After the Chinese table tennis association put forward "the third business plan", training methods used for adolescent table tennis players has changed considerably to strengthen international exchanges and cultural dissemination of table tennis (Chen and Huang, 2014). The traditional all-day training method was replaced by a "sports-education combination" training approach, which aims to cultivate student-players to help them become well-rounded for their moral, intellectual, physical, and aesthetic qualities (Hu, 2016; Zhang, 2013). Due to this change in approach, the training time given to adolescent table tennis players has shortened. Therefore, table tennis coaches and researchers face the challenge of improving the training efficiency of adolescent table tennis players. The goal of this study was to examine the effectiveness of new training methods that improve current practices.
Multi-ball table tennis training method has been used for more than three decades among players of all ages and levels. The term 'multi-ball exercise' refers to training in which a coach or robot continues to supply a number of balls to a player within a certain period of time. The type, duration, direction, and frequency of ball injection depend on the design of the training process (Katsikadelis et al., 2017). Multi-ball training is one of the common methods used in adolescent table tennis training and it is always used to promote formation and fixation of players' technical movements, improve their training effects (Jiang, 2005; Katsikadelis et al., 2014; Shen, 2012; Wang and Qiao, 2011). Multi-ball training with different means of spin, power, speed, placement, and arcs can improve training efficiency and help players strengthen a variety of difficult movements, therefore, improving the player's capabilities. Players can use different placements and different speeds of the ball to improve the combination of different ball techniques (Zheng and Jin, 2016).
The forehand drive shot is fundamental and crucial in table tennis, and the spin is an important key factor in winning table tennis competitions. Table tennis players perform a greater lateral impulse to cause the trunk rotation and accelerate the body movement to implement a forehand drive shot. Players also exert greater ground reaction force on the right foot in receiving backspin than receiving topspin serves (Chen et al., 2012). Players increase the racket tilt angle for a forehand drive and raise the forward trajectory angle to return a backspin serve compared with returning the topspin serve. Table tennis players would increase the upswing path angle to perform the forehand drive in receiving backspin serve (Tsai et al., 2010). The players exert greater muscular activity in the wrist extensors, the biceps and the triceps for the backspin serve forehand drive than when returning the topspin serve forehand drive (Tsai et al., 2012). More rotation of the body and more strength should be exerted for table tennis players to perform forehand drive in returning backspin serve than topspin serve.
Weighted training is a training exercise designed to increase muscle strength and volume. The workout involves using different types of skeletal muscle contraction (concentric or eccentric contraction) to generate power against gravity, usually with respect to the weight of a dumbbell, barbell, etc. or some other form of resistance. Weighted swing training is a form of special physical training in table tennis. After continuous practice, it can improve the strength and coordination of upper limb muscles (Guo and Zhang, 2004; Trzaskoma et al., 2010). Table tennis coaches can utilize weighted training to develop their players' muscle strength in both upper and lower limbs since its benefits are better than traditional strength training (Sofiene et al., 2016).
In this study, weighted swing training requires players' eyes to be closed. Keeping the eyes closed allows players to engage in motor imagery training. Motor imagery (MI) training is a mindfulness method that involves players recalling and reproducing the correct technical actions they have completed in the past in order to evoke the player's feeling of presence (Tian, 2000). MI has received much attention for its role in athletic performance and is often included in athletes' psychological skills training packages to complement their regular training programs(Beauchamp et al., 1996). Hall (2001) suggested that imagery training should be treated similarly to physical practice since research has suggested that they are functionally equivalent (Gabriele et al., 1989).
Because training constitutes a large part of the daily work of athletes, motor imagery training enables athletes to recall actions accurately (Starkes, 2000). Cumming and Hall (2002) found that the amount of imagery practice that an athlete engages in is monotonically related to their competitive standard. Furthermore, they found that athletes perceive imagery as being highly relevant to improving their performance and thereby require a great deal of concentration (Hall, 2001). Elite athletes can use MI to improve movement efficiency and strength if combined with physical training (Lebon et al., 2010).
In addition, several studies have demonstrated that schoolchildren who use motor imagery effectively learn motor skills. Bohan et al. (1999) found that motor imagery is more beneficial in the early learning stages of a motor task. Imagery is twice as effective in the development of motor skills in young children (Bohan et al., 1999; Doussoulin and Rehbein, 2011).
Previous studies only investigated the effects of eye-closed and weighted training methods on participants respectively (Ducher et al., 2011; Guillot et al., 2015; Mac-Kelvie et al., 2002; Robin et al., 2007). The target population of this study was adolescent table tennis players whose technique was in its formative stage. It aims to explore the stroke effect of eye-closed and weighted training on adolescent player's stroke accuracy and stability through the experiment, which combined the weighted training and training to intervene the adolescents' table tennis skill and offer coaches reference when formulating daily training plans.
We hypothesized that: 1) both the traditional training method and EWMT method can improve the stroke effect of adolescent table tennis players. 2) Compared with the traditional training method, EWMT method can significantly improve the stroke effect of adolescent table tennis players. 3) Compared with the topspin, the EWMT method can significantly improve the backspin's stroke effect.
This study was approved by the ethics committee of Shanghai University of Sport. Participants were randomly selected from China Table Tennis College. The inclusion criteria included; table tennis player, 9 to 12 years of age, right-hitting arm, shake-hands grip. Forty-eight adolescent players ([M.sub.age] = 10.25 [+ or -] 1.12; [M.sub.height] = 1.50 [+ or -] 0.06 m; [M.sub.weight] = 41.15 [+ or -] 5.84 kg) who met the inclusion criteria were chosen to attend this study. The average training time for them was 3.85 [+ or -] 1.23 years, and the table tennis technical grade (General Administration of Sport of China, 2013) of them has been to the same level. All subjects were informed prior to the experiment about the study procedures, and they provided written informed consent.
The 48 players were randomly divided into two groups: 24 players (12 male and 12 female) were assigned to the experimental group (EG), in which they engaged in eye-closed and weighted swing exercises for 10 weeks; the other 24 players (14 male and 10 female) were assigned to a control group (CG), in which they received normal training without eye-closed and weighted swing intervention. There was no significant difference between EG and CG in age, training year, technical grade, height, weight, and other study variables (p > 0.05) (Table 1).
(1) The experimental ball was DHS D40+ (3-star) of Double Happiness Company (DHS). The table and rackets used in this experiment were Rainbow table made by DHS, and Timo Boll-ZLCacrbon, separately. The racket was wrapped with red rubber on one side while black on the other side.
(2) Radar speed detector: SPEEDSTER radar produced by BUSHNELL Company (the United States), located approximately 3 1/2 - 4 meters away from the player (directly facing the player).
(3) Dumbbells: 1 kg and 2 kg dumbbells manufactured by Langwei Company, China.
(4) Serving machine: V-989H Serving machine was manufactured by Nittaku Company, Japan. The parameter settings used for the study included the following: when serving topspin, the serving machine's upper wheel speed was level 7 (10 levels in total, the higher level the faster speed), the bottom wheel speed was level...