Bench press is a common weight training method for upper extremity muscle groups. In this research, real-time electromyogram (EMG) was used to detect upper extremity muscle activations on three grip widths (narrow, middle and wide grips) and two exercise phases (barbell ascent phase and descent phase) during the dynamic periods of bench press. In the study, it was found that there was no significant different in upper extremity muscle activations when changing grip widths during the ascent phase of bench press exercise. Also, there was no significant different in upper extremity muscle activations when changing grip widths during the descent phase of bench press exercise. Therefore, the activations of upper extremity muscle groups were not apparently related to the grip width of bench press. However, when comparing muscle activations during ascent and descent phases, upper extremity muscle activations were found significantly greater in the ascent phase than descent phase. In the ascent phase, the pectoralis major、anterior deltoid, and triceps increase the muscle concentric contraction activation to overcome the gravity. Latissimus dorsi、posterior deltoid, and biceps also increase their muscle activation to support the stability of joints.