Tennis Shoulder Injury, Sharapova injury, Sharapova reverse forehand, Sharapova shoulder injury, Australian Open, heat, hard courts
Tennis is one of the most popular sports in the world. Unfortunately, both recreational and elite players often must curtail their play at an early age following apparent "overuse" injuries. [1,2]
Shoulder injuries are particularly debilitating. Elite players whose careers have been hampered by shoulder injuries in recent years include Rafael Nadal, Maria Sharapova, Rodger Federer, Pete Sampras, and Patrick Rafter. For a review of shoulder anatomy and common injuries see sites at the WTA and USTA.
The exact origin of the injury for a particular athlete is rarely identified. Most often the athlete's serve and the frequency of play are the suspected culprits, ideas apparently borrowed from anecdotal comparisons between the tennis serve and major league pitching. [5,6] Curiously, there is no definitive clinical evidence to confirm either of those hypotheses.
While any mechanical device, including the joints of the human body, can certainly be pushed beyond their limits, the repeated invocation of "overuse" is an interesting hypothesis, but hardly a credible medical diagnosis unless it is confirmed by predictive scientific data. No such study has been performed to date.  Simply saying the word "overuse" over and over does not make it so, nor is it necessarily helpful to athlete or coach.
It seems particularly odd to target a properly constructed serve as the offending stroke. The serve has been the subject of multiple modeling efforts attempting to understand how it might produce shoulder degeneration. [6-10] While it is certainly one of the most frequently employed strokes [9, 11, 12, ,14, 18] it is also the stroke where balance, preparation, and execution are most easily controlled by the athlete and coach. It is the one stroke where few surprises occur.
The same cannot be said of another stroke, one that has become increasingly popular - the reverse forehand (Figure 1).
Figure 1. During a running right-handed reverse forehand, the direction of the forces stressing the shoulder joint shift suddenly from along the flight of the ball at the start (far left) and middle of the stroke to vertical at the completion (far right). The arrows depict the upper arm rotating in the ball and socket shoulder joint. The arrows emphasize the dramatic direction change of the upper arm in the final moments of the stroke. The upper arm directional arrows are reproduced in a cluster in the upper right corner of the image.
Figure 2. During an aggressive topspin forehand the upper arm moves smoothly in a single arc with no sudden change in direction such as that seen at the precipitous termination of the reverse forehand.
Figure 3. The driving forehand (left) produces a stroke characterized by an uncomplicated single smooth arc for the racquet head. The reverse forehand (right) generates a more complex racquet head motion that starts along an arc (green) similar to that of the driving forehand, transitions to a vertical arc (yellow), and finally enters a terminal reverse loop (red).
A Coach Ahead of His Time
I grew up in the 1950's fortunate to spend each afternoon with a wonderful Frenchman who taught kids about a strange thing (in that era) called the Western grip. We played on both green and red clay and spent lots of time talking about balance and momentum. I can still hear coach tell me softly again and again that a true tennis stroke begins with the legs and acquires its power by channeling the momentum of the whole body into the head of the racquet. What he said sounded right but, since I was a skeptical teenager and complete physics nerd, I went back to my high school physics classes and looked mathematically at the fundamental mechanics of producing the perfect tennis stroke. It seemed to me both then and now that Mr. Bouchet was quite correct.
A tennis stroke is composed of a linked series of neuromuscular events we now call the kinetic chain.  For example, to produce my coach's favorite shot, a driving topspin forehand using a Western grip (Figure 2), the generation of power begins in the foot and ankle with the balanced contraction of extensors and flexors crossing the ankle joint. Next the same balanced contraction of muscle groups crossing the knee increases leg extension. Athletes know this coordinated effort involving ankle and knee joints as "pushing off".
Next, the hips begin to rotate, trunk muscles fire, and the shoulders start to rotate around an imaginary axis going down the spine. As the shoulders rotate around that central axis, flexor and extensor muscle groups at the shoulder, elbow, and wrist joints fire accelerating the racquet head towards the ball.
Ideally, the stroke culminates with all the rotational force of legs, trunk, and shoulders transferring momentum along the arm and wrist into the racquet. Electromyographic and imaging studies demonstrate that elite players generate higher racquet head velocity by increasing the horizontal rotation of the shoulders around the spine.  As the racquet head moves through the ball, the shoulder bears the burden of decelerating the racquet, withstanding all of the momentum not transferred into the acceleration of the ball.
In a forehand ending across the body, the rotation of the pelvis and shoulders around that central axis takes some of the rotational and deceleration burden off of the shoulder joint.
At the end of the match it was acceptable to coach for my legs to be tired, but never my arms. If an arm was tired it meant I was trying to "hit hard" by asking my shoulder and arm muscles to make up for the laziness of not getting my feet and body in position to do their part. Coach estimated that in a proper stroke about ninety percent of the power came from leg extension and torso rotation leaving the shoulder to handle the remaining ten percent.
Beyond the math, there was one intangible coach taught. If a stroke was correctly produced, at the time of contact with the ball the racquet head should feel like it had taken on a life of its own, moving through the ball smoothly with no sense of strain or jarring impact on arm or shoulder. It took a good bit of what we would now call kinetic imaging and even more practice, but it worked. Through high school and college years I do not recall ever hearing that someone trained by Mr. Bouchet retired from a match as a result of a shoulder injury.
The Reverse Forehand - Ignoring Fundamental Physics
It is because of that early training, and analysis of the fundamental physics, that I find the current fascination with the "reverse forehand" concerning. As I watch the numerous slow motion videos available on the web depicting the reverse forehand of otherwise excellent athletes, the frequent complete disconnect in the kinetic chain during execution of the stroke is quite obvious. 
Frequently there is no evidence of any transfer of momentum from legs and body into the heart of the stroke. The shoulders and pelvis often do not rotate at all or rotate late, pulled along by the shoulder and the momentum of the racquet. As a result of the tardiness or even absence of hip and trunk rotation, the shoulder muscles and tendons must labor alone to initially accelerate and then decelerate the stroke.
In addition, instead of the raquet head moving in a smooth single arc through the ball and toward the opponent, the initial horizontal and forward rotation of the shoulder joint during the early part of the stroke suddenly changes to vertical finally ending above and behind the head (Figure 1). This dramatic change in the direction component of the forces acting on the shoulder joint will produce a significant strain on the muscles, tendons, and bone insertions of this complex and delicately balanced joint.
Kinetic chain mechanics can be reliably monitored using high speed video techniques that are readily available to athletes, coaches, and medical personnel. But these techniques only allow us to infer the probable stresses on the joint. Metabolic demands can be monitored using thermal infrared imaging technology that is becoming increasingly available.
Figure 4 depicts the thermal energy generated by the tissues surrounding the shoulder joint before and after execution of 30 reverse forehand exercises using 8 pound weights. The images were obtained with a compact hand-held FLIR ThermaCam EX320 provided courtesy of the Spitzer Space Telescope, California Institute of Technology. The system can identify temperature shifts of 0.14oF. In this example, cool temperatures appear black or blue, mid-range temperatures appear red, higher temperatures yellow, and the highest temperatures appear white. Peak temperatures increased by 2.8oF following execution of the reverse forehands.
Figure 4. Infrared thermal imaging of the right shoulder before (left frame) and after (right frame) 30 reverse forehand. The camera detects a temperature increase of 2.8oF.
"Back of the envelope" calculations suggest that the reverse forehand subjects the shoulder joint to excessively high torques as the rotation vector changes from horizontal to vertical even under optimal conditions.
To make matters worse, the shot seems to be most often employed when the athlete is behind, not ready for the shot, and off balance. In addition, the stroke seems to appear more often late in a match, when fatigue makes it more likely that any stroke will be incorrectly executed particularly when used as a defensive, off-balance attempt for a quick kill.
The high torque characteristic of the stroke can easily be made more severe if a fatigued athlete makes sudden last moment corrections to compensate for poor court positioning. The stroke seems ideally suited to produce the type of subclinical late match "perfect storm" injury that may be ignored at first, but will later surface in the athlete's life as career-changing muscle, bone, and tendon defects ascribed to "overuse". The appearance of a bone bruise on MRI may be a tipoff that a recent traumatic injury has occurred, but MRI's are expensive and rarely obtained for "a simple bruise". 
If the cause of the injury is summarily ascribed to "overuse" and the primary culprit is thought to be the serve, the athlete trying to return to competition following surgical repair is left standing alone at the service line wondering if a once best friend in combat betrayed him... and is about to do it again.
At present no formal data exist to test the hypothesis that the reverse forehand leads to increased numbers of shoulder injuries. But both field studies and computer simulation experiments are perfectly feasible with current video analysis software and infrared imaging techniques. We in the medical community owe it to our children and our athletes to perform these studies as soon as possible.
REFERENCES and NOTES
1. Bahamonde, R.E. and D. Knudson, Kinetics of the upper extremity in the open and square stance tennis forehand. Journal of Science and Medicine in Sport, 2003. 6(1): p. 88-101.
2. Marx, R.G., J.W. Sperling, and F.A. Cordasco, Overuse injuries of the upper extremity in tennis players. Clinics in Sports Medicine, 2001. 20(3): p. 439.
3. Richardson, A.B., Overuse syndromes in baseball, tennis, gymnastics, and swimming. Clinics in Sports Medicine, 1983. 2(2): p. 379-90.
4. Winge, S., U. Jorgensen, and A.L. Nielsen, Epidemiology of injuries in Danish championship tennis. International Journal of Sports Medicine, 1989. 10(5): p. 368-371.
5. Hill, J.A., Epidemiologic perspective on shoulder injuries. Clinics in Sports Medicine, 1983. 2(2): p. 241-6.
6. Kibler, W.B., A. Sciascia, and S. Moore, An acute throwing episode decreases shoulder internal rotation. Clinical Orthopaedics and Related Research, 2012. 470(6): p. 1545-1551.
7. Abrams, G.D., A.L. Sheets, T.P. Andriacchi, and M.R. Safran, Review of tennis serve motion analysis and the biomechanics of three serve types with implications for injury. Sports Biomechanics, 2011. 10(4): p. 378-390.
8. Elliott, B., Biomechanics and tennis. Br. J. of Sports Medicine, 2006. 40(5): p. 392-396.
9. Kibler, W.B., T.J. Chandler, B.P. Livingston, and E.P. Roetert, Shoulder range of motion in elite tennis players. Effect of age and years of tournament play. The American Journal of Sports Medicine, 1996. 24(3): p. 279-85.
10. Perry, J., Anatomy and biomechanics of the shoulder in throwing, swimming, gymnastics, and tennis. Clinics in Sports Medicine, 1983. 2(2): p. 247-70.
11. Filipčič, T., A. Filipčič, and T. Berendijaš, Comparison of game characteristics of male and female tennis players at Roland Garros. ACTA Univ. Palacki. Olomuc., Gymn, 2008. 38(3): p. 21-28.
12. Norton, P. and S.R. Clarke, Serving up some grand slam tennis statistics, in Proceedings 6th Australian Conference on Mathematics and Computers in Sport 2002: Bond University, Queensland, Australia. p. 202-209.
13. Pluim, B.M., J.B. Staal, G.E. Windler, and N. Jayanthi, Tennis injuries: occurrence, aetiology, and prevention. Br. J. Sports Med., 2006. 40: p. 415-423.
14. Johnson, C.D. and M.P. McHugh, Performance demands of professional male tennis players. Br. J. Sports Med., 2006. 40: p. 696–699.
15. Roetert, E.P., M. Kovacs, D. Knudson, and J.L. Groppel, Biomechanics of the tennis groundstrokes: Implications for strength training. Strength and Conditioning Journal 2009. 31(4): p. 41-49.
16. Landlinger, J., S. Lindinger, T. Stoggl, H. Wagner, and E. Muller, Key factors and timing patterns in the tennis forehand of different skill levels. Journal of Sports Science and Medicine, 2010. 9(4): p. 643-651.
17. Kibler, W.B., Biomechanical analysis of the shoulder during tennis activities. Clinics in sports medicine, 1995. 14(1): p. 79-85.
18. Anzilotti, K.F., M.E. Schweitzer, M. Oliveri, and P.J. Marone, Rotator cuff strain: A post-traumatic mimicker of tendonitis on MRI. Skeletal Radiology, 1996. 25(6): p. 555-558.
19. High definition video examples of the reverse forehand as it is used in match play by Rafael Nadal, Maria Sharapova, Rodger Federer, and Pete Sampras are currently available on Youtube. For a discussion of inappropriate use of the reverse forehand see Miguel Cervantes' article: "The Reverse Forehand: Don't Try It!", Long Island Tennis Magazine, September 29, 2011.
M. C. Storrie-Lombardi, M.D.
November 27, 2012
1 (626) 379-9331