Although tennis-specific literature is available related to biomechanics and tennis performance (13,15,16,26,30) as well as the sport's physiological demands (3,17,19,27) and strength and conditioning program design (4,5,20,21,29,31,32), articles blending these areas are not as readily available. This article focuses on the design of tennis-specific exercises based on the physical demands of the tennis serve.
The technique of the tennis serve has received more attention in the literature than the other strokes, probably because it is the easiest stroke to study, as it is initiated from a stationary position, and the only stroke over which the player has total control. As can be seen in Figure 1, the serve involves a summation of forces sequenced in a largely proximal to distal (legs, trunk, and arm/racquet) fashion. This requires a coordinated sequence of movements with proper timing of each segment (11).
Figures 1b-c feature the knee flexion required to initiate the ground reaction forces, representing the first stage of the stroke's kinetic link. A growing body of research is available on the ground reaction forces produced by tennis players (2,13,37). For example, Girard et al. (15) recently measured the lower limb electromyogram and ground reaction force profiles characterizing the tennis serve and found that more refined neuromuscular coordination patterns distinguished the serves of elite players from those of their lower level counterparts. Indeed, the importance of knee flexion to the serve has been consistently advocated by researchers and coaches such that 110 to 120° of flexion has become a commonly referenced norm. Contemporary work has also underlined the importance of knee extension for the development of high serving velocities (25). In fact, Elliott (10) found that power serves require the segments of the kinetic chain to move in a coordinated fashion starting with ground reaction forces and leg drive, leading to an upper arm external rotation of approximately 170°. It may seem obvious and intuitive (i.e., players who flex their knees must extend), but coaches and physical conditioning specialists will often observe otherwise. Significantly, with Federer-one of the world's premier servers-we see aggressive knee flexion and extension of both legs (Figure 1d). This supports the value of closed chain exercise prescription that promotes powerful knee-but more so sequential triple lower limb (ankle, knee, and hip)-joint extension as part of advanced strength and conditioning programming. In these exercises, as with the serve, the sequential recruitment of the gastrocnemius, soleus, quadriceps, and gluteals initially acts eccentrically (preparation phase), then concentrically (acceleration phase), and finally eccentrically again (follow-through/landing phase).
Ground reaction forces transferred up through the kinetic chain have been shown to contribute between 50 and 60% of the total force from the proximal segments of the chain (19). Kibler determined that during the tennis serve approximately 51% of kinetic energy was produced in the trunk/legs with the shoulder contributing 13%, elbow 21%, and wrist 15% (19).
HIP AND TRUNK ROTATION
Figures 1c-e highlight the hip and trunk rotation, which represents the next link in the sequence. In particular, Figure 1c illustrates the preparatory or throwing position common to many elite servers. The alignment of the shoulders is rotated beyond that of the hips in the transverse plane, creating a horizontal separation angle of ∼20° (27). Similar to where the above knee flexion places the quadriceps on eccentric stretch in preparation for an explosive concentric drive through the forward swing to impact, this horizontal twisting helps to elicit a stretch-shortening cycle response with the musculature of the trunk. The exaggerated vertical tilt of the shoulders (lateral flexion) compared with the hips helps position Federer to laterally flex during the forward swing, a form of trunk rotation known to increase along with serve speed (1). The extent to which Federer rotates in the sagittal plane becomes evident in Figure 1d, upon initiation of his leg drive forward and upward. From the perspective of the strength and conditioning coach, knowledge of these 3 types of trunk rotation and their involvement in the serve is key. Often, there is a misconception that player preparation needs to only address rotation in the transverse and sagittal planes, yet the importance of lateral trunk flexion to the serve is such that it too needs to be challenged. Here, a parallel can be drawn with the typical core stability exercises prescribed to players, which predominantly train explosive trunk action through the transverse and sagittal planes. Inclusion of exercises for core stability that include transverse, coronal, and sagittal plane stresses is an important part of a comprehensive strength and conditioning program for tennis.
In light of this prominent role of the trunk in the stroke, a strong core is clearly important from both performance and injury prevention points of view. Roetert et al. (33) identified a strength imbalance between the muscles of the abdominal area and the lower back. The abdominal muscles work to accelerate and stabilize the trunk during the serve (Figures 1b-e), while the muscles in the lower back decelerate and stabilize the trunk during the follow-through (Figure 1f). Kovacs et al. (24) stated that training for tennis requires a strong understanding not only of the acceleration aspects of movement (as described above with respect to the trunk) but also the need for tennis-specific deceleration. Because of the aforementioned anterior-posterior imbalance in tennis players as well as a potential imbalance between left and right sides, a series of low back exercises (including exercises for the left and right erector spinae) need to be incorporated into training programs along with abdominal exercises. Training the temporal and strength characteristics of transversus abdominis, lumbar multifidus, and the diaphragm can also help to provide a base upon which other muscles can effectively contract. Ensuring that a core stability program does not overemphasize the abdominal or trunk flexor musculature is important in light of this known disproportionate trunk development. As forces are transferred from the trunk to the arm, we start to see an integration of proximal force generation and distal movement (long-axis rotation), a motion defined by coupled scapular stabilization, glenohumeral rotation, and forearm pronation (20) creating a composite motion.
In Figures 1d-f, we can observe the vigorous internal rotation of the shoulder that contributes to a high resultant racquet head speed. In fact, during the acceleration phase of the tennis serve, the shoulder internal rotation angular velocity can reach values greater than 2,500 (°/s) (14). The magnitude of this internal rotation can be indirectly appreciated through the positions in which the shoulder and the upper arm find themselves in Figure 1d. These positions are extreme and provide further insight into the shoulder joint kinetics that punctuate the serve, where joint loading is related to serve speed (27,35). The type of service backswing that a player employs is also known to affect the activation of the shoulder musculature (18,29) and the loading profile of the shoulder joint (10), which in turn has implications for strength and conditioning program design.
The muscles in front of the chest and trunk (pectorals, abdominals, quadriceps, and biceps) act as the primary accelerators of the upper arm and therefore racquet rotation to impact, while the muscles in the back of the body (rotator cuff, trapezius, rhomboids, and back extensors) act to decelerate this racquet-upper limb system during the follow-through (9). Isokinetic testing results identify the resultant imbalances between internal and external rotator strength on the dominant side of the body (6,8). Elliott et al. (11) indicated that training must be such that the muscles surrounding these joints are strengthened both in eccentric and concentric movement patterns to help protect the region from injury (7).
Finally, the muscles responsible for controlling forearm pronation and wrist flexion, the most distal joint actions in the forward swing sequence, also need to be conditioned for repeated high speed performance. Appropriate exercises for wrist and forearm strengthening include standard wrist curls for flexion/extension and radial and ulnar deviation as well as forearm rotations for pronation and supination with a counterbalanced weight (bar with weight at one end only).
APPLICATION OF TENNIS BIOMECHANICS TO TRAINING
The information presented earlier in this article provides the framework for the development of strength and conditioning programs specific to tennis. The aim of this final section of the article will be to discuss and recommend strength and conditioning exercises that train key muscles used during the tennis serve and that would have an impact on improving the players' ability to accelerate and promote explosiveness for optimal serve performance. As has been discussed, these exercises provide a training stimulus for the entire body as the lower body, trunk, and upper extremities all form links within the body's kinetic chain. While it is beyond the scope of this article to provide a complete list and discussion of strength and conditioning exercises for tennis, the reader is referred to 2 sources for more extensive development on this topic (23,30).
Figure 2a-c shows a whole body exercise commonly referred to as a medicine ball squat thrust that can be used on or off court with a medicine ball. This exercise simulates the knee flexion and subsequent extension that elite players use during the serve and emphasizes the concept of eccentric preload of the quadriceps followed by explosive knee extension. In addition to the lower-body work that is performed, the player is aggressively extending the hip and lower back by activating the gluteal and erector spinae musculature. Multiple sets of this exercise are recommended with an emphasis on proper body positioning as pictured and the coordination (and therefore summation) of the lower-body, trunk, and upper-body segments to launch the medicine ball as high and straight up into the air as possible.
One of the key regions of the body requiring emphasis in the tennis player is the core. As mentioned earlier, core training should encompass all 3 planes: sagittal, coronal (frontal), and transverse. An example of a core exercise sequence that would encompass these planes includes the sit-up with medicine ball pass (sagittal), seated medicine ball rotations on exercise ball (transverse), side plank (coronal), and side plank with unilateral simultaneous row. Significantly, these 4 exercises also provide a training stimulus, using movement patterns that closely simulate the tennis serve.
The sit-up with medicine ball pass (Figure 3) uses a medicine ball and partner and involves the player doing a sit-up with a medicine ball while passing the ball to a partner to provide an overload concentrically to the abdominal muscles. Upon returning to the starting position, the player catches the return toss from the partner providing an eccentric overload to the abdominals as the player decelerates back to the starting position of the exercise. This exercise can be performed with many variations including the use of a physioball to provide an unstable training surface, as well as the involvement of a diagonal or rotational component to stress the oblique musculature.
Figure 4 shows the seated medicine ball rotation exercise that places the player in a seated position on a physioball. The player, along with a partner, performs rotational tosses emphasizing the preload of the trunk musculature during the backswing of the exercise immediately followed by explosive acceleration of the ball back to the partner. Both a forehand and a backhand motion can be used with crosscourt (diagonal) and down the line (straight ahead) tosses being employed as appropriate.
Figures 5 and 6 show the side plank and side plank with unilateral row exercises. These exercises engage critically important muscles that stabilize the trunk and lower and upper extremities. The athlete assumes the side plank position as pictured, ensuring a proper straight-line alignment without bowing or inferior dipping of the midsection. By resting on the elbow, the athlete uses high levels of scapular stabilization in addition to the core and gluteal activation required to stabilize the rest of the body. Figure 6 shows a variation where a unilateral row is performed while the body remains in the stabilized plank position.
Recent research has examined the range of motion (excursion) of the trunk and angular velocity of the trunk during core exercise performance in athletes (36). They found exercises similar to the sit-up with rotation and seated medicine ball toss use 50 to 60° of differential rotation, which is similar to the rotation used during overhead throwing sport activities. Additionally, velocities used during these exercises were approximately 50% of the velocity of the pelvis and lower torso used during throwing. Additional research also guides the strength and conditioning professional on proper timing of core exercises in the training program. A common belief has been that core training, and for that matter rotator cuff and scapular training, should not be performed prior to skill training (i.e., on-court training) due to the possible interference with optimal on-court performance due to fatigue of key stabilizing musculature. Research has provided support to the notion of performing core exercises after a workout rather than prior to exercise. Navalta and Hrncir (26) have shown that the performance of core exercises during a recovery period following training can actually serve to clear lactate and provide a useful benefit to the elite tennis player following an extended period of on-court training. Thus, the benefit of core muscle activation and strength training can be coupled with lactate clearance and facilitation of recovery, underlining this sequence as preferable for these important exercises in a tennis players program.
For the upper extremity, biomechanical research has identified the powerful concentric muscle activation required to produce racquet head acceleration on the serve. This includes explosive internal rotation of the shoulder, forearm pronation, and wrist flexion. To simulate this portion of the service motion, a 90/90 wall plyometric can be used, focusing on rapid internal rotation of the shoulder and wrist flexion to propel the medicine ball into the wall explosively (Figure 7). While this drill activates the subscapularis, latissimus dorsi, pectoralis major, and teres major, the key internal rotators of the shoulder, it also loads the wrist flexors and pronators (25). It is critically important to point out that this exercise does focus on internal rotation strength development. Research has repeatedly shown that tennis players do have very high levels of internal shoulder rotation strength and that often a tennis-specific imbalance is present in the dominant shoulder whereby the internal rotators are overly developed relative to the posterior rotator cuff (external rotators) (6,8). Care must be taken to ensure that internal rotation exercises such as the 90/90 wall plyometric are used only among players with optimal external/internal rotation muscular balance.
Finally, to improve distal upper extremity muscular explosiveness, the wrist snap plyometric is recommended in addition to the standard wrist curl program to provide a rapid concentric and eccentric training stimulus. This exercise (Figure 8) consists of placing the player in a seated position with a small hand-sized medicine ball. The ball is rapidly thrown using only a wrist flexion motion (the elbow remains fixed in approximately 90° of elbow flexion) toward the ground whereby the player catches the ball and quickly returns it back to the floor using a snapping type motion.
This article has summarized key biomechanical variables inherent in an elite-level tennis serve. It has highlighted the key movement patterns and muscle activations of the serve and in so doing provided the framework for the exercises recommended for the tennis player. Inclusion of these key training exercises in a tennis player's strength and conditioning program can ensure development of the key muscles responsible for force development and transfer in the elite-level tennis serve.
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Keywords:© 2009 by the National Strength & Conditioning Association
biomechanics; tennis; serve