During rest, and light to moderate exercise, even if prolonged, 95% of ATP is used for muscle activity 1. Requires oxygen and mitochondria, and involves chemical reactions that break bonds of fuel molecules that release energy to make ATP 2. Begins with glycolysis and is followed by reactions that happen in mitochondria (Glucose + oxygen= carbon dioxide + water + ATP 3. Carbon Dioxide released diffuses out of muscle into blood, removed by lungs