustained isometric contraction, by means of frequency analysis of the sur­face EMG. Fifty-eight fatigue studies were done on 12 muscles in seven normal sub­jects, four women and three men, aged 18-33. Fatigability was estimated by use of contractions of half-to-two-thirds of maximum voluntary contraction (MVC) strength. The increase in slow wave ra­tio, or percentage of integrated amplitude in the frequency component at below 60 Hz to total activity, was calculated. The triceps brachii was found to be more fatigable than the biceps brachii and brachioradialis, the extensor carpi ulnaris more than the flexor carpi ulnaris, the semi-tendinosus more than the rcctus femoris, and tibialis anterior more than the gas-trocnemius and soleus. There was no sig­nificant difference between the rectus femoris, vastus medialis and vastus lateralis. In general, the lower limb muscles, especially the knee extensors and calf muscles, were less fatigable than , the upper limb muscles. In the upper limb, the extensors were more fatigable than the flexors. No consistent difference in fatigability was noted between one-joint and two-joint muscles. Differential muscle fatigability could be due to differences in muscle fiber com­position.5 Success in numerous activi­ties appears related to fiber-type distri­bution in skeletal muscles. Changes in fiber composition have not been demon­strated to occur with any form of train­ing. Long contraction times appear to correspond to fibers rich in mitochondria, or slow-twitch fibers. Rehab protocols would be wise to provide additional sets of workloads on those groups of muscu­lature that fatigue quicker if endurance gains are required. In the rehab clinic, fatigability of se­lected musculature that requires rehabili­tation must be carefully observed.6 In order for injured musculature that fa­tigues quickly to regain normal strength and endurance, it is necessary to perform protocols that augment such strength and endurance loss. A common practice is to perform the strength-endurance ex­ercise at the beginning and end of the exercise session. Kim D. Christensen, DC, CCSP, DACRB, is co-director of the SportsMedicine & Rehab Clinics of Washington. He is a popular speaker, and participates as a team physician and consultant to high school and uni­versity athletic programs. He is cur­rently a postgraduate faculty member of numerous chiropractic colleges and is the current president of the American Chiropractic Association (ACA) Rehab Council. Dr. Christensen is the author of numerous publications and texts on musculoskeletal rehabilitation and nu­trition. He can be reached at Chiroprac­tic Rehabilitation Assoc,18604 NW 64th Avenue, Ridgefield, WA 98642. References Kondo N, el al., "Regional differences in the effect of exercise intensity on thermoregu- latory sweating and cutaneous vasodilata- tion". Ada Physiol Stand 1998; I64(I):7I- 78. Costill DL. "Sweating: its composition and effects on body fluids". Ann N. Y. Acad Sci 1977; 301:160-174. Roberts MF, Wenger CB. "Control of skin circulation during exercise and heat stress". Med Sci Sports 1979; ll(l):36-41. Sato H. Hayami A. J Orthop Spoils Phvs Ther 1980; l(Winter): 153-158. Hautier CA. Belli A, Lacour JR. "A method for assessing muscle fatigue during sprint ex­ ercise in humans using a friction-loaded cycle ergomcter". Eur J Appl Phvsiol Occup Physiol 1998; 78(3):23 1-235. Dugan SA, Frontera WR, Muscle fatigue and muscle injury. Phys Med Rehabil Clin N Am 2000; ll(2):385-403.