Shortening-induced force depression in electrically activated human quadriceps femoris following stretch-shortening cycle exercise

Abstract

When muscle is allowed to shorten during an active contraction, the maximum force that redevelops after shortening is smaller than the isometric force at the same muscle length without prior shortening. This phenomenon is referred to as force depression (FD) following shortening contractions. The purpose of this study was to investigate the effects of muscle fatigue on shortening-induced FD in electrically activated human quadriceps muscle. Healthy untrained men (n=8) performed isometric reference contractions (ISOM) and isometric–concentric–isometric (ICI) contractions, using maximal electrical stimulation. FD was assessed by comparing the steady-state isometric forces produced following active muscle shortening with the purely isometric reference forces obtained at the corresponding muscle length. In order to test for effects of fatigue on FD stretch-shortening cycle exercise (SSC) was carried out consisting of 100 drop jumps performed from 0.5 m with counter-movement to 90° in the knee with immediate maximal rebound, with 30 s interval between the jumps. Besides, muscle soreness as well as CK activity up to 168 h after SSC exercise was determined. SSC exercise induced very intense muscle pain, the CK activity in the blood within 72 h after SSC exercise had increased approximately up to 1350 U/L-1 (p<0.05, as compared to the pre-exercise value). Thus, muscle damage (as judged from indirect indicators) was severe. Although FD was observed in all subjects in fresh and fatigued muscle it was significantly reduced only at 10 min following SSC exercise (p<0.05) and later in recovery there were no changes in FD comparing to pre-exercise values. The detailed mechanisms underlying FD are still debated. This study adds the result that, the relative amount of FD does not appear to be affected by the force level in fatigued muscle. Therefore, it was concluded that in unfatigued and fatigued muscle FD was similar in electrically activated human quadriceps muscle. Much research will have to be performed before we understand how FD influences everyday life.