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Vertical and radial motions of the center of mass during the takeoff phase of high jumping
Abstracts
899
VERTICAL AND RADIAL MOTIONS OF THE CENTER OF MASS DURING THE TAKEOFF PHASE OF HIGH JUMPING Jesus Dapena and Chul S. Chung Department of Physical Education, Indiana University, Bloomington, IN 47405, USA. By placing the muscles of the takeoff leg in faster eccentric or slower concentric conditions, a high jumper can increase the ground reaction force and the height of the jump. Film analysis of seven high jumpers showed that the radial velocity of the center of mass (c.m.) with respect to the supporting foot was more negative or less positive than the vertical velocity throughout the takeoff phase. This favored faster eccentric or slower concentric contractions of the leg muaclea. The radial distance from the hip of the takeoff leg to the c.m. (R jH) first decreased by 0.030 m, due to negative radial motions of the arms and of the SWEnging leg. This contributed to a smaller negative radial velocity of the hip (V ), and thus to slower eccentric contractions of the muscles of the takeoff leg. Htferefore,it may have helped to cushion the initial impact with the ground. Subsequently, R jH increased by 0.120 m due to positive radial velocities of the arms, the swingEng leg and the head-and-trunk. This contributed first to larger negative (and later to smaller positive) VBB values, and thus to faster eccentric and slower concentric contractions of the muscles of the takeoff leg. (This project was supported by a grant from the USOC and TAC.)
JERK-COST DURING THE LEARNING OF UNRESTRAINED RAPID ARM MOVEMENTS Klaus Schneider, Tim Hart, and Ronald F. Zemicke Department of Kinesiology University of California, Los Angeles, CA 90024-1568, USA Using their nondominant left arm. four male human subjects performed arm movements between two targets. During the motion subjects had to circumnavigate a barrier located midway between the targets and extended outward from the vertical plane of the targets. The two targets and the barrier placed boundary constraints on hand trajectories, but otherwise the motion was unrestrained. The goal of the task was to minimize movement time. After 100 practice trials, subjects repeated arm movements across a spectrum of movement times. Arm movements were recorded on high-speed cinC film, and linear kinematical data were obtained for all arm segments, and jerk-cost was calculated for hand motion. Total jerk-cost was partitioned into jerk-cost resulting from magnitudinal changes of the hand acceleration vector and jerk-cost resulting from directional changes of the hand acceleration vector. Pronounced changes occured in hand velocity and acceleration magnitude, direction, and timing. Total jerk-cost, including magnitude and direction, was significantly lower for the movements performed after practice than during practice. The decrease in jerk-cost was consistent with an increased smoothness of the movements repeated after practice.
THE EFFECT OF SEAT HEIGHT ON MUSCLE LENGTHS, VELOCITIES AND MOMENT AIL"I LENGTHS DURING CYCLING S.G. Rugg and R.J. Gregor. Human Biomechanics Laboratory, Department of Kinesiology, UCLA, Los Angeles, CA 90024 The purpose of this study was to calculate muscle lengths, velocities and moment arm lengths for the vasti, hamstring, gastrocnemius and soleus muscles to orovide additional insight into lower extremity kinetics during cycling. Five skilled cyclists were filmed (100 fps) pedaling at 90 rpm on a windload simulator. Seat heights of 100, 105. 110 and 115% of crotch height were used. Variations in moment arm lengths acting about the hip, knee and ankle were estimated from the data of Nemeth and Ohlsen (J. Biomech. 18: 129-140, 1985). Smidt (J. Biomech. 6: 79-92, 1973) and MRI of each subject, respectively. Joint angles and moment arm lengths were used to calculate muscle lengths with velocities obtained via finite differences. Absolute length for the vasti and soleus tended to decrease, but increase for the hamstrings and gastrocnemius with increased seat height. Average moment arm lengths at the knee for the vast1 and hamstrings increased by 10 and 22X, respectively, with an increase in seat height. For all conditions, maximum vasti shortening velocities were recorded at 108 degrees from TDC and ranged from 9 cm/set to 23 cm/see from the lowest to highest seat heights tested. The major compensations for a change in seat height occurred at the knee and ankle. The small deviations observed in the muscle mechanics acting about the hip appear to support the kinetic data indicating the importance of hip extension during the propulsive phase of cycling regardless of seat height.
899
VERTICAL AND RADIAL MOTIONS OF THE CENTER OF MASS DURING THE TAKEOFF PHASE OF HIGH JUMPING Jesus Dapena and Chul S. Chung Department of Physical Education, Indiana University, Bloomington, IN 47405, USA. By placing the muscles of the takeoff leg in faster eccentric or slower concentric conditions, a high jumper can increase the ground reaction force and the height of the jump. Film analysis of seven high jumpers showed that the radial velocity of the center of mass (c.m.) with respect to the supporting foot was more negative or less positive than the vertical velocity throughout the takeoff phase. This favored faster eccentric or slower concentric contractions of the leg muaclea. The radial distance from the hip of the takeoff leg to the c.m. (R jH) first decreased by 0.030 m, due to negative radial motions of the arms and of the SWEnging leg. This contributed to a smaller negative radial velocity of the hip (V ), and thus to slower eccentric contractions of the muscles of the takeoff leg. Htferefore,it may have helped to cushion the initial impact with the ground. Subsequently, R jH increased by 0.120 m due to positive radial velocities of the arms, the swingEng leg and the head-and-trunk. This contributed first to larger negative (and later to smaller positive) VBB values, and thus to faster eccentric and slower concentric contractions of the muscles of the takeoff leg. (This project was supported by a grant from the USOC and TAC.)
JERK-COST DURING THE LEARNING OF UNRESTRAINED RAPID ARM MOVEMENTS Klaus Schneider, Tim Hart, and Ronald F. Zemicke Department of Kinesiology University of California, Los Angeles, CA 90024-1568, USA Using their nondominant left arm. four male human subjects performed arm movements between two targets. During the motion subjects had to circumnavigate a barrier located midway between the targets and extended outward from the vertical plane of the targets. The two targets and the barrier placed boundary constraints on hand trajectories, but otherwise the motion was unrestrained. The goal of the task was to minimize movement time. After 100 practice trials, subjects repeated arm movements across a spectrum of movement times. Arm movements were recorded on high-speed cinC film, and linear kinematical data were obtained for all arm segments, and jerk-cost was calculated for hand motion. Total jerk-cost was partitioned into jerk-cost resulting from magnitudinal changes of the hand acceleration vector and jerk-cost resulting from directional changes of the hand acceleration vector. Pronounced changes occured in hand velocity and acceleration magnitude, direction, and timing. Total jerk-cost, including magnitude and direction, was significantly lower for the movements performed after practice than during practice. The decrease in jerk-cost was consistent with an increased smoothness of the movements repeated after practice.
THE EFFECT OF SEAT HEIGHT ON MUSCLE LENGTHS, VELOCITIES AND MOMENT AIL"I LENGTHS DURING CYCLING S.G. Rugg and R.J. Gregor. Human Biomechanics Laboratory, Department of Kinesiology, UCLA, Los Angeles, CA 90024 The purpose of this study was to calculate muscle lengths, velocities and moment arm lengths for the vasti, hamstring, gastrocnemius and soleus muscles to orovide additional insight into lower extremity kinetics during cycling. Five skilled cyclists were filmed (100 fps) pedaling at 90 rpm on a windload simulator. Seat heights of 100, 105. 110 and 115% of crotch height were used. Variations in moment arm lengths acting about the hip, knee and ankle were estimated from the data of Nemeth and Ohlsen (J. Biomech. 18: 129-140, 1985). Smidt (J. Biomech. 6: 79-92, 1973) and MRI of each subject, respectively. Joint angles and moment arm lengths were used to calculate muscle lengths with velocities obtained via finite differences. Absolute length for the vasti and soleus tended to decrease, but increase for the hamstrings and gastrocnemius with increased seat height. Average moment arm lengths at the knee for the vast1 and hamstrings increased by 10 and 22X, respectively, with an increase in seat height. For all conditions, maximum vasti shortening velocities were recorded at 108 degrees from TDC and ranged from 9 cm/set to 23 cm/see from the lowest to highest seat heights tested. The major compensations for a change in seat height occurred at the knee and ankle. The small deviations observed in the muscle mechanics acting about the hip appear to support the kinetic data indicating the importance of hip extension during the propulsive phase of cycling regardless of seat height.