Final answer:
Immediate energy for muscular activities is primarily stored in creatine phosphate, which powers muscles for the first 8-15 seconds, and glycogen, which continues to provide energy for about 90 seconds.
Step-by-step explanation:
Immediate energy for muscular fitness is stored in creatine phosphate and glycogen. Creatine phosphate provides energy very quickly, powering muscle contraction for the first 8-15 seconds during high-rate activities, like sprinting. Once depleted, glycogen stored in muscles is used to produce ATP, which can last for about 90 seconds of intense activity. Making ATP from glycogen does not require oxygen initially, which is beneficial since increased oxygen supply takes some time to reach the muscles. This anaerobic production of ATP does result in lactic acid buildup, causing muscle fatigue and soreness.
Dietary fats and sugars consumed just before exercise are used immediately, and excess glucose is stored in the liver and muscles as glycogen. When needed, glycogen is broken down to provide a longer-term energy source during continued exercise.