The cross bridge cycle• The sliding of the filaments depends on mechanical work performed by the globular heads of myosin, work that uses chemical energy contained in adenosine triphosphate (ATP). • The actions of the myosin heads are regulated by the cross bridge cycle, a sequence of detachment, activation, and attachment • In each cycle a globular head undergoes a displacement of 5 to 10 nm. Contractile activity continues as long as Ca2+ and ATP are present in the cytoplasm in sufficient amounts.