Badminton is an open skill sport where the shuttlecock can not touch the ground during the game. To hit the shuttlecock successfully, players must rapidly change their moving direction (Kuntze et al., 2010; Manrique and Gonzalez-Badillo, 2003; Shariff et al., 2009). Atypical actions and hitting movements often occur during the game; therefore, many special footwork methods have been developed to facilitate the rapid movements required to hit the shuttlecock, including quick turning and jumping and quick directional change movements. Studies have shown that the majority of badminton sport injuries occur in the lower extremity joints of athletes (Goh et al., 2013; Reeves et al., 2015). Badminton players most commonly face knee injury, followed by back, ankle joint, thigh, and calf injuries (Goh et al., 2013; Miyake et al., 2016; Reeves et al., 2015; Yung et al., 2007). Injury of the anterior cruciate ligament is the most common knee joint injury, and 70% of anterior cruciate ligament injuries are noncontact injuries. The major mechanisms of action for these injuries are landing actions and quick directional changes (Dai et al., 2015; Wang, 2011).
Because badminton is a one-sided hitting sport, its action characteristics will produce asymmetric phenomena. Therefore, the lower limber joints mechanisms and risks of injury in both of the forehand side and backhand side are different (Kimura et al., 2012). Lower extremity injuries in badminton players usually occur during jump landing in a lateral (LL) or poster lateral direction (PL) on the play court (Kimura et al., 2010; 2012; Shuhei et al., 2018). During backhand lateral hitting, players will use the rotation, flexion and extension of the torso to achieve the best hitting position. Executing backhand lateral hitting will produce larger hip joint abduction. It is considered that this action strategy will increase the lower extremity joint load (Kimura et al., 2012; Sasaki et al., 2018). Except for studies on the different types of hitting positions, few studies have evaluated action strategies of the same hitting position in different action situations.
A player's reaction ability is important movement ability in badminton. Badminton players should possess excellent selective reaction abilities to assist moving and hitting on the court. Visual reception has a critical impact on selective reaction. Dynamic vision is an important factor affecting the selective reaction of badminton players (Loureiro Jr and Freitas, 2012). The players must rapidly determine the hitting position after the opponent hits the shuttlecock. It has been pointed out that the visual processing system of excellent athletes is better than that of general athletes (Berman, 1988; Abernethy and Neal, 1999; Laby et al., 2011). If players can predict the hitting position in advance during play, the moving load can be reduced and the active hitting efficiency can be increased to increase the chance of scoring. However, studies on badminton have rarely used selective reaction as a variable to investigate player performance and lower extremity load and injury.
Joint torque and joint stiffness are important indicators for evaluating lower extremity load and injury. Studies that combine imaging and force plates can perform inverse dynamics to estimate the joint torque. Previous studies have shown that the magnitude of knee joint torque during landing is closely associated with the mechanism of anterior cruciate ligament injury (Chappell et al., 2002; Wang et al., 2010; Yu et al., 2006). In addition to joint torque, joint stiffness is also an important reference parameter. Joint stiffness is the adjustment reaction of individuals to the process of dynamic movement and environmental changes. Therefore, joint stiffness is an important parameter for evaluating sport performance and injury risks. Excessively high lower extremity stiffness during movement may cause injuries to the skeletal system (Milner et al., 2007), whereas an excessively low joint stiffness may increase the risk of soft tissue injuries (Granata et al., 2002; Williams et al., 2001). The calculation method of joint stiffness combines joint angle changes and torques; therefore, the analysis of joint stiffness can be used to further investigate sport performance and the mechanism of lower extremity sport injuries.
Previous studies evaluating athletic performance and sports injuries used badminton footwork action, but previous studies have not yet analyzed the actual hitting footwork action. If the actual hitting footwork action were difference of the shadow action, will be reduced the effect of the badminton footwork assessment. Inverse dynamics analysis can more clearly evaluate the risk produced by the action. In view of this, the purpose of this study was to investigate the influences of hitting motion and unanticipated hitting direction on landing mechanics after backhand lateral jump smashing and landing to analyze joint stiffness and torque changes in three lower extremity joints.
Sixteen elite Taiwanese male badminton players was recruited; all the players were right handed. All the participants had won in the quarterfinals of tournaments conducted at the national level in Taiwan. The average age, height, body weight, and experience in badminton of the players were 21.1 [+ or -] 1.9 years, 1.74 [+ or -] 0.04 m, 68.4 [+ or -] 6.7 kg, and 10.5 [+ or -] 2.4 years, respectively. The participants reported no history of surgery on the lower limbs or musculoskeletal disorders in a span of 1 year prior to data collection. All procedures were approved by the Institutional Review Board of Fu Jen Catholic University.
Eight high-speed infrared cameras (Vicon MX-T20-S+, Oxford Metrics, UK) were set up in the badminton court, (Figure 1). The image capturing frequency was set at 300 Hz. In the court, the front edge of a Kistler force platform (Kistler 9821, Kistler Instrument, Inc., Swiss) was embedded 150 cm in front of the backcourt boundary. The lateral edge of the platform was aligned with the service line set for playing doubles. The capture frequency of the Kistler force platform was set at 1500 Hz. The racket (Yonex) had a string tension of 29 pounds. The participants used the shuttlecock (Victor) and shoe (Yonex). Reflective markers were affixed on the rackets and shuttlecocks. The markers were used to determine the heights of the strokes. Vicon Nexus Version 1.8.5 software were used simultaneously to capture the kinematic and kinetic parameters while performing the lateral jump smash footwork on the backhand side.
This study analyzed lateral jump smash footwork actions in four different badminton training situations. The swing actions were divided into two situations, shadow (footwork and racket swinging practice without targets, Figure 2) and hitting actions (footwork and stroke shuttlecock...