4. Results
An example of contribution of each term to the generation of racket head speed is shown in Figure 3(a). The
contribution of motion dependent term increased exponentially right before the impact. The shaft restoring torque
contributed to the racket face speed positively in the first half and negatively in the latter half of the forward swing
motion. The contribution of active joint torques increased from 0.14 s to 0.23 s in the forward swing motion. The
contribution of shoulder joint forces increased positively from 0.1 s to 0.15 s and decreased from 0.15 s to 0.25 s.
In addition, the contribution of joint torques was larger than joint forces significantly at the impact.
The contributions to the motion dependent term in Figure 3(a) are shown in Figure 3(b). The contribution of
shaft restoring torques to generation of the motion dependent term increased from 0.1 s to 0.18 s and decreased till
0.23 s in the forward swing motion. The contribution of active joint torques decreased from 0.13 s to 0.18 s and
then increased to the impact. The external force term contributed positively from 0.13 s to the impact.
5. Discussion
The results showed that the motion dependent term, the shaft restoring torque term and joint torque terms are
large positive or negative contributors to the generation of racket head speed during the forward swing phase. The
contributions of joint torque term and shaft restoring torque term tended to show opposite sign value. After
converting the motion dependent term into other terms, the results indicate that the motion dependent term is
mainly caused by shaft restoring torque term and cancelled by negative contribution of joint torque term around
0.05 s before the impact. Since the deformation of racket shaft is caused by the joint torques, a new algorithm
which converts the shaft restoring torque term into the joint torque term should be required to investigate head
speed generating mechanism.
6. Conclusion
We have developed a method of quantifying the contribution of the racket-side upper limb joint torques and
shaft restoring torque on the head speed generation mechanism during the badminton smash motion. The results
showed that 1) the motion dependent term was the largest contributor to the generation of head speed right before
the impact, and 2) the shaft restoring torque term was positive contributor to the generation of motion dependent
term over the forward swing period in the badminton smash motion. Although this study focused on analysis of
badminton smash motion, this method can be used to analyse other types of badminton swing motions, such as
drive and high clear shots. Additionally, this method can be readily applied to the analyses of other hitting motion
using a hitting tool that has flexible elements such as a golf club.