1. Myosin heads cannot bind to actin filaments
troponin
tropomyosin
actin filament
ADP
myosin complex Pi
myosin filament
At rest, the actin-myosin binding site is blocked by tropomyosin, held in place by troponin
Myosin heads cannot bind to actin filaments

2. Ca2+ binds to troponin, changing its shape
Tropomyosin is pulled out of the binding site
Myosin head can bind – bond is an actin-myosin cross bridge

3. Ca2+ activates ATPase, breaking down ATP to ADP + Pi
Energy provided moves myosin head, pulling the actin filament along in a ratchet motion

4. Free ATP binds to head, changing shape
Actin-myosin cross bridge breaks
ATP is hydrolysed, and head returns to original shape

5. With continued stimulation the cycle is repeated

6. If stimulation ceases, Ca2+ is pumped back into sarcoplasmic reticulum
Troponin and tropomyosin return to original positions
Muscle fibre is relaxed

7. Supplying energy for muscle contraction
Energy is required for the movement of the myosin heads that make the actin filaments slide and for the return of calcium ions into the reticulum of the sarcoplasm.
The energy is provided by the breakdown of ATP to ADP and inorganic phosphate. Resting muscles only store enough ATP to maintain contraction for a short time.
Muscle fibres contain large numbers of mitochondria that can generate ATP by respiration of glucose but this process is relatively slow. (even anaerobic respiration takes time to supply ATP)
Muscle fibres also store phosphocreatine.
This can be used to produce ATP very rapidly.

8. Using phosphocreatine to generate ATP
Phosphocreatine is able to transfer a phosphate ion to ADP and is therefore able to replace the the ATP that has been broken down.
ADP + Phosphocreatine  ATP + Creatine
The amount of phospho creatine available is limited but there is enough to keep the muscles going until there is more ATP available from the mitochondria.
Nevertheless this means that really intense muscle activity can only be maintained for a short time eg 10 secs for a 100m sprint.