James M. Kohler, Sean P. Flanagan and William C. Whiting

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Purpose: To evaluate muscle activity of the prime movers and core stabilizers while lifting stable and unstable loads on stable and unstable surfaces during the seated overhead shoulder press exercise.

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Conclusions: Little support was found for training with a lighter load using unstable loads or unstable surfaces.

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IN PLAIN ENGLISH: To better target a muscle or group of muscles, put the body in a position to maximize the use of that/those muscles (read: become MORE stable) as opposed to a position of instability which will lessen the potential of overload sought.