KINETIC ENERGY OF BODY SEGMENTS IN DROP JUMP

Abstract

INTRODUCTION - In sport movements, such as sprinting and jumping, where the ability to produce explosive movement is extremely important, the elastic characteristics of the muscle-skeletal system are decisive. Drop jump (DJ) exercises are often used as a training method to develop the mechanical and control capabilities of the neuromuscular system. DJ has also been used to obtain very important data about the degree of adaptability of the locomotor system to increasing amounts of mechanical load, using different dropping heights. The purpose of this study is to analyse the changes of the kinetic energy of body segments (KE) in relation to the increase o! mechanical load, both in the stretching and push-off phases. These changes are related to the angular kinematics of joints and to the relative length changes on some of the extensor muscles. METHODS - Four elite sprinters (height 17855 cm, body mass 69.34.5 kg) performed 6 DJ from 25, 40, 55 and 70 cm. Ground reaction forces were recorded at 500Hz and vertical peak and vertical net impulse were calculated. Simultaneously, angular kinematic data of the ankle, knee and hip joints were calculated using video analysis system (120 Hz). We used Visser et al. (1990) model to calculate the relative length changes (%L) of gastrocnemius (G), rectus femoris (RF) and biceps femoris (BF) muscles. The joint angular velocity (~)) and relative muscle length changes velocity (%L.s-') were also calculated. The KE energy of the foot, lower leg, thigh and upper body mass centres were calculated from the kinematic and anthropometric data. RESULTS - The basic results are showed on the table, AVG and SD values, both for the stretching and shortening phases of the ground contact period. CONCLUSION - The KE of the body segments at the end of push-off was higher on DJ25 and DJ40, Wen compared with the KE at touchdown, presenting a positive energetic balance. At DJ55, the energetic balance was null and at DJ70 was negative. The stretching and shortening velocities increased in all muscles with the increase of DJ height. Nevertheless, no differences were found on vertical jumping achievement between DJ executed from different heights. The subjects were able to reduce the increasing stretching load producing higher vertical net impulses during the downward movement. REFERENCES Visser J.J.. Hoogkamer, J., Bobbert, M.F. and Huijing, P.A.(1990). Eur J Appl Physiol, 61: 453-460