We use the energy generated from splitting ATP (Adenosine Triphosphate) for all functions of the body e.g. nervous transmission, cellular function, muscle contraction, breathing and circulation.
When we exercise the need for ATP increases. Our ATP demands increase with higher intensity exercise as does our oxygen requirements. When exercising at very high intensities our demand for oxygen cannot be met, so the body uses anaerobic energy pathways to re-make or re-synthesise ATP.
The energy released in the breakdown of CP (creative phosphate), which is naturally stored in the body will quickly replace a phosphate onto an adenosine di-phosphate without oxygen. When the intensity of the exercise is between 90-100% of max, the need for ATP is very high, but CP stores only last about 10 seconds. The fast twitch type 11 B fibres will contract quickly for fast movement, and are mainly fuelled by the CP energy pathway (phosphate energy system).
Events that epitomize the phosphate energy pathway and focus on fast twitch muscle fibre contraction (very high intensities) are power lifting, sprinting, high jump, shot putt, hammer throws etc. If ATP is needed for longer the intensity of the exercise (speed or power output) will decrease slightly (e.g. to 70-90%) and a different chemical pathway will release energy to rebuild ATP.
Carbohydrates will be digested and then broken down into glucose in the blood stream and then even further. The energy released will fuel the rebuilding of ATP at moderate to high intensity exercise. The oxygen demand will still be high: oxygen demands in the muscles cannot be met with normal respiratory function. Thus this energy pathway is anaerobic too.
The painful result of carbohydrate break down is H+ ions and lactic acid; this hurts – or feels like a burn in the muscles. Very quickly the muscles will fatigue and slow down, or even stop contracting. I’ve seen a 400m runner collapse at the 300m as the legs were too wobbly. The 400m race is the classic event for the lactate energy pathway as it takes between 45-50 seconds, and is at about 80% of max. The lactate pathways can keep rebuilding ATP for up to 2 minutes in a trained athlete. Other events are the 100m freestyle or a “sport aerobic” competition routine (lasting exactly 2 minutes).
If the duration of exercise is longer than 2 minutes the muscles contract slower. As the muscles contract at a slower rate the slow twitch fibres are activated. They have the endurance to keep on contracting. Oxygen needs can be met though our normal breathing.
The aerobic pathway is quite complex. It’s called the Krebs cycle. Carbohydrates or fats can be broken down with oxygen and water to release energy to rebuild ATP slowly, for a long time. The marathon, triathlons, long power walks or even 30 mins on the cross trainer will use the aerobic pathway to fuel the re-synthesis of ATP. As to the proportion of carbohydrates VS fat metabolism by the aerobic system, it depends on the athlete, the intensity, the duration and the amount of substrates (carbs and fat) available!