Topic 9.2 · Core + Supplement
Exercise and Heart Rate
This section is really two skills stacked on top of each other: designing a fair investigation into heart rate, and then explaining — properly, in terms of respiration — why the result comes out the way it does. Cambridge tests both, and a strong answer keeps them separate rather than blending description into explanation.
Designing a defensible investigation
A workable investigation measures a resting heart ratefirst, as a genuine baseline, before any activity takes place. A defined physical activity is then carried out for a controlled duration or intensity, and pulse rate is measured immediately afterwards. If recovery is being studied, further measurements are taken at fixed time intervals afterwards. Crucially, the participant should be allowed to return close to their resting rate before another exercise condition is attempted — comparing a fresh trial against one where the previous exercise hasn’t fully worn off produces a biased, unfair result. Repeating measurements and comparing repeat values, or a mean, improves reliability.
The independent variable is the physical-activity condition — its duration or its intensity. The dependent variable is heart or pulse rate, usually expressed in beats per minute. Everything else that could affect the result should be held as constant as possible between trials: the same person, the same type of exercise, the same method and timing of measuring the pulse, and similar environmental conditions.
Pulse rate itself is measured by counting pulses over a fixed, known interval and converting that count to beats per minute — the interval has to be fixed and applied consistently, or the resulting rate cannot be compared fairly between trials.
What the data typically shows
A typical pattern runs: a resting value first, then a rise in heart rate once activity begins, reaching a raised value during the activity itself, followed by a gradual fall back towards the resting level once activity stops. Plotted as a graph, time sits on the x-axis and heart rate (in beats per minute) sits on the y-axis; from a graph like this you can compare the peak rate reached, how long that peak takes to appear, and how long recovery takes before the rate returns close to its starting point. A more intense or longer activity can produce a higher peak, or a slower return to resting rate — but that comparison is only fair between trials that were otherwise controlled in the same way.
Why heart rate rises during activity
The explanation Cambridge wants is a causal chain, not a single sentence in isolation. During physical activity, muscles contract more and require more energy, so their rate of respiration increases. That increased respiration means active muscle needs oxygen and glucose delivered faster, and needs carbon dioxide removed faster as it is produced. Heart rate rises specifically to meet that increased transport demand — a faster heart circulates blood more quickly, so more oxygen and glucose reach active tissue per minute, and carbon dioxide is carried away per minute more quickly too. Once activity stops and that demand falls again, heart rate gradually returns towards its resting level.
A common way to lose marks here is stopping the explanation halfway: “the muscles need more blood” on its own doesn’t say why, and “breathing is faster so more oxygen gets in” skips straight past the actual cause, which is the rise in muscle respiration creating the demand in the first place. A complete answer names respiration explicitly as the reason transport demand has increased, then links that demand to the rise in heart rate.
The detail of how muscles respire, and what happens when oxygen supply can’t keep up with demand during strenuous exercise, belongs to the respiration topic rather than this one — useful context for recovery time, but not a substitute for the transport-based explanation this section actually tests.