There is a need within human movement sciences for a markerless motion capture system, which is easy to use and sufficiently accurate to evaluate motor performance. This study aims to develop a 3D markerless motion capture technique, using OpenPose with multiple synchronized video cameras, and examine its accuracy in comparison with optical marker-based motion capture. Participants performed three motor tasks (walking, countermovement jumping, and ball throwing), and these movements measured using both marker-based optical motion capture and OpenPose-based markerless motion capture. The differences in corresponding joint positions, estimated from the two different methods throughout the analysis, were presented as a mean absolute error (MAE). The results demonstrated that, qualitatively, 3D pose estimation using markerless motion capture could correctly reproduce the movements of participants. Quantitatively, of all the mean absolute errors calculated, approximately 47% were <20 mm, and 80% were <30 mm. However, 10% were >40 mm. The primary reason for mean absolute errors exceeding 40 mm was that OpenPose failed to track the participant's pose in 2D images owing to failures, such as recognition of an object as a human body segment or replacing one segment with another depending on the image of each frame. In conclusion, this study demonstrates that, if an algorithm that corrects all apparently wrong tracking can be incorporated into the system, OpenPose-based markerless motion capture can be used for human movement science with an accuracy of 30 mm or less.

The purpose of this study was to investigate whether performance level and ball spin affect arm and racket kinematics of the table tennis topspin forehand. Nine advanced and eight intermediate male table tennis players hit topspin forehands against light and heavy backspins. Five high-speed video cameras were used to record their strokes at 200 fps. Contributions of joint rotations to the racket speed, the racket kinematics at ball impact, the time required for racket acceleration and the maximum slope of the racket speed-time curve (s(max)) were determined. The advanced players showed a significantly larger contribution of lower trunk axial rotation to the racket speed at impact and a significantly larger value of s(max), and tended to require a less time for racket acceleration than the intermediate players. The racket speed at impact was not significantly different between the two player groups. The players adjusted the racket face angle rather than the inclination of the racket path at impact to the different ball spins. The results suggest that the ability to accelerate the racket in less time in the topspin forehand against backspin balls may be an important factor that affects the performance level.

The purpose of this study was to determine the significance of mechanical energy generation and transfer in the upper limb in generating the racket speed during table tennis topspin forehands. Nine advanced and eight intermediate table tennis players performed the forehand stroke at maximum effort against light and heavy backspin balls. Five high-speed video cameras operating at 200 fps were used to record the motions of the upper body of the players. The joint forces and torques of the racket arm were determined with inverse dynamics, and the amount of mechanical energy generated and transferred in the arm was determined. The shoulder internal rotation torque exerted by advanced players was significantly larger than that exerted by the intermediate players. Owing to a larger shoulder internal rotation torque, the advanced players transferred mechanical energy from the trunk of the body to the upper arm at a higher rate than the intermediate players could. Regression of the racket speed at ball impact on the energy transfer to the upper arm suggests that increase in the energy transfer may be an important factor for enabling intermediate players to generate a higher racket speed at impact in topspin forehands.