Initial Changes in Jump Height and Lower Extremity EMG across Five Consecutive Days in Novices

Abstract

Practice is an important variable for improving motor skill performance; however, little is known about either the time course or causal mechanisms underlying changes in motor skill performance with practice. PURPOSE: The purpose was to investigate the time course of changes in vertical jump height during countermovement jumps and determine if changes in jump height were associated with alterations in either muscular activity or the mechanical parameters associated with jumping performance. METHODS: Ten novice (mean Day 1 jump height = 24.9 cm) college-age participants performed 10 jumps over five consecutive days from two Kistler force plates (1 kHz). Knowledge of results was given after each trial in an effort to evoke maximum efforts. Jump height, average vertical ground reaction force (VGRF), and duration of the takeoff phase (tTAKEOFF) were determined for the six intermediate trials daily and analyzed using separate 5×6 (days × trials) repeated measures ANO VAs. EMG data were simultaneously collected on days 1, 2 and 5 from five leg muscles (gluteus maximus, rectus femoris, vastus lateralis, biceps femoris and gastrocnemius) using a Noraxon telemetered EMG system (1 kHz) in accordance with ISEK standards. EMG onset relative to takeoff was determined for each muscle in each trial using visual estimation techniques and multiple experimenters. The EMG onset data were analyzed using separate 3x6 repeated measures ANO VAs. RESULTS: Jump height increased across days (F = 3.72, p<0.05) and post hoc analysis (pairwise t-tests) indicated that jump height on days 2 through 5 significantly (p<0.05) differed from Day 1 (average increase = 3.8%). Average VGRF and tTAKEOFF did not differ across days or trials. Vastus lateralis activity occurred earlier in the takeoff phase across days (Day 1 = 75%, Day 2 = 78% & Day 5 = 81% before takeoff, respectively); however, this change was significant only at the p<0.10 level. The onset of EMG activity did not change across days for the other muscles. Day by Trial interactions were not present for any variable. CONCLUSIONS: Significant increases in jump height occurred during the first 24 hours of the study, but no further increases in jump height occurred. As average VGRF, tTAKEOFF and the onset of muscle activity for four of the five muscles tested did not change across days, a common mechanism for increased jump height was not present. The results suggest that the subjects adapted differently across days to improve their jump performance.