SPINAL SHRINKAGE IN UNLOADED AND LOADED DROP-JUMPING

Plyometric activities, engaging the muscle in a stretch-shortening cyc le, are widely used in athletic training. one such plyometric exercise is drop-jumping, where the athlete drops from a raised platform and i mmediately on landing performs a maximal vertical jump. These actions are also performed with the athlete externally loaded by the addition of weights to provide greater resistance. Exercises which involve repe ated impacts have been shown to give rise to a loss of stature (shrink age) which can be measured by means of a sensitive stadiometer. This s tudy examined the shrinkage induced by unloaded and loaded drop-jumpin g from a height of 26 cm. Eight male subjects, aged 20-24 years, perfo rmed the test protocol three times, at the same time of day on each oc casion. Fifty drop-jumps from a height of 26 cm were performed with no additional load and with a load of 8-5 kg carried in a weighted vest. The third condition was a standing trial where the subject stood for 10 min (the time taken to perform the jumps) wearing the weighted vest . Stature was measured before exercise, immediately after exercise and after a 20 min standing recovery. On a separate occasion the regimen was performed and the vertical reaction force was measured using a Kis tler force platform. The mean change in stature for the two jump condi tions showed shrinkages of 0.62 (+/- 0.43) mm for unloaded and 2.14 (/- 1.56) mm for the loaded (p < 0.05). The variance in shrinkage was g reater in the loaded case compared to the unloaded condition (p < 0.05 ) indicating a wider range of jumping strategies. Force platform data showed average peak vertical reaction forces of 3.90 (+/- 0.66) x body weight and 4.11 (+/- 0.54) x body weight for the unloaded and loaded conditions, respectively (p < 0.05). The rate of force loading rose fr om 20742 (+/- 4336) Ns-1 to 28089 (+/- 5116) Ns-1 (p < 0.05) when the weighted vest was added. There was no correlation between force data a nd the degree of shrinkage. Results reflect the greater physical stres s of loaded drop-jumping compared to unloaded.