The effects of four time-varying factors on the mean frequency of a myoelectric signal

Daily activities involve dynamic muscle contractions that yield nonstationa ry myoelectric signals (MESs). The purpose of this work was to determine th e individual effects of four time-varying factors (the number and firing ra te of active motor units, muscle force and joint angle) on the mean frequen cy of a MES. Previous theoretical and experimental work revealed that altho ugh changes in the number and firing rate of active motor units contribute to the nonstationarities of the signal, they do not significantly affect th e mean frequency. In the experimental work, 12 subjects performed 25 static contractions, one for each force (20, 30, 40, 50 and 60% of maximum volunt ary contraction) and elbow joint angle (50, 70, 90, 110 and 130 degrees ext ension) combination. A MES was recorded from the surface of the biceps brac hii during each contraction. The results indicated that muscle force only w eakly affects the mean frequency. Also shown was that alteration in muscle geometry resulting from changes in elbow joint angle does significantly aff ect the mean frequency. Knowing this is important for the assessment of mus cle fatigue during dynamic contractions. (C) 2001 Elsevier Science Ltd. All rights reserved.