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

The forces produced during stretches of passive and activated muscles, and isometric force deficits after stretching of activated muscles were examined 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 the production of collagen cross-links. The relative weights of the plantar flexor 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 chromatography. At an ankle position of 90 degrees, groups had similar forces at 5, 10, 20, 40, 60 and 80 Hz before stretching. Forces at 40 degrees with stretches of the passive muscles (five times from 90 degrees to 40 degrees) were lower for all stretches in BAPN-injected rats (P < 0.05). Isometric force deficits resulting from stretches of activated muscles (80 Hz, 20 times from 90 degrees to 40 degrees, rest intervals 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-injected 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 force deficits only at low stimulation frequencies.