Department of Kinesiology

Jesus Dapena

RESEARCH

BIOMECHANICS OF HUMAN PERFORMANCE

    The basis for the field of Biomechanics is that the laws of mechanics apply to living organisms just as well as they do to inanimate objects. The structural stresses of a tree, the swimming of a shark and the takeoff of a long jumper are all subject to the laws of mechanics in the same way as a square block of wood sliding down an inclined plane in the classical physics experiment.

    Biomechanics is a diverse interdisciplinary field with branches in disciplines as varied as Zoology, Botany, Physical Anthropology, Orthopedics, Bioengineering and Human Performance. In all of these disciplines the general purpose of Biomechanics is the same: to understand the mechanical cause-effect relationships that determine the motions of living organisms. However, within each discipline Biomechanics tries to solve problems specific to that discipline: In Zoology, Botany and Physical Anthropology the main goal is the understanding of the relationships between structure and function; in Orthopedics and Bioengineering the main focus is also on structure and function, but with a special added emphasis on practical applications, such as the development of prosthetic devices. In Human Performance, Biomechanics contributes to the description, explanation, and prediction of the mechanical aspects of human exercise, sport and play.

    Biomechanics research can be said to have two main facets within Human Performance: "basic science" and "applied science." In its basic science facet, Biomechanics tries to understand how the human body functions mechanically in a situation of maximum effort. When an athlete achieves a good performance, much of it depends on the fitness of the athlete (strength, quickness, endurance). But part of the value of the performance is also determined by the efficiency with which the body is used by the athlete. This efficiency can be improved through the coordination of the motions of different body parts in ways that permit the exertion of larger forces by the muscles, more appropriate timing of stretch reflex mechanisms, longer ranges of motion of certain limbs, etc. Thus, the value of a performance depends on the fitness of the athlete, but also on the technique used. The main objective of the basic science facet of biomechanics is to understand the cause-effect mechanisms that make some techniques better than others, and ultimately to find the optimum technique: what sequence of movements would lead to the best possible performance.

    The basic science facet outlined above has an immediate bearing on applications: Once the optimum technique (or elements of it) are known, it should be possible to examine individual athletes, and determine what technique defects are preventing them from reaching their ultimate potential. Subsequent correction of these defects should lead to improved performance.

BASIC RESEARCH

    There are two major approaches to basic research in Biomechanics. In the "inverse approach" the investigator quantifies the movements that occurred during a recorded sports event, and calculates the causal factors (momentum, acceleration, and ultimately, muscle force) that led to the observed movements. This process is generally carried out through film or video analysis, making extensive use of computers.

    In the "direct approach" the researcher starts from causal factors (for instance, muscle tensions or ground reaction forces measured directly with a force plate), and subsequently calculates the accelerations, velocities and changes in location that result. An important subarea within the direct approach is computer simulation, in which the researcher makes hypothetical alterations in some of the causal factors, and finds out what effects these alterations would have had on the motions of the subject, and consequently on the value of the performance.

    One of my two main lines of research is the mechanics of high jumping. The other one is my work on the mechanics of throwing events in track and field. I first started working primarily on the hammer throw, but I have now changed my emphasis to the discus throw. My ultimate goal is to reach a complete (or almost complete) understanding of the mechanisms involved in the techniques of these sport events. This will allow me to diagnose with a high degree of confidence the technique defects of individual athletes. The subsequent correction of these defects should lead to improved performance results and also to greater safety during the execution of the performance.

    In a different facet of my work, I contribute to improve the methodology used for research in Biomechanics. I do not have a special interest in the development of methodology, but I have found it necessary for the advancement of my research on human motion itself. In the past, I have worked on the development of three-dimensional (3D) film analysis, and also on computer simulation methods. I am currently developing computer graphics applications. In the future, I plan to resume my work on computer simulation, because this methodology will probably be very useful for the study of the high jump takeoff.

    My graduate students have been involved in some of my work on methodology. I have also contributed significantly to my students' research on human motion. My students' projects on human motion do not follow a single line of research, because each student has different interests. Still, the students' projects could be grouped into two categories: the study of flail-like motions, and an analysis of hurdling technique.

Main Lines of Basic Research

    High jumping

        When I was a graduate student my main interest was in the biomechanics of high jumping. This area remains one of my two main areas of research (together with track and field throwing events). When I was a graduate student, I made the first 3D analysis of high jumping. (For that, I first had to develop the necessary methodology.) Later on, I went into deeper analyses. These studies focused separately on various aspects of the run-up, takeoff and bar clearance phases of the jump. I have also recently completed a multi-part project on the twist rotation in high jumping which is currently in various stages of manuscript preparation and submission for publication.

        Today, the high jump is one of the sports activities that is best understood from a biomechanical standpoint. We now understand very well how the rotation over the bar is produced; the mechanisms involved in the run-up and takeoff are also much better understood than before, although not nearly as well as the bar clearance. In the future, I plan to complete the study of the airborne phase. After that, I will concentrate on the takeoff. I will use computer simulation as one of my main research tools to study the takeoff phase. I have done a lot of work in the simulation of airborne motions, but the simulation of the takeoff phase will be quite a different undertaking which will require a large amount of work in the development of new algorithms and computer programs.

        (To see some interesting movies of the support foot during the takeoff phase of a high jump, go here; to see computer animations of a high jump and of an artificial high jump generated using computer simulation, go here.)

      Hammer throwing

        The hammer throw involves very interesting mechanical processes, but its complexity, together with its marked 3D nature has made biomechanists shy away from it. In the early 1980's, I had the appropriate research tools and experience to study the hammer throw, and I started a long-term comprehensive research project on this event.

        The beginning of the project was difficult. Unlike high jumping, which had always attracted researchers, biomechanical information on the hammer throw virtually did not exist. A variety of approaches had to be tried, to check what parameters would provide information leading to a better conceptual understanding of the event. The "pieces of the puzzle" finally fell in place. I found that the preliminary winds that the athlete makes in the back of the throwing circle during the early part of the throw, as well as the single-foot support phases that occur later on during the full turns are very important for the result. This contradicted the widely held belief of coaches that these parts of the throw do not contribute much to the distance of the throw. I also found that hammer throwers are unable to use a technique that coaches often recommend ("countering with the hips", which in theory should be advantageous) because it would require a tremendous amount of strength in a group of muscles (the latissimus dorsi) that throwers generally neglect in their training.

        Through my work on the hammer throw, I gradually developed an interest in the discus throw, in which I am now concentrating much of my attention.

      Discus throwing

        A few years ago, my research experience on the hammer throw, together with the local availability of an elite group of discus throwers, got me interested in the mechanical analysis of the discus throw. As in the hammer throw, little or no research had been done on the discus using 3D methods. But both of these throws involve a marked rotational motion about the vertical axis, and partly because of this similarity my previous work on the hammer throw greatly facilitated my work on the discus.

        At this point I have completed a pilot study on the discus throw, based on a small group of subjects. The data show that the main interaction between the athlete and the ground occurs in the early part of the throw, while the main interaction between the athlete and the discus occurs in the late part of the throw. This specificity of purpose in the different parts of the throw was not known previously. Most coaches and athletes believe that the final part of the throw is the only important one, but my results indicate that the practitioners need to pay more attention to the early part of the throw, because it plays a key role in the generation of momentum for the combined athlete-plus-discus system. At this point the results are still preliminary, because they are based on a small number of subjects. My next step will be to repeat the study with a larger number of elite discus throwers. I have a grant from USA Track & Field to carry out this project in 1996.

        A line of theoretical reasoning suggests that it would be beneficial to release the discus while the feet are still in contact with the ground, but many throwers release the discus after the feet have left the ground. After I complete the previous study, I will investigate the reasons for this apparent discrepancy between theory and practice in the discus throw.

    Other Lines of Basic Research

      Methodology

      (a: Film analysis methods)

        Film (or video) analysis is the main research tool in most Sport Biomechanics laboratories, including ours. Athletes are filmed with high-speed movie cameras during competitions. The films are processed, and then projected, one frame at a time, onto the surface of a digitizing tablet. An operator uses a cursor to measure ("digitize") the positions of a series of body landmarks (usually 21) in each photograph. The coordinates are entered directly into a computer. (A similar process can also be carried out with video cameras.) Until recently, most laboratories have used two-dimensional analysis methods, in which the coordinates taken from the digitizer are simply multiplied by a scale factor to obtain real-life coordinates of the performer. However, this two-dimensional simplification of three-dimensional motions has obvious limitations, and lately many laboratories have turned to 3D analysis, which involves filming with two cameras simultaneously. The digitized data taken from the films of the two cameras are used to compute the 3D coordinates of the body landmarks.

        In the early 1980's, our lab was one of the first to adopt a standard method for 3D film analysis (Abdel-Aziz & Karara's "Direct Linear Transformation" method) for regular use. Earlier, I had worked myself on the development of methodology for 3D film analysis. Together with one of my students (Rosa Angulo), I have also compared the accuracy of video-based versus film-based 3D analysis.

        The development of 3D film analysis methodology is an area in which I am not currently working. I feel that this tool is already very useful in its present state of development, and that it is time to use it to investigate human motion, rather than to keep developing the methodology.

      (b: Computer graphics)

        The 3D body landmark coordinates computed from the film data serve as input for computer programs that use a mathematical model of the human body to calculate a variety of mechanical parameters. They can also be used to produce computer graphics that help in the interpretation of the data and later also to communicate the results to other researchers and especially to practitioners. I am currently deeply involved in the development of animation using computer graphics. (To see a rather unusual computer graphic, press here.)

      (c: Computer simulation of airborne motion)
        Although most of my research has followed the "inverse approach," part of it has focused on the "direct approach" (or simulation). I started with the development of a method for the simulation of airborne movements, my Ph.D. dissertation topic. The methodology that I developed then was a key tool for my latest research on the twist rotation of high jumping. (See above.)

      (d: Computer simulation of ground-supported motion)

        I subsequently expanded the airborne simulation method to handle the two-dimensional (and later the three-dimensional) simulation of pole vaulting. The motions of the limbs relative to each other affect how the hands of the vaulter press on the pole, and consequently the subsequent motions of the athlete. This computer model can be used to investigate the effects that diverse motions of the arms and legs have on the overall motion of the vaulter, and thus on performance. The method required the parallel development of a mathematical model of the vaulting pole. This extended project resulted in two publications with one of my students (Teresa Braff) and a Ph.D. dissertation by another student (Orly Nicklass). In the future, I plan to use simulation to study the high jump takeoff (see above). This will require the development of a different method from the one used for pole vaulting, due to the different nature of these two events. However, the experience gained in the development of the pole vault simulation method will be very helpful for the development of the method for the simulation of the high jump takeoff.

      (e: Computer simulation of sprinting)

        I also used simulation to quantify the effects of altitude and wind on the times of track and field sprinters. It is widely known that a tailwind and the rarified air found at high altitude help sprinters to achieve better times by reducing air resistance. I developed a mathematical model of the sprinter that was incorporated into a simple computer simulation program. This program calculates, from the time, altitude, and wind conditions of any 100-meter race the approximate time that the same sprinter would have achieved in "standard" conditions (zero altitude and wind). The method can be used to adjust all sprinting times to standard conditions, thus helping to provide a more objective assessment of the value of the performance of each sprinter.

      Other Projects with Students

      

      (f: Flail-like motions)

        Some flail-like motions of parts of the body make the end of the distal segment reach very large velocities (typical examples are a karate chop or a tennis serve). In some cases, flail-like motions are planar (e.g. in a straight forward kick), while in others they are markedly three-dimensional (e.g. in a baseball pitch). A variety of flail-like motions (baseball pitch, volleyball spike, tennis serve and soccer kick) have been studied by graduate students at our lab under my guidance (Michael Feltner, Chul-Soo Chung, Rafael Bahamonde and Jacob Levanon, respectively), and I have been a co-author in some of the published papers. As a result of this work, we now understand very clearly what makes some flail-like motions be planar and others markedly three-dimensional. Further work should be directed to improve our understanding of what enables the tip of the distal segment to reach large velocities in flail-like motions. I will probably continue to leave this work mainly to the initiative of my graduate students; my role will be to provide guidance and support.

      (g: Hurdling)

        With one of my students (Craig McDonald) I analyzed the mechanics of track and field hurdling. The women's hurdles are lower than the men's, but we found that the women traveled higher over their hurdle than the men, and therefore the women did not benefit fully from the lower hurdle. However, if the women reduced the height of their jump over the hurdle they would have less time available in the air to prepare the legs for the landing, and the result would be a braking of the forward motion during the landing. We also found that the motion of the trailing leg in the air plays an important role in the preparation for the landing: A far backward position of this leg during the hurdle clearance facilitates a landing with less loss of forward momentum.

    APPLIED RESEARCH

        My basic research on high jumping is complemented by applied research work that I have been doing regularly since 1982 for USA Track & Field (the governing body for track and field athletics in the United States), and sometimes also for the United States Olympic Committee and the International Olympic Committee. In this work, members of our lab film the top American athletes during competitions. The films are then digitized, and relevant mechanical parameters of the motions are calculated. After the results are interpreted, a report is sent to each athlete.

        These reports start with a description of the optimum technique in high jumping. Then, numerical data and graphical information are used to describe the strong and weak aspects of each athlete's technique, and to give recommendations on possible changes that might lead to improvements in performance. Since 1994 the reports are accompanied by videotapes of solid-figure computer animations that explain to the athletes the defects in their techniques. The animations also show what the athletes' jumps would look like after the implementation of changes to correct the defects. The athletes can use these videotape animations to visualize the changes that they need to make in their techniques in order to improve performance.

        This applied research work has been a good complement to our basic research. By following the development of top-caliber athletes from one year to the next, the technique alterations associated with changes in performance can be monitored, and this contributes to a better understanding of the cause-effect mechanisms of these sports events. In addition, many of our graduate students have gained valuable field experience.

        The applied research activities go hand in hand with the publication of papers directed to coaches and athletes, my participation as a lecturer in clinics, and other activities that should be considered service rather than research.

        Grant funding from USA Track & Field and the U.S. Olympic Committee (and in one occasion from the International Olympic Committee) has contributed to support these applied research projects. The help that our laboratory has provided to American athletes is appreciated by USA Track & Field and the U.S. Olympic Committee, and has greatly encouraged these sport governing bodies to provide us with grant funding to support not only our applied research but also our basic research. This is a valuable source of funding for us.

      RESEARCH: PLANS FOR THE FUTURE

        I plan to continue my lines of basic research on high jumping and on the discus throw. For this, I will probably need to work also on the development of research methods, in particular in the area of computer simulation. My students will have freedom to choose the directions of their work, and I will keep guiding them through their research projects. I will also maintain my solid relationship with USA Track & Field and the U.S. Olympic Committee. My applied research will continue to be centered primarily on high jumping, with some occasional "forays" into other sport events.

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      Last updated: December 2000
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