The science of exercise

Muscles run on a molecule called adenosine triphosphate, or ATP. The body stores only enough ATP for a few seconds of hard effort, so it must constantly regenerate it using three energy systems that overlap.

For a few seconds of explosive effort, such as a sprint start, muscles use stored creatine phosphate. For efforts lasting up to a couple of minutes, glycolysis breaks down glucose quickly without needing oxygen, but produces by-products associated with fatigue. For longer, steadier activity, aerobic metabolism uses oxygen to break down carbohydrates and fats, producing large amounts of ATP.

During exercise, breathing speeds up and deepens to bring in more oxygen and remove carbon dioxide. Heart rate and the volume of blood pumped with each beat increase, sending more oxygen-rich blood to working muscles.

With regular endurance training, the heart becomes stronger and pumps more blood per beat, so resting heart rate often falls. Muscles grow more capillaries and more mitochondria, the structures in cells that carry out aerobic metabolism, allowing them to use oxygen more efficiently.

Resistance training, such as lifting weights, causes tiny amounts of stress and damage to muscle fibres. During recovery the body repairs and strengthens them, and over weeks and months muscle fibres can grow larger, a process called hypertrophy.

Much of the early strength gain in a new training programme comes from the nervous system, which learns to recruit more muscle fibres and coordinate them better. Adequate protein, sleep and rest days are important for recovery and adaptation.

• Overload: the body adapts only when challenged beyond its usual level.
• Progression: training load should increase gradually to keep improving and avoid injury.
• Specificity: the body adapts to the specific type of exercise done, so marathon training builds endurance rather than sprint speed.
• Recovery: adaptation happens during rest, not during the workout itself.
• Reversibility: fitness gains fade if training stops.

Regular physical activity is one of the most powerful things people can do for their health. Large studies link it to lower risks of heart disease, stroke, type 2 diabetes, several cancers, depression and dementia, and to stronger bones and longer life.

Exercise also improves mood and sleep. It can reduce symptoms of anxiety and depression, and many people experience a lift in mood after activity. Benefits begin at modest levels: even people who move from no activity to some activity see meaningful health gains.

The World Health Organization recommends that adults do at least 150 to 300 minutes of moderate-intensity aerobic activity per week, such as brisk walking, or 75 to 150 minutes of vigorous activity, such as running, along with muscle-strengthening activities on two or more days a week. It also advises reducing time spent sitting.

Children and adolescents are advised to average at least 60 minutes a day of moderate to vigorous activity. People with health conditions, or those starting after a long period of inactivity, should consider talking to a health professional about what is suitable for them.