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Biomechanics In Tennis

Jani Macari Pallis, 
Ph.D. Photo
Jani Macari Pallis, Ph.D.

About five years ago, I had several discussions with colleagues regarding the status of biomechanics in tennis. I was interested in knowing: who were the "movers and the shakers" in the field, who was really on the leading edge, what were the best techniques for motion capture and what were the best software tools for performance analysis.

Let's back up first. What is biomechanics and what to biomechanists do?

Biomechanics is the study of how living things move using the sciences of biology and mechanics. It is part of a broader field called kinesiology which is the study of human movement. The study of kinesiology includes exercise physiology, motor development, motor learning and biomechanics.

Biomechanics is an interdisciplinary field combining several different sciences. Clearly, the human performance aspect of biomechanics requires a good understanding of anatomy (the science of the structure and parts of a living organism) or physiology (the biological study of the functions of living organisms). However, biomechanics also includes that part of physics known as mechanics. Mechanics analyzes how forces affect objects both at rest (statics) and in motion (dynamics). Both external and internal (including muscles) forces create human movement in sport and exercise. Putting it all together, sports biomechanics is the study of how internal and external forces affect the motion and performance of the human body in an athletic endeavor.

There also are biomechanists that study human motion in industrial settings or work in ergonomics. These biomechanists study equipment design in the workplace and focus on improving worker performance by reducing fatigue or discomfort. There are scientists that study animal and even plant biomechanics. For example, studies have been conducted on how a tree's structure develops or how fluid flows through a tomato.

Biomechanists also need to be knowledgeable and comfortable with the tools of the trade: math, computer science, and statistics.

Who uses and investigates sports related biomechanics? Studies are often conducted by university researchers and professors. Biomechanics is also important to the national governing boards of sports, like the USTA for tennis, and all the other sports represented in the Olympics, like skiing and swimming.

Biomechanic studies have been used to:

  • Enhance performance;
  • Correct problems;
  • Recognize and create new playing techniques;
  • Injury prevention;
  • Injury recovery;
  • Develop athlete physical training programs.
  • The U.S. Olympic committee has a sports science group with biomechanists on staff. Not all sports focus on biomechanic studies. Team or contact sports are difficult to study. In wrestling, for example, there are simply a lot of body parts hidden from view.

    Coaches need to understand biomechanics as well as the players themselves. Professional players benefit by understanding areas of improvement. For beginners, biomechanics helps them understand the basic stroke production.

    Knowledge of biomechanics is important in the sports industry for equipment designers. Groups like the ASTM (American Society for Testing and Materials) and NOCSAE (pronounced Noxsey, National Operating Committee on Standards for Athletic Equipment) determine the standards and testing for athletic equipment from bike helmets to running shoes to racquets.

    What are the types of questions related to tennis biomechanists might study?

  • Is it a good idea to imitate the performance of a tennis champion?
  • Are there techniques that limit injury?
  • What are the best techniques to teach tennis?
  • Why should I teach tennis differently than the way I learned it?
  • How many hours should you train a day?
  • For example: What's an optimal training schedule? Many pros train 4 hours a day, 6 days per week. Jimmy Connors trained only 1 1/2 hours a day, but it was focused and concentrated. Which is better? In general, is one training method superior to another? Is it dependent on the individual or the skill level?

    A professional tennis player may compete an average of 7 years. What if we could extend that longevity to 8-9 years? We could increase the time that they can compete injury free. There's a monetary value that can be placed on that additional 1-2 years of professional play. The information can be used to support the longevity for any player, not just the professionals.

    Studies have been conducted that confirm that tighter strings create more control, while looser strings create more power. That's not "intuitive." You might actually think that the ball has a longer "dwell time" (remains on the strings longer) if the strings are tighter, but that isn't so.

    Should you imitate a champion? Not necessarily. Although a tremendous amount can be learned from watching them, their strength, agility and endurance is probably far greater than yours.

    Information learned from this research is then applied to coaching and training techniques. Swing patterns have changed, racquets have changed, so have the position or stance that the athletes use as they play. However, it is very common to find coaches that teach the way they were taught years ago and those older coaching and training techniques do not necessarily support the "modern game of tennis." The USTA has summarized some of its research in biomechanics in a video tape production for coaches.

    Biomechanic studies are conducted to determine: how to learn the skill, what works best and what's safest. Another area of interest is to understand the racquet strings and its effect on player performance. As the equipment changes or rules change we need to be asking: What are the ramifications to the game's techniques, the players' performance, training and potential for injury?"

    There are many ways to capture information in scientific studies. An entire industry exists dedicated to instrumentation (collection of information with scientific equipment and instruments). A variety of equipment exists to calibrate, accumulate and analyze motion data.

    In sports, equipment such as in-sole sensors or force plates measure the pressure that the athlete imposes on the ground as he or she performs. It's not surprising that equipment used to measure the gaits and motions of the most elite athletes are also used in the medical and rehabilitative fields to analyze the motions of individuals with limited and restrictive movements. The same equipment monitors and looks for improvements as the person recovers from an injury.

    There are four major methods of motion capture: optical, electromagnetic, magnetic, and "non-invasive" video systems. Motion capture techniques are not only used to study athletes, but used in animated films and video games.

    Today, computer animated characters can look very realistic. How do they do that? One technique uses small reflective balls. The balls are attached to key joints and points on a person's body. Infra-red cameras are set up around the athlete or performer and can track the movement of these little balls. If one of the balls is out of view of the cameras, its position can be determined by the other cameras. This is called an "optical" system and is typically limited to an indoor studio setting.

    Another technique uses magnetic sensors instead of balls. There are wires leading from these sensors into computer systems that collect the motion information. This is called a "magnetic" system and although the motion capture is not limited to a studio, the wires restrict very complicated motions.

    There are also methods that are "electromagnetic." These systems use body suits. These motions are collected by a computer and then used with animated or "synthetic" characters.

    While these systems do an excellent job at motion capture and analysis, they are invasive methods of motion capture. Clearly tennis equipment and players could not be tagged with these devices during a professional tournament.

    Subsequently, some researchers use video or film cameras to record motions. Your standard home video camera collects 30 frames/second. Researchers will often use high speed cameras at 250 or 500 frames/second (sometimes even higher).

    Motion capture and analysis has been a tool used by the military, universities, and research groups from industry (groups of people that conduct scientific studies to develop new products or techniques).

    The military has used motion capture and analysis to study ballistics (the flight of objects that are not self-propelled, for example bullets). These objects have a shelf-life (a length of time that an object can be kept without deteriorating). Researchers wanted to understand if projectiles built today would still fly the same way several years later. They used high speed cameras to study the performance of these objects over a period of time and observed differences.

    Researchers and students at universities use motion capture and analysis to study both biomechanics (the science of how a living organism moves) and fluid (liquid or gas) flow.

    One very interesting project was funded by the Navy and conducted at a university. The Navy and other branches of the Department of Defense are interested in unmanned vehicles. The researchers used cameras under the water to study how sea creatures maneuver on the ocean floor. How does a crab move over the edge of a cliff? By understanding how these creatures move over this difficult terrain, better underwater droids (robots) can be constructed.

    High speed cameras can be used whenever motion is so fast that the motion can not be comprehended by our eyes and brain. One example from industry was an electronic toothbrush study. The cameras were used to determine if and how the bristles of the toothbrush reached the nooks and cracks on and between the teeth and gums.

    Today, advances in technology are making machines and processes faster and smaller. Motion capture and analysis is used in "quality control" (techniques used to assure high quality products). Cameras are used to monitor assembly lines for products like raisin cereal and soft drinks. For example, soda cans move quickly down an assembly line as they are filled and packaged. Automated camera systems watch the process and are able to determine when problems occur (dented cans, messy labels). A high speed camera is used to pinpoint where the damage occurs on the assembly line.

    One of the biggest users of motion capture and analysis is the automobile industry and not just to watch the performance of fast race cars! Motion capture and analysis is used in airbag performance and safety studies. Engineers and scientists study how airbags deploy. They study the impact of the airbag against a person's body. They want to understand how the airbag affects people of all ages and sizes, especially small children.

    Of course high speed cameras are used in sports to capture biomechanics. Motion capture and analysis has been used extensively in sports such as golf. Researchers at companies that manufacture golf equipment study how the contact between the golf club and the ball affect the flight of the ball. There are even training systems developed that videotape and analyze your motion so that you can become a better player.

    These last systems will be the focus of the next column. I'd like to try and incorporate your questions on biomechanics into this series of columns on biomechanics in tennis. Don't hesitate to write me using this form with your questions.

    Until Next Month ... Jani

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    This column is copyrighted by Jani Macari Pallis, Ph.D., all rights reserved.

    Dr. Jani Macari Pallis is the founder and CEO of Cislunar Aerospace, Inc., an engineering and research firm in San Francisco. In addition to her engineering practice, she has led two collaborations between NASA and Cislunar, creating educational materials on the aerodynamics of sports for pre-college students and educators. As the head of NASA's "Aerodynamics in Sports" project, she has led a team of researchers investigating the aerodynamics, physics and biomechanics of tennis. The group has conducted high speed video data capture at the US Open and research of ball/court interaction, footwork, serve speeds, trajectories and ball aerodynamics. Pallis received a BS and MS from the Georgia Institute of Technology, an MS in mechanical engineering from the University of California, Berkeley and a Ph.D. in mechanical and aeronautical engineering from the University of California, Davis. She is a member of the Executive Committee of The International Sports Engineering Association.

    Questions and comments about these columns can be directed to Jani by using this form.


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