This dissertation investigated the influence of extra mass in open kinetic chain motions and applied results of fundamental studies to a new form of baseball bat swing warm-up (i.e., the overloaded arm warm-up). Fundamental studies of the horizontal arm swing (Study 1 and 2) provided mechanical justification for using extra mass. Study 1 (kinematic approach) found the forearm extra mass reduced the arm swing speed significantly but the upper arm extra mass did not affect it much according to the increased extra mass. The change of the moment of inertia (MOI) was speculated to be a main factor determining the performance of limb dynamics. Study 2 (kinetic approach) illustrated the different control strategy of the CNS at each joint and provided justification on results of Study 1. The detrimental effect of the forearm extra mass was primarily attributed to the counteractive interaction torque and the significantly increased MOI due to its location. However, the upper arm extra mass would be beneficial to the arm swing speed when a decent amount (25 to 50% of upper arm mass) was decided. In Study 3, even though the after-effect of the overloaded arm warm-up was not statistically significant, it showed the possibility of the bat speed improvement. The overloaded bat was not beneficial to the bat speed. Baseball players should be careful on a significant bat speed drop at the first trial immediately after loaded warm-ups.