Smilios, I, Häkkinen, K, and Tokmakidis, SP. Power output and electromyographic activity during and after a moderate load muscular endurance session. J Strength Cond Res 24(8): 2122-2131, 2010-The purpose of this study was to examine (a) the mechanical power and the electromyographic (EMG) activity during a moderate load muscular endurance session and (b) the maximal mechanical power output and EMG activity using a light load and a heavy load afterward. Sixteen men (age: 20.7 ± 1.1 years) performed 4 sets of 20 repetitions with an initial load of 50% of 1 repetition maximum (1RM), and 2 minutes of rest, in the squat exercise. Furthermore, the subjects performed 4 repetitions with loads of 40 and 80% of 1RM before, immediately after, and 30 minutes after the end of the session. Average power and EMG activity from vastus medialis (VM), vastus lateralis (VL), and rectus femoris (RF) were recorded during the concentric phase of the lift. Average power did not change during the first 2 sets, while it decreased (p < 0.05) during the third and the fourth set. Average quadriceps (AQ), VM, VL, and RF activity increased (p < 0.05) until the 11th repetition, approximately, during each set while it increased gradually from set to set. Maximal power and AQ, VM, and VL activity with the loads of 40 and 80% of 1RM were decreased (p < 0.05) after the session. Blood lactate reached 10.2 ± 2.5 and 13.1 ± 4.1 mmol·L−1 after the second and the fourth set, respectively. It appears that during a muscular endurance session with submaximal power output, mechanical performance may gradually decrease, probably due to metabolic fatigue, while muscle electrical activity may increase. Following this type of a session, maximal power output and muscle activation with a light and especially with a heavy load are reduced.