How hard: effort you can measure

Lesson 1 gave you minutes: a weekly amount of moderate activity, a smaller amount of vigorous, and strength work on two or more days. Every one of those minutes carries an adjective, and the adjective is the hard part. Is your brisk walk moderate? Is it moderate for you and for the friend who walks with you? When a plan tells you to train hard, how hard is that? This lesson gives you the ways people answer: a table of activities, a 0 to 10 scale, the talk test, heart rate, and for lifting, repetitions in reserve. It tells you how far each one can be trusted. Two of the numbers you're most likely to be handed, a maximum heart rate from a formula and the calories on a watch, turn out to be the ones you should trust least for yourself.

Two meanings of intensity

There are two different questions hiding in the word intensity, and most confusion about effort comes from answering one when you meant the other.

The first is absolute intensity: how much energy the activity costs, whoever does it. Walking at a set speed on flat ground costs about the same for everyone of a given size, so you can put a number on the activity itself.

The second is relative intensity: how big a share of your capacity the activity takes. The same walk that barely registers for a runner can leave someone who's been ill for a year out of breath. The activity has not changed. The person has.

The guidelines lesson 1 described use both. The US Centers for Disease Control and Prevention (CDC) page on measuring intensity gives an absolute definition and a relative one side by side, and says plainly why it needs both: "an activity that is vigorous for a person of average fitness may only be moderate for a very fit person."² Hold on to that sentence. The rest of the lesson is mostly about what it costs you to forget it.

METs: putting a number on the activity

The unit for absolute intensity is the MET, short for metabolic equivalent. The CDC's plain version: "One MET equals the energy or oxygen used while sitting quietly."² An activity rated at 4 METs costs about four times what you would use sitting still.

The numbers come from the 2024 Adult Compendium of Physical Activities, a systematic review that pulled 2356 measured energy costs out of 701 papers and turned them into a table of 1114 activities.¹ Its bands (the CDC uses the same moderate and vigorous cut-offs):¹²

And some entries from its tables, speeds as the Compendium prints them in miles an hour, with kilometres an hour in brackets:¹

• Walking, 2.5 mph (4.0 km/h), level: 3.0 METs
• Walking, 2.8 to 3.4 mph (4.5 to 5.5 km/h), moderate pace: 3.8
• Walking, 3.5 to 3.9 mph (5.6 to 6.3 km/h), brisk, walking for exercise: 4.8
• Bicycling under 10 mph (16 km/h), leisure: 4.0
• Jogging, general, self-selected pace: 7.5
• Running, 5.0 to 5.2 mph (8.0 to 8.4 km/h), a 12-minute mile: 8.5
• Bicycling, 12 to 13.9 mph (19 to 22 km/h), moderate effort: 8.0

That's why the UK guidelines give brisk walking as their example of moderate and running as their example of vigorous.¹⁸ The table also shows why the guidelines can treat a vigorous minute as worth about two moderate ones (the arithmetic is this course's). The Compendium puts the US weekly target at "∼500–1000 MET-min/week".¹ A MET-minute is just METs times minutes: 150 minutes of brisk walking at 4.8 is 720 MET-minutes, and 75 minutes of running at 8.5 is about 640. Roughly half the time at roughly twice the cost lands in the same place.

What the Compendium says about itself

Read this sentence twice, because it is the Compendium's own warning about its own numbers: "the Compendium can serve as a starting point for prescribing individual activities but does not reflect precise individual EE values."¹ EE is energy expenditure.

The reason is built into the unit. A MET assumes everybody burns the same amount at rest, a standardised 3.5 millilitres of oxygen per kilogram of body weight per minute. The paper itself notes that resting rate varies with age, sex, height and body composition, and gives the example that women with obesity used about 8 to 15 per cent more energy than women without obesity walking at the same treadmill speeds. The authors add that the activity still fell in the same intensity band, which is the point of the example: the band travels from person to person, the exact number does not. They also say the tables are meant for adults without conditions that affect metabolism, and because resting rate runs lower in older adults, there is now a separate Compendium for people aged 60 and over.¹

The relative scales: your own effort

The CDC gives a relative version of the same bands on a scale from 0 to 10, "where 0 is the level of effort of sitting, and 10 is maximal effort". Moderate is "a level of effort of 5 or 6", and vigorous "begins at a 7 or 8 on this scale."² Both WHO and the US guidelines tell older adults to judge effort relative to their own fitness. The US wording is "Older adults should determine their level of effort for physical activity relative to their level of fitness."¹⁶¹⁷

Scales like this are called ratings of perceived exertion, or RPE, and the best-known ones carry Borg's name. This course read only the abstract of his 1982 paper, which describes a "Borg Scale for ratings of perceived exertion" and a newer category-ratio method, but prints neither scale.⁷ So the anchors for the classic 6 to 20 version come from the Generation 100 trial you met in lesson 1, which describes its Borg scale as running "from 6 for no exertion to 20 for maximum exertion".⁶ The CDC doesn't call its 0 to 10 scale Borg's, and nor does this lesson.

Generation 100 is also a useful look at how researchers tie the scales together. Its interval sessions were set at about 90 per cent of peak heart rate (peak meaning the highest measured in testing), which it matched with a Borg rating of about 16, and its steady sessions at about 70 per cent, a rating of about 13.⁶ Those are the trial's own rough pairings for its own participants, who were 70 to 77.

You'll often hear that a 6 to 20 rating times ten gives your heart rate, so a 13 means about 130 beats a minute. This course found no source for that rule, so it cannot tell you whether it holds. It can make one rough check of its own, and it is this course's arithmetic, not anyone's finding. Generation 100 paired a rating of about 13 with about 70 per cent of peak heart rate. For a 73-year-old, the formula you will meet below predicts a peak near 157, and 70 per cent of that is about 110, not 130. On that estimate the rule runs about 20 beats high at that age, and an estimate is all it is.

The talk test

The most practical of the relative measures needs no scale and no device. The CDC's version, "As a rule of thumb, a person doing moderate-intensity aerobic activity can talk, but not sing, during the activity. A person doing vigorous-intensity activity cannot say more than a few words without pausing for a breath."²

That settles something a MET value can't. Picture your friend of 72 again, on her bike at the 4.0-MET pace, and her son of 40 riding beside her (both are this course's construction). She can talk but not sing. He's singing. The activity is the same, and so is its MET value, so in absolute terms both are doing moderate activity. Relative to each of them, the talk test says the ride is moderate for her and easier than moderate for him. Nothing about the bike changed; the people did.

The talk test sounds like folk wisdom, but it's been tested against gas analysis in a laboratory.

In 2004 Persinger and colleagues, a group that included Foster and Porcari, put 16 healthy volunteers through tests where the effort rose step by step on a treadmill and a bike, measured their breathing gases, and checked their speech as the effort rose. The abstract, which is all this course read, reports: "At the point where speech first became difficult, exercise intensity was almost exactly equivalent to ventilatory threshold. When speech was not comfortable, exercise intensity was consistently above ventilatory threshold."⁸

The ventilatory threshold needs a word of explanation, and this is background the course is supplying rather than something the abstracts define. As effort climbs steadily, there's a point where your breathing starts to rise faster than the work does. You've felt it: the moment a climb stops being something you can chat through. Speech uses the same air you're trying to breathe, so it is squeezed out right about there. That's why the test tracks something real in your body rather than something about the activity.

A 2014 review by Reed and Pipe (also read at abstract level) says the same holds across walking, jogging, cycling, an elliptical and a stair stepper, and adds a limit: "It may not be practical for high-intensity interval training."⁹ A 2023 study of 17 healthy young adults reciting a set passage on a treadmill found each stage of the test tracked heart rate, oxygen use and breathing.¹⁰

Physiologists often express effort as a share of your heart-rate reserve: the gap between your resting heart rate and your maximum. (That definition is background this course supplies; the abstract uses the term without defining it.) Seventy per cent of reserve is a higher heart rate than it sounds, because it counts up from rest, not from zero.

The caveat: all the validation samples are small, 16 and 17 people here, and the classic studies come from one research group.⁸¹⁰ That's a reason to hold the talk test as a good tool rather than a precise one. It is free, it needs no maximum, and the next section shows how shaky the usual way of getting a maximum is.

Heart rate, and the formula on every gym machine

Heart rate is a relative measure too, if you know what your maximum is. The trouble is that almost nobody does, so they estimate it, and the estimate most people meet is 220 minus age.

Robergs and Landwehr's verdict is blunt, and it is their judgement in a commentary, not a consensus statement: "Consequently, the formula HRmax=220-age has no scientific merit for use in exercise physiology and related fields."³ They went further, "Currently, there is no acceptable method to estimate HRmax." They did set a looser bar for training than for research, "errors ≤8 b/min are likely to be acceptable", and then said that "it is likely that current equations used to estimate HRmax are not accurate enough" to meet it for many people.³

The best-known replacement came a year earlier. Tanaka and colleagues pooled group averages from 351 studies, 18,712 people in all, and then checked the result on 514 people in their own lab. This course read the abstract only. It reports: "In the meta-analysis, HRmax was strongly related to age (r = -0.90), using the equation of 208 - 0.7 x age."⁴ The line didn't differ between men and women or with activity levels, and the authors concluded that "the currently used equation underestimates HRmax in older adults."⁴

Read the r = -0.90 carefully. It was computed on the averages of groups, not on individuals, and averages hide the scatter between people. That's this course's point rather than the paper's, and Robergs and Landwehr's table of equations notes the same gap from another side: it lists Tanaka's result with no error figure at all.³

How far off, for one person?

A group average can be excellent and still miss you by a lot. The clearest recent number comes from Martin and colleagues in 2025, in an open-access paper that compared seven formulas against the maximum heart rate actually measured on a treadmill in 230 adults aged 18 to 68.⁵ Tanaka's formula had the smallest average error, about 7 beats. The 220 formula showed no significant bias on average in their sample. They also found Tanaka's predictions drifted low for people with higher maximums, while 220 minus age did not, and their own practical conclusion is that 220 minus age "exhibited arguably the most consistent performance across the sample" and supports "its continued use as a practical and generalizable option".⁵ So the newest data do not crown a winner: Tanaka's is a little closer on average, the old formula is steadier, and both miss individuals by the same wide margin.

That margin is the result that matters for you: "LOA were wide across all equations (approximately ±18–24 bpm), indicating substantial individual-level variability."⁵ LOA are limits of agreement, the range either side of a formula's prediction within which 95 per cent of individuals' measured maximums fell. The 18 to 24 is the spread across all seven formulas; for each one on its own, the authors put it at "approximately ±20 bpm". Their typical error, about 7 to 10 beats, runs from just under to well past the 8 beats Robergs and Landwehr would accept for training ranges. The authors' conclusion is that "none demonstrated high individual level accuracy".⁵

The scope matters. Their sample was 174 men and 56 women, mostly recreational athletes, tested in one university lab, and the authors call the analysis exploratory.⁵ It says nothing specific about older or sedentary beginners.

Working one through

Take a 50-year-old, a person this course has made up to show the arithmetic.

• 220 minus age: 220 − 50 = 170 beats a minute.
• Tanaka: 208 − (0.7 × 50) = 208 − 35 = 173.

Three beats apart. The formulas nearly agree, which feels reassuring. Now put Martin's limits around Tanaka's 173. For each formula they were about 20 beats either side,⁵ so this person's real maximum could plausibly sit anywhere from about 153 to about 193, and the formula has no way to say where.

Why does that matter? Because nobody trains at their maximum; they train at a percentage of it. Say a plan tells this person to work at 70 per cent of maximum (not of reserve), about where Generation 100 set its steady sessions.⁶ On the prediction that's 0.7 × 173 ≈ 121 beats.

Both kinds of number so far, METs and formula maximums, work like that: an average of many people, handed to one person as if it were theirs.

One group should not use heart-rate targets at all without advice. Martin's authors note that medicines such as beta-blockers "can blunt" the heart's response to exercise, "rendering HR-based zones misleading".⁵ If you take a medicine for your heart or blood pressure, the talk test and the 0 to 10 scale are the measures to use, and your doctor is the person to ask about targets.

Effort in the weight room: repetitions in reserve

The talk test isn't built for a set of squats, and nothing this course read tested it there; the set is over in well under a minute. Lifters use a different relative measure, repetitions in reserve, or RIR: when you finish a set, how many more good repetitions could you have done? Zero means you couldn't have done another. Three means you stopped with three left.

A scale built on this was published in 2016 by Zourdos and colleagues, pairing each RIR with a rating: "(RPE-10 = 0-RIR, RPE-9 = 1-RIR, and so forth)".¹¹ It was tested in 29 people, 15 experienced and 14 novice squatters, in the squat only. This course read the abstract only.¹¹

Why this matters for training: the 2026 position stand from the American College of Sports Medicine (ACSM), which this course read in full and which lessons 4 and 5 use heavily, recommends that healthy adults lift with "high effort", and says that enough effort can come from sets taken to near failure or "a target of 2–3 repetitions in reserve (RIR)".¹³ It also says that "there is insufficient evidence to quantify exact RIR and perceived exertion targets".¹³ So RIR is a common tool, and ACSM gives a range rather than a precise dose.

The obvious question is whether people can judge it at all.

Halperin's review found guesses slightly better closer to failure and in sets of 12 reps or fewer, and only trivially better in later sets, and that "training status did not seem to influence prediction accuracy". Its conclusion is carefully worded: "Participants were imperfect in their ability to predict proximity to task failure independent of their training background. It remains to be determined whether the observed degree of inaccuracy should be considered acceptable."¹²

Two practical readings follow, and both are this course's inference from those findings rather than anything the review recommends. A long, light set, say 30 wall press-ups, is exactly where your guess will be worst, so for practising RIR pick a version of an exercise you can only do about 8 to 12 times. And if you aim for 2 to 3 in reserve and tend to underpredict like the average person, you're probably still inside or just above the range, which is a forgiving direction to be wrong in.

What your watch can and cannot tell you

Wrist devices report a heart rate and an estimate of calories burned. The two figures behave very differently.

A 2017 laboratory study by Shcherbina and colleagues tested seven devices on 60 volunteers against a 12-lead ECG (the full hospital heart trace) and measured gas exchange while they sat, walked, ran and cycled.¹⁴ For heart rate, the median error ran from 1.8 per cent in cycling to 5.5 per cent in walking. For energy expenditure it was another story: "No device achieved an error in EE below 20 percent", with median errors from 27 per cent for the best device to 93 per cent for the worst. "Device error was higher for males, greater body mass index, darker skin tone, and walking."¹⁴ The authors' conclusion: "most wrist-worn devices adequately measure HR in laboratory-based activities, but poorly estimate EE, suggesting caution in the use of EE measurements as part of health improvement programs."¹⁴

A 2020 systematic review by Fuller and colleagues, covering 158 publications to May 2019, found the same shape. For heart rate, "56.5% (n=100) were within ±3% measurement error". For energy, "For energy expenditure estimates, no brand of wearable was within ±3% measurement error more than 13% of the time".¹⁵

Keep the dates and settings attached. These are 2017 devices in a lab, and a review whose newest study predates May 2019; the Fuller review itself says devices keep being redesigned and newer research is needed.¹⁵ This course has not read how any device computes its calorie figure, so it can't tell you why the errors are so large, only that they were.

What people get wrong about effort

"Maximum heart rate is 220 minus your age." It's an estimate with no study behind its origin, and it still did about as well as newer formulas in 2025; every formula tried had individual limits of about 18 to 24 beats either way.³⁵ Use it as a rough centre, not a fact about you.

"So a formula number is worthless." The mirror-image mistake. Martin's authors call age-based formulas "useful for population-level benchmarking and for setting initial training targets".⁵ A formula gives you a sensible place to start; what it cannot give you is your own number.

"My watch knows how many calories I burned." On the evidence read, it's a fair pulse monitor and a poor calorie counter.¹⁴¹⁵

"If you can talk, you're not working." Talking but not singing is the CDC's own description of moderate intensity,² and moderate minutes count in full toward every guideline lesson 1 covered. The mirror-image mistake is thinking a session only counts if you're gasping.

"A brisk walk is moderate, full stop." It is moderate in METs.¹ Whether it is moderate for you depends on your fitness, which is what the relative scales are for.

Practice

These two exercises are a walk and one easy set. Lesson 1 covered who should talk to a doctor before starting; if that's you, do that first. The callout at the top still applies: stop at once for chest pain or pressure, dizziness, palpitations or breathlessness out of proportion to the effort.

Connections

Back. Lesson 1, How much, and what it rests on, gave you moderate and vigorous minutes; this lesson gives the adjectives a meaning you can check on yourself. Hold lesson 1's curve in mind while you read the numbers above: the biggest gain on it comes from moving from none to some, and WHO's good practice statement is "Doing some physical activity is better than doing none."¹⁶ Getting the band exactly right matters far less than moving at all. Sleep lesson 1 taught how to judge a device against a reference instrument, and Nutrition lesson 3 what constrained energy expenditure means for exercise.

There's a safety connection too. The authors of a case series on five previously fit people admitted to hospital in Singapore after their first spin class put this on their prevention list: "Self-awareness of level of exertion especially when new to spin", alongside telling the instructor you're new.¹⁹ The talk test and the 0 to 10 scale help with the first. They measure your breathing, though, not what an unfamiliar hour is doing to your muscles, so being fit enough to chat is no protection. That last point is this course's reading of the case series: the patients were fit, and what they had in common was a first, hour-long class.

Forward. Lesson 3 uses the talk test to build a first month of walking or running. Lesson 4 asks of the ACSM stand's evidence the course's question: trained whom, for how long, measured on what? Lesson 5 uses RIR to set how hard each set of a lifting session should be.

Go deeper

• How to Measure Physical Activity Intensity, CDC. Read in full here. Short, free, and the clearest single statement of absolute and relative intensity and the talk test.
• The surprising history of the "HRmax=220-age" equation, Robergs and Landwehr, 2002. Read in full here. A free PDF and a good piece of detective work; its table of 43 published equations shows how many attempts there have been.
• Exploratory analysis of the accuracy of age-based maximal heart rate equations, Martin and colleagues, 2025. Read in full here. Open access, with the individual error stated plainly in its results and a practical section that defends 220 minus age as a starting point; keep its sample in mind.
• 2024 Adult Compendium of Physical Activities, Herrmann and colleagues. Read in full here, with the website's walking, running and cycling tables. Look up an activity you actually do.

Sources

1. S. D. Herrmann, E. A. Willis, B. E. Ainsworth and colleagues, "2024 Adult Compendium of Physical Activities: a third update of the energy costs of human activities", Journal of Sport and Health Science 13(1), 2024, pp. 6 to 12, doi 10.1016/j.jshs.2023.10.010 . Read level: full text, plus the activity tables on pacompendium.com for running, walking and bicycling. Supports: the review's size, the MET bands, the example activity values as printed on the website, the MET-minute range, the limitation sentence, the example of women with obesity, the scope, the 3.5 mL/kg/min standard, the authors' note that the obesity example stayed in the same intensity band, the 19 to 59 age range, and the separate Older Adult Compendium with the paper's reason for it. The km/h conversions and the MET-minute arithmetic are this course's.
2. Centers for Disease Control and Prevention, "How to Measure Physical Activity Intensity", page dated 4 December 2025, https://www.cdc.gov/physical-activity-basics/measuring/index.html . Read level: full page, via browser text extraction (curl was refused by the server). Supports: the definition of a MET, the moderate and vigorous cut-offs, the 0 to 10 relative scale, the fit-person sentence and the talk test.
3. R. A. Robergs and R. Landwehr, "The surprising history of the 'HRmax=220-age' equation", Journal of Exercise Physiology Online 5(2), 2002, pp. 1 to 10 . Read level: full text. Supports: the absence of a research record, the conversations with Karvonen and Åstrand and Åstrand's comment, the 1971 origin, the re-fit's error of about 21 beats, the quotations, and the table listing Tanaka's equation with no error figure. The point about correlations on group means is this course's, as the body says.
4. H. Tanaka, K. D. Monahan and D. R. Seals, "Age-predicted maximal heart rate revisited", Journal of the American College of Cardiology 37(1), 2001, pp. 153 to 156, doi 10.1016/S0735-1097(00)01054-8 . Read level: abstract only. Supports: the study counts, the equation and r, the finding on sex and activity, and the conclusion about older adults. No individual error is given from this paper, because the course didn't read the part that reports it.
5. J. Martin, B. Lindsey, C. Gerrity and J. Ambegaonkar, "Exploratory analysis of the accuracy of age-based maximal heart rate equations across cardiorespiratory fitness levels", PLoS One 20(10), 2025, e0335842, doi 10.1371/journal.pone.0335842 . Read level: full text. Supports: the sample, the seven formulas, Tanaka's mean absolute error and its proportional bias, the absence of mean or proportional bias for the 220 formula, the authors' practical case for it, the limits of agreement across all seven formulas and for each, the typical error, the training-zones sentence, the note on beta-blockers, the sentence on population-level benchmarking and the conclusion. The 50-year-old and 65-year-old ranges are this course's arithmetic on the paper's ±20.
6. D. Stensvold and colleagues, "Effect of exercise training for five years on all cause mortality in older adults: the Generation 100 study", BMJ 371, 2020, m3485, doi 10.1136/bmj.m3485. **Read level: full text.** Supports: the Borg 6 to 20 anchors as the trial describes them, the pairings of about 90 per cent of peak heart rate with about 16 and about 70 per cent with about 13, and the participants' ages.
7. G. A. Borg, "Psychophysical bases of perceived exertion", Medicine and Science in Sports and Exercise 14(5), 1982, pp. 377 to 381, doi 10.1249/00005768-198205000-00012 . Read level: abstract only. Supports: the phrase quoted and the paraphrase of the category-ratio method. The abstract prints neither scale, which the body says.
8. R. Persinger, C. Foster, M. Gibson, D. C. Fater and J. P. Porcari, "Consistency of the talk test for exercise prescription", Medicine and Science in Sports and Exercise 36(9), 2004, pp. 1632 to 1636, PMID 15354048 . Read level: abstract only. Supports: the 16 volunteers, the design, the ventilatory threshold quotation, the 84 and 74 per cent figures at the threshold, the 80 and 81 per cent figures at the talk test's equivocal stage, and the finding that they did not differ significantly. The definition of heart-rate reserve is this course's background.
9. J. L. Reed and A. L. Pipe, "The talk test: a useful tool for prescribing and monitoring exercise intensity", Current Opinion in Cardiology 29(5), 2014, pp. 475 to 480, doi 10.1097/HCO.0000000000000097 . Read level: abstract only. Supports: the consistency across five kinds of exercise and the quotation about interval training.
10. Y. Kwon, K. W. Kang and J. S. Chang, "The talk test as a useful tool to monitor aerobic exercise intensity in healthy population", Journal of Exercise Rehabilitation 19(3), 2023, pp. 163 to 169, PMC10331140 . Read level: full text. Supports: the 17 young adults, the recited passage and the stages tracking heart rate, oxygen uptake and ventilation.
11. M. C. Zourdos, A. Klemp, C. Dolan and colleagues, "Novel resistance training-specific rating of perceived exertion scale measuring repetitions in reserve", Journal of Strength and Conditioning Research 30(1), 2016, pp. 267 to 275, doi 10.1519/JSC.0000000000001049. **Read level: abstract only.** Supports: the pairing of ratings with RIR and the sample.
12. I. Halperin, T. Malleron, I. Har-Nir and colleagues, "Accuracy in predicting repetitions to task failure in resistance exercise: a scoping review and exploratory meta-analysis", Sports Medicine 52(2), 2022, pp. 377 to 390, doi 10.1007/s40279-021-01559-x . Read level: abstract only. Supports: the 12 studies and 414 participants, the underprediction figure, the conditions under which accuracy was slightly or trivially better, the training-status finding and the conclusion. That the underprediction figure hides large differences between studies is from the abstract's heterogeneity figure.
13. B. S. Currier and colleagues, "Resistance training prescription for muscle function, hypertrophy, and physical performance in healthy adults: an overview of reviews", ACSM position stand, Medicine and Science in Sports and Exercise 58(4), 2026, pp. 851 to 872, doi 10.1249/MSS.0000000000003897 . Read level: full text of the article (Europe PMC XML: introduction, methods, all results sections, Table 1 to Table 6 as rendered, discussion, limitations, disclosures). Supplemental appendices not opened. Supports: "high effort", the 2 to 3 RIR target and the sentence on insufficient evidence for exact targets.
14. A. Shcherbina, C. M. Mattsson, D. Waggott and colleagues, "Accuracy in wrist-worn, sensor-based measurements of heart rate and energy expenditure in a diverse cohort", Journal of Personalized Medicine 7(2), 2017, 3, doi 10.3390/jpm7020003 . Read level: full text. Supports: the design, the heart-rate and energy errors, the factors linked to higher error and the conclusion.
15. D. Fuller, E. Colwell, J. Low and colleagues, "Reliability and validity of commercially available wearable devices for measuring steps, energy expenditure, and heart rate: systematic review", JMIR mHealth and uHealth 8(9), 2020, e18694, doi 10.2196/18694 . Read level: full text. Supports: the 158 publications to May 2019, the heart-rate and energy figures, and the authors' note that devices keep changing.
16. World Health Organization, WHO guidelines on physical activity and sedentary behaviour, 2020, ISBN 9789240015128 . Read level: full text downloaded; the adult, older-adult, pregnancy and chronic-condition recommendation boxes and good practice statements read closely. Supports: the good practice statement on some activity, and the older-adult statement on adjusting effort relative to fitness.
17. US Department of Health and Human Services, Physical Activity Guidelines for Americans, 2nd edition, 2018 . Read level: full text downloaded; the key guidelines read. Supports: the older-adult sentence on judging effort relative to fitness.
18. Department of Health and Social Care, UK Chief Medical Officers' physical activity guidelines, first published 2019, updated 10 July 2026 . Read level: full text of the current (2026) HTML version: foreword, executive summary, adult and older adult guidelines and supporting evidence. Supports: brisk walking and running as the examples of moderate and vigorous activity.
19. Two sources on exertional rhabdomyolysis . F. G. O'Connor, P. Deuster, J. Leggit and colleagues, Clinical Practice Guideline for the Management of Exertional Rhabdomyolysis in Warfighters 2020, Consortium for Health and Military Performance. Read level: full text of the 2020 version. Supports: the contributing-factors quotation. And L. P. S. Yow, H. Y. Ho, I. Y. W. Lum and I. M. Hanif, "Exercise-induced rhabdomyolysis: a case series of spin-related rhabdomyolysis", Cureus 13(7), 2021, e16352, doi 10.7759/cureus.16352. Read level: full text. Supports: the five previously fit adults admitted after a first one-hour spin class, and the prevention list, including telling the instructor you're new. That breathing-based measures don't guard against this is this course's reading, as the body says.
20. J. D. MacDougall, D. Tuxen, D. G. Sale, J. R. Moroz and J. R. Sutton, "Arterial blood pressure response to heavy resistance exercise", Journal of Applied Physiology 58(3), 1985, pp. 785 to 790, doi 10.1152/jappl.1985.58.3.785 . Read level: abstract only. Supports: the sharp rise in blood pressure when the breath is held under load, measured in five experienced bodybuilders lifting to failure. Lesson 6 covers it and the guidance on it.