Effects of ambient temperature on mechanomyography of resting quadriceps muscle

It has been speculated that resting muscle mechanical activity, also known as minor tremor, microvibration, and thermoregulatory tonus, has evolved to maintain core temperature in homeotherms, and may play a role in nonshivering thermogenesis. This experiment was done to determine whether resting muscle mechanical activity increases with decreasing ambient temperature. We cooled 20 healthy, human, resting, supine subjects from an ambient temperature of 40 to 12 degrees C over 65 min. Core temperature, midquadriceps mechanomyography, surface electromyography, and oxygen consumption ((V) over dotO(2)) were recorded. Resting muscle mechanical and electrical activity in the absence of shivering increased significantly at temperatures below 21.5 degrees C. Women defended core temperature more effectively than men, and showed increased resting muscle activity earlier than men. Metabolism measured by (V) over dotO(2) correlated with resting muscle mechanical activity (R = 0.65; p = 0.01). Resting muscle mechanical activity may have evolved, in part, to maintain core temperature in the face of mild cooling.