Three different steps in the contraction relaxation require ATP :

1. Splitting of ATP by myosin ATPase provides the energy for the power stroke of the cross bridge.2. Binding (but not splitting) of a fresh molecule of ATP to myosin permits detachment of the bridge from the actin filament at the end of a power stroke so that the cycle can be repeated. This ATP is subsequently split to provide energy for the next stroke of the cross bridge.3. The active transport of Ca back into sacroplasmic reticulum during relaxation depends on energy derived from the breakdown of ATP.

Muscle fibers have alternate pathways for formatting ATP.

Becouse ATP is only energy source that can be directly used for these activities , ATP must constantly be supplied for contractile activity to continue. Only limited stores of ATP are immediately available in muscle tissue, but three pathways supply additional ATP as needed during muscle contraction. (1) transfer of a high-energy phosphate from creatin phosphate to ADP, (2) axidative phosphorylation (the citric acid cycle and electron transport system), and (3) glycolysis.

Creatine phosphate

Creatine phosphate is the first energy storehouse tapped at the onset of contractile activity (gambar). Like ATP , creatine phosphate contains a high-energy phosphate group, which can be donated directly to ADP to from ATP. Just as energy is released when the terminal phosphate bond in ATP is split, similarly energy is released when bond between phosphate and creatine phosphate, along with the phosphate , can be donated directly to ADP to from ATP . this reaction , which is catalyzed by the muscle-cell enzyme creatine kinase, is reversible, energy and phosphate from ATP can be transferred to creatine to ftom creatine phosphate.

(bagan)

As the energy reserves are built up in a resting muscle, the increased concentration of ATP favors the transfer of the high-energy phosphate group to creatine phosphate, in accordance with the law of mass action . A rested muscle contains about five time as much creatine phosphate as ATP . thus, most energy is stored in muscle in creatine phosphate pools. At the onset of contraction, when the meager reserves of ATPare rapidly used, the reaction is reversed. Additional ATP is quickly formed by the transfer of energy and phosphate from quickly formed by the transfer of energy and phosphate from creatine phosphate to ADP. Because only one enzymatic reaction is involved in this energy transfer, ATP can be formed rapidly 9within a faction of a second) by using creatine phosphate.Thus creatine phosphate is the first source for supplying additional ATP when exercise begins. Muscle ATP level actually remain fairly constant early in contraction , but creatine phosphate stores become depleted . in fact, short bursts of high-intensity contractile effort, such as high jumps, sprints, or weight lifting , are supported primarily by ATP derived at the expense of creatine phosphate . other energy system do not have a chance to become operable before the activity is over . creatine phosphate stores typically power the first minute or less of exercise.