To investigate the effects of base of support (BOS) and external loads on electromyographic (EMG) onset in the shoulder muscles during push-up exercises. Methods: Two levels of external load were applied at two levels of BOS stability during push-up exercises. EMG onset in six shoulder muscles was measured in 30 healthy participants. Results: With load set at 4 of body weight (BW), EMG onset in the lower trapezius (LT) (. P = 0.003) and biceps brachia (BB) (. P = 0.001) was significantly decreased with no load. Conversely, in other muscles (the upper trapezius (UT), teres major (TM), seratus anterior (SA) and deltoid posterior (DP)), time to EMG onset did not change significantly. No significant changes in EMG onset were observed with load at 2 of BW. The average time to EMG onset was significantly decreased for different stages of BOS instability in the LT (. P = 0.04) and UT (. P = 0.001). Conclusion: Both load and BOS instability reduce time to EMG onset, but BOS instability produces greater reductions.