Acute changes in jump-test scores after a submaximal aerobic exercise bout

The percentage of skeletal muscle fast-twitch fibers (FTF) positively correlates with the scores of squat jump (SJ) and counter movement jump (CMJ) power-tests (Bosco and Komi, 1976). Exercise-induced selective glycogen depletion of slow-twitch fibers (STF) and an enhanced recruitment of FTF during subsequent dynamic exercise has been recently demonstrated (Krustrup, 2004). Since few studies evaluated changes in jump-test scores following a single glycogen-depleting exercise bout, the aim of this study was to compare acute changes of SJ and CMJ test scores before and after a 40-min exercise bout performed at an intensity promoting a high selective glycogen depletion of STF. Seven well trained (at least 3 years; 7 training sessions each week) middle distance runners (26.6±6 yr; 66.7±7 kg) underwent a maximal incremental running test to determine lactate threshold speed (LTS). Treadmill speed was plotted against blood lactate concentration [La-] obtained during the test and speed at the blood [La-] of 4 mmol•L-1 (Mader, 1976) was recorded as LTS. Subjects rested for 3 days, then underwent the SJ and the CMJ tests, before (t0) and immediately after (t1) an acute 40-min exercise bout performed at their own LTS. LTS was chosen as the optimal exercise intensity because: i) have been stated (see Weineck, 2001) that endurance trained athletes reach their LTS at approximately 80% of maximal oxygen consumption (V’O2max); and ii) have been reported (Gollnick, 1974) that endurance exercises performed at nearby 85% of V’O2max triggers selective glycogen depletion of STF. The Wilcoxon (matched-pair) non-parametric statistics was used to compare t0 vs. t1 test results. A p<.05 was considered significant. Results evidenced, at t1, significant increases in both SJ (+8.1%) and CMJ (+5.6%) power-tests. In conclusion, an acute exercise bout, depleting selectively STF glycogen content, increases explosive strength, possibly via an enhanced recruitment of FTF.