Final answer:
After about 15 seconds of skeletal muscle contraction, continued, sustained contraction is maintained by energy from ATP stored in the muscle (Option C).
Step-by-step explanation:
The source of energy that is used to power the movement of contraction in working muscles is adenosine triphosphate (ATP) – the body's biochemical way to store and transport energy. ATP provides the energy for cross-bridge formation and filament sliding. However, ATP is not stored to a great extent in cells.
Creatine phosphate is a molecule that can store energy in its phosphate bonds. In a resting muscle, excess ATP transfers its energy to creatine, producing ADP and creatine phosphate. This acts as an energy reserve that can be used to quickly create more ATP. When the muscle starts to contract and needs energy, creatine phosphate transfers its phosphate back to ADP to form ATP and creatine. This reaction is catalyzed by the enzyme creatine kinase and occurs very quickly; thus, creatine phosphate-derived ATP powers the first few seconds of muscle contraction. However, creatine phosphate can only provide approximately 15 seconds worth of energy, at which point another energy source has to be used (Figure 10.12).
After about 15 seconds of skeletal muscle contraction, continued, sustained contraction is maintained by energy from ATP stored in the muscle.