The purpose of this study was to investigate the patterns of motions of the serving arm and the underlying muscular activity responsible for the motions. Stroboscopic photography and three-dimensional (3D) cinematography were used to study five male, right-handed, collegiate tennis players while performing two types of tennis serves (flat and slice). Film analysis procedures were used to calculate 3D coordinates of the landmarks. The serving arm was modeled as a three-link kinetic chain composed of the racket/hand, forearm, and upper arm. Rigid body kinematics and kinetics were used to calculate the following parameters: temporal phases of the serves, ball velocity after impact, linear velocities of the joints, joint angles and angular velocities, and resultant joint torques and forces at the shoulder, elbow, and wrist joints. In the slice serves, the mediolateral horizontal components of the linear velocities of the wrist and racket were directed more toward the right, and there was a horizontal abduction angular velocity of the upper arm between maximum external rotation and impact. These motions allowed the subjects to swing the racket more toward the right which produced sidespin on the ball. There were only minor differences between the joint torques of the two types of serves. The role of the shoulder muscles during the early stages of the forward swing was to resist the forced external rotation of the upper arm produced by the translation and rotation of the trunk. In the latter stages of the forward swing, the shoulder muscles were used to sequentially elevate, internally rotate, and extend the upper arm. The varus torque exerted at the elbow joint, through tensile forces on the medial side and bone-on-bone compressive forces at the lateral side, prevented the valgus rotation of the forearm and was associated with the internal rotation torque at the shoulder. The rapid movements of the forearm near impact were produced by pronation and extension torques at the elbow. The wrist torques played an important role during the forward swing of the serve. These torques were first used to prevent the inertial lag of the racket/hand segment and later to produce flexion and ulnar deviation of the wrist. The stresses placed on the pronator and wrist muscles were very large and could lead to the development of medial epicondylitis (tennis elbow). The development of large torques at the shoulder, elbow, and wrist and the repetitive nature of the tennis serve make these articulations vulnerable to injuries of the musculoskeletal system.
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