The role of the second myosin head. 

 

It has generally been believed that either the just one head binds to actin at a time or that the two heads bind independently.  While both heads are bound in rigor muscle, the evidence for similar binding in active muscle or in motility assays is equivocal. The most likely scenario in muscle fibers is that faster attachment of the second head may occur immediately after first head bound to actin detaches. However, some evidence suggests that simultanous binding of both myosin heads can be maintained in protocols where only one head is expected to make a working stroke. There is also new evidence that the second head attaches fairly rapidly to actin after a step stretch. This finding supports the hypothesis that distortion of the head during lengthening is sufficient to promote binding of the second head.