Force deficits after stretches of activated rat muscle-tendon complex withreduced collagen cross-linking

The forces produced during stretches of passive and activated muscles, and isometric force deficits after stretching of activated muscles were examine d in rat plantor flexor muscle-tendon complexes with reduced collagen cross -links (pyridinoline). Female Sprague-Dawley rats (n = 6, age 87 days) were injected twice daily for 43 days with beta -aminopropionitrile (BAPN, 333 mg/kg/day i.p.), an inhibitor of lysyl oxidase, which is responsible for th e production of collagen cross-links. The relative weights of the plantar f lexor muscles were similar for BAPN and saline-injected (control, C) rats ( n = 6). Pyridinoline was lower in the tendon (22.9%), and in the plantaris (17.1%), and soleus (7.4%) muscles (P < 0.05), with no changes observed in collagen content (hydroxyproline), as determined by high-pressure liquid ch romatography. At an ankle position of 90, groups had similar forces at 5, 1 0, 20, 40, 60 and 80 Hz before stretching. Forces at 40 with stretches of t he passive muscles (five times from 90 to 40) were lower for all stretches in BAPN-injected rats (P < 0.05). Isometric force deficits resulting from s tretches of activated muscles (80 Hz, 20 times from 90 to 40, rest interval s 3 min) followed similar courses for BAPN-injected and C rats, and were 51 .1 (2.4)% (C) and 54.7 (4.6) % (BAPN) before the last stretch. After 1 h of rest, isometric force deficits were 26% and 29% larger at 10 Hz and 5 Hz, respectively, in BAPN-treated rats (P < 0.05). The reduction in BAPN-inject ed collagen cross-linking of the skeletal muscle-tendon complex reduced the forces produced during stretches without muscle stimulation (i.e. passive stretch), and stretching of activated muscles produced larger isometric for ce deficits only at low stimulation frequencies.