The invention is directed to a muscle trainer and method for exercising a weaker of two opposing muscle groups of a person swinging an implement, such as a golf club, wherein, if the two opposing muscle groups were of appropriate strength, the two muscle groups would desirably apply forces in opposite directions to the implement to assist in maintaining the implement in an ideal club shaft plane. The contemplated muscle trainer of this invention includes a body having a grip surface located thereon, and a force generator positioned at a prescribed location on the body, which is spaced from the grip surface, for urging the prescribed location of the body in a direction away from a force direction which the weaker muscle would normally apply to the implement in the swinging thereof by the person. The invention further contemplates a method exercising at least a dominated muscle of two opposing muscles typically used by the golfer when attempting to swing a golf club in an ideal club shaft plane, where the dominated muscle applies a first swing force to the golf club in a non-dominating swing force direction, and a dominating muscle applies a second swing force in a dominating swing force direction to the golf club which is opposite the direction, and exceeds the force, of the non-dominating swing force.