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Human Kinetics Publishers, Inc.

HUMAN KINETICS

Excerpts

Exercise Performance and Cold Air Exposure*

By Dr. Lawrence Armstrong


Several sports are contested outdoors during winter months, including ice hockey, road or cross country footraces, snowboarding, alpine or cross-country skiing, and ice-skating. Other sports and recreational activities sometimes occur when the air temperature is cold; these include American football, soccer, baseball, hiking, and rock climbing. Performance in these sports and activities may be negatively affected if deep body temperature decreases.

Because preexercise warm-up prepares muscles and the cardiovascular system, helps to prevent injury, and promotes metabolic efficiency, it is important that you understand a study conducted in Japan in 1992. The purpose of that research was to observe physiological responses to different warm-up protocols when ambient temperature was 10°C (50°F). The test subjects were trained skiers who performed five warm-up runs on different days: 15 min of exercise at 70% .VO2max; 15 min at 50% .VO2max; no warm-up; 30 min at 70%
.VO2max; and 30 min at 50% .VO2max. Test results showed that the optimal preparation for exercise occurred after the latter test, involving moderate exercise intensity and longer duration. Warming up with a short duration, high-intensity protocol was ineffective. The following paragraphs describe other physiological studies that complement this finding.

Exercise and work performance, as well as acclimatization to cold (see below), depend on more than the temperature of the air or water surrounding the body. If several layers of insulative clothing are worn, for example, it is possible that core body temperature might even rise during exercise in the cold. Therefore, regardless of the severity of cold exposure, you should focus on the effects on physical performance of a fall in core temperature or muscle temperature.

Cardiorespiratory endurance will likely decrease if the whole body cools, and this is more likely to occur if exercise in cold air is prolonged. Laboratory studies indicate that this decrease in endurance performance is due to at least three distinct adaptive responses, which may differ depending on the environmental conditions and exercise protocol utilized. First, maximal heart rate decreases with body cooling. Because stroke volume does not increase, cardiac output falls (cardiac output = heart rate 3 stroke volume; see chapter 2, page 22), as does the ability to sustain strenuous exercise. Second, when blood temperature decreases below 37°C (98.6°F), less O2 is delivered to skeletal muscle and other tissues because hemoglobin’s molecular structure binds to O2 molecules more tightly. When both cardiac output and the dissociation of oxygen from red blood cells fall, .VO2max decreases. Third, blood flow to skeletal muscle during exercise decreases when the body is cooled. Because this reduces O2 delivery to muscles, more energy must be produced via anaerobic metabolism, increasing lactate levels in blood and muscle. Of these three adaptive responses, the first has the greatest impact.

Interestingly, a recent investigation conducted in Scotland demonstrated that cardiovascular endurance performance in cold air declined, even when body temperature was elevated. The top panel of figure 3.8 (page 92) depicts the influence of air temperature on the cycling time to volitional exhaustion of eight subjects. Clearly, the 11°C (52°F) air temperature provided the least stressful condition for endurance performance. The 21 and 31°C (70 and 87°F) environments decreased exercise time for the reasons described in chapter 2 (see “Heat and Physical Performance,” page 24). Regarding the present topic, cooling the air from 11 to 4°C (52 to 39°F) caused a decrease of 12.1 min in cycling time to exhaustion. However, the bottom panel in figure 3.8 shows that the average rectal temperature was elevated to 39°C (102°F) at the end of the 4°C (39°F) trial and was higher in all other trials (i.e., 40°C [104°F] in the 31°C [87°F] environment). Thus, since body temperature did not decrease, the mechanisms of this decrease in performance were different from those described in the previous paragraph. The authors of this investigation proposed a few possible reasons for the decline in cycling endurance time, but could offer no definitive explanations based upon their data. For example, it is known that pulmonary ventilation ( .VE) and oxygen consumption ( .VO2) are affected differently by cooling the airways, the body’s core, and the skin. The effects of airway cooling during exercise depend on the intensity of cold-air stress; .VO2 may increase, decrease, or remain constant. If central body temperature decreases, .VE and .VO2 fall, until shivering thermogenesis occurs; these factors subsequently increase. When the exercise intensity is great enough to maintain or elevate core temperature, but skin temperature decreases due to cold-air exposure, .VE and .VO2 increase. Figure 3.9 illustrates this and complements figure 3.8, in that all data in these figures were collected during the same investigation. Figure 3.9 demonstrates that .VO2 increased the most, and that the skin temperature was lowest, during the 4°C (39°F) trials; yet, rectal temperature was elevated to a level that prevented shivering from occurring (figure 3.8). These increases in .VE and .VO2 may have been caused by stimulation of temperature receptors in the skin, or increases in muscle tension and/or muscle metabolism.

Muscular endurance, defined as the ability to sustain continuous contractions at submaximal intensity, may be altered by cold-air exposure. As was the case with cardiovascular endurance, muscular endurance depends more on the decline in muscle temperature than changes in the surrounding air temperature. This effect also depends on the degree of cooling. When slight cooling of muscle is employed, fatigue develops at a slower rate. However, when muscle temperature falls below 27°C (82°F), muscular endurance decreases. This effect may be due to either reduced nerve conduction velocity or recruitment of fewer muscle fibers, especially those near the muscle surface.

Maximal muscular strength and peak muscular power also decrease at lowered muscle temperatures., This negatively affects dynamic physical performance in virtually all sports and occupations. Specific decrements, subsequent to muscle cooling, occur in jumping and sprinting events. The loss of performance in these events amounted to 4-5% per degree Celsius. There are several possible reasons why cooling may inhibit force production and power output. First, there may be an increase in the time it takes muscle fibers to reach maximal tension. This may involve a slower rate at which actin and myosin cross-bridges (i.e., the microfilaments in skeletal muscle) break and reattach. Second, the viscosity of the fluid inside muscle fibers (sarcoplasm) may increase as the muscle is cooled, increasing the resistance to movement of the cross-bridges and actin. Third, it is known that the rate of chemical reactions in muscle slows as the temperature drops, primarily because muscle enzyme activity and the production of high-energy phosphates (e.g., ATP) decrease. Due to these changes in maximal strength and power, the most susceptible types of exercise are those that are brief and dynamic, utilizing fast movement velocities and the elastic properties of active muscles.

* To reprint this excerpt with permission from Human Kinetics Publishers, Inc., please contact the publicity department at 1-800-747-4457 or publicity@hkusa.com.




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