Persistent mechanical effects of decreasing length during isometric contraction of ovarian ligament smooth muscle.

When isometrically-contracting strips of ovarian ligament smooth muscle were suddenly shortened by 10-20% of their length, force fell rapidly and then redeveloped along an exponential time course. The amount of force recovered fell short of that expected in an isometric contraction at the new length, and this force deficit was proportional to the magnitude of the length step (approximately 80% of force was recovered after a 10% shortening). A sudden imposed decrease in length was more effective in reducing subsequent force than was isotonic shortening. Early in the recovery phase the stiffness of the muscle was decreased to less than its expected value; stiffness recovered to expected levels on an exponential time course approximately three to four times faster than force recovery itself. Force-velocity curves made during the redevelopment phase showed a reduced maximal force (Fmax) and an increased maximal shortening velocity (Vmax) when compared with control contractions matched in force, time and length. The curves crossed at approximately 10% of Fmax. During isometric relaxation the muscles showed an increase in their expected stiffness; prior imposed shortening (as above) reduced the relaxation stiffness increase in proportion to the prior force deficit. The persistent effects of early events on the later phases of the contraction, as well as the increase in shortening velocity with very light loads, are consistent with the hypothesis that the sudden shortening detaches crossbridges and that same fail to reattach during force recovery. During isotonic shortening of unperturbed muscle some slowly-cycling crossbridges may act as an internal load and reduce shortening velocity.