Leg muscle metabolism during exercise in the heat and cold

In an effort to assess the effects of environmental heat stress on muscle metabolism during exercise, 6 men performed work in the heat (T _db = 41° C, RH = 15%) and cold (T _db = 9° C, RH = 55%). Exercise consisted of three 15-min cycling bouts at 70 to 85% \(\dot V_{O_2 \max } \) , with 10-min rest between each. Muscle biopsies obtained from the vastus lateralis before and after each work bout were analyzed for glycogen and triglyceride content. Venous blood samples drawn before and after exercise were assayed for lactate, glucose, free fatty acids, hemoglobin, and hematocrit. Oxygen uptake, heart rates and rectal temperatures were all significantly higher during exercise in the heat. Blood lactate concentration was roughly twice as great during the heat experiments as that measured in the 9° C environment. Muscle glycogen utilization per 60 min was significantly greater in the heat (−74 m moles/kg-wet muscle) as compared to the cold exercise (−42 m moles/kg-wet muscle.) On the average, muscle triglyceride declined 23% during exercise in the cold and 11% in the heat. The findings of an enhanced glycolysis during exercise in the heat is compatible with earlier studies which demonstrate a decreased availability of oxygen due to a reduction in muscle blood flow.