Muscle contraction cannot occur without calcium ions and the ATP that provides energy. When the nerve impulse reaches the neuromuscular junction, it travels through the sarcolemma into the transverse tubule system, triggering the channels for calcium ions release to open: CA2+ enters into the sarcoplasm, binding the troponin molecules in the thin filaments of the muscle fiber. The troponim changes shape and moves tropomyosin away from the myosin-binding sites on actin, leaving them uncovered. The myosin heads also contain ATPase that splits ATP into ADP and P. This splitting process generates mechanical energy of motion to the myosin heads which in turn start acting like crossbridges, rotating, then detaching from actin and getting ready to bind again, acting like springs. This is the sliding process. In other words, when the level of calcium ions is high enough and there is enough ATP, the myosin heads of the thick filaments pull on the thin filaments inside the muscle fiber, and the thin filaments slide toward the center of a sarcomere, narrowing the I bands and H zones until they disappear. This is the moment of maximum contraction. The contraction goes on as long as there is enough calcium and ATP.
Tortora, G.J., Derrickson B. (2010). Introduction to the Human Body: The Essentials of Anatomy and Physiology (8th Ed.).
: John Wiley & Sons, Inc. Hoboken, NJ