A Comparison of Different Methods for Assessing Plyometric Ability During Jumps

Abstract

PURPOSE: To compare different methods for assessing plyometric ability during countermovement (CMJ) and drop jumps (DJ) from different heights. METHODS: Twelve resistance-trained men (age: 21.8 ± 1.7 years; height: 1.81 ± 0.06 m; mass: 85.1 ± 8.6 kg) performed CMJ and DJ from heights of 0.40 m, 0.60 m, and 0.80 m. Force plates recorded the ground reaction force from which the descent (absorption phase) and ascent (propulsive phase) of the center of mass during ground contact was determined. Jump height (JH), vertical stiffness (VSTIFF) and normalized work (WNORM), power output (PONORM), and impulse (INORM) during the absorption and propulsion phases were calculated. Plyometric ability was assessed using the modified reactive strength index (RSIMOD) and four indices using propulsion time (PTI), propulsive work (PWI), propulsive power (PPI), and propulsive impulse (PII). Analysis of variance was used to assess the differences in the mechanical variables and the plyometric indices across the four jump conditions. Correlations were used to assess the relationships between the plyometric indices and JH. RESULTS: JH (mean differences: 0.03 – 0.06 m; p=0.007) and VSTIFF (mean differences: 0.69 – 0.93 kN/m, p<0.001) were greater during CMJ compared to the DJ conditions. The mechanical variables during the absorption phase were greatest during the highest DJ (WNORM mean differences: 1.6 – 10.0 J/kg, p<0.001; PONORM mean differences: 4.9 – 31.3 W/kg, p<0.001; INORM mean differences: 0.36 – 2.52 m/s, p<0.001), while WNORM and INORM during the propulsive phase were greatest during the CMJ (WNORM mean differences: 0.2 – 1.1 J/kg, p=0.020; INORM mean differences: 0.09 – 0.21 m/s, p<0.001) with no differences in PONORM (p>0.05). RSIMOD increased across the four jumps and was greatest at the highest DJ condition (mean differences: 0.003 – 0.150, p<0.001). The greatest values for the other indices were reported for the CMJ (p<0.001). The largest correlations with JH were found for PII (r = 0.958 – 0.993). CONCLUSION: RSIMOD does not reflect the changes in mechanical variables during the propulsion phases of CMJ and DJ and may not therefore provide an accurate assessment of the ability to utilize the stretch-shortening cycle during different jumps. Practitioners should consider using PII as a measure of plyometric ability.