This is the second story in a three-part series examining the analytics boom in tennis. Part 1 examined the use of performance data in match preparation. This story looks at wearables, tracking technology, load management and training.
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In Brent Salazar’s previous jobs as a strength and conditioning coach in the NFL, his players wore GPS devices that tracked their workloads during every minicamp, training camp and mid-week practice. On game days, their shoulder pads were outfitted with Zebra’s RFID tags. How far and how fast each player moved was never in question.
When he became director of performance at the U.S. Tennis Association, a newly created position that he stepped into two summers ago, Salazar was surprised to find that tennis’ embrace of technology was “at the infant stage.” The sport, he says, is just now beginning to adopt the more sophisticated European sport science approach of really knowing the “true demands of what a match is going to take” and training accordingly.
Darren McMurtrie, Tennis Australia’s performance analysis manager: “Tennis is probably one of the last sports in the world to monitor its players properly.”
Last year, the USTA installed the Kinexon ultra-wideband technology at its player development center in Orlando, bringing the NBA’s most-used data tracking system to tennis. Only this summer did the International Tennis Federation approve Catapult Sports’ OptimEye S5 as the first GPS device that can be worn in competition, but the men’s ATP and women’s WTA tours still haven’t signaled their affirmation, limiting for now the number of matches in which the wearables can actually be used to the Grand Slams, the Davis Cup and the Fed Cup. Hawk-Eye’s optical tracking cameras can provide movement information from most competitions, but the raw data is not readily accessible to all players.
“Tennis is probably one of the last sports in the world to monitor its players properly,” says Darren McMurtrie, Tennis Australia’s performance analysis manager, who uses a mix of Catapult and Hawk-Eye with players. “It’s the nature of the sport. Everyone’s individual.”
Even when there is support from a national governing body, players are regularly globetrotting to tournaments. Among American players, for instance, many of the elites don’t train at the USTA National Campus on Lake Nona in Orlando. And those who make it their primary training base might only be on campus 15 weeks a year.
But the USTA is doing its best to understand the physical demands of the sport. Two months ago, IBM went online with its Coach Adviser portal to help American players and coaches quantify exertion in matches that have Hawk-Eye data, which can help establish baselines and set training goals.
“When you’re really playing the right way, according to your game style, this is the energy level that you’re exerting,” says Martin Blackman, the USTA’s general manager of player development. “So we have to make sure that we get you ready for that with your conditioning program and that we replicate that in your on-court training.”
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A tennis player in a singles match must defend a court that is 39-feet long by 27-feet wide. Those confines might seem small until one plays 422 total points, as Novak Djokovic and Roger Federer did in their epic five-set Wimbledon final in July. But just how taxing was that on their bodies?
Craig O’Shannessy, Djokovic’s analyst and the proprietor of Brain Game Tennis, says he’s seen data suggesting that players might move an average of eight to 12 meters per point. While no one seems to have studied this in depth, several other analysts concur that 10 meters is a reasonable estimate for a pro player.
Using that as a benchmark, Djokovic and Federer each logged about 2.6 miles in their match, but that hardly tells the whole story given the myriad accelerations, decelerations and changes of direction. O’Shannessy calls the distance a player runs “interesting” rather than “important” because “there’s no set data that says the player that runs the least in a match wins more or loses more.”
Salazar has worked with Kinexon’s VP of sports performance, Paul Robbins, to adapt the wearable technology for tennis. The speed zones, for instance, vary greatly from basketball. Tennis players may spend a majority of their time moving slower than three miles per hour—walking speed—and almost never more than 16 mph. “But then we take a look at what our change of directions are for a set,” Salazar says.
Hard courts require additional conditioning to prepare joints and tendons for quick changes of direction, whereas clay courts place more strain on the groin as players slide and play from a wider base.
Players might change direction a few times per point and more than a thousand times per match. To quantify the physical impact of how often each athlete reverses course, IBM and the USTA developed a metric called Red Steps, or redirect steps. This metric is the foundation for IBM’s enhanced player workload monitoring in the latest iteration of its Watson-powered tennis tool. Coach Advisor tracks both overall physiological load and shorter-burst mechanical intensity. Players returning serve generally “are working harder because they’re running and tend to have less control of the game,” says Elizabeth O’Brien, IBM program director for sports partnerships.
The sheer quantity of redirections is not the only metric that matters. “It sounds weird, but it’s the direction of the change of direction,” says Dan Kant-Hull, Marquette’s assistant director of sports performance. Kant-Hull used to oversee the use of Catapult GPS devices with the NFL’s Cleveland Browns and, for the past three seasons, did the same with Marquette’s men’s and women’s tennis programs. Right-handed tennis players, he learned, are more apt to go from left to right because they want to set up their forehand. Knowing the split can inform training programs. Another takeaway: “There’s more of vertical component than we ever give tennis credit for,” Kant-Hull says. While rotational movements dominate the game, tennis players get airborne on serves and forehands more than one would naturally expect.
Karl Hogan, Catapult’s commercial director, says the company’s sport science team has developed algorithms to identify specific shot types. He acknowledges that the GPS component of the device in tennis is less material than the inertial measurements gleaned by the accelerometer, gyroscope and magnetometer because players exert themselves in bursts. “They are some of the most impressive athletes in the world, and it’s hard to understand what the physical demands are when you watch it on TV,” Hogan says.
Exertion isn’t just limited to match play. Hogan says one player realized his pre-match ritual was unduly taxing: “He was doing the equivalent of one set in his warm-up session.”
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While athletes in some field sports might toggle between natural grass and artificial turf, tennis has three surfaces in regular rotation: clay, grass and hard courts. The ball bounces differently on each, and bodies perform differently on each surface type. Hogan says those differences in force are visible in Catapult’s data as clusters of “micro-movements” that can impact a player over the aggregate of three or five sets.
Salazar says hard courts require additional conditioning to prepare joints and tendons for quick changes of direction, whereas clay courts place more strain on the groin as players slide and play from a wider base. One of the ways the USTA has sought to account for ground forces is by using Plantiga’s instrumented insoles, which act as a force plate for the foot. “I get excited every time I use it,” Salazar says. “There’s so much that we’re going to be able to get out of this.” One player returning from injury had a 3% imbalance in the forces generated by each foot. The asymmetry resulted from a poor heel lift, suggesting recovery was not yet complete.
Monitoring rehab is a critical piece of a wearable’s value. Forbes documented how Andy Murray wore a Catapult device under the direction of his strength coach, Matt Little, while returning from hip surgery in January 2019. Kant-Hull supervised a women’s player returning from an injury with Catapult, which she was able to wear in matches last year as NCAA players are not subject to the same restrictions as pros. Not all drills or practice sets require the same effort, so Kant-Hull monitored her workload in real-time to ensure she added appropriate increments of effort.
“Any other sport outside of track and field, nobody knows exactly how much you run or move or jump or throw,” he says. “Now, we have a really good snapshot of what you’re actually doing. It removes the guesswork.”
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