The topspin forehand is considered to be the most common table tennis stroke. It is also regarded as the most effective stroke (Malagoli Lanzoni et al., 2010; 2014; Iino and Kojima 2011; Mocanu and Negolescu, 2018). The topspin forehand is a complex, multi-joint movement that is performed in proximal to distal sequences, with multiple muscles working in different phases in different ways within a coordinated kinematic chain. The stroke allows the racket to make contact with the ball at a high speed and acceleration, which causes the ball to both move with a speed directed forward and rotate around its axis. Studies related to the evaluation of kinematic and kinetic parameters of this stroke have demonstrated that at impact, the ball is hit by a racket moving forward and upwards at a resultant speed of approximately 20 m/s. The acceleration of the racket at the time of contact is approximately 180 m/[s.sup.2], whereas the ball flying away from the racket may rotate up to 140 times per second and travel at a speed of 40 m/s (Hudetz, 2005). The values of the above parameters and other parameters related to the kinematics and kinetics of the topspin movement may vary depending on factors related to the stroke, such as the force involved, the conditions created by the opponent (initial ball parameters), the direction of movement, and the way the ball is hit (Bankosz and Winiarski, 2017). A number of studies on stroke parameters have focused on the relationships between a movement and the work done or force generated, between the force and racket speed, or between the kinetics of the upper limbs and other body segments (Iino and Kojima 2009; 2011; Qian, et al., 2016; Bankosz and Winiarski 2018a; 2018b; Iino, 2018). The highly complex and coordinated movements, the multitude of variations and the large inter-individual diversity lead to large movement variability performed during the topspin forehand stroke. Together with the phenomenon of functional variability, these factors can result in a large number of possible solutions for the completion of the stroke. This large number of possible solutions, in turn, causes an enormous amount of information confusion in the recommendations to coaches and players about how the topspin forehand should be performed. The range of movement variability and invariant elements during this stroke seem to be very interesting factors. Evaluations of these factors can be very useful for providing instructions on performing the topspin forehand technique. The literature on movement variability is quite rich. Most often, inter-individual variability results from the psychophysical characteristics of a player, such as his or her body height, the size of his or her individual body segments, and his or her preferences, and is described in textbooks and materials for players and coaches. Some researchers who have analyzed this problem approached movement variability as movement "noise", which comprises unintended movements resulting from complex multi-joint movements (Bartlett et al., 2007). However, intra-individual variability has been considered an essential element of normal, healthy function, as it offers flexibility in adapting to difficulties and impediments (Hamill et al., 1999; van Emmerik and van Wegen, 2000). Variability is therefore defined as functional changeability or intentional change that results from different situations and conditions of an athlete's tasks, e.g., the parameters of a flying ball, the actions of the opponent, unexpected changes in the situation, and actions to avoid an injury (Bartlett et al., 2007). However, some researchers have emphasized that consistency and repeatability are needed for certain parameters, especially at critical moments, such as when the ball makes contact with the racket, whereas the magnitudes of kinematic parameters are correlated with, e.g., the speed and accuracy of the stroke during a serve (Whiteside et al., 2013). The differences in the magnitudes of movement parameters are due to compensation mechanisms. For example, a change in the range of motion of one joint is compensated by a change in the range of motion of another joint (Dupuy et al., 2000; Smeets et al., 2002; Davids et al., 2003, Mullineaux and Uhl, 2010; Horan et al., 2011). Studies have shown that the variability of a movement decreases when the movement is accompanied by increased mental effort focused on a given aspect of the activity (Carson et al., 2014). It has also been found that the functional variability of movements also transforms and develops with the age and experience of players (Busquets et al., 2016). Some studies in the literature have also stressed that motor variability occurs even when one is maintaining a standing position, as there are compensation mechanisms associated with performing breathing movements (Kuznetsov and Riley, 2012).
Movement variability in table tennis has not been described extensively. Bootsma and van Wieringen (1990) evaluated the diversity of racket movements during the forehand drive stroke and confirmed the occurrence of functional variability in the range of the racket's kinematic parameters. The authors also noted that there is less variability in the spatial parameters (direction of racket motion) at the moment the racket makes contact with the ball. Similar findings were published by Sheppard and Li (2007), who described "funneling" as the phenomenon of reduced differentiation in some parameters of racket motion at the moment the racket makes contact with the ball. Recent studies conducted by Iino et al. (2017) also showed that the ability to use the redundancy in the joint configuration to stabilize the racket at a vertical face angle at impact may be a critical factor that affects performance level. A deeper understanding of the mechanisms of movement variability in table tennis seems to be needed. Understanding movement variability, the possibility of its occurrence and its range may also be important for the practice of this sport, especially for teaching the complex techniques of different strokes. The awareness of coaches and players concerning the phenomenon of movement variability and its purpose, range and functionality may facilitate the training process. This awareness may also be important in monitoring and correcting techniques and developing improvement plans for individual players. It is also important to understand which parameters (and to what extent) can change when different modifications of a given technique are used, for example, when players use different values of parameters and different directions of forces. Therefore, the purpose of this study was to (1) determine the values of and differences between calculated kinematic parameters (select body segment angles for chronologically arranged events and the acceleration of the hand when the racket makes contact with the ball) in two modifications of the topspin forehand stroke and (2) assess inter-individual and intra-individual variability of the parameters in two modifications of the topspin forehand stroke. Based on a literature review and the findings of previous studies (Bankosz and Winiarski, 2017; 2019), the hypothesis was that majority of the angular and kinematic parameters are different between the two modifications of the topspin forehand in the whole group and individual players. The second hypothesis was that inter- and intra-individual variability of the calculated parameters is high, i.e., most of the variability index values are high. Moreover, the variability in the event when the racket makes contact with the ball is smaller than that in other events.
The study examined seven top-ranked (international level) Polish adult male table tennis players, who had a mean body height of 1.78 m (SD = 0.03) and a mean body weight of 76.5 kg (SD = 8.0). All the players were ranked in the top 10 Polish senior athletes, and their mean age was 23 y (SD = 2). Six participants were right-handed, and one was left-handed. Each participant was informed about the course, benefits and risks of the research prior to signing an institutionally approved informed consent document to participate in the study, and they signed it. The study was approved by an institutional ethics board.
The participants performed 2 tasks that represented modifications of the topspin forehand--topspin after a topspin ball (TF1) and topspin after a backspin ball (TF2). Kinematic parameters were measured using the master edition of the MR3 myoMuscle system (Noraxon, USA). Inertial sensors were located on the body of the study participant to record the accelerations (Figure 1). Sensors were attached with special straps and elastic self-adhesive tape. The sensors were placed symmetrically so that the positive x-coordinate on the sensor label corresponded to a superior orientation for the trunk, head, and pelvis. For the limb segment sensors, the positive x-coordinate corresponded to a proximal orientation. For the foot sensor, the x-coordinate was directed distally (to the toes). Following the manufacturer's protocol, the sensors were placed in following locations:1) the head sensor - in the middle of the back of the head, 2) the upper thoracic sensor - below C7 in line with the spinal column, 3) the lower thoracic sensor - on the lower ribs in the front, in line with the spinal column at the L1/T12 level, 4) the pelvic sensor - centrally on the sacrum; upper arm sensor - midway between the shoulder and elbow joints, lateral to the bone axis, 5) the forearm sensor -posteriorly and distally on the forearm, where there is a low amount of muscle tissue; the hand sensor - centrally and dorsally on the hand, 6) the thigh sensor - on the frontal and distal half of the thigh, where there is a low amount of muscle tissue, 7) the shank sensor - in the front and slightly medial to the tibia, and 8) the foot sensor - on...