School › Exercise and the body › Ages 15–16
Exercise physiology: heart, muscles and energy
Cardiac output and how it changes in exercise, maximum heart rate, how muscles get ATP from creatine phosphate, glycolysis and aerobic respiration, lactic acid and oxygen debt, and what the evidence says about activity and health. With a heart-rate recovery investigation.
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Cardiac output is the volume of blood pumped by one ventricle per minute: CO = HR × SV. At rest, a typical heart rate is about 75 beats per minute (range 60–100) and a typical stroke volume about 70 mL, giving CO ≈ 5.25 L/min.[6]
In exercise, heart rate in healthy young people may rise to about 150 bpm and stroke volume to about 130 mL, so CO rises to about 19.5 L/min, 4–5 times the resting value. Maximum heart rate can be estimated as 220 − age. Trained athletes typically have very low resting heart rates.[6]
Muscles run on ATP. Creatine phosphate powers the first few seconds of contraction (about 15 seconds' worth). Glycolysis breaks down glucose without oxygen and yields a little ATP; aerobic respiration breaks down glucose in the presence of oxygen to carbon dioxide, water and about 36 ATP per glucose, compared with 4 from glycolysis.[7]
When oxygen cannot be delivered fast enough, pyruvic acid is converted to lactic acid, which may contribute to muscle fatigue. The oxygen debt is the oxygen needed to make up for ATP made without oxygen, which is why breathing stays faster for a while after exercise stops.[7]
Guidelines: ages 5 to 18 should average at least 60 minutes of moderate or vigorous activity a day; adults at least 150 minutes of moderate activity a week. Worldwide, about 31% of adults are insufficiently active, and insufficiently active people have a 20–30% increased risk of death compared with active people.[1][2]
Evidence: in young people, activity is linked to better bone health, fitness, cardiometabolic and mental health and lower body fat, and to better attention and memory. Many of these findings are associations from studies of groups of people, so they do not show on their own that activity is the only cause.[2][3]
Try it: Heart-rate recovery after step exercise
You need
- A stable, low step or gym bench fixed against a wall
- Stopwatch
- Record sheet and graph paper or a spreadsheet
Safety
Use a stable, low step on a non-slip floor. Participation is voluntary; pupils with a health condition follow their care plan or take the role of timekeeper or recorder. Stop at once if anyone feels dizzy, short of breath beyond normal, or has chest pain, and tell the teacher. Data are personal: no comparing or ranking pupils.
What to do
- Sit quietly for 3 minutes, then measure your resting heart rate: count your pulse for 30 s and double it. Repeat once and take the mean.
- Step up and down on the step at a steady pace you can keep (up-up-down-down) for 2 minutes.
- Immediately sit down and measure your pulse (30 s × 2). Repeat every minute for 5 minutes.
- Plot heart rate (bpm) against time after exercise and mark your resting value with a horizontal line.
- Calculate the percentage increase from rest to the first reading, and the recovery time to within 10 bpm of resting.
- Estimate cardiac output at rest and at your first reading, assuming SV = 70 mL at rest. Evaluate: what is the uncertainty if you can miscount by 1 beat in 30 s?
What happens, and why?
During exercise the muscles need more oxygen for aerobic respiration, so heart rate and stroke volume rise and cardiac output increases several times. After exercise the heart rate falls back over a few minutes; breathing and oxygen use stay raised for a while because of the oxygen debt. Counting for 30 s and doubling means a miscount of 1 beat gives an error of ±2 bpm.[6][7]
Play: Aerobic or anaerobic?
Each statement is about how muscles make ATP. Does it describe aerobic respiration (with oxygen) or anaerobic energy release (without oxygen)?
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Quiz
Other ages: Ages 6–7 · Ages 10–11