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Measurement issues in human movement

This is an excerpt from Biomechanics of Sport and Exercise 4th Edition With Web Resource-Loose-Leaf Edition by Peter M. McGinnis.

Quantitative biomechanical analyses of human movement involve measurements of biomechanical variables. The variables measured may be temporal (timing), kinematic (position, displacement, velocity, acceleration), or kinetic (force, energy, work, power). In any case, some sort of instrument is used to measure the variable. The instrument itself and the setting that it is used in may affect the performance of the athlete, patient, or client. The process of measuring something influences the parameter being measured. The validity of the measured parameter is thus threatened by the measurement process. Measurement technology that minimizes the measurement effects on the performer is preferred.

Besides minimizing measurement effects on the performer, other desirable attributes of measurement technology used in biomechanics are accuracy and precision, which are both related to measurement error. How much measurement error is acceptable depends on the setting in which the measurement will be used. In research, the highest accuracy and precision are required; in clinical applications, more error is tolerable; and in sport competition or training, even more error is acceptable.

Laboratory Data Collection

Ideally, the environment in which you measure the performance should be carefully controlled. Most data for quantitative biomechanical analyses are collected in a biomechanics laboratory, where the environment can be controlled. The drawback is that a biomechanics laboratory is not the setting in which the athlete, patient, or client normally moves or performs. The novelty of the environment may influence the movements being measured. The laboratory should be set up to duplicate as closely as possible the environment in which the movements normally occur.

The benefit of data collection in a laboratory is control of the environment. This control helps minimize measurement error. The cameras, lights, temperature, and so forth are always the same. The subject thus performs in the same conditions during each evaluation. Much of the instrumentation is permanently set in position so the time needed to calibrate and prepare the instrumentation for data collection is minimal. In addition, sensors, markers, or other data collection devices can be attached to the performer.

The drawback is that the environment is not the same as the real-life environment in which the athlete, patient, or client usually moves. A baseball pitch thrown in the lab may be very different from the same type of pitch thrown in a game. The lights and cameras and the technicians watching and measuring the patient's movements may make the patient self-conscious and alter the movement. The attachment of markers, sensors, or other gear to the performer will have some effect on the movements being measured. In laboratory data collection, it is very important for the subject to become familiar with the equipment and the laboratory environment before data collection begins.

In-the-Field Data Collection

An actual athletic competition may be the best environment for measuring the biomechanics of an athlete's performance because it is one in which the athlete normally performs (see figure 16.1a). The competition setting, however, may not be the best for the biomechanist. Most biomechanics technology is not very portable. To record ground reaction forces, force plates would have to be mounted in the competition setting. To record muscle activity, electrodes would have to be attached to the athlete's body and signals from these sent to a receiver for recording. Alternatively, a device for recording the signals could be attached to the athlete. The type of data regularly collected in athletic competitions is kinematic data. Technology for measuring kinematic data includes electronic timing devices and video recordings and their computerized analysis systems, radar or laser velocity-measuring devices, and inertial measurement units (IMUs) that are attached to, or worn by, the athlete. With the exception of IMUs, most of these measurement devices or systems are relatively noninvasive. The performance of the athlete is minimally affected by their use. The athletes and the directors of the athletic competitions will be more likely to grant biomechanists permission to use these noninvasive types of measurement equipment than the more invasive types during competitions.

The major drawback of collecting biomechanical data during an athletic competition is lack of control of the environment. The biomechanist has no control over the performer or the factors influencing the performance, and the positions of the data collection instruments may be restricted. The film or video cameras and the radar or laser velocity-measuring devices all require direct views of the performance. The radar or laser velocity-measuring devices require setup along the line of the performer's motion. Officials, spectators, other athletes (see figure 16.1b), and so on may block these views during periods of data recording. Changes in lighting may limit the use of film or video cameras. Inclement weather may also limit the use of the equipment (e.g., too cold, too hot, too wet). All these devices require electrical power. Multiple batteries must be on hand, or an accessible power source must be found at the athletic venue. Transporting expensive and fragile electronic equipment to and from the competition venue exposes it to risks of damage or theft. The larger the space in which the activity of interest occurs, the more cameras are required to record the activity and the more difficult will be the calibration of the space. The preplanning and setup time for data collection at an athletic competition is also extensive. Additionally, it is difficult to duplicate the use of the given cameras in exactly the same positions from one competition to the next. In spite of these drawbacks, sport biomechanists regularly collect biomechanical data at the Olympic Games and world and national championships for a variety of sports.

At least one professional sport organization has embraced the measurement of biomechanical data during competition and thus eliminated many of the problems with control of the environment. Since 2006, image-based or radar-based kinematic measurement systems have been permanently installed in every major league baseball (MLB) stadium in North America. From 2006 through 2016, the Pitchf/x system was used in each stadium. Using motion capture data from several fixed video cameras mounted in the stadium, the Pitchf/x system tracked the motion of every pitched baseball from the pitcher's hand to the plate during games. Almost instantaneously, Pitchf/x computed a variety of kinematic measures of the pitch including velocity, spin, break, and ball position in the strike zone. The official MLB measurements of pitch speeds were provided by Pitchf/x until 2017. Since 2017, Statcast has provided the official MLB pitch speed measurements. Statcast was installed in all MLB stadiums in 2015. Similar to Pitchf/x, it uses multiple fixed video cameras mounted throughout the stadium, but it also uses Trackman, a Doppler radar tracking device, to measure the trajectories of pitched, batted, and thrown balls. Not only is the ball tracked, but all the players on the field are also tracked by the Statcast system. MLB stadiums have become biomechanics laboratories.