Having examined the molecular structure of the myofibril, let us know consider the series of events by which the increase in sarcoplasmic calcium (produced by the muscle action potential) causes myofibril to contract. The general consensus of opinion is that the process involves the repetitive formation and then breakdown of bonds between the thin filaments and the cross-bridges on the thick filaments. Of course this all occurs at the molecular level, so we cannot observe the process directly. Therefore, the molecular changes underlying some of the steps remain speculative (which is why the process is sometimes referred to as the sliding-filament theory).
When a muscle fiber contracts, the overlapping thick and thin filaments in each sarcomere 'slide' past each other, propelled by movements of the myosin cross-bridges.
During this shortening of the sarcomeres, there is actually no change in the lengths of either the thick or thin filaments.
Rather, as a result of the filaments sliding past each other, the entire sarcomere shortens in length. Typically duringmuscle movement, one end of the muscle remains at a fixed position, while the other shortens toward the fixed position.
As for excitation-contraction coupling, we will consider the sliding-filament mechanism in a number of distinct stages. Use the key below to identify all the elements in the subsequent figures.
A. Rest:
Prior to any action potential arriving in the muscle fibre the sarcoplasmic Ca2+ concentration is 
very low. Tropomyosin molecules cover the myosin-binding sites on the actin molecules, thereby preventing the myosin cross-bridges from binding to actin.
The cross-bridges are, however, in an energised or 'primed' state as a result of the splitting of ATP molecules. The products of this reaction (ADP and inorganic phosphate) remain bound to the myosin cross-bridges.
This energy storage in myosin is analogous to the storage of potential chemical energy in a stretched spring (i.e., energy has been expended in stretching the spring and remains stored in the spring).
B. Binding:
As a result of the release of calcium from the sarcoplasmic reticulum, the sarcoplasmic Ca2+concentration increases to 10-4M and some of this Ca2+ binds to the calcium binding sites on the troponin complex.
This binding changes the shape of the troponin complex, which in turn displaces the tropomyosin molecules from the myosin-binding sites on the actin molecules.
This process then allows the myosin cross-bridges to bind to the thin filaments. The thick and thin filaments are now coupled.
C. Power Stroke:
The myosin cross-bridge swings over, and binds weakly to a new actin molecule, and is now at an ange of 90 degree relative to the thin filaments. Inorganic phosphate released in this process initiates the 'power stroke', whereby the myosin cross-bridge rotates on its hinge, pushing the actin filament past it.
The net effect of this step is that the sarcomere length (the distance between two Z-lines) has decreased because the same process is repeated on both ends of the thick filament.
At the end of this stage the ADP (which is still bound to the ATPase) is released.
D. Detachment:
During the cross-bridge movement, myosin is bound very firmly to actin. This linkage must be broken to allow the cross-bridge to be primed again and repeat the cycle.
The release of the ADP from the ATPase allows a new ATP molecule to bind to myosin, which in turn breaks the link between actin and myosin, allowing them to separate.
If there is still Ca2+ in the sarcoplasm then the tropomyosin filament will still be displaced, the cross bridges are able to bind again to the thin filaments, and the whole cycle is repeated again.
E. Relaxation:
However if there is no Ca2+ available for binding, the troponin complex will return to its original shape and the tropomyosin filaments will resume their blocking position over the myosin binding sites on the thin filaments.
As a consequence, the thick and thin filaments are prevented from binding and will simply slide past each other resulting in the sarcomere length increasing. Note that this means that whereas the contraction of the myofibril requires energy, the relaxation of the myofibril is passive (i.e., does not require energy).
The animation on the right shows ONE complete cross-bridge cycle which begins with the influx of Ca2+ and finishes with the cross-bridge being "primed" again by the break down of ATP. In reality, each cycle only decreases the sarcomere length by approximately 20 nm (around 1% of its total length), whereas a single action potential causes a sarcomere to shorten by around 40%. Obviously this means that the cross-bridge cycle is repeated many times following just a single action potential.
After describing the rather intricate technique that skeletal muscle fibres use to contract, it perhaps seems a little out of place to talk about what happens after we die and start pushing up daisies. However, what happens to our skeletal muscles after we die does highlight some very important features of the molecular mechanism of muscle contraction.
Rigor mortis is the stiffness which skeletal muscles begin to exhibit about four hours after death, reaches a peak after about 12 hours, and subsides after about 2-3 days.
The stiffness is thought to be due to the leakage of Ca2+ into the sarcoplasm from both the extracellular fluid and the sarcoplasmic reticulum, as the cells die and are unable to maintain the rather large differences in ion concentrations which are present in these compartments during life.
Some of this calcium binds to the troponin complex, displaces the tropomyosin filament, allows binding of the cross bridges and initiates the power stroke which results in the muscle fibres shortening.
However you will recall from the description of the sliding-filament mechanism that at this stage ATP is required to bind to the cross bridges to detach the thick filaments from the thin filaments.
Of course when you are dead there isn't any ATP lying around, and so the muscle fibres remain in this contracted state until the molecular structure of the filaments starts to decompose. Not very pleasant I guess but it does highlight (1) the importance of Ca2+ in triggering the process, (2) the fact that the cross bridges are primed before cross-bridge binding occurs and (3) that ATP is required for cross bridge detachment.