Every fact in the three exercise lessons is listed here with where it comes from. The numbers in [brackets] on the lesson pages link to this list.
How we rate sources
High: space agencies (NASA), peer-reviewed open textbooks (OpenStax, Rice University) and professional scientific societies (American Physical Society).
Medium-high: reputable science news reporting directly on a primary document.
Medium: single-author essays and popular history sites. We only use these for history details, and never on their own: each one is paired with a second source.
Wikipedia was not used as a source. Numbers are rounded (for example 9.8 m/s² for Earth's gravity). The text was drafted with help from AI (Claude) and then checked against these sources. Corrections: hello@echoesof.science.
Where the facts are simplified or less certain: the 60-minutes-a-day guideline is the UK guideline for ages 5 to 18 [1]; other countries word their guidelines slightly differently. The resting and exercise values for heart rate, stroke volume and cardiac output are typical textbook figures [6]; individuals differ widely. The '220 minus age' rule only estimates maximum heart rate [6]. The figure of about 36 ATP per glucose is a textbook approximation [7]. The WHO statistics (for example 86% of adolescents not meeting the recommended activity levels, 31% of adults insufficiently active, a 20–30% higher risk of death for insufficiently active people) [2] come from surveys and studies and are estimates. The health benefits listed come from studies that mostly show associations. These lessons are general education, not medical advice: pupils with a health condition should follow their care plan and ask a doctor or the school nurse what activity is right for them.
All sources
[1] Physical activity guidelines for children and young people
NHS (UK National Health Service) · Reliability: High
Type: National health service
Children and young people aged 5 to 18 should aim for an average of at least 60 minutes of moderate or vigorous intensity physical activity a day across the week, and do a variety of activities to build movement skills, muscles and bones. Moderate activity raises your heart rate, makes you breathe faster and feel warmer; you can still talk but not sing. Examples: walking to school, playground games, PE, football, swimming, skipping, dancing, cycling. Muscle and bone strengthening: gymnastics, football, jumping, martial arts, sit-ups and press-ups. Reduce time spent sitting and break up long periods of not moving.
World Health Organization (WHO) · Reliability: High
Type: International health agency
Physical activity is any bodily movement produced by skeletal muscles that requires energy. In children and adolescents it promotes bone health, muscle growth and motor and cognitive development and is linked to better fitness, cardiometabolic and mental health and lower body fat. Adults: at least 150 minutes of moderate activity a week. 31% of adults (about 1.8 billion) are insufficiently active; 86% of adolescents do not meet the recommended levels. Insufficiently active people have a 20–30% increased risk of death.
[3] Health Benefits of Physical Activity for Children
Centers for Disease Control and Prevention (CDC) · Reliability: High
Type: National public health agency
Physical activity improves attention and memory, reduces the risk of depression, builds strong muscles and endurance, improves blood pressure and aerobic fitness, helps maintain normal blood sugar levels, reduces the risk of type 2 diabetes and obesity, strengthens bones, and helps regulate body weight and reduce body fat.
National Heart, Lung, and Blood Institute (NHLBI, NIH) · Reliability: High
Type: National health research institute
Physical activity is any body movement that works your muscles and requires more energy than resting; exercise is a planned, structured kind of physical activity. Being physically active is one of the best ways to keep your heart and lungs healthy; even modest amounts are good for your health.
National Heart, Lung, and Blood Institute (NHLBI, NIH) · Reliability: High
Type: National health research institute
Aerobic activity makes your heart beat faster than usual and you breathe harder (walking, jogging, running, cycling, skateboarding, rollerblading, jumping rope). Moderate: you can talk but not sing; vigorous: you can't say more than a few words without stopping for a breath. Muscle-strengthening activities improve strength, power and endurance (push-ups, sit-ups, lifting weights, climbing stairs, digging). Bone-strengthening: your muscles push against your bones (running, walking, jumping rope). Balance: standing on one leg, walking heel-to-toe, walking backward. Flexibility: stretching, touching your toes, yoga.
[6] Anatomy and Physiology, 19.4 Cardiac Physiology
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Cardiac output CO = HR × SV. Resting: stroke volume about 70 mL (55–100 mL), heart rate about 75 bpm (60–100), CO about 5.25 L/min (4.0–8.0). In exercise, HR may rise to 150 bpm in healthy young people and SV to about 130 mL, giving CO of about 19.5 L/min, 4–5 times the resting rate. Maximum heart rate can be estimated as 220 minus age. Trained athletes typically have very low heart rates.
[7] Anatomy and Physiology, 10.3 Muscle Fiber Contraction and Relaxation
OpenStax, Rice University · Reliability: High
Type: Peer-reviewed open textbook
Muscles use ATP. Creatine phosphate powers the first few seconds of contraction (about 15 seconds' worth). Glycolysis is anaerobic and breaks down glucose to make ATP. Aerobic respiration breaks down glucose or other nutrients in the presence of oxygen to produce carbon dioxide, water and ATP: about 36 ATP per glucose versus four from glycolysis. Without oxygen, pyruvic acid is turned into lactic acid, which may contribute to muscle fatigue. Oxygen debt: oxygen intake stays raised after exercise until it is repaid.