Today’s blog will focus on exercise and movement retraining to optimize the loading phase of the tennis serve. Serve quality and protection of the shoulder joint is predicated on the “health” of the entire kinetic chain. The kinetic chain is the notion that joints and segments (leg, hip, trunk, shoulder, elbow, and wrist) influence one another during movement. As the body moves, a chain of events occur that affect the movement of neighboring joints and segments. During the serve, optimal positioning of each body segment during each phase is achieved by a functioning kinetic chain. The segments of the body must have adequate mobility, flexibility, strength/endurance and control to harness, generate and transfer the forces from the ground (during loading phase) up through the kinetic chain and out into the ball. You can think of the legs, hips and trunk muscles as the force generators; the shoulder as a channel to funnel and regulate the energy, and the elbow wrist and hand as the force delivery mechanism. Per Hoeven & Kibler, to achieve an optimal service motion, the following prerequisites are necessary: (1) an intact functional kinetic chain, (2) normal scapular function, and (3) adequate dynamic and static stabilization of the shoulder.
Phase of Tennis Serve
The loading phase starts when the ball is in the air and ends when the elbow is at its lowest vertical position and the knees are at maximum knee flexion. This phase is critical as it sets up the development of force production in the later phases. During the loading phase, the powerful legs and trunk muscles play a critical role in absorbing forces eccentrically (muscles lengthen under tension) which enables the muscles to generate immense power. Furthermore, the torso winds up like a corkscrew to harness power/energy in joints, muscles, tendons and ligaments. The body/torso is then uncoiled during the acceleration and contact phase transferring the energy through the shoulder, elbow, wrist, and to the ball. If any of the links in the chain are not synchronized and performed with good mechanics, the outcome of the serve will not be optimal. This will result in decreased power, velocity, efficiency, and ultimately an increased risk of shoulder injury. In Short, all segments (leg, hip, trunk, shoulder, elbow, and wrist) of the kinetic chain must be optimal to produce an effective, efficient and powerful serve that the shoulder can tolerate.
Key Components of Loading Phase
1.Dissociation between the torso and pelvis “aka X-Factor or pelvis-trunk seperation): This pertains to axial rotation observed in the loading phase and cocking phase of serve as well as ground strokes. It is measured by a line connecting the hip joint centers and a line connecting the shoulder joint centers. Pro players show an average of 25-30 degrees separation. The separation angle allows the torso to act like a cork screw to harness and generate force. Further discussion of the X-Factor will be presented on a later date!
Exercise: to help Trunk & Hip Dissociation
2. The back leg provides most, but not all, of the upward and forward push, whereas the front leg provides a stable post to allow rotational momentum.
3. Elite servers have greater vertical and horizontal force production to drive up and out toward the ball and earlier activation of the major lower body muscle.
4. The shoulder & Pelvis are positioned in a rear lateral tilt (shown in green arrows in adjacent figure). The tilted alignment facilitates the development of angular momentum through lateral trunk flexion during the forward swing: a critical factor in a high-velocity serve.
5. Shoulder over shoulder position increases forces on the racquet side which are absorbed in the lower limb and then transferred into the racquet and ball during contact.
Leg Power & Strength for Serve Velocity & Shoulder Protection
To highlight the imperative role the muscles of the legs, hips and trunk during loading phase have on power development and protection of the shoulder, several studies will be utilized.
- Kibler et al demonstrated that > 50% of the total energy come from the legs, hips and trunk during the loading phase.
- Bahamonde et al discoveredservice velocity to be directly correlated with > muscle force during loading phase.
- Kibler et al, revealed that a 20% ↓in kinetic energy from the trunk requires a 34% increase in velocity to achieve the same kinetic energy to the hand.
- Elliot et al displayed that players who flexed the front knee by 7.6 ̊ in the loading phase of the serve had an increase demand on the shoulder internal rotators and an varus demand at the elbow during the point of maximal external rotation (MER) of the shoulder when compared with those who flexed by 14.7°. In other words, players who have more knee flexion or a better “leg drive” use their legs to drive the trunk upwards which in turn places the arm in an externally rotated position. A less effective leg drive (↓knee flexion) requires the player to use the shoulder external rotators to achieve MER which requires greater concentric demand on the shoulder internal rotators to reverse this action during the acceleration phase/contact phase.
- Van der Hoeven, H., & Kibler, W. B. (2006). Shoulder injuries in tennis players. British journal of sports medicine, 40(5), 435-440.
- Bahamonde, R. (1997). Joint power production during flat and slice tennis serves. In ISBS-Conference Proceedings Archive(Vol. 1, No. 1).
- Kibler, W. B. (1995). Biomechanical analysis of the shoulder during tennis activities. Clinics in sports medicine, 14(1), 79-85.
- Elliott B, Fleisig G, Nicholls R, Escamilia R. Technique effects on upper limb loading in the tennis serve. J Sci Med Sport. 2003;6(1):76-87.
In conclusion, players should:
- Develop an effective “leg- drive” during loading phase.
- Develop an optimal kinetic chain and control during service as “breakage” in the proximal kinetic chain (legs, hips and trunk) will result in ↑demand at the distal segments (shoulder, elbow and hand) to make up for the loss of force production. This will ultimately result in overuse shoulder injury.
- Develop adequate T/S mobility, oblique and abdominal muscle function, lower extremity power for optimal potential energy absorption during loading phase.