The purpose of this study was to compare the kinematic characteristics of the lower limbs between the DS and SS. The main findings of the study were as follows. DS showed a significantly less time-spending compared with SS during both the BS and FS phases. Meanwhile, DS showed significantly larger ankle internal rotation and inversion while smaller knee abduction and external rotation during the BS. DS showed significantly larger knee extension while smaller hip adduction and knee internal rotation compared with SS during the FS. However, SS showed significantly larger ROM of ankle plantar flexion external rotation, and significantly larger ROM of knee extension. Moreover, SS showed significantly larger knee internal rotation compared with DS. DS showed a significantly greater ROM in dorsiflexion and internal rotation of the ankle joint during the BS phase, with a significantly larger ankle eversion at BE. This means that players who perform DS need to perform more warm-up techniques and prepare themselves earlier. Meanwhile, DS has a longer path of movement and the ankle joint seems to absorb more energy, which could help players better adjust for the forward swing action. This is partly consistent with a previous study, Wang et al. (2018) compared the kinematics and electromyographic characteristics during table tennis topspin loop against backspin movement between two different level athletes. Moreover, our findings showed that the motion time of DS was less than SS, which is also an important factor for increasing ball speed (Iino and Kojima, 2009). Malagoli Lanzoni et al. (2018) compared the biomechanical characteristics of cross-court (CC) or long-line (LL) during a table tennis top spin shot. They reported that the lower limb body showed more rotation with respect to the table when playing the LL shot. This finding is supported by this study. Furthermore, no significant difference between shot executions was observed for knee activity on the sagittal plane in this study. This is also consistent with the above study. Therefore, it is speculated that the pelvis and shoulder joints may perform additional compensatory work. In addition, DS showed a significantly greater angular changing rate of ankle eversion and hip flexion, and a significantly larger ankle eversion at BE. Based on the theory of stretch-shortening cycle (Komi and Bosco, 1978; Walshe et al., 1998; Jiang, 2020), the increased ankle eversion and hip flexion may enhance muscle output of the tibialis anterior muscle and gluteus maximus (Zhang et al., 2017), which is a potential factor to increase racket back velocity. This has also been manifested in dissimilar experimental set-ups (Finni et al., 2003; Gregor et al., 1988; Komi, 2000; Stevens, 1993). Meanwhile, Abrams et al. (2012) reported the occurrence rate and prevalence of injuries in tennis athletes. The general tendency was that acute injuries were more common in the lower extremities, while chronic and overuse injuries were more familiar in extremities and trunk of the upper body. The constructed ratio proposed by Croisier and Crielaard (2000) combines two extremely different velocities. Hamstring strains usually occur during joint movements at high speeds. Hewett et al. (2005) measured the neuromuscular control of kinematics and joint loads of 205 female athletes (9 athletes suffered ACL injuries) in high-risk sports such as volleyball, football, and basketball. Compared with the healthy athletes, the knee abduction angle was enhanced when the side-bending athletes landed. The movement, strength, and torque of injured athletes enhanced more rapidly than that of healthy athletes. This could mean that, compared with the SS, the athletes have a higher risk of knee and ankle injuries during the DS. SS show significantly larger knee joint external rotation and adduction at BE, and significantly larger angular changing rate of knee external rotation in the backward swing phase. This may contribute to the stretching of the internal rotator, resulting in enhanced contraction effects during the forward swing (Zhang et al., 2017). The significantly larger ROM of knee extension with larger ankle external rotation of SS at the forward swing phase may contribute to greater weight transfer range to facilitate momentum generation (Ball and Best, 2007; Zhang et al., 2017). In addition, compared with SS, the movement of hip and ankle joints during the DS was larger. Similarly, the movement of the knee joint of SS is larger than DS. This may mean that players should prepare for a more high-intensity activity for short periods during DS. The knee external rotation and internal rotation of SS was larger than that of DS at BE and FE, respectively, which may result in knee injury, such as anterior cruciate ligament rupture. The ability of the knee joint to remain stable when subjected to the rapidly changing loads during activity relates to dynamic knee stability (Williams et al., 2001). The musculature surrounding the knee is critical for maintaining joint stability (Sanna and Connor, 2008), so we speculate strengthening the muscles around the knee joint can improve the dynamic stability of the knee joint and reduce the risk of a knee injury. Zhang et al. (2017) observed similar results, and suggested that it was beneficial for table tennis players to be able to enhance the speed of wielding the racket in less time for hard shots, because a limited time is allowed to execute a stroke in table tennis. The ability to accelerate the racket in less time in the topspin forehand may be an important factor that affects performance levels during the game. The elite players tended to require less time to enhance the speed of wielding the racket and the contribution of lower trunk axial rotation was significantly smaller for the intermediate players. Some limitations of this study must be mentioned. The differences in the biomechanical characteristics between bilateral lower limbs were not compared in the study. In addition, information about the variables considered during the racket-ball impact was not included in this study. Kinematics and kinetics data of the lower limbs should be measured simultaneously, and the process of racket-ball impact should be considered in future studies. |