Effects of Trunk and Foot Positions on Electromyographic Activity and Co-contraction of Selected Lower Extremity Muscles During Leg-Press Resistance Training

The purpose of this study was to determine the effects of trunk and foot positions on electromyographic activity and co-contraction of selected lower extremity muscles during leg-press resistance training. 12 male powerlifters performed leg-press movement while the backrest of leg-press machine was adjusted at 15°, 20°, 25°, and 30° angles relative to the horizon. The feet were placed in three different positions namely, top, middle, and bottom of the foot-plate. Electromyography activity of the rectus femoris (RF), vastus medialis (VM), vastus lateralis (VL), biceps femoris (BF), and semitendinosus (ST) muscles were recorded while performing leg-press task at different trunk-feet conditions. The muscle co-contraction at the knee joint was also calculated. When the feet were placed on top of the foot-plate, RF had greater activity at 15°, 20°, and 25° compared to 30° backrest position. VM showed greater activity at 20° and 25° compared to 30°. VL showed greater activity at 20° and 25° compared to 30°. It was also more active at 20° than at 25° of backrest position. The ST and BF were more active at 20° compared to other positions. There was a slight co-contraction ratio difference at 20°-tap degrees condition compared to other conditions. According to the results, it is recommended that, for the recruitment of more motor units when using the 45-degree leg-press machine, the backrest of the machine be adjusted at a 20° angle with the feet placed at the top of the foot-plate.