Kinetic and Electromyographic Analysis of Single Repetition Constant and Variable Resistance Leg Press Actions

During a multi-joint leg press action, maximal force capabilities increase with increasing knee angle. In typical resistance exercises, constant resistance is used, which does not stress the neuromuscular system at large knee angles. The purpose of this study was to compare constant and variable resistance settings using a range of contraction loads and velocities. Nine healthy, untrained males performed single contractions using four different resistance settings (constant resistance, two settings that increased resistance at large knee angles and one setting that reduced resistance at large knee angles). Single contractions using 40%, 60%, and 80% of one repetition maximum with both steady-paced and explosive contractions were assessed. Concentric force, velocity, power, and EMG (from the vastus lateralis, vastus medialis, rectus femoris, and biceps femoris muscles) were assessed at 20° segments throughout the leg press range of motion (∼60-180°). Results showed that force and rmsEMG activity was greater (P<0.05) by increasing resistance during steady-paced contractions. During explosive contractions, higher velocities were producible (P<0.05) when resistance was reduced at large knee angles. The results of this study show that different resistance settings affect kinetic and electromyographic characteristics, which may lead to specific long term training-induced adaptations.