Explosive Leg Press Stretch-Shortening Potentiation

Abstract

The importance of understanding how the stretch-shortening cycle (SSC) may enhance muscle performance over concentric-only (CO) contractions has great relevance to sport scientists, coaches, and athletes. Potentiation is the enhancement that occurs in the concentric phase of the SSC as compared to performing concentric-only contractions (contractions with no immediate preceding eccentric contractions). PURPOSE: To determine ballistic leg press potentiation of force, acceleration, velocity, and power at six leg press sled time intervals across the concentric range of motion (ROM) and at ten time intervals across the initial 200 ms of the concentric phase of contraction. We also sought to determine relationships between specific potentiation variables and the time of the amortization phase and eccentric force during the last 100 ms of the eccentric phase of the SSC. METHODS: Twenty-one male competitive distance runners (age: 31 ± 5 yrs; weight: 80 ± 11 kg) performed SSC and CO leg press ballistic throws with a load of 150% bodyweight on a 35 degree angle leg press machine. The machine connected to a cable-extension linear position transducer that was interfaced with a data acquisition system utilizing custom software. After familiarization, warm-ups, and practice trials subjects performed 3 SSC and 3 CO leg press ballistic throws with a large amplitude concentric ROM at the knee from 90 to 0 degrees. Test-retest evaluations of muscle performance variables for this study have a coefficient of variation ranging from 1.7% to 4.3%. RESULTS: Across the full concentric ROM, differences from zero (p<0.05) existed with 1) potentiated mean force and acceleration only at the first 1/6th time interval over the concentric phase, and 2) potentiated mean velocity and power at all leg press time intervals over the entire concentric phase with the exception of power over the last 1/6 time interval (closest to full knee extension). Across the initial 200ms of the concentric phase, differences from zero (p<0.05) existed with 1) potentiated mean force and acceleration up until the time intervals greater than 160 ms, and 2) potentiated mean velocity and power for all time intervals over the first 200ms of the concentric phase. There were significant correlations between eccentric mean force measured during the last 100ms of the eccentric phase and potentiated average mean force during the initial 200ms of the concentric phase (r = 0.44, p<0.05) and potentiated average mean power across the full concentric ROM (r = 0.62, p < 0.01). CONCLUSIONS: Findings indicate large amplitude SSC leg press ballistic throws potentiate mean force, acceleration, velocity, and power early in the concentric ROM, and that potentiated mean velocity and power are conserved throughout the full concentric ROM with the exception of power at the very end of the range. Eccentric mean force measured during the last 100ms of the stretching phase is positively related to potentiated average mean force during the initial 200ms of the concentric phase and potentiated average mean power across the full concentric ROM. Practical Applications: To facilitate positive effects on potentiated concentric force and power during large amplitude SSC movements like an explosive leg press exercise, it may be beneficial to specifically train to improve force late in the eccentric phase of the SSC.