What Powers Muscle Contraction?

In order for muscle fibers to contract, a lot of ATP must be used (Figure 2.8). Some of the energy released from ATP is used to power the contraction. Interestingly, ATP is also necessary for a contracted muscle cell to “relax” as well. When the muscle is no longer being stimulated, ATP helps the thick and thin filaments to dissociate from each other so

that each sarcomere can return to a relaxed (or unstimulated) position. In addition, ATP is necessary to pump calcium out of intracellular fluid of the muscle fiber. Calcium is either pumped out of the cell or more likely into sarcoplasmic reticulum organelles.
 

If ATP is deficient, muscle fibers become locked in a contracted state called rigor. Rigor mortis occurs when the human body dies as the integrity of muscle cell membranes decrease. This allows calcium to leak into the contracting regions of muscle fibers from the extracellular fluid and from within the sarcoplasmic reticulum. As a result, calcium bathes myofibrils and contraction is invoked. Usually there is enough ATP in these dying cells to power the contraction. The dying cell then remains locked in a contracted state.