The effect of 4-weeks of plyometric training on Reactive Strength Index and leg stiffness on cross fit athletes

BACKGROUND: Plyometric exercises, the main feature of which is stretching the muscle before its contraction, are known as jumping exercises and are based on maximizing muscle strength production in the shortest possible time in order to improve muscle speed and strength. The purpose of this study was to examine the effect of 4-week intervention program of plyometric exercises on Reactive Strength Index (RSI) and vertical stiffness on cross fit athletes.METHODS: Thirty-three participants with a mean age 30.41±7.05 years, body mass 75.32±11.91 kg, and body height 174.55±7.27 cm volunteered to participate in this study. Participants were randomly separated in two groups, experimental (EG=14) and control (CG=15) group. The experimental group consisted of athletes who followed the regular fitness training program and who also performed a specific program of plyometric exercises, while the control group included athletes who performed only the standard training program. One day before the inception of the intervention program, the participants were evaluated on the RSI and the vertical stiffness, performing five jumps with the maximum possible effort with the shortest possible contact time during the execution of the jumps, while the vertical stiffness was evaluated with the execution of 20 jumps of maximum intensity at a specific rate of 2.5 Hz on a Chronojump platform (Chronojump, Barcelona, Spain). The intervention training program lasted four weeks, three sessions per week, every 2 days always the same hours and included thirteen different plyometric exercises.RESULTS: No significant interaction effect was found for RSI (F=1.295, P=0.265, η2=0.046), and no significant main effect for group (P>0.05). On the contrary, significant main effect was found for time (F=9.114, P=0.005, η2=0.252). EG showed a slight decrease in RSI (P>0.05), however, CG revealed a significant reduction after the end of intervention program by 17.41% (P<0.022). No significant interaction effect was found for vertical stiffness (P>0.05). However, significant main effect was found for time (F=11.182, P=0.002) and significant main effect for group (F=4.828, P=0.037). Percentage improvement was 10.59% and 6.52% for the EG ad CG, respectively.CONCLUSIONS: The application of plyometric exercises causes an improvement in the explosive strength of the lower limbs in CrossFit athletes.