The aims of this study were to determine kinematic parameters, to evaluate interindividual and intraindividual variability in the range of the calculated values and to identify differences in the parameters between two modifications of the topspin forehand stroke. The parameters included joint angles in different events, such as the ready position and the backswing, contact and forward events, and the acceleration of the hand at the moment when the racket made contact with the ball. Many table tennis researchers have evaluated different kinematic parameters of the forehand and backhand strokes [Iino and Kojima,2009;2011;2016a;2016b; Qian et al.,2016; Malagoli Lanzoni et al.,2018] and the relationships of the parameters with speed, force generation, and other factors. Few papers have been devoted to the presentation of models of stroke movements, which may not only be important knowledge but also helpful in developing practical guidelines for coaches and players on how the movement is to be coordinated or how it is performed at the level of individual joints. The angles in most joints calculated in the study reveal how the movement in individual segments of the player's body should be performed. An observation of the average results of the group is obviously insufficient to provide such information. This is because each player has a different way of performing the technique, as evidenced by the high CV values for the whole group in most joints in all events. When searching for information about movement coordination, it is better to refer to the results of individual players. For example, for the player presented inTable 5, the mean value for the lumbar spine [LumbRot] was, in individual events in TF1, -10.8° in the ready position. Then, the player rotates this part of the spine externally to -4.6° [backswing]. Afterwards, he rotates the segment internally to -7.5° in the hitting movement [which is the contact event]. The movement is finished by additional rotation, ending with an internally rotated position and an angle of -15.7° for this segment [forward]. In TF2, these values were, -30.7°, -27.4°, -30.4° and -35.6°, respectively [Table 6]. As observed, all strokes were performed with an internally rotated position of this segment [negative values]. Obviously, the values given are the averages of 15 strokes, but the CV values, especially for the backswing and forward events, are small. Similarly, important movement sequences can be observed in the elbow joint [ElFlex]. The movement in other exemplary players [Table 3] is composed of flexion to 64.7° [ready position], extension to 26.1° [backswing], and flexion to 45.0° [contact] and 58.1° [forward]. It is therefore possible to identify the extension movement in the backswing phase and the flexion movement in the forward phase [which is from the backswing event to the moment of the forward event, including the contact event]. During this period of time, the flexion angles for the shoulder joint were 25.2° [ready position], 16.2° [backswing], 43.8° [contact], and 85.1° [forward]. These results demonstrate a substantial range of flexion [approximately 70°] in the shoulder joint in the entire forward phase [from the backswing event to the forward event]. In TF2 [Table 4], the range of this movement was even larger, and the angular values in the individual events reached 29.3° [ready position], 5.7° [backswing], 27.9° [contact], and 95.3° [forward]; however, there was high variability in the backswing event. The large range observed for the wrist supination movement was noticeable. In most players, there was an increase in supination during TF2 [compared to TF1] in this joint, but the difference was not confirmed by statistical significance. The difference was especially evident at the moment of the backswing but was also evident at the contact and ready position [Cohens d=0.80], which is undoubtedly due to a difference in the rotation of the flying ball and adjustments in the direction of movement and the angle of the racket. This difference was also evident in the results of the whole group for this movement between TF1 and TF2. The increase in wrist supination in TF2 may be considered practical information for coaches and players concerning adjustments in the rackets face to different incoming balls. Interestingly, there are substantially larger and significant differences between the angles in particular events.
The values of hand acceleration at the moment of contact differed significantly between the two strokes. In the whole group, the statistically confirmed difference was approximately 40 m/s2 between the means [Table 1,Table 2, d-Cohens = 0.95]. Therefore, the acceleration of the playing hand at the moment the racket made contact with the ball, together with the parameters assessed in previous studies [Bańkosz and Winiarski2017], may be considered an important factor in differentiating topspin forehand strokes.
Another element evaluated in the study was the variability of the parameters. This was determined using the coefficient of variation [CV]. It is very interesting that the analysis of the variability of the whole group in terms of angular parameters showed high CV values, which was assumed to be indicative of large variability. This result can be considered a manifestation of different coordination patterns in the stroke movements between TF1 and TF2. It can be concluded that the athletes present different methods of performing the modifications of the topspin forehand stroke. There can be many reasons for this, e.g., differences in training [players were coached at different centers in the country], morphological differences between players, and specific physical and mental characteristics. Such differences are common in sports [Komar et al.,2015]. Despite this difference in coordination, the players achieved very similar acceleration values at the moment of contact [low variability in the acceleration values]. This result can be considered a manifestation of the phenomenon ofequifinalityin table tennis, which is described in the literature in relation to various aspects of movement [Jaric et al.,1999, Reiser et al.,2011]. Based on the observations made in the study, it can be concluded that even though the players used different methods of performing the movement, they obtained similar values for some parameters, as shown for acceleration.
The observation of the CV values in the whole group of players mostly revealed small and moderate values in the knee joints. It must be noted, however, that many of the calculated CVs were very large for parameters with small SDs and small ranges of motion that spanned a few degrees. This was the case for the joints of the spine and wrist. For example, in the wrist, the CV was very high, but the range of movement was only a few degrees; sometimes, the mean value was also close to 0°. Perhaps the CV as a measure of movement variability should not be treated and interpreted in absolute terms and should be considered with the range of motion and the standard deviation.
It is also worth noting that there were many parameters with small amounts of variability in individual players. Perhaps, as stated in some previous studies [Bootsma and Wieringen,1990; Sheppard and Li,2007], each player performs tasks in a similar and reproducible way, especially in critical moments, for example, in the contact event. Some authors have emphasized that in sports, constancy and repeatability are needed for specific parameters [Whiteside et al.,2013]. Some authors also strongly emphasize that an improvement in technique probably leads to a reduction in movement diversity [Dai et al.,2012]. It is also likely that each player, who has his or her own way of performing and coordinating the movement, performed their tasks in an automated, perfect way. Undoubtedly, this concept is related to the extensive training in which the players had been involved. Nevertheless, it cannot be stated that the repetitions were performed in an identical way. Bartlett et al. [2007], when reviewing studies on interindividual and intraindividual variability in several sports, showed that even the best athletes [e.g., with similar results] do not perfectly reproduce the same movements [the same parameters, such as the range of motion or coordination]. The likely reasons for this phenomenon are movement functionality and functional variability, which also manifest in the possibility of compensation, so changes in the angle of a joint, range of motion or other parameters are compensated by changes in other parameters. This phenomenon can also enable adaptation to the conditions and requirements of the task [Komar et al.,2015]. This has been demonstrated in studies on many sports and activities, such as basketball [Mullineaux and Uhl,2010; Sevrez and Bourdin,2015], throws [Dupuy et al.2000], and darts [Smeets et al.,2002]. It was found that even respiratory movements cause changes that manifest in discrete joint movements when one is maintaining balance in a certain posture and center of gravity [Kuznetzov and Riley,2012]. Some authors emphasize that random variability characterizes novice motor performance, whereas active functional variability may exemplify expert motor performance [Schorer et al.,2007].
Interestingly, the specific variability did not change despite the change in the nature of the stroke: a comparison of the variability in TF1 and TF2 did not show significant differences.
Our research did not reveal significant differences between the angular parameters of the two modifications of the topspin stroke in the whole group of players. These differences were found only in particular participants. The most likely reason for this result may be the large and very large variability of the angular parameters in the whole group.
The results of this study may also be interesting and useful for coaches and players. The range of inter-individual variability of kinematic parameters shows that the technique of the movement of participants is very individual. This finding is probably the result of differences that exist between players [morphological, functional, psychological, etc.]. Therefore, coaches and players must face this fact in the training process and must be very careful when creating or adopting a model of a technique for individual players. The above coordination of movement during the topspin forehand may also be considered a practical value of this research.
Regarding the limitations of our research, the previously discussed coefficient of variation [CV] should not be interpreted in absolute terms because when the differences in performance are of several degrees and considered small angular values, the calculated CV is several tens of percent or higher. Therefore, the standard deviation may also be a good indicator when there is a small arithmetic mean [close to 0].
It should also be noted that in our study, we evaluated events based on the movement of the playing hand. Due to the principle of "proximal to distal sequences" occurring in topspin movements [Bańkosz and Winiarski2017,2018a], angles in medial segments [spine, body trunk] may not correspond to the initial or final values in a given movement phase and a given segment. Thus, data interpretation may be difficult. It should also be noted that the top players from the ranking lists in Poland who were included in the study came from one country and are important players in Europe but are not among the top world athletes.