Evaluation of stretch load capacity and utilization of stored elastic energy in leg extensor muscles during vertical jumps

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He, Qin, 1957-
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Thesis (M.A.)
School of Physical Education
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An alternating cycle of negative-positive work phases in the lower legs during vertical jumps represents a sequence where storage and utilization of elastic energy takes place.It is possible that energy storage and its utilization depend on the stretch load capacity of activated muscles in legs. It is also possible that there may be a difference between trained and untrained subjects in stretch load capacity. To investigate these assumed differences and to evaluate the leg stretch load capacity, fourteen male subjects were assigned to either a trained (N = 7) or untrained (N = 7) group. All subjects performed two vertical jumps, drop jump and counter-movement jump, at four different heights (20, 35, 50 and 65 cm). A Locam 16 mm high speed camera was used to film the all movements during two vertical jump tests. A Sonic digitizer interfaced to VAX was used to digitize the film data in this study. As expected, the mechanical work, vertical force and vertical velocity during landing phase increased with the increase of dropping height for both of the groups. But, these selected biomechanical variables, including positive work, force, kinetic energy and jumping performance, during take-off phase for trained subjects increased when dropping height was increased from 20 to 50 cm,and then decreased when dropping height increased up to 65 cm; for untrained subjects increased when dropping height increased from 20 to 35 cm, and then decreased when dropping height up to 50 or 65 cm. In counter-movement test, the mechanical energy output during positive phase increased with increase of jumped heights. The results of present study revealed that the main effects of drop jump as one of the most popular plyometric drills to influence jumping performance are significant.