Like other tissues, skeletal muscle tissue consists of cells (muscle fibers). The size of these cells ranges from 5 to 100 μm in diameter, and they are up to several centimeters long (Randall et al. 2002). Their parallel arrangement allows for the fibers in a muscle to pull together in a specific direction to exert force. This muscle force is achieved by parallel subunits inside muscle fibers (myofibrils) which in turn consist of sarcomeres, the longitudinally repeated functional units of muscle. The importance of the sarcomere is that its arrangement helps us understand the molecular basis for muscle function. Each sarcomere contains two proteins arranged in a particular geometric pattern, namely actin and myosin, and the interaction between these two proteins explains how a muscle is able to contract (current mechanism known as the sliding-filament theory; see Fig. 1). This contraction requires energy and it is obtained from glucose and lipids that are in turn transformed into adenosine triphosphate (ATP), the energy currency of the cell. Both ATP production (see how chemistry fuels running) and hydrolysis release heat, and it is this heat that contributes to the body’s temperature.
ATP is necessary for muscle work because of two major processes that are energy-dependent. The first process has to do with the cycling of attachment and detachment of myosin cross-bridges to actin, which is mediated by an enzyme called actomyosin ATPase. Without ATP, the myosin heads cannot detach from the actin filament for a new cycle. The second process involves the pumping of Ca2+ (calcium ions) back into the sarcoplasmic reticulum of the muscle fiber by an enzyme called sarcoplamic reticulum ATPase. Free Ca2+ induces muscle contraction by binding to troponin (a muscle protein) which changes the configuration of another protein, tropomyosin, allowing for the myosin heads to access myosin binding sites on the actin filament. During strenuous exercise, muscle energy consumption can surpass that of a resting muscle 100-fold and achieve rates of energy consumption greater than 1.5 kg ATP per minute of activity (McIntosh et al. 2006). If we reflect on the facts that skeletal muscle constitutes about 40% of our total body weight and that muscle contraction is only ~25% efficient, we immediately realize that a lot of expended energy must be released as heat (Sawka and Young 2006). In fact, about 3 joules of energy are released as heat for every joule of chemical energy that is converted into mechanical work4. This extra heat produced during activity is added to the heat generated by our basal metabolism and increases body temperature (2-4°C increase in core temperature is common after strenuous exercise; Randall et al. 2002). Excessively high body temperatures threaten enzymatic activity and thus, avoiding excessive heat storage is of paramount importance when exercising.
Athletes performing different types of exercise rely on the strength of specific muscles. This strength depends on muscle morphology and architecture and myosin isoform composition (see genetic aspects in How Much do Genes Affect Your Athletic Potential?). The phenotypic profiles of muscle fibers cannot only be affected by neuromuscular activity, hormones and aging, but also by the athletic training a person undergoes. Broadly speaking we can define two types of exercise: high-resistance exercise and endurance exercise. The first one results in greater muscle mass and strength by involving some form of high-intensity weightlifting for a short duration (8-12 repetitions) two to three days a week. In contrast, muscular endurance is achieved by low-intensity exercise regimes during 30-60 minutes on an almost daily basis (McCarthy and Esser 2012). Thus, athletes usually bear this in mind when exercising. But, irrespective of the type of exercise, we always feel an increase in body temperature associated with activity. How is it then that our body copes with the excess heat generated during sports or other physically-demanding activities?
Metabolic heat generated by active muscles is transferred to the bloodstream and then to the body core. Whether this heat increases our body temperature or not will depend on different environmental variables, particularly ambient temperature. For example, in cold climates we can suffer from heat loss, which will lower our body temperature and cause our metabolic rate to slow down. If body heat generation cannot keep up with the dissipation of heat to the environment, our body temperature will eventually decrease to dangerous low levels, and may even end in death. The opposite can happen in hot climates. If we cannot dissipate enough heat, we accumulate heat causing our metabolic rate to increase (which generates even more heat) and leads to overheating. This can also end in death. Fortunately, since most of us do not experience extreme weather on a regular basis, our bodies are able to handle this interplay between internal and external temperature in different ways.
Humans can be classified as endotherms, which means that our own energy metabolism produces the heat that determines our body temperature. Endothermy allows us to also be homeotherms, because our body temperature is relatively constant and independent of ambient temperature (core body temperature in humans is about 37°C). Thermoregulation in homeotherms occurs through two collaborative processes, namely behavioral and physiological temperature regulation. The first consists of conscious and unconscious behavioral changes influencing heat storage, like modifying activity levels, seeking shade or sunlight, reducing surface area for heat exchange and even changing clothes. Physiological temperature regulation encompasses responses that are unconscious. Our bodies can control the rate of metabolic heat we produce (e.g., by shivering), as well as heat loss by sweating and blood flow distribution (e.g., cutaneous vasodilatation and vasoconstriction). This last mechanism facilitates heat transfer from the skin to the surrounding air or water and is highly dependent on environmental temperature, air humidity, air or water motion, radiation and clothing (Gavin 2003). Biophysically speaking, this heat transfer can be achieved by non-evaporative avenues (conduction, convection, and radiation) called “dry heat exchange” or via evaporative cooling (see Fig. 2). Evaporation is induced by sweating (it can begin after just 2 seconds of engaging in heavy physical work; Randall et al. 2002) when we exercise, and it is the only known mechanism for dissipating heat against a thermal gradient. For example, on a hot day in the desert, our exocrine glands can produce over 12 liters of sweat, effectively cooling our bodies to tolerable temperatures (Jablonski 2006). However, the effectiveness of sweating is low in very humid environments, making it extremely difficult to get rid of excess heat.
Usually our motivation to win or complete a certain task leads us to ignore effective thermoregulatory strategies, and this usually causes lower performance, injuries and/or heat related illnesses. On top of thermoregulatory strategies during activity, there are also several pre- and post-exercise strategies that aid us in performing better and avoiding injuries (Noonan et al. 2012, Ross et al. 2013, Very et al. 2013). For example, warming up and stretching prior to exercising has been shown to deter muscular injuries. In general, warm-up is defined as activities that make us sweat mildly but do not fatigue us, with the purpose of improving muscle dynamics and preparing us for more stringent demands of subsequent exercise. There are two types of warm-up, active and passive. Active warm-up is the most common type of warm-up for both amateur and professional athletes and involves some kind of non-specific body movement (e.g., jogging, cycling or callisthenics). In contrast, passive warm-up results from the increase of muscle temperature or core body temperature by external means, like heating pads, vibrational devices (Cochran 2013), hot showers, saunas, etc. Stretching, as part of warm-up, is recommended within 15 minutes prior to activity to obtain the best results (Woods et al. 2007). Post-exercise strategies are more commonly used by professional athletes after very demanding activities that may lead to muscular fatigue and injuries. These usually involve some kind of muscle cooling technique (DeGroot 2013). Finally, every athlete knows that good nutrition (see Putting Protein in Its Place) and hydration (see Hydration and Sports Beverages) are essential for safe and effective exercising. In particular, hydration is strongly linked to thermoregulation. Although sweating allows us to get rid of excessive heat efficiently, it also presents the risk of dehydration, if not enough water is consumed.
Regular physical activity enhances and maintains health, but we need to take special considerations (like the ones we saw above) when engaging in sports or other vigorous physical activities. This is particularly true in hot and humid weather, which cannot only lead to poor athletic performance but also to heat stress and even death. Besides inadequate hydration, excessive heat retention can be caused by physical exertion, insufficient recovery time in-between activities, and inappropriate clothing. Heat stress can come in the form of heat cramps (painful cramps in abdominal muscles and muscles of the extremities), heat syncopes (weakness, fatigue and fainting), heat exhaustion due to water and/or salt depletion (causing exhaustion, muscle cramps, nausea, vomiting, dizziness, elevated temperature, weakness headaches, etc.), and heat strokes due to failing thermoregulation (causing nausea, seizures, disorientation, and in severe cases unconsciousness or comatosis). There are also serious risks associated with exercising in cold weather (Castellani and Young 2012). For example, sprains and strains are common, and in very cold-weather, frostbite and hypothermia (core body temperature dropping below that required for normal metabolism) can present a challenge to unprepared athletes.
All of these health related issues can be avoided by learning on the one hand, about the different strategies that our bodies use to control body temperature and, on the other hand, ways of helping our bodies to thermoregulate when external conditions are too harsh. We should always reduce the risks of heat-related illness, by hydrating and eating appropriately, adjusting exercise activity levels according to our current fitness status, having adequate recovery periods between bouts of exercises, and realizing when it is better to cancel athletics and stay home (Heat-Related Illnesses 2014).
By Jose G. Crespo
Jose G. Crespo is a researcher in the field of animal physiology and behavior with an emphasis on insect thermoregulation and neuroscience. He is currently a Postdoctoral researcher at the University of Utah – Department of Biology.
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Articles by Jose G. Crespo.