RESUMEN
The biceps femoris was analyzed in 10 volunteers during knee extension and flexion with the foot in normal plantar flexion and dorsal flexion positions. Hewlett-Packard surface electrodes, an electromyographic signal amplifier, a computer equipped with an A/D conversion plaque (Model CAD 10/26), software especially designed to record and analyze the signals, Horizontal Leg Press equipped with four platforms for the feet (2 superior and 2 inferior) which acted as a support for the movements and electrogoniometers were used. The biceps femoris muscle acted as an antagonist in knee extension, primary motor of hip extension and flexor of knee joint. The upper platforms of the Leg Press required the strongest activity from the biceps femoris in relation to the lower platforms. As for bilateral action, the activity of the right biceps femoris predominated over the contralateral muscle action in all plataforms. Foot positions did not interfere in biceps femoris action.