- NEUROLOGICAL ASPECT OF SPLIT STEP – Decisions made from the perception or anticipation of the hitter’s movement around the set-up phase (loading at the impact zone)
One of the greatest challenges of the dynamic tennis environment is a creation of a quick and accurate decision of the physical response (movement) corresponding to the technical-tactical intentions of the opponents’ striking actions. By prioritizing the needs of high-speed reactions, expert tennis players tend to decide of the movement direction and intensity while still being above the ground, at the air phase of the split step, as proposed by Uzu et al. (2009). From there the decision of the movement intensity and direction corresponds to receiver gathering information while increasing the much-needed muscle spindle sensitivity of the leg muscles before the ground contact, moving their center of mass away towards the edges of their support base (Torres-Luque et al., 2011).
The decision of how the body weight will be distributed at the time of the landing presents the crucial decision as the landing efficiency depends on how the reaction force impulses can be magnified during the landing.
In this phase of the split step, while being in the air, receiver gathers relevant information about the direction of the incoming ball. Information can be potential (anticipatory) and based on the hitter’s kinetics at the set-up or actual (reactional) based on the incoming ball information following the contact. Usually, players that use more predictive information initiate earlier response (Aviles et al., 2002) and cover more court space, have more time to make optimal technical-tactical decisions by being earlier prepared to set up. These situations enable engaging more muscle groups in creating more efficient stroke momentum for the sake of creating more advantageous point situations. Shim et al., (2005) showed that relying on the predictive reaction increase the movement competency in terms of the quickness of the movement (earlier initiation) and court coverage. By comparing receiver’s reaction in two different mode of ball delivery: with observable movement mechanics (person as a feeder) or without observable movement (feeding machine) they pointed that the absence of kinetic information sources leads to delays in movement initiation. The time of the ball direction recognition was significantly slower in the case of the machine as a feeder highly influencing the time of the movement initiation (human feed 127 ms vs machine feed 197 ms). This time difference could potentially allow player to increase the efficiency of the stroke execution by initiating movement and stroke preparation earlier, with additional court coverage by as much as 1.2 meters (0.6 meters in both directions).
Perceptual skills were shown to be one of the crucial aspects of the high-level performances in interceptive sports, having experts relying more on abilities of detecting action relevant cues than to a speed of a response (Ripoll et al., 1995). When planning a neuro-motor reaction, skilled performers use their knowledge and experience to selective allocate their attention to relevant sources of information (Piras et al, 2014; Williams, Ward, & Chapman, 2002). By acknowledging the relevant and ignoring the non-relevant ones, expert players create conditions of spatiotemporal affordances by decreasing their reaction time and increasing the available time for decisions and movement (Vaeyens, Lenoir, Williams, Mazyn & Philippaerts, 2007). Filtering information by their relevance and minimizing the processing time, the cognitive time can be used in a more effective way, simplifying the decision-making process. By creating a timely decision of the weight distribution during the landing players can highly intensify their split step landing momentum (direction and intensity).
At the split step, before the landing, as there is an increase in spatio-temporal pressure and high demand of the timely and accurate decision, experts tend to make anticipatory responses based on the kinetics of the hitter player. On the contrary, decisions made from the actual ball flight information, as the most reliable, can be classified as reactionary as the response was initiated following the contact. In any case, the pre-motor reaction time (RT) is of crucial importance. Due to latency in processing of sensory information, timing of the split step based on the reliable information from the ball flight does not fully enable receiver player to be prepared to successfully intercept the ball, especially when facing more powerful hitters. When not being decisive of the hitter’s intention, receivers carry out neutral preparatory vertical jump keeping the center of the mass in the middle and body weight being equally distributed to both sides of the body (without lateral direction). This type of landing cannot be considered as efficient as it does not have intensified momentum to the direction of the impact zone.
Having in mind that it takes 50 ms (Lee et al., 1983) – 200 ms (Benguigui et al. (2003) for the player to detect information and use it to begin or correct a movement due to the visual motor delay (Tresilian, 1993), there is clearly a need for speeding up the reaction time, initiating decisions of movement earlier. In addition, it was shown that efficiency of performing a split step as a preparation step does not have to rely on the muscular structures. Aviles et al. (2002) showed that expertise in returns of the serve differs as an efficiency of a movement response relies on an ability to adapt to the opponent’s stroke execution. Although highly ranked national players demonstrated earlier unweighting from the split step (average –28 ms vs. –5 ms in relation to the impact), professional players initiated much earlier movementfrom the split step to the ball (on average 130 ms vs. 160 ms after impact).Their quality of response was related to the efficiency of the perceptual-cognitive systems, and how was the air phase of the split step used to process information in predicting the initial intention of the server.
MUSCULAR ASPECT – MUSCLE TENSION
Split step as an essential preparation movement (Groppel, 1992) enhances muscle pre-activity increasing the muscle spindle sensitivity for the stretch shortening reflex (Kovacs M.S., 2009) with the sole purpose of initiating an explosive movement when in contact with the ground (Uzu R, Shinya M, Oda S. A, 2009). During the air phase, joints of the lower extremity approach full extension and then flex just prior to the contact while during landing, forces generated by muscle-tendon units and reaction forces are applied to the lower muscle extremities (McNitt-Gray 1993).
The mechanical goal of a split step is to effectively convert the total body momentum at the landing (J.L. MCNITT-GRAY, Biomechanics in Sport: Performance Enhancement and Injury Prevention) into the efficient lateral movement. Stretching of the quadriceps and calf muscles stores the the kinetic energy (Elliot, 2006) to be released into kinetic energy of the subsequent lateral step at the moment of the landing (Uzu et al., 2009), enhancing a faster reaction with faster contraction (Tatsuo M et al., 2005; Bernardi et al., 1998)).
By performing a split step, player’s possibilities of enhancing the speed of movement increases approximately by 15-20% (Knudson & Elliott, 2004), needed for the efficiency of movement no matter of type of hitter or a stroke receiver is facing. The point of using the split step with the whole-body weight elevated and then landing is initiating the movement with the ground pushing back with equal and opposite force (Groppel, 1989). GRF can add up to 2.5 times of the body weight (van Gheluwe and Deporte, 1992) intensifying the movement, if used efficiently. In addition, by modifying multijoint control strategy, players can enhance the external loading experienced during impact by as much as eight times their body weight magnitude (McNitt-Gray et al., 1990).
AIR PHASE EXAMPLES
To create wider and more intensified first step off the split step (usually a foot pronation), experts tend to initiate the movement firstly with the ankle supination (inwards the center of the mass) during the air phase enabling wider and more explosive movements (FIGURE, images left in both players) outwards during the pronation. The goal should be intensifying the movement momentum by pivoting the leading foot towards the ball direction an creating the rotations of the whole unit of the body (toe-knee-hip-shoulder) towards the impact zone (Vaverka et al., 2003). Obviously, if the receiver has intention of intensifying the movement by pronating while landing, the decision of supination at the pre-contact phase of the stroke while being in the air should be definitely based on the perception of the kinetic cues of the hitter at the stroke set-up phase, or even before. Therefore, the whole SUP movement should be considered as anticipatory, with decision of its execution made based on the early observation and prediction.