Possible Cause of Nonlinear Tension Rise in Activated Muscle Fiber during Stretching.

Tension in contracting muscle fiber under conditions of ramp stretching rapidly increases, but after reaching a critical stretch P _c sharply decreases. To find out the cause of these changes in muscle fiber tension, we stopped stretching before and after reaching P _c and left the fiber stretched for 50 msec. After rapid tension drop, the transient tension rise not accompanied by fiber stiffness increase was observed only in fibers heated to 25°C and stretched to P _c . Under other experimental conditions, this growth was absent. We suppose that stretch of the fiber to P _c induces transition of stereo-specifically attached myosin heads to pre-power stroke state and when the stretching is stopped, they make their step on actin and generate force. When the tension reaches P _c , all stereospecifically attached myosin heads turn out to be non-stereospecifically, or weakly attached to actin, and are unable to make the force-generating step.