ATP prepares myosin for binding with actin by moving it to a
higher-energy state and a "cocked" position.
Once the myosin forms a cross-bridge with actin, the Pi disassociates
and the myosin undergoes the power stroke, reaching a lower energy
state when the sarcomere shortens.
ATP must bind to myosin to break the cross-bridge and enable the
myosin to rebind to actin at the next muscle contraction.
The muscle contraction cycle is triggered by calcium ions binding to
the protein complex troponin, exposing the active-binding sites on the
actin. As soon as the actin-binding sites are uncovered, the
high-energy myosin head bridges the gap, forming a cross-bridge. Once
myosin binds to the actin, the Pi is released, and the myosin
undergoes a conformational change to a lower energy state. As myosin
expends the energy, it moves through the "power stroke," pulling the
actin filament toward the M-line. When the actin is pulled
approximately 10 nm toward the M-line, the sarcomere shortens and the
muscle contracts. At the end of the power stroke, the myosin is in a
After the power stroke, ADP is released, but the cross-bridge formed
is still in place. ATP then binds to myosin, moving the myosin to its
high-energy state, releasing the myosin head from the actin active
site. ATP can then attach to myosin, which allows the cross-bridge
cycle to start again; further muscle contraction can occur. Therefore,
without ATP, muscles would remain in their contracted state, rather
than their relaxed state.
Summary from: https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-musculoskeletal-system-38/muscle-contraction-and-locomotion-218/atp-and-muscle-contraction-826-12069/
Essentially look up the cross bridge cycle (http://muscle.ucsd.edu/musintro/bridge.shtml)
I would also suggest reading it from the master himself here:
Huxley, A. F. "Muscular contraction." The Journal of physiology 243.1 (1974): 1-43.