Final answer:
During the first 1 to 2 minutes of moderate to heavy exercise, the body relies on creatine phosphate for ATP production. After, it switches to glycolysis - an anaerobic process that breaks down glucose to produce ATP.
Step-by-step explanation:
During the first 1 to 2 minutes of moderate to heavy exercise, the body relies on creatine phosphate for ATP production. Creatine phosphate is a high-energy molecule that can rapidly generate ATP, providing the initial burst of energy for muscle contraction. However, creatine phosphate can only sustain ATP production for about 8-10 seconds. After that, the body switches to glycolysis - an anaerobic process that breaks down glucose to produce ATP. Glycolysis can supply energy for about 90 seconds, but it is slower than creatine phosphate for ATP generation. It does not require oxygen, making it useful during the initial phase of exercise when the heart and lungs need time to supply sufficient oxygen to the muscles. The by-product of glycolysis without oxygen is lactic acid, which can cause tiredness and soreness in muscles.