The Relationship between Muscle Temperature, MUAP Conduc- tion Velocity and the Amplitude and Frequency Components of the Surface EMG During Isometric Contractions

The relationship between the tension developed during brief isometric contractions and the frequency and amplitude components of the surface EMG were recorded in 5 male subjects after immersion of their arms and legs in water at 24, 27, 34 and 37 deg. C. for 20 min. The muscles examined were the handgrip, biceps, quadriceps, and gastrocnemius muscles. For the three warmest water temperatures, the muscle strength was unaffected by the water bath temperature. Muscle strength was 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. The frequency components of the EMG as assessed by Fourier Analysis of the raw EMG and MUAP conduction velocity, were largely unaffected by muscle tension but was dramatically affected by muscle temperature. During fatiguing contractions at a tension of 40% MVC, the shift in frequency was similar for the warmest three muscle temperatures while for the coldest temperature, the shift in center frequency was much less. The results show the feasibility of use of EMG amplitude to assess muscle use in most physiological conditions but draws doubt on using the frequency components of the EMG as a measure of fatigue under circumstances where muscle temperature may vary.