Tennis is a sport with a goal of creating a movement of certain pace and intensity to reach a certain interception point as a target determined by space and time (Renshaw & Davids, 2004). Tennis performance is highly affected by the ways how players apply their movement momentum to the ball that is approaching towards the future impact point.
The striking action in tennis is influenced by the preparatory phase which includes the efficiency of the receivers’ reaction and movement response. Players constantly adapt to the highly challenging situations and dynamics of the game imposed by opponents. The most important aspect of the adaptation is the reaction time. Players with a quicker reaction can make a quicker decision about the orientation and initiation of their movement, enabling balanced and timely set up to store the potential energy for efficient stroke movement.
MOMENTUM AS DETERMINING FACTOR FOR STROKES POSSIBILITES
Momentum measures the ‘motion content’ of an object and is based on the product of an object’s mass and velocity. For players to perform accurate strokes with high velocity, there should be an optimal combination of linear and angular momentum. The optimal combination depends on the urgency of movement, level of balance, perception of the situation and even level of confidence. With a creation of a larger momentum, greater forces and abilities are required from a hitter to decelerate to set up and execute a successful stroke.
In competitive tennis, forward momentum is necessary when a player tries to pressure (force) a receiver to make an error. These errors are caused by imposing the spatio-temporal pressure to the opponent, such are placing him/her to the disadvantageous position with higher technical-tactical limitations for the stroke production. The forward momentum is the most efficient way to generate an aggressive stroke while simultaneously maintaining balanced and strong posture. Forward momentum enables hitters to intercept the ball earlier and further in front and to make the time and the distance between the players and the ball flight shorter consequentially creating higher spatial and temporal pressure to the receiver. For these reasons, expert players tend to anticipate and move earlier to position themselves into the hitting position so they could use the forward momentum as much as possible.
Depending on the speed of the reaction and anticipatory response, the receivers can create different intensity and direction of movement momentum to set up (Figure 1). The momentum at the set up influence the stroke momentum and creation of the forwards momentum throughout the impact zone.
Figure 1. Hitter’s Movement Momentum intercepting the Ball Momentum (blue arrow)
Figure 1 shows three lateral movements of a hitter and the most possible stroke direction in each case.
Red lines: Moving in lateral direction with defensive preferences limits stroke options to the cross-court as momentum is mostly kept in lateral direction with less possibilities in transferring it forward. As the red dotted arrow points, the direction of the ball is expected to be in a cross-court direction but closer to the middle than sideline due to the limitation in stroke production, having a player choosing safety over penetration.
Yellow lines: Moving down the yellow line in lateral direction limits stroke options as momentum are kept in lateral direction but with more possibilities of successful deceleration to set up. This could enable more successful transfer of the momentum forward compared to the red line. As the yellow dotted arrow points, the direction of the ball is expected to be in a cross-court direction closer to the sideline with more penetrating characteristics and less limitations in stroke production compared to the case of the movement following the red line.
Green lines: Moving down the green line presents more proactive approach when intercepting the ball, and for the player to be able to do so, earliest possible anticipatory movement is needed. The green stroke has far more possibilities of transferring the momentum forwards and in two different directions, making ISI indeterminate and therefore much more difficult to anticipate.
Receivers carefully observe the movement momentum of the hitters in order to anticipate their stroke possibilities. As Figure 2 shows, following their stroke (images A1, B1), receivers tend to closely observe the movement momentum of the hitters (images A2, B2) from their split step position (images A1, B1) to the impact point (images A3, B3). The two receivers (players beyond the net in both A and B) initiate their movement as early as possible towards the most optimal position at the court (lateral or forward, green arrows, center images) to keep the advantage over the point dynamics. Their responsive movement is initiated based on the observation of the hitter’s movement momentum towards the impact point and by judging its limitations. In both cases, as the hitters had higher urgency of movement towards the impact point towards the corners of the court (red arrows, center images), their stroke potentials were judged as with lack of the forward momentum (limited). The stroke was anticipated by the receivers as weaker and in the cross-court (yellow dotted arrow), early initiated to the cross-court area (green arrow, Djokovic, image A2) or forwards (green arrow, Federer, image B2).
FIGURE 2. Anticipation based on the relation between the movement momentum of the hitter and receiver