In table tennis one's level depends on one's technical skills, such as ball hitting, setting up, proper footwork, but also on tactical, mental and motor skills. But table tennis is a sport in which the level of technical preparation is a crucial factor in high performance. Having high technical ability means that one is able to play using coordinated movements, with controlled power and able to impart adequate speed and spin to the ball. A high level of technical preparation involves perfect placement of the ball and being able to make the game almost unreadable to one's opponent. Analysing table tennis technique can help to identify the key factors underlying effective play (Zhang, et al. 2013). Most published biomechanical analyses of table tennis techniques have examined topspin strokes, especially the topspin forehand, viewed by many as the most frequent and effective stroke in the game in the last few decades (Iino and Kojima, 2011; Kondric et al., 2007; Neal, 1991; Yoshida et al., 1996; Zhang et al., 2013). Many publications on technical preparation of topspin strokes have been comparative analyses (Fu et al., 2016; Iino and Kojima, 2009, 2011; Kasai and Mori, 1998). Kasai and Mori (1998) emphasised that topspin technique varied with skill. Highly skilled players tended to use their whole body, rotating their upper body and using the knee joints effectively. Fu et al. (2016) investigated the characteristics of the centre of foot pressure trajectory during the table tennis topspin fore hands. They detected that high-level players had better foot drive technique and better control of foot motion during the forehand loop. Iino and Kojima (2009) demonstrated the importance of racket acceleration and upper body rotation to performance of topspin strokes. The same researchers compared topspin forehand strokes in players of varying skill level and found that advanced players exhibited greater internal rotation and torque of the shoulder than intermediate players (Iino & Kojima, 2011). They also concluded that an increase in energy transfer may be an important factor in enabling intermediate players to generate a higher racket speed at impact when playing topspin fore hands.
Only a few studies have explored the effect of certain factors on racket kinematics. Iino and Kojima (2015) investigated the effects of racket mass and stroke rate on the kinematics and kinetics of the trunk and the racket arm in the table tennis topspin backhand. They found, that the racket mass had no effect on the kinematic parameters of the trunk, playing arm that they examined and no effect on most of the kinetic parameters of the playing arm (the exception was that peak torque during wrist dorsiflexion increased with racket mass). Iino et al. (2008) evaluated the effect of movements in individual upper limb joints on racket velocity during performance of two variants of topspin backhand strokes and suggested that changes depend on the arrangement of individual upper limb segments rather than on angular velocities in individual joints of the limb holding the racket. Qian et al. (2016) investigated the role of lower limb kinematics in topspin forehands. They concluded that hip flexion and internal rotation were important, as well as the ability to using "lower limb drive". Bankosz and Winiarski (2017, 2018) carried out extensive analysis of biomechanical and kinematic parameters in relation to topspin strokes, but it is difficult to find studies that offer comprehensive analysis of the relationship between the kinematics of individual body segments and racket kinematics. Racket kinematics, especially velocity of the racket when hitting the ball, are critical to the quality of the stroke. Increasing racket velocity at impact can affect the ball speed or rotation of the ball (Neal, 1991). Determining the body segments, directions and ranges of movement and angular velocities that have a significant effect on racket velocity may be critical to improving training efficiency, especially in terms of movement technique. Changing the range or direction of movement, the angular velocity of a particular joint and, especially, racket velocity enables the player to change or adjust the parameters of the stroke, for example the angle, velocity and force of the stroke. Despite considerable effort by researchers and coaches, the rules of changing and adjusting movements during table tennis strokes are not fully understood. The aim of this study was, therefore, to calculate correlations between individual joints velocities and racket velocity (maximum velocity and velocity on impact with the ball) for variants of topspin forehand and backhand strokes in table tennis. The pattern of correlations should suggest how changes in body segment velocities influence racket velocity. Knowledge of such correlations may help coaches and players to improve topspin technique. We hypothesised that angular velocities of trunk and arm correlate with the resultant racket velocity because these segments propel the distal body segments. Based on the literature and our coaching experience, we assumed that the increase in the velocities of trunk rotation, knee extension and elbow flexion-extension are the primary contributors to racket velocity.
The study participants were 10 elite, female, junior table tennis players, numbered amongst the top 16 junior players in Poland. Their mean age was 16.0 years ([+ or -] 2.5 y), and they had a mean body mass of 54.4 ([+ or -] 3.2 kg) and mean body height of 1.65 ([+ or -] 0.06 m). All players provided writ ten, informed consent to participation in the study. The local ethics committee (the Senate Research Ethics Commit tee at the University School of Physical Education) approved the research.
The experiment was performed in certified biomechanical laboratory settings using a BTS Smart-E (BTS Bi engineering, Milan, Italy) motion analysis system. The system consisted of six digital cameras operating in the infrared spectrum (1.1[micro]m) at a frequency of 120 Hz, in addition to two NetworkCam cameras operating in the visible range with a frequency of 20 Hz (Figure 1).
The data were collected through a digital USB/PC input and processed using the BTS Smart Analyser software. Thirty-four passive reflective markers were attached to the player's body with double-sided adhesive tape in specific locations featured in the biomechanical model. We used the human gait model for the lower extremities that was based on an extension of a published model (Davis et al., 1991) and an innovative model developed by us that includes additional points for the upper extremities (Figure 2): the acromion, lateral and medial epicondyle of the humerus, the styloid processes of the radius and ulna, and the head of the third metacarpal. The trunk segment was represented by the shoulder girdle (acromions and C7) and pelvic girdle (ASISs, greater and lesser trochanters and sacral point--Pietraszewski et al., 2012). This arrangement enabled the use of a local reference system and determination of the centre of gravity for each segment: head, trunk and both lower and upper extremities (Teu et al., 2005). Furthermore, three markers were also placed on the edge of the racket. An acoustic sensor--a piezo-electric (PZT 5H) ceramic sound transducer was also attached to the racket's surface to measure the exact time when the ball made contact with the racket. The racket was Donic Persson Powerplay with the 2.1mm rubber of Butterfly Tenergy 05. The marker and measurement protocol was pre-tested in a quasi-static simulation trial.
Prior to the measurement procedure players followed standard (15-minute) and technical (20-minute) warm-up routines. Each player presented six tasks, three for forehand strokes and three for backhand strokes, with different levels of force and speed: FH1 (and BH1)--top-spin forehand (and topspin backhand) against a no-spin ball, played with force, with a velocity and rotation of about 75%; FH2 (and BH2)--topspin forehands and top spin backhands against a backspin ball, played with force, with a velocity and rotation of around 75%; FH3 (and BH3)--topspin forehand (and topspin backhand) against a ball with no spin, played with force and with a velocity near to maximum for the stroke--"heavy topspin". The force of topspin strokes was estimated by the participants them selves, they were only asked to differentiate the power of the stroke. The task consisted of 15 strokes of the type specified. All balls were delivered to the player by the same person (coach) from a basket, one after another, at a frequency of around 50 balls per minute. Right-handed play ers performed topspin strokes diagonally from the middle of the right and left sides of the table for forehands and backhands respectively. Racket displacements and the development of an acoustic signal in a cycle of strokes were used to identify the moment when the racket made contact with the ball and evaluate the maximum linear velocity of the racket (resultant--VRmax) and the linear velocity at the moment of contact (resultant--VRcont). Angular velocities in the joints involved in the forward phase of topspin strokes were also evaluated: the highest velocity in the joint (Vmax and Vmin, velocities of two opposite directions) and angular velocity at the moment of contact with the ball (Vcont). The angular velocities of following joints and directions of all limbs were calculated: wrist joint (W): for abduction--adduction (AA) and flexion--extension (FE), elbow joint (E): FE and pronation-supination (PS), shoulder joint (Sh): FE, internal--external rotation (IE) and AA, ankle joint (A): FE, knee joint (K): FE, hip joint (H): FE, IE and AA.
We also measured the spatial orientation of the pelvis (P) and shoulder girdle (S): obliquity (Obli), tilt (Tilt) and rotation (Rot). The directions of individual movements are represented by the sign of the...