Search Results

You are looking at 1 - 4 of 4 items for

  • Author: Warren G. Darling x
Clear All Modify Search
Restricted access

Jan M. Hondzinski and Warren G. Darling

Experiments were designed to examine the visual contributions to performance of back aerial double somersaults by collegiate acrobats. Somersaults were performed on a trampoline under three visual conditions: (a) NORMAL acuity; (b) REDUCED acuity (subjects wore special contacts that blocked light reflected onto the central retina); and (c) NO VISION. Videotaped skill performances were rated by two NCAA judges and digitized for kinematic analyses. Subjects' performance scores were similar in NORMAL and REDUCED conditions and lowest in the NO VISION condition. Control of body movement, indicated by time-to-contact, was most variable in the NO VISION condition. Profiles of angular head and neck velocity revealed that when subjects could see, they slowed their heads prior to touchdown in time to process optical flow information and prepare for landing. There was not always enough time to process vision associated with object identification and prepare for touchdown. It was concluded that collegiate acrobats do not need to identify objects for their best back aerial double somersault performance.

Restricted access

John W. Chow, Warren G. Darling and James C. Ehrhardt

The purpose of this study was to determine the coordinates of the origin and insertion, muscle volumes, lengths, lines of action, and effective moment arm of the quadriceps muscles in vivo using magnetic resonance imaging (MRI) and radiography for a pilot study involving musculoskeletal modeling. Two magnetic resonance scans were performed, and axial images were obtained for the left thigh of a female subject in the anatomical position to measure muscle volume, coordinates of the origin and insertion, and muscle belly length at the anatomical position of each quadriceps muscle. Six knee radiographs were used to determine the effective moment arm of the quadriceps force at different knee flexion angles. A combination of MRI and radiography data was used to compute the muscle lengths at different knee flexion angles. The coordinates of the vastus lateralis, muscle volumes of individual quadriceps muscles, and effective moment arms were clearly different from the corresponding values from cadaver data reported in the literature. These comparisons demonstrate the advantages of using personalized muscle parameters instead of those collected from cadavers and dry-bone specimens.

Restricted access

John W. Chow, Warren G. Darling and James C. Ehrhardt

The purpose of this study was to determine the maximum muscle stress (σ), defined as the maximum isometric force divided by the physiological cross-sectional area, of the quadriceps muscles for a pilot study involving musculoskeletal modeling. One female subject performed maximum effort isometric knee extension exercises on an isokinetic dynamometer at different attachment arm angles. The gravitational effect was taken into consideration when determining the isometric resultant knee torques at different knee flexion angles. The anatomical and geometric parameters of the quadriceps muscles were obtained from radiography and magnetic resonance imaging taken from the subject. The σ value was computed using me measured knee torques, musculoskeletal parameters data, and information reported in the literature. The computation procedures used in this study represented the first attempt to incorporate the concept of optimal muscle length in the determination of maximum muscle stress. The σ values obtained from the data for nine different knee flexion angles ranged from 21.4 to 30.5 N/cm2. The average value of 25.6 ± 2.6 N/cm2 is notably smaller than the human σ values reported in the literature, but is comparable to the σ values obtained from isolated muscles.

Restricted access

John W. Chow, Warren G. Darling, James G. Hay and James G. Andrews

The purpose of this study was to propose and evaluate a method for the in vivo determination of the force-length-velocity relations of individual quadriceps muscles. One female subject performed maximum effort knee extensions on an isokinetic dynamometer. The gravitational and inertial effects were taken into consideration when determining the resultant knee torque. Selected anatomical and geometric parameters of the quadriceps muscles were obtained from radiography and magnetic resonance imaging (MRI). Hill’s (1938) mechanical model was used to represent the force-velocity relation of a muscle at a given length, and the constants in Hill’s model were assumed to vary with muscle length. Experimentally determined knee torque and muscle shortening velocity data were used to determine the unknown parameters in the muscle model. The relation between each muscle parameter and muscle length for each muscle was obtained using regression analysis. On average, the muscle model overestimated the knee torque by 15.5 ± 5.1%. The overestimations may have resulted from the lack of low torque-high velocity data for the determination of muscle model parameters. When a set of fixed Hill constants was used, the knee torque was underestimated by 29.0 ± 10.6%. The results demonstrate the feasibility of the method proposed in this study.