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Arm swing improves vertical jump height but the underlying mechanism is not well understood. We assume that the negative acceleration of arm swing can increase the load and tension of the lower limb’s extensor muscles enhancing force output and resulting in higher jump. The purpose of this study was to examine how arm swing affects the EMG activity of the lower limb muscles during vertical jumping in relation to the arm swing kinematics. Sixteen physically active male college students were asked to perform squat (SJ) and countermovement jumps (CMJ) with arms akimbo and with arm swing. Jumps were carried out on a force platform while kinematic data and EMG activity of the right lower limb were measured. Jumping height increased by 33% in SJ and by 18.9% in CMJ when performed with arm swing. In SJ with arm swing the vertical ground reaction force (GRF) was larger during the negative acceleration of the arm (71–91% of the jump, p < 0.001) with significantly slower relative joint extension. Similarly, in CMJAS during the negative acceleration of the arm vertical GRF was larger (89–97% of the jump, p < 0.001) compared to CMJ. Arm swing increased the jump time, net impulse and take off velocity in both jumps. No differences were detected in EMG between jumps carried out with and without arm swing. When jumps carried out with arm swing, we can observe higher vertical GRF during the negative acceleration of the arm swing that indicate higher load on the lower limb, but we cannot confirm our initial idea with the results of the EMG comparison. Increase in jump height can be explained by the extended jump time resulting in greater mechanical variables in jumps with arm swing.