Ratanapinunchai, J and Madeeyoh, I. Differences in electromygraphic activity of the middle and the lower serratus anterior fibers during open and closed kinematics exercises. J Strength Cond Res 36(7): 1788–1792, 2022—This paper seeks to determine differences in muscle activation between the middle (MSA) and the lower serratus anterior (LSA) fibers during 3 exercises. Fifteen healthy men were recruited for the study. Bench push-up plus (BPUP), unstable BPUP (UPUP), and resisted shoulder flexion (RSF) with 10 repetitions at maximum weight were tested. The normalized electromyographic (EMG) activity of the MSA and the LSA was recorded during isometric scapular protraction for BPUP and UPUP or at 150° shoulder flexion for RSF. Normalization was calculated using maximum voluntary isometric scapular protraction activity (%EMG). The results showed that the ratio of %EMG of MSA:LSA was higher than one in BPUP (1.41 ± 0.36) and UPUP (1.81 ± 0.47), but less than one (0.86 ± 0.27) in RSF. These ratios were statistically significantly different (p < 0.05). The activity of MSA and LSA in RSF was significantly (p < 0.002) higher than that of BPUP and UPUP (%EMG of the MSA = 80.53 ± 26.07, 64.75 ± 23.19 and 55.62 ± 25.56 for the RSF, BPUP, and UPUP, respectively; %EMG of LSA = 95.54 ± 24.15, 50.71 ± 26.32, and 32.26 ± 16.14 for the RSF, BPUP, and UPUP, respectively). There was no difference in the %EMG of MSA and LSA between BPUP and UPUP. The MSA worked more than the LSA in the push-up plus (PUP), but the opposite was found in the RSF exercise. The study suggests different functions of the MSA and the LSA. Different exercises should be used, including a PUP for the MSA and RSF for the LSA, or a combination to emphasize both parts of the SA.