6) The myosin head releases its ADP and is again tightly bound to actin (as in step 1) ready for the next ATP and the next contraction cycle. Complete shortening of the muscle fiber is accomplished by repeated cycles of 1-6 to successively pull in the thin filaments (like pulling a rope hand over hand). At any one time during the contraction, some of the myosin heads are bound to G-actin and some are not so that the filaments are not likely to totally separate from each other. How are tropomyosin and troponin involved in regulating muscle contraction?