Short-Term Effects of Exercise
The Body’s Immediate Response to Exercise
The moment you start exercising, your body begins a coordinated series of adjustments to match the increased energy demand of working muscles. These short-term effects happen within seconds to minutes of activity beginning — and many begin even before you start, as the body anticipates exercise. Understanding what changes, why it changes, and why it matters to a performer is the focus of this lesson.
Heart Rate Response
At rest, heart rate is typically 60–80 beats per minute (bpm). As exercise begins, heart rate rises almost immediately due to signals from the nervous system and hormones such as adrenaline. This anticipatory rise — technically called the anticipatory rise — can increase heart rate even before any physical exertion, as observed in athletes waiting on the start line.
Once exercise is underway, heart rate continues to climb in proportion to intensity. At light intensity, heart rate may stabilise at 100–120 bpm. At moderate intensity (aerobic zone), it sits between 120 and 160 bpm. At maximal intensity (all-out sprinting), heart rate can reach 180–200 bpm. The maximum heart rate is estimated by 220 minus age: a 16-year-old has a maximum of approximately 204 bpm.
The reason for the increase is straightforward: working muscles demand more oxygen. The heart compensates by beating faster, increasing the volume of blood — and therefore oxygen — delivered per minute. Cardiac output rises from roughly 5 litres per minute at rest to over 20 litres per minute during maximal exercise, driven by increases in both heart rate and stroke volume.
Short-term cardiovascular response: heart rate rises immediately due to nervous system signals and adrenaline; anticipatory rise occurs before exercise starts; during exercise, HR increases with intensity; both HR and stroke volume increase, raising cardiac output from ~5 L/min at rest to over 20 L/min at maximal exercise.
Breathing Rate and Depth
Alongside the cardiovascular response, the respiratory system rapidly increases both the rate and depth of breathing. At rest, a typical adult breathes about 12–20 times per minute with a tidal volume of around 0.5 litres. During maximal exercise, breathing rate can rise to 40–60 breaths per minute and tidal volume can reach 3 litres, increasing total air moved per minute (minute ventilation) from roughly 6 L/min at rest to over 100 L/min during maximal exercise.
The increased rate and depth of breathing serves two purposes: it brings more oxygen to the alveoli for gas exchange, and it removes the increased carbon dioxide produced by working muscles. CO2 is a key stimulus for breathing: receptors in the brain and blood vessels detect rising CO2 levels and signal the respiratory centre in the brainstem to increase the rate and depth of breathing automatically.
Both changes (rate and depth) are important. Increasing rate alone would result in shallow breathing with lots of air moving in the trachea and bronchi — the “dead space” that never reaches the alveoli. Increasing depth ensures that each breath reaches the alveoli where gas exchange can actually occur. In well-trained athletes, the body preferentially increases depth rather than rate up to moderate intensities, making breathing more efficient.
Lactate Accumulation
During low to moderate exercise, lactic acid is produced in small amounts and is cleared by the aerobic system as fast as it is made. As intensity increases past the lactate threshold, production exceeds removal and lactic acid begins to accumulate in the muscle and blood. This accumulation has several consequences for the performer: muscles begin to feel heavy and painful; coordination can deteriorate; and the performer must reduce intensity or stop.
For a GCSE performer, understanding lactate accumulation is directly relevant to race strategy and training. A 400 m runner who goes out too fast will exceed their lactate threshold early, accumulating lactic acid that impairs their ability to maintain speed in the final straight. A well-trained distance runner has a higher lactate threshold, meaning they can run faster before lactic acid begins to accumulate — one of the key benefits of regular aerobic training.
Muscle fatigue during intense exercise is partly caused by lactic acid accumulation and partly by the depletion of energy stores (particularly glycogen). After exercise ends, breathing rate remains elevated for a period — this is the excess post-exercise oxygen consumption (EPOC), sometimes called oxygen debt. During this recovery phase, oxygen is used to break down lactic acid, restore muscle glycogen, replenish the oxygen stores in haemoglobin and myoglobin, and return body temperature and heart rate to resting values.
Three key short-term effects: (1) Heart rate and cardiac output increase — to deliver more oxygen to working muscles. (2) Breathing rate and depth increase — to supply more oxygen to alveoli and remove more CO2. (3) Lactate accumulates if intensity exceeds the lactate threshold — causing muscle fatigue and a limit on sustained high-intensity effort.
Heart rate is not the only cardiovascular adjustment during exercise. Stroke volume also increases. As muscles contract during exercise, they squeeze the veins running through them (the “muscle pump”), propelling blood back to the heart faster. This increased venous return stretches the ventricles and causes them to contract more forcefully (Starling’s law of the heart), ejecting more blood per beat. At rest, stroke volume is around 70 ml; during exercise, it can rise to 110–130 ml in a trained person. The combined effect of increased HR and increased SV dramatically multiplies cardiac output and oxygen delivery.
Data questions often present a graph or table of heart rate, stroke volume or cardiac output at rest and during exercise. To score full marks on an interpret data question, you need to: (1) quote a specific figure from the data with units (e.g. “heart rate rose from 72 bpm to 168 bpm”); (2) identify the trend (rising, plateauing, falling); and (3) link the trend to a physiological reason or sporting implication (e.g. “the rise in HR increases cardiac output, ensuring more oxygen reaches the working leg muscles during the running phase”). Students who only quote a figure without explaining its significance, or who explain the physiology without quoting data, typically score half marks.