Does force enhancement occur during the stretch-shorten cycle?

Abstract

Recently, research into mechanisms for muscle force enhancement has gained popularity. Force enhancement is a term used for the increase in force output of muscle above that predicted by the muscle tension-length relationship, which results from active muscle stretch. Due to its dependence on active stretch it is plausible that the force enhancement property of muscle may play a role in stretch-shorten cycle movements, specifically vertical jump. Therefore the purpose of this project was: 1) To establish a curve relating force and torque to the magnitude of leg extension and lower extremity joint angles, respectively, in upright multi-articular leg extension, 2) To evaluate endpoint force enhancement as a result of a novel countermovement, the traditional countermovement, and no-countermovement, 3) To evaluate torque enhancement in each stretch-shorten cycle (SSC) condition, 4) To evaluate electromechanical delay (EMD) during each SSC condition, and 5) To examine jump height, take-off velocity, rate of force development, and impulse in each SSC condition. Results indicated that force enhancement but not torque enhancement occurs in SSC conditions. EMD was not changed even with higher rate of force development (RFD) in the self-induced drop (SD) condition. Peak normalized electromyography (EMG) was also similar across conditions. Finally, countermovement (CM) outperformed the SD and squat jump (SJ) conditions in jump height, take-off velocity and impulse, but not SD in RFD. Results suggest that high eccentric load does not necessarily alter torque enhancement, EMD or performance in SSC movements.