Sarcomere overextension reduces stretch-induced tension loss in myofibrils of rabbit psoas

Stretch-induced damage to skeletal muscles results in loss of isometric tension. Although there is no direct evidence, loss of tension has been implicitly assumed to be the consequence of permanent loss of myofilament overlap in some sarcomeres ('sarcomere overextension'). Using isolated myofibrils of rabbit psoas muscle (n=38; 6 control and 32 test specimens) at 12-15°C, we directly tested the idea that loss of tension following stretch is caused by sarcomere overextension. Experimental myofibrils were maximally activated at the edge of the descending limb (sarcomere length ～ 2.9 μm) of the sarcomere length-tension relationship and then stretched by 1 μm sarcomere(-1) at a constant speed of 0.1 μms(-1)sarcomere(-1) to result in an average strain of 33.6 ± 0.9% (mean ± 1 SE). Myofibrils were immediately returned to the original lengths and relaxed. Isometric tension measured in a subsequent re-activation 3-5 min later was reduced by 24.6 ± 1.5% from its original value. In 22 out of the 32 test specimens, all sarcomeres maintained myofilament overlap, while in 10 myofibrils one or two sarcomeres were stretched permanently beyond myofilament overlap (>4.0 μm), and thus exhibited overextended sarcomeres. Loss of tension following stretch was significantly smaller in myofibrils with overextended sarcomeres compared to myofibrils with no overextended sarcomeres (19.5 ± 2.3% and 27.1 ± 1.8%, respectively; p=0.017). Combined, these results suggest that the loss of tension associated with stretch-induced damage can occur in the absence of sarcomere overextension and that sarcomere overextension limits rather than causes stretch-induced tension loss.