Final answer:
Sustained skeletal muscle contraction is initially maintained by ATP from creatine phosphate and subsequently through anaerobic glycolysis, with aerobic respiration providing energy for longer-term activity.
Step-by-step explanation:
After the initial few seconds of skeletal muscle contraction, continued, sustained contraction is maintained by energy primarily from anaerobic glycolysis and creatine phosphate, and eventually aerobic respiration if oxygen is available. At the onset of muscle contraction, ATP stored in the muscle is used up within seconds. Subsequently, creatine phosphate donates a phosphate group to ADP to rapidly regenerate ATP, but this source is also quickly depleted in about 15 seconds. Thereafter, anaerobic glycolysis takes over to produce ATP and pyruvic acid which can lead to the formation of lactic acid under anaerobic conditions, contributing to muscle fatigue during high-intensity exercises. For extended periods of muscle activity, aerobic respiration becomes the main ATP source, producing much more ATP and allowing the muscle to work for longer durations without fatiguing quickly.