Energy Systems and Metabolism

Because muscle holds only enough ATP for a second or two of hard work, it must be resynthesised as fast as it is used. Three systems do this, and all contribute from the moment exercise begins, with their relative share set by the intensity and duration of the effort, along with training and recovery. The phosphagen system responds fastest: creatine phosphate stored in the muscle donates a phosphate to regenerate ATP through the enzyme creatine kinase, giving very high power, and it predominates in brief, maximal efforts such as a sprint, jump or throw. The glycolytic system breaks down glucose and muscle glycogen to pyruvate, a rapid process that does not directly use oxygen, producing ATP quickly and generating lactate, and it contributes most during hard efforts of short to moderate duration. The oxidative system is aerobic: in the mitochondria, carbohydrate and fat are fully oxidised through the citric acid cycle and the electron transport chain, which responds more slowly but has an enormous capacity and supplies most of the ATP as exercise is sustained.

The systems also differ greatly in how much ATP they yield: rapid glycolysis produces only a small amount of ATP per glucose molecule but very quickly, whereas full oxidation of the same glucose in the mitochondria yields far more, though more slowly. Creatine phosphate stores are refilled aerobically during recovery, which is why the ability to repeat sprints depends partly on aerobic fitness. These systems form a continuum, and the mix shifts with the intensity and duration of exercise. Short, explosive efforts draw mainly on the phosphagen and glycolytic systems, while longer, steadier work relies on aerobic metabolism.

Under otherwise comparable conditions, the fuel the aerobic system burns shifts with intensity. At rest and during easy exercise, fat provides much of the energy, but as intensity rises the balance moves towards carbohydrate, which delivers more energy per litre of oxygen and becomes the dominant fuel at high intensity. This balance also depends on the duration of exercise, training status, sex and recent nutrition, and protein contributes only a little. Carbohydrate is stored as glycogen in muscle and liver, and these stores are limited, whereas fat stores are effectively vast, so prolonged endurance exercise is often limited by how much glycogen the muscle and liver hold.

Lactate is often misunderstood. It is produced whenever glycolysis is rapid, including under fully aerobic conditions, and rather than being a waste product it is a useful fuel that can be taken up and oxidised by other muscle fibres and the heart, or used by the liver to make glucose. Blood lactate reflects the balance of its production, transport, oxidation and clearance, and the point often called the lactate threshold depends on the testing protocol used.

Metabolism adapts to the type of training done. Endurance training increases the number and size of mitochondria and the density of capillaries, improving the muscle's capacity to use oxygen and to burn fat, and it raises the amount of glycogen the muscle can store, all of which delay the point at which fatigue sets in. Sprint and high-intensity training instead enhance the phosphagen and glycolytic systems and the muscle's tolerance of hard glycolysis. Fatigue in prolonged exercise is multifactorial: falling muscle glycogen and the effort of maintaining blood glucose play a large part, while central fatigue, pacing and environmental factors such as heat also contribute, together underlying 'hitting the wall'. After hard exercise the phosphagen stores refill within minutes, while replacing depleted muscle glycogen takes many hours and depends on carbohydrate intake, which shapes how quickly an athlete can recover between sessions. This is the physiology that sensible fuelling and pacing are designed to manage.

This physiology shows up across sport and exercise medicine. Underfuelling for the training load causes fatigue and poor recovery, and in endurance events the sudden flagging of glycogen depletion. Sustained low energy availability, where intake does not match the energy demand of training, can develop into relative energy deficiency in sport (RED-S), which affects many body systems and needs recognition rather than being dismissed as simply hard training. Rarely, exercise intolerance with muscle pain, cramping or dark urine that is out of proportion to the effort points to a metabolic muscle disorder or, after extreme exertion, to rhabdomyolysis, a breakdown of muscle that needs urgent care. Matching fuel to the training load underlies both performance and recovery.

• •Muscle stores little ATP, so it is resynthesised continuously during exercise.
• •All three systems contribute from the start; their share depends on intensity and duration.
• •Phosphagen system: creatine phosphate, the highest and briefest power.
• •Glycolytic system: quick ATP and lactate, for short to moderate efforts.
• •Oxidative system: most ATP for sustained exercise, burning carbohydrate and fat.
• •Lactate is a recyclable fuel; blood lactate reflects production and clearance.