Biomedical subjects

A Baca

Publications and source records attributed to A Baca.

4 recordsLinked to original sources

PubMed1999

A comparison of methods for analyzing drop jump performance.

PURPOSE: Drop jumping is a popular form of plyometric training. Different techniques are applied to determine parameter values quantifying drop jumps, such as the jump height or the durations of the phases of downward and upward movements of the center of mass (CM) during foot contact with the ground after dropping. The flight-time method estimates the jump height from the time between the instant of leaving the ground and the instant of landing. In video-based methods, markers are placed on the skin of the subject to define the positions of the body segments. The time-dependent positions of the CM and parameter values are then calculated utilizing models of the human body. If the vertical velocity of the CM can be estimated at one instant, the parameter values can be calculated from the vertical ground reaction forces. METHODS: The purpose of this study was to find out which technique yields the lowest errors compared with the results obtained by the double force plate technique. In this investigation, two force plates were used, one located under the drop platform. Twenty-five drop jumps were analyzed with eight different methods. There were large differences between the reference method and other methods. Using the height of the drop platform (0.39 m) to estimate the velocity at the end of the free fall, in conjunction with data from one force plate, resulted in a mean difference of 4.2% (SD: 9.6%) in the calculated jump height. Using video information to estimate the time that the velocity of the CM fell to zero after the drop phase, in conjunction with data from one force plate, resulted in differences in the jump height of up to 17%. RESULTS: Differences between the reference method and video based methods were comparatively small (mean value of differences in jump height: -0.007 m, SD: 0.013 m for the best of these methods) but not negligible. CONCLUSIONS: Nevertheless, video based methods turned out to be the most promising alternative to the reference method to determine accurate variables concerning drop jump performance.

Adult↗

PubMed1998

Application of computer animation techniques for presenting biomechanical research results.

Computer animation is becoming a widely accepted method for presenting results of biomechanical analyses and simulations. After summarizing the main aspects of three-dimensional and video overlaid animation, two applications of these techniques are presented. A parameterized graphical model of the tennis racket has been developed, which can be used for visualizing the simulated ball-racket impact phase. In the second example, high-speed video sequences of recorded drop jumps are overlaid with graphical elements to compare methods for determining the jumping height and related parameters. These graphical elements represent quantities, which are calculated applying the different methods. Differences can vividly be demonstrated.

Biomechanical Phenomena↗

PubMed1997

Spatial reconstruction of marker trajectories from high-speed video image sequences.

A novel method is presented for the spatial reconstruction of marker trajectories, which describe the motion of articulating segments of the human or animal body or of a technical construction. In human motion analysis, such markers may be attached to landmark points defining the configuration of the subject. A high-speed video-recording system comprising two cameras is used for motion data acquisition. The particular images are composed of the synchronous recordings of both cameras; the resolution of one image is 239 x 192 pixels, which is comparatively low. To obtain high precision, a new centre estimation method has been developed to calculate the image coordinates of the centres of the recorded spherical markers to subpixel precision. Image coordinates of the centres in subsequent frames are obtained by first applying a tracing algorithm, which calculates the position of a marker to pixel precision, and then using the new centre estimation method. The reconstruction of the spatial coordinates from the image coordinates is based on a three-dimensional photogrammetric calibration and results in deviations of about 0.1%. The instantaneous acceleration of a falling ball has been calculated. The average deviation from the gravitational constant was comparatively small. From the accuracy of the results, it can be concluded that the novel method is applicable in connection with the high-speed video-recording system presently used. Moreover, the techniques are suitable for reconstructing three-dimensional marker trajectories from any low-resolution video image sequences recorded simultaneous with at least two cameras.

Algorithms↗

PubMed1996

Precise determination of anthropometric dimensions by means of image processing methods for estimating human body segment parameter values.

A method has been developed for the precise determination of anthropometric dimensions from the video images of four different body configurations. High precision is achieved by incorporating techniques for finding the location of object boundaries with sub-pixel accuracy, the implementation of calibration algorithms, and by taking into account the varying distances of the body segments from the recording camera. The system allows automatic segment boundary identification from the video image, if the boundaries are marked on the subject by black ribbons. In connection with the mathematical finite-mass-element segment model of Hatze, body segment parameters (volumes, masses, the three principal moments of inertia, the three local coordinates of the segmental mass centers etc.) can be computed by using the anthropometric data determined videometrically as input data. Compared to other, recently published video-based systems for the estimation of the inertial properties of body segments, the present algorithms reduce errors originating from optical distortions, inaccurate edge-detection procedures, and user-specified upper and lower segment boundaries or threshold levels for the edge-detection. The video-based estimation of human body segment parameters is especially useful in situations where ease of application and rapid availability of comparatively precise parameter values are of importance.

Adult↗