Hip flexion angle affects longitudinal muscle activity of the rectus femoris in leg extension exercise

Purpose We investigated the effect of the hip flexion angle (HFA) on the longitudinal muscle activity of the rectus femoris (RF) during leg extension exercise (LEE). Methods We conducted an acute study in a specific population. Nine male bodybuilders performed isotonic LEE using a leg extension machine at three different HFAs: 0°, 40°, and 80°. Participants extended their knees from 90° to 0° at each HFA setting for four sets of ten repetitions at 70% of the one-repetition maximum. The transverse relaxation time (T2) of the RF was measured before and after LEE using magnetic resonance imaging. We analyzed the rate of change in the T2 value in the proximal, middle, and distal regions of the RF. The subjective sensation of muscle contraction of the quadriceps was measured using a numerical rating scale (NRS) and compared with the T2 value which was the objective index. Results At 80°, the T2 value in the middle RF was lower than that in the distal RF ( p