Purpose Previous evidence from surface electromyograms (EMGs) suggests that exercise-induced muscle damage (EIMD)
may manifest unevenly within the muscle. Here we investigated whether these regional changes were indeed associated with
EIMD or if they were attributed to spurious factors often afecting EMGs.
Methods Ten healthy male subjects performed 3×10 eccentric elbow fexions. Maximal voluntary contraction (MVC),
muscle soreness and ultrasound images from biceps brachii distal and proximal regions were measured immediately before
(baseline) and during each of the following 4 days after the exercise. Moreover, 64 monopolar surface EMGs were detected
while 10 supramaximal pulses were applied to the musculocutaneous nerve. The innervation zone (IZ), the number of
electrodes detecting largest M-waves and their centroid longitudinal coordinates were assessed to characterize the spatial
distribution of the M-waves amplitude.
Results The MVC torque decreased (~25%; P<0.001) while the perceived muscle soreness scale increased (~4 cm; 0 cm for
no soreness and 10 cm for highest imaginable soreness; P<0.005) across days. The echo intensity of the ultrasound images
increased at 48 h (71%), 72 h (95%) and 96 h (112%) for both muscle regions (P<0.005), while no diferences between
regions were observed (P=0.136). The IZ location did not change (P=0.283). The number of channels detecting the greatest M-waves signifcantly decreased (up to 10.7%; P<0.027) and the centroid longitudinal coordinate shifted distally at 24,
48 and 72 h after EIMD (P<0.041).
Conclusion EIMD consistently changed supramaximal M-waves that were detected mainly proximally from the biceps
brachii, suggesting that EIMD takes place locally within the biceps brachii