The tennis serve is arguably the most important stroke in the sport. It is also one of the most complex movements in sports. In order to offer clarity to those studying or teaching the serve, biomechanical expert and BodiTrak advisor Mark Kovacs (along with Todd Ellenbecker) published a series of papers on the 8 Stage Model for Evaluating the Serve.
The model can be separated into three distinct phases:
Mark recently shared a trace of a tennis serve on BodiTrak, which we wanted to break down stage-by-stage.
As with all striking sports, the movement starts with the ground. The best tennis servers in the world are able to create energy by pushing to the ground and transfer it through the kinetic chain and deliver to the racquet or ball. In fact, research from Dr. Ben Kibler found that 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 (“The 4000 Watt Tennis Player”). Kibler determined that during the tennis serve only 13% of the kinetic energy was produced by the shoulder, 21% by the elbow and 15% by the wrist. The trunk and lower body were responsible for a staggering 51% of kinetic energy produced in the tennis serve.
Here’s a breakdown of the 8 Stages that will hopefully provide some insight into how a tennis player uses the ground to generate racquet speed.
1. Start: This describes everything from the initiation to the point that the ball and racquet are at rest. In the BodiTrak capture, you’ll notice that the vast majority of pressure is on the front leg (98% v 2%).
2. Release: This stage encompasses the period from the end of the start phase to the point that the ball is released from the non-racquet hand. The release stage is not only important for tossing the ball, but initiating the coil (torso rotation). You’ll notice in the BodiTrak trace that the athlete has rocked back onto their back leg (6% v 94%).
3. Loading: As the name suggests, the Loading phase is where the athlete prepares to exert force into the ground, similar to top of the backswing in golf or leg kick in baseball. This phase begins at ball release and lasts until the point of maximum knee flexion. You’ll notice that the rear foot has moved towards the target. The new RacquetFit Certification (essentially TPI for tennis) discusses the importance of this shift for power and efficiency:
Good lower body power is paramount for proper weight shift and loading. Limited weight shift toward the front leg can reduce the lower body’s contribution to power generation during the serve. Therefore, athletes will try to produce excessive power in their upper body by throwing the racquet early.
This doesn’t mean there isn’t significant contribution from the back leg, which can be a common misconception among amateur tennis players. While it’s important for tennis players to shift to the front side, contribution from the back leg is critical in generating power. As you can see BodiTrak DVF graph above, the athlete is prepared to jump towards the ball in the loading stage, exerting as much as 1.5 – 2x their bodyweight of vertical force into the ground by pushing with both legs. The magnitude of their vertical thrust has a meaningful impact on the velocity of the serve produced. According to research from Kovacs and Ellenbecker (2011), the velocity of a serve has a correlation with greater muscle force during the loading stage.
4. Cocking: This refers to the end of the loading stage and to the point of maximal shoulder external rotation.
5. Acceleration: The acceleration phase spans the end of the cocking stage to the point of contact.
6. Contact: As the name suggests, this is the moment when the racquet contacts the ball. Similar to the moment of impact in golf, you’ll notice that the athlete is not in contact with the ground. We’ve highlighted this in numerous sports (below). While interaction with the ground is responsible for generating over half of the velocity imparted to the tennis ball, it is not important at the actual point of impact.
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7. Deceleration: This is following contact and ends when the upper and lower body have finished decelerating. The athlete re-contacts the ground in the deceleration stage.
8. Finish: From the end of the deceleration to the point of initial movement to prepare for the next stroke.
Like other striking or throwing sports, efficiently transfer energy through the kinetic chain is critical for maximizing power. How and when a tennis player pushes into the ground is just as important as how hard they push.
Mark Kovacs, PhD is the CEO of the Kovacs Institute (www.kovacsinstitute.com) , the Executive Director of the International Tennis Performance Association (www.itpa-tennis.org ) and Head of the Racquetfit Tennis Education Advisory Board (www.racquetfit.com ). He has trained more than two dozen Top 100 ATP and WTA professional tennis players and hundreds of top junior and collegiate tennis athletes.