MECHANICAL FACTORS IN THE INITIATION OF ECCENTRIC CONTRACTION-INDUCEDINJURY IN RAT SOLEUS MUSCLE

1. Mechanical factor(s) associated with the initiation of eccentric co ntraction-induced muscle injury were investigated in isolated rat sole us muscles (n = 180; 42 protocols with 4-6 muscles per protocol). Five eccentric contractions were performed with 4 min between contractions . Three levels of peak eccentric contraction force (100, 125 and 150% of pre-injury maximal isometric tetanic tension, P0), length change (0 .1, 0.2 and 0.3 muscle length, L0) and lengthening velocity (0.5, 1-0 and 1.5 L0/s) were utilized. Force was varied with stimulation frequen cy (10-150 Hz). The eccentric contractions were initiated at muscle le ngths of 0.85 or 0.90 L0. Following the fifth eccentric contraction, t he muscle was incubated in Krebs-Ringer buffer for 60 min. Peak isomet ric twitch tension (P(T)), P0, maximal rate of tension development (+d P/dt), maximal rate of relaxation (-dP/dt), and creatine kinase (CK) r elease were measured prior to the five eccentric contractions and at 1 5 min intervals during the incubation period. Total muscle [Ca2+] was measured after 60 min incubation. 2. The mean (+/-S.E.M.) initial decl ine in P0 for the muscles performing the most injurious protocol was 1 3.6 +/- 4-8 % (n = 6); P0 in control muscles immediately following per formance of five isometric contractions was elevated 1-2 +/- 1.0 % (n = 8). These means were different at probability, p = 0.005. Mean [ATP] in muscles immediately following the isometric control and most injur ious protocols, respectively, were 16.30 +/- 1.49 and 19.84 +/- 1.38 m umol/g dry wt (p = 0-229). 3. Decrements in P0, P(T), + dP/dt, and - d P/dt immediately after the injury protocol were related most closely t o the peak forces produced during the eccentric contractions; greater initial declines in P0, + dP/dt and - dP/dt were also observed at high er lengthening velocities independent of peak force. Slow declines in P0 and - dP/dt during the 60 min incubation following the injury proto col were greatest for muscles performing contractions at the longer in itial length. CK release was independent of all mechanical factors wit h the exception of lengthening velocity. CK activity at 45 and 60 min into the incubation period was greater for muscles lengthened at the h ighest velocity used (1.5 L0/s). Mean total muscle [Ca2+] for muscles performing the eccentric contractions was elevated by 38 % over isomet ric control muscles but the elevation was unrelated to any of the four mechanical factors. 4. These data support the hypothesis that eccentr ic contraction-induced injury is initiated by mechanical factors, with muscle tension playing the dominant role. They also demonstrate that specific mechanical factors differentially affect the various injury c riteria, i.e. reductions in contractile performance were most related to produced forces, and CK release was most related to lengthening vel ocity.