Performance of plantar flexor muscles with eccentric and isometric contractions in intact rats

Purpose: To examine the changes in performance of active plantar flexor mus cles of rats by controlled dorsiflexion (i.e., stretching of muscles) at tw o angular velocities. Methods: Repeated stretches (30) at two velocities of ankle rotation [slow stretch (0.87 rad.s(-1) (i.e., 50 degrees.s(-1))), fa st stretch (10.47 rad.s(-1) (i.e., 600 degrees s(-1)))] were superimposed o n maximally active muscles from an ankle position of 1.57 rad to 0.70 rad ( i.e., from 90 degrees to 40 degrees). Repeated isometric contractions (30) of the same duration (1900 ms) and rest interval (3 min) were performed at 1.13 rad (i.e., 65 degrees). Performance was assessed by measuring the isom etric torque at ankle positions of 1.57 and 0.70 rad, work during concentri c contractions [range of motion 1.22 rad (i.e., 70 degrees)], and the time to produce 50% of the maximal isometric torque. Results: Thirty isometric c ontractions resulted in a linear reduction in torque (total deficit of 13.8 % at 1.57 rad), whereas for slow and fast stretches, half of the total, non linear deficit at 1.57 rad (about 30%) was completed after six stretches. I ncreases in half contraction times were larger for stretches than for isome tric contractions. Reductions in isometric torque were greater at an ankle position of 1.57 rad than at 0.70 rad. One hour of rest after the repeated stretches and isometric contractions did not restore muscle performance. Co nclusions: Isometric contractions of skeletal muscle can create a torque de ficit which is much less than that after stretches. Repeated fast and slow stretches resulted in similar torque deficits which did not recover after a rest period of 1 h.