Muscle temperature and EMG amplitude and frequency during isometric exercise

While muscle temperature is known to vary with environmental temperature and the insulation provided by clothing, little has been published examining the interrelationships between the amplitude and frequency of the electromyogram (EMG), muscle tension, muscle fatigue, and muscle temperature.

Seven male subjects immersed their arms and legs in water at 24, 27, 34, and 37 degrees C for 20 min. Muscle temperature, strength (maximal voluntary contraction; MVC), endurance for a fatiguing contraction at 40% MVC, and EMG were assessed in the handgrip, biceps brachii, quadriceps, and gastrocnemius muscles.

MVC was 44.8% lower for all muscles examined at the coldest muscle temperature. For all temperatures, the relationship between EMG amplitude and tension for brief isometric contractions was nearly linear; however, the increase in the amplitude of the EMG with muscle fatigue was reduced for the coldest muscle temperatures. The frequency components of the EMG and motor unit conduction velocity were largely unaffected by muscle tension but were inversely related to muscle temperature, with a 10 degrees C reduction in temperature resulting in a 32 Hz reduction in the center frequency. During fatiguing contractions at a tension of 40% MVC, the percent reduction in frequency was similar for all muscle temperatures, being reduced by about 20% from the beginning to the end of the contractions.

EMG amplitude can be used to assess muscle use in most physiological conditions, but the frequency components of the EMG are so related to temperature as to make its use more restricted.