The myosin heads have attached, ADP and Pi, products of the breakdown of ATP [step 3 in cycle at bottom], which remain bound to the heads, preventing further ATPase activity in the resting state (end product inhibition). The myosin heads can combine with weakly reactive sites on the actin (thin) filaments (pale blue below) [see step 4 of cycle at bottom]. Adjacent, strongly reactive sites (red) are blocked by the tropomyosin molecule (black), which is pulled and held in position over the active sites by the troponin molecules (which bind to both tropomyosin and actin) (See B below).
With the rise in sarcoplasmic concentration, Ca2+ combines with troponin causing it to release the tropomyosin from its blocking position over the active sites on the actin chain (C at left). The myosin heads are now free to combine with the strongly reactive sites on the actin filaments. Binding of any myosin heads synergistically enhance exposure of the strong binding sites (D at left). [See step 5 of cycle].
The combination of the myosin head with the strongly reactive sites on the actin, reduces the affinity of the myosin heads for the Pi molecules, which dissociate from the myosin heads, causing the heads to rotate on their necks, pulling the attached actin filaments inward by a small amount (1% of the total possible shortening distance). [See step 6 of cycle]
Next, at a rate inversely proportional to the force on the muscle, the ADP dissociates from the active site [steps 7 & 8 of cycle], permitting the myosin head to bind to any available ATP. Binding of ATP to the actin-myosin head complex, causes the release of the myosin head from the actin binding site, and its return to its original angle [steps 1 & 2 of cycle]. The actin is however, held in position by other myosin heads since all do not release at once. The released ATP-myosin head complex, now can't recombine with another actin site, until the ATP has been cleaved to form ADP and Pi [step 3]. One factor that governs the speed of shortening, therefore, will be the rate at which the myosin head can break down ATP (i.e. its ATPase potency). Clearly, it will take about 100 pulls of many heads in parallel, with an ATP being used up on each cycle of each head, for a sarcomere to shorten fully. This cycling will continue as long as the Ca2+ concentration remains high in the sarcoplasm, and ATP is available. As soon as the Ca2+ levels fall sufficiently, Ca2+ will dissociate from the troponin, and the tropomyosin/troponin complex will return to the blocking position. Relaxation will then occur as the myosin heads with ADP and Pi attached are unable to access the active sites on the actin.
Note that ATP is required to cleave the avidly formed bond between actin and myosin. In the absence of ATP the cross linking becomes fixed, and the muscle becomes rigid and inextensible. This condition is seen shortly after death when the muscles' energy supply is used up, and is known as rigor mortis.