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Training for Health, Training to Improve
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In one pass Moving for health and training to get better are two different goals.
Educational content, not medical advice — consult a clinician.
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Chapter 1
Health asks for regular movement
Moving for health and training to get better are two different goals. Health asks the body for a set of adaptations: muscle that takes up blood sugar more readily, a healthier inner lining of the blood vessels, steadier blood pressure, and less muscle loss with age. Most of these do not require training very hard.
One reason is that many of the benefits come from each session itself, and they last only a while. When a muscle contracts, it moves its glucose transporter, , to the cell surface and takes up sugar without needing insulin. After one bout of endurance exercise, blood pressure can stay lower for up to about 22 hours. Blood flow sweeping again and again over the vessel lining changes how the vessels work and how their walls are built. So for health, moving regularly matters more than now and then pushing to your limit.
The WHO 2020 guidelines put it this way: some activity is better than none and more is better, but the higher you go, the less each extra amount adds, and no one can say exactly where that tapering begins.
If you get chest pain or tightness, or faint, during exercise, stop and call emergency services now; see Balancing health and progress.
One reason is that many of the benefits come from each session itself, and they last only a while. When a muscle contracts, it moves its glucose transporter, , to the cell surface and takes up sugar without needing insulin. After one bout of endurance exercise, blood pressure can stay lower for up to about 22 hours. Blood flow sweeping again and again over the vessel lining changes how the vessels work and how their walls are built. So for health, moving regularly matters more than now and then pushing to your limit.
The WHO 2020 guidelines put it this way: some activity is better than none and more is better, but the higher you go, the less each extra amount adds, and no one can say exactly where that tapering begins.
If you get chest pain or tightness, or faint, during exercise, stop and call emergency services now; see Balancing health and progress.
Numbers · Why the dose curve flattens
Population data can only be background, but it says the same thing as the mechanism above. Arem 2015 pooled 6 prospective cohorts with 661,137 adults, followed for a median of 14.2 years, and compared self-reported leisure-time activity with the risk of death. Compared with people who did no leisure activity, those below the recommended minimum had a 20% lower risk of death; those at 1 to 2 times the minimum, 31% lower; at 2 to 3 times, 37% lower; and at 3 to 5 times the curve leveled off, at 39% lower. Going from the minimum to several times the minimum bought only a few more percentage points. People at 10 or more times the minimum showed no rise in risk either. These are observed associations and cannot prove that the amount of activity caused them.The WHO 2020 guidelines give adults a range of 150–300 minutes a week of moderate-intensity, or 75–150 minutes of vigorous-intensity, aerobic activity, or an equivalent mix, plus strength training that works all the major muscle groups on 2 or more days a week. The guidelines say that more than 300 minutes of moderate activity brings additional benefit, but that recommendation is only conditional, because no one can say where the gains start to shrink. For strength training, the guidelines found no evidence that doing more brings more health benefit.
So the curve says this: going from inactive to regularly active gains the most; going from regular to a great deal gains less, but it does not turn into harm.
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Mechanism · What one session changes in the body
Thompson 2001's review puts it plainly: a single exercise session can temporarily lower , raise the so-called good cholesterol, high-density lipoprotein () cholesterol, lower blood pressure, and improve insulin sensitivity and blood sugar control. The authors suggest that part of what people credit to long-term training actually comes from the last few sessions. The blood-pressure effect has a low threshold and has been seen after exercise at about 40% of maximal capacity; the effect on blood sugar seems to need exercise near 70% of maximal capacity, but that has not been carefully studied.On blood pressure, the American College of Sports Medicine (ACSM) 2004 position stand says that in people with high blood pressure, one bout of endurance exercise, or sustained training, lowers blood pressure by about 5–7 mmHg; after a single session the drop lasts up to about 22 hours, and it is largest in people whose starting pressure is highest.
Long-term training adds another layer. The muscle makes more of the glucose transporter itself, so its capacity to take up sugar grows. Blood flow rubbing again and again against the artery lining, a frictional force called shear stress, changes how the arteries work, how wide they are and how thick their walls are; Green 2017's review counts this among the direct reasons exercise lowers cardiovascular risk.
Following the mechanism, this is why regularity matters more than intensity: part of the benefit has to be refreshed session after session. That step is an inference; no trial has compared it directly.
Chapter 2
Improving keeps asking for more
Running faster, lifting heavier and winning games is a different goal. The body adapts only to a stimulus beyond what it is used to, and it builds what you train: long endurance training makes muscle build more mitochondria, and heavy strength training makes it build more contractile protein, so strength and muscle grow together.
The catch is that once the body has adapted, the old stimulus is no longer beyond what it is used to. To keep improving, you have to keep raising the demand: more weight, more volume, more sessions, or more sport-specific work. The American College of Sports Medicine position stand on strength training says that continued adaptation requires progressive training, and the training frequency it gives rises from 2–3 days a week for beginners to 4–5 days for advanced lifters.
Set this against health and the difference is clear. The WHO guidelines find strength training on 2 or more days a week good for health, but found no evidence that doing more brings more health benefit; muscle size, by contrast, keeps growing with more weekly sets. The extra training done for performance is not more health; it is a separate investment. It brings progress, and it also brings a higher risk of injury; see How hard training leads to injury.
The catch is that once the body has adapted, the old stimulus is no longer beyond what it is used to. To keep improving, you have to keep raising the demand: more weight, more volume, more sessions, or more sport-specific work. The American College of Sports Medicine position stand on strength training says that continued adaptation requires progressive training, and the training frequency it gives rises from 2–3 days a week for beginners to 4–5 days for advanced lifters.
Set this against health and the difference is clear. The WHO guidelines find strength training on 2 or more days a week good for health, but found no evidence that doing more brings more health benefit; muscle size, by contrast, keeps growing with more weekly sets. The extra training done for performance is not more health; it is a separate investment. It brings progress, and it also brings a higher risk of injury; see How hard training leads to injury.
Evidence · How much strength work is enough
For the same strength training, the answer depends on whether you are asking about health or about muscle.On health: the WHO 2020 guidelines strongly recommend (moderate-certainty evidence) that adults do moderate- or higher-intensity strength training that works all the major muscle groups on 2 or more days a week, and they state that there is no evidence that larger volumes of strength training bring more health benefit.
On muscle: Schoenfeld 2017's pooled 34 training groups from 15 studies; each additional weekly set was linked with a 0.37% larger gain in muscle size on average, and high- and low-volume groups differed by 3.9%. The abstract reports no plateau or ceiling, but when volume was compared in three bands (fewer than 5, 5–9, and 10 or more sets a week), the difference was only a trend.
On strength: the American College of Sports Medicine 2009 position stand is a progression framework for coaches. Beginners use a weight they can lift about 8–12 times; experienced lifters cycle through weights they can lift 1–12 times, shifting gradually toward heavy loads; when they can do one or two repetitions more than the target, the weight goes up by 2–10%. Training frequency rises from 2–3 days a week to 4–5.
Put the three together: the health line is reached at a modest amount, while the performance line keeps asking for more. That does not mean more training is harmful; it means more training buys something else.
Mechanism · Why you build what you train
Coffey and Hawley's 2007 review takes training adaptation down to the molecular level: muscle changes which proteins it makes, and how much, according to the stimulus. The result is set by training volume, intensity and frequency together with the half-life of each protein, and many features are specific to the type of stimulus. Long endurance training brings more mitochondria, a shift of muscle fibers toward the slow type and changes in how fuels are used; heavy strength training brings more synthesis of contractile protein, muscle growth and higher maximal force. The molecular signals behind the two paths are different.This is specificity: whichever capacity is challenged again and again is the one that grows most. To get better at something, you have to keep giving that thing a stimulus beyond what it is used to; the practical ways to apply progressive overload are covered in Progressive Overload.
Hawley 2018's review adds that the methods athletes use to amplify endurance adaptation all rest on a larger metabolic load and a larger disturbance inside the cell, repeated over months and years. In other words, the higher your level, the more each further step costs. If you want both kinds of training, they also hold each other back a little (see The Interference Effect).
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Chapter 3
How hard training leads to injury
When hard training harms you, it is usually not that the heart and lungs have been worn out; the load has risen faster than the slowest tissues. Muscle and cardiovascular fitness can catch up with a new training load within weeks, while tendon, bone and cartilage are much slower. Measuring adult Achilles tendons with the carbon-14 that nuclear tests left in the air showed that the tendon core is barely renewed for decades, while muscle keeps renewing itself. After a single session, the breakdown of collagen in a tendon also peaks earlier than its synthesis.
So the danger is not training a lot; it is suddenly training a lot more. An International Olympic Committee (IOC) consensus lists rapid changes in training and competition load as a major risk factor for injury, and athletes who carry high loads steadily, year in, year out, may actually get injured less (see Training injuries).
The heart follows a similar pattern. In a study that followed more than twenty thousand male doctors, the of sudden cardiac death rose sharply during vigorous exertion and the 30 minutes after it, yet the absolute risk of any single session was extremely low, and in people who already exercised vigorously on a regular basis the risk rose less.
So the danger is not training a lot; it is suddenly training a lot more. An International Olympic Committee (IOC) consensus lists rapid changes in training and competition load as a major risk factor for injury, and athletes who carry high loads steadily, year in, year out, may actually get injured less (see Training injuries).
The heart follows a similar pattern. In a study that followed more than twenty thousand male doctors, the of sudden cardiac death rose sharply during vigorous exertion and the 30 minutes after it, yet the absolute risk of any single session was extremely low, and in people who already exercised vigorously on a regular basis the risk rose less.
Mechanism · Why tendon and bone lag behind
The gap comes from how fast each tissue renews itself. Heinemeier 2013 used the carbon-14 that mid-20th-century nuclear tests left in the atmosphere as a label and measured 28 samples of adult Achilles tendon core. Their carbon-14 levels matched the air of several decades before sampling, which means this tissue is barely renewed; the muscle samples measured alongside kept renewing.Tendons can adapt; they just keep a different rhythm. Magnusson 2010's review says that loading makes a tendon build more collagen; synthesis peaks about 24 hours after exercise and stays raised for about 3 days, while collagen breakdown also rises and peaks earlier than synthesis. Following that timeline, in the period right after training the tendon is mostly taking old material apart, and only later building new.
Bohm 2015's pooled 27 training studies lasting at least 8 weeks. Tendons do become stiffer and stronger, mainly according to how heavy the load is rather than the type of contraction; studies of 12 weeks or more showed somewhat larger effects, though the difference was not statistically significant. Bone also strengthens under load, because the loading signal makes bone-building cells more active. And recreational runners actually have lower rates of hip and knee osteoarthritis than sedentary people; see the chapter on why athletes' joints don't fall apart (see joints).
So when a few weeks of training make you feel you can do more now, that signal comes from muscle and the heart and lungs; the bill from tendon and bone is counted in months. Each tissue's timetable is covered in detail in Training injuries.
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Evidence · Can a load ratio predict injury
Many teams and fitness apps compute an acute:chronic workload ratio (ACWR): the last week's training load divided by the average of the past few weeks. It comes from a 2016 review by Gabbett, who proposed a training-injury prevention paradox: athletes used to high loads get injured less, and excessive, rapid increases in load may be behind a large share of non-contact soft-tissue injuries. He put the ratio forward as a predictor of injury.The ratio has since drawn direct criticism. Impellizzeri 2020 points out that no study has properly estimated a causal link between it and injury, so changing the ratio to lower injury rates remains conjecture, and that the ratio's statistical properties are poor and create artifacts. Their conclusion: there is no evidence supporting the use of the ACWR to manage training load or to guide injury prevention.
What survives is the mechanism behind it, not the number: a tissue bears load relative to what it is already used to. A specific numerical rule fares no better: in a randomized trial of novice runners, a plan that limited the weekly increase to 10% did not reduce injuries (see Training injuries).
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Numbers · Vigorous exercise and the heart
Albert 2000 followed 21,481 male doctors in the US Physicians' Health Study who had no self-reported cardiovascular disease at the start, and confirmed 122 sudden deaths over 12 years. During vigorous exertion and the 30 minutes after it, the of sudden cardiac death was 16.9 times that of other times, yet the absolute risk was extremely low: about 1 death per 1.51 million episodes of vigorous exertion. In people who exercised vigorously on a regular basis, this relative risk rose less. The American College of Sports Medicine 2015 consensus on pre-exercise screening adds that exercise-related cardiovascular events are often preceded by warning symptoms, and that the cardiovascular risk of exercise falls as people become more active and fit.For most people, these numbers point to one approach: raise intensity from the level you are used to, rather than jumping from barely moving straight to all-out.
Another common question is whether years of high-level training can harm the heart. Newman 2021's of case-control and cohort studies found that athletes had 2.46 times the odds of atrial fibrillation (an irregular heart rhythm) compared with non-athletes; the odds were higher in mixed sports than in endurance sports, and higher in athletes under 55. These are observational studies; they cannot show that training volume caused it, or how much training counts as too much. They are about athletes. In the general population, in Arem 2015's pooled data, people active at 10 or more times the recommended minimum still had a lower risk of death than people who did no leisure activity ( 0.69).
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Chapter 4
Why hard exercise feels so good
The great mood after a hard game is not the same as the pleasant feeling of a walk. During hard exercise, the sympathetic nerves and the adrenal glands release two catecholamines, adrenaline and noradrenaline (the body's stress hormones), into the blood. Their levels can climb to 1.5 to more than 20 times the usual amount, and the harder and longer the exercise, the higher they go. The heart speeds up and you feel keyed up.
The runner's high used to be credited to endorphins; the evidence now leans toward endocannabinoids, signaling molecules the body makes itself that act on cannabinoid receptors. In a randomized trial of 63 people, 45 minutes of moderate-intensity running left them happier and less anxious, with higher endocannabinoids in the blood; blocking opioid receptors with a drug did not stop any of these changes.
With low-intensity exercise such as walking, most people feel good while they are doing it; once intensity passes a threshold, how people feel during exercise generally gets worse. A match adds competition, teamwork and winning or losing, and their effect on mood has not been measured separately. Feeling good does not mean better for your health: endocannabinoids changed clearly only after moderate intensity, not after very hard exercise.
The runner's high used to be credited to endorphins; the evidence now leans toward endocannabinoids, signaling molecules the body makes itself that act on cannabinoid receptors. In a randomized trial of 63 people, 45 minutes of moderate-intensity running left them happier and less anxious, with higher endocannabinoids in the blood; blocking opioid receptors with a drug did not stop any of these changes.
With low-intensity exercise such as walking, most people feel good while they are doing it; once intensity passes a threshold, how people feel during exercise generally gets worse. A match adds competition, teamwork and winning or losing, and their effect on mood has not been measured separately. Feeling good does not mean better for your health: endocannabinoids changed clearly only after moderate intensity, not after very hard exercise.
Evidence · Endorphins or endocannabinoids
On this question, the human and animal evidence have to be read separately.In people: Boecker 2008 scanned 10 athletes with PET and found that after a 2-hour run, more opioid receptors were occupied in the prefrontal and limbic areas of the brain, and the stronger the euphoria, the more were occupied. That supports a role for endogenous opioids such as endorphins. Siebers 2021 ran a double-blind : 63 people took either the opioid-blocking drug naltrexone or a placebo, then ran for 45 minutes. Both groups became more euphoric and less anxious, endocannabinoids rose just the same, and a similar share in each group reported a runner's high. So in people, the high does not depend on opioid signaling.
In mice: Fuss 2015 found that the anxiety relief and pain relief after running depend on cannabinoid receptors and disappear when those receptors are blocked; euphoria itself, however, cannot be studied in mice.
Intensity matters too. Raichlen 2013 had runners run at four intensities; only after moderate intensity did endocannabinoids in the blood change clearly, not at very low or very high intensity.
Read the limits carefully. The human evidence shows that endocannabinoids rise and that blocking opioid receptors does not affect the high. What has been shown directly in mice is that the anxiety relief and pain relief after running depend on cannabinoid receptors; euphoria itself cannot be measured in mice. So in people, endocannabinoids are the strongest candidate, not a settled answer.
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Evidence · Team sports and mood
Chekroud 2018 analyzed survey data from 1,237,194 US adults: people who exercised had 1.49 fewer days of poor mental health in the past month (43.2% fewer) than similar people who did not. Every type of exercise was associated with fewer such days; the largest associations were for popular team sports (22.3% fewer), cycling (21.6% fewer), and aerobic and gym activities (20.1% fewer), and sessions of about 45 minutes three to five times a week showed the strongest association. The authors wrote one line in particular: more exercise was not always better.This is a cross-sectional study: it asks about exercise and mood at the same moment, so it cannot tell which came first, and it is just as plausible that people in a good mood are more willing to go and play. So it shows that team sports and good mood often go together; it cannot show that playing a sport improves mood more than walking does.
Ekkekakis 2011's review supplies the other half: below the ventilatory or lactate threshold, most people feel pleasant during exercise; near the threshold, people differ widely; above it, how people feel during exercise generally gets worse. Letting people choose their own intensity often makes higher intensities easier to accept.
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Chapter 5
Balancing health and progress
Health and progress do not conflict, but they need to be planned separately. Health is the floor: the WHO 2020 guidelines give adults a range of 150–300 minutes a week of moderate-intensity, or 75–150 minutes of vigorous-intensity, aerobic activity, plus strength training on 2 or more days a week, and bouts of any length count. Above the floor sits the part you add for progress: raise the demand step by step, and plan lighter stretches so tendon and bone can catch up (see Recovery science).
Exercising on just two days at the weekend counts too. Two large observational studies found that for the same total amount, packing it into one or two days or spreading it across the week was associated with similar risks of death and cardiovascular disease.
Legs that are sore for a few days and hurt less each time are usually adapting normally (see DOMS). Performance that keeps falling for weeks and does not recover however much you rest should make you think of overtraining (see The Interference Effect).
If you get sudden chest pain or tightness during or after training, call emergency services now. If one calf suddenly becomes swollen, warm and painful, do not wait: seek medical care right away, and call emergency services if chest pain or breathlessness comes with it. If you faint during exercise, call emergency services now.
Exercising on just two days at the weekend counts too. Two large observational studies found that for the same total amount, packing it into one or two days or spreading it across the week was associated with similar risks of death and cardiovascular disease.
Legs that are sore for a few days and hurt less each time are usually adapting normally (see DOMS). Performance that keeps falling for weeks and does not recover however much you rest should make you think of overtraining (see The Interference Effect).
If you get sudden chest pain or tightness during or after training, call emergency services now. If one calf suddenly becomes swollen, warm and painful, do not wait: seek medical care right away, and call emergency services if chest pain or breathlessness comes with it. If you faint during exercise, call emergency services now.
Evidence · Is a weekend-only routine enough
dos Santos 2022 followed 350,978 US adults for a median of 10.4 years, with activity self-reported. Compared with inactive people, weekend warriors (1 to 2 sessions a week) had an all-cause mortality of 0.92 (95% 0.83–1.02, an interval that crosses 1), and the regularly active (3 or more sessions a week) an HR of 0.85. For the same total amount of activity, the two patterns had similar risks of death (HR 1.08, 95% CI 0.97–1.20).Khurshid 2023 used wrist accelerometers to record a week of activity in 89,573 people in the UK Biobank. People who did at least 150 minutes a week of moderate-to-vigorous activity with more than half of it on one or two days, and people who spread it out, both had similarly lower risks of atrial fibrillation, heart attack, heart failure and stroke than inactive people.
The WHO 2020 guidelines changed their wording too: bouts no longer have to last at least 10 minutes, and bouts of any length count toward the total.
All of these are observational studies. They answer whether the total amount is enough; they do not answer injury. Following the mechanism, concentrating activity has two costs. Part of what each session brings lasts only a while; the blood-pressure drop after one bout of endurance exercise, for example, lasts up to about 22 hours. And sitting still all week and then going hard at the weekend is exactly the situation of a sudden rise in load, which neither the tissues nor the heart are used to; see How hard training leads to injury. So if the weekend is the time you have, go ahead and train, raising intensity slowly from the level you are used to; any movement on other days also adds to the total.
In practice · Is constant soreness overtraining
Say you play a sport four times a week and your legs are always sore. First work out which kind of soreness it is.Delayed-onset muscle soreness () is the kind that comes a day or two after a new movement or a sudden jump in load and fades within a few days; when the same exercise is repeated a few times, the soreness gets milder each time (see DOMS). If the soreness clears between games and your performance is holding up, your body is most likely adapting.
Overtraining shows a different set of signs. A joint consensus of the European College of Sport Science and the American College of Sports Medicine splits it into three stages: functional overreaching, a short hard stretch after which you come back stronger with rest; non-functional overreaching, where recovery drags on and you do not come out stronger; and the most severe, overtraining syndrome, in which performance keeps falling, no amount of rest brings it back, and low mood, worse sleep and frequent illness often come with it. Lasting fatigue and falling performance can also come from other problems such as anemia or infection, so have a doctor rule those out first (see The Interference Effect).
To keep improving while getting hurt less, common practice is to let total volume and intensity follow your own recovery and to plan lighter stretches in cycles; to raise only one thing at a time and give tendon and bone time measured in months; and to stop when pain is sharp, pinpoint, or gets worse the more you train (see Training injuries).
Red flag · When to stop and get help now
The signs below are not overtraining and not an ordinary injury; they need action right away. This site does not diagnose; if they appear, stop first.Sudden chest pain or tightness during or after training that feels like pressure or squeezing, may spread to the arm, neck or jaw, and comes with breathlessness or a cold sweat: it may be a heart emergency, so call emergency services now.Fainting during exercise: call emergency services now. Anyone who has fainted, even once, should see a doctor to find the cause.One calf suddenly swollen, painful and warm, especially after a long journey or long sitting: think of deep vein thrombosis and seek medical care right away, without massaging or stretching it; if chest pain or breathlessness comes with it, call emergency services now.Marked breathlessness, a racing heart or feeling about to faint during exercise: stop at once and do not push through; if the symptoms do not settle, seek medical care right away.
The American College of Sports Medicine consensus on pre-exercise screening notes that exercise-related cardiovascular events are often preceded by warning symptoms. So once these signs appear, do not finish the set or the game.
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References · 24
- Thompson, P. D., Crouse, S. F., Goodpaster, B., Kelley, D., Moyna, N., & Pescatello, L. (2001). The acute versus the chronic response to exercise. Medicine and Science in Sports and Exercise, 33(6 Suppl), S438-S445. 'There is strong and consistent evidence that a single exercise session can acutely reduce triglycerides and increase high-density lipoprotein (HDL) cholesterol (HDL-C), reduce blood pressure, and improve insulin sensitivity and glucose homeostasis. Such observations suggest that at least some of the effects on atherosclerotic cardiovascular disease (ASCVD) risk factors attributed to exercise training may be the result of recent exercise.' Training increases exercise capacity, permitting larger acute effects. 'The acute effect of exercise on blood pressure is a low threshold phenomenon and has been observed after energy expenditures requiring only 40% maximal capacity. The acute effect of exercise on glucose metabolism appears to require exercise near 70% maximal, but this issue has not been carefully examined.' Conclusion: exercise has definite acute effects on lipids, blood pressure and glucose homeostasis; 'Considerable additional research is required to define the threshold of exercise required to produce these putatively beneficial effects' (abstract, PMID 11427768). 10.1097/00005768-200106001-00012
- Pescatello, L. S., Franklin, B. A., Fagard, R., Farquhar, W. B., Kelley, G. A., & Ray, C. A. (2004). American College of Sports Medicine position stand: Exercise and hypertension. Medicine & Science in Sports & Exercise, 36(3), 533-553. Documents post-exercise hypotension persisting ~22 h after a single moderate-intensity session; recommends timing training to coincide with morning BP peak. 10.1249/01.MSS.0000115224.88514.3A
- Richter, E. A., & Hargreaves, M. (2013). Exercise, GLUT4, and skeletal muscle glucose uptake. Physiological Reviews, 93(3), 993-1017. Muscle contraction activates AMPK which translocates GLUT4 to the sarcolemma independently of insulin; this effect persists for hours post-exercise. 10.1152/physrev.00038.2012
- Green, D. J., Hopman, M. T. E., Padilla, J., Laughlin, M. H., & Thijssen, D. H. J. (2017). Vascular adaptation to exercise in humans: role of hemodynamic stimuli. Physiological Reviews, 97(2), 495-528. Reviews the functional and structural arterial adaptations that follow repeated episodic exposure to the hemodynamic forces of exercise: 'Exercise modifies blood flow, luminal shear stress, arterial pressure, and tangential wall stress, all of which can transduce changes in arterial function, diameter, and wall thickness.' Implications include atherosclerotic risk in conduit arteries, blood pressure control in resistance vessels and microvascular health. Conclusion: 'Exercise training studies have demonstrated that direct hemodynamic impacts on the health of the artery wall contribute to the well-established decrease in cardiovascular risk attributed to physical activity' (abstract, PMID 28151424). 10.1152/physrev.00014.2016
- Bull, F. C., Al-Ansari, S. S., Biddle, S., Borodulin, K., Buman, M. P., Cardon, G., et al. (2020). World Health Organization 2020 guidelines on physical activity and sedentary behaviour. British Journal of Sports Medicine, 54(24), 1451-1462. 'All adults should undertake 150-300 min of moderate-intensity, or 75-150 min of vigorous-intensity physical activity, or some equivalent combination of moderate-intensity and vigorous-intensity aerobic physical activity, per week'; the guidelines recommend regular muscle-strengthening activity for all age groups. Conclusion: 'They reaffirm messages that some physical activity is better than none, that more physical activity is better for optimal health outcomes and provide a new recommendation on reducing sedentary behaviours.' Full text (PMC7719906): adults should also do muscle-strengthening activities at moderate or greater intensity involving all major muscle groups on 2 or more days a week (strong recommendation, moderate-certainty evidence), and 'There was no evidence to support a dose-response association with higher volumes of muscle-strengthening activities.' 'MVPA bouts of any duration now count towards these recommendations, reflecting new evidence to support the value of total physical activity volume, regardless of bout length.' There is moderate-certainty evidence of a curvilinear dose-response for all-cause and CVD mortality and incident cancer and diabetes: 'More physical activity is better, although the relative benefits tend to diminish at higher levels of physical activity. However, it is not possible to specify the physical activity levels where diminishing returns begin.' Hence the recommendation that more than 300 min moderate (or 150 min vigorous) a week has additional health benefits is rated conditional. 10.1136/bjsports-2020-102955
- NHS. (2026). Fainting (page last reviewed 17 August 2026). Fainting is when you pass out for a short time; it is not usually serious, but anyone who has fainted should see a GP to find out what might have caused it. Causes can include standing up too quickly (which could be a sign of low blood pressure), not eating or drinking enough, being too hot, being very upset or in severe pain, heart problems, and taking drugs or drinking too much alcohol. Call 999 if someone is not breathing, cannot be woken up within 1 minute, has not fully recovered or has difficulty with speech or movement, has chest pain or a pounding, fluttering or irregular heartbeat (palpitations), has seriously hurt themselves before or after fainting, is shaking or jerking (a seizure), fainted while exercising, or fainted while lying down; do not drive yourself to A&E. If you feel about to faint: lie down with your legs raised, or if you cannot, sit with your head lowered between your knees; drink some water; cross your legs while standing up or rock up and down on your toes; clench your fists. If you see someone faint: check whether they respond by gently shaking their shoulders and asking loudly; if not, shout for help and tilt back the head and lift the chin; listen for breathing for at least 10 seconds; if they are breathing normally, lay them on their back and raise their legs (on their side if pregnant, especially over 28 weeks); they usually wake up within 30 seconds. www.nhs.uk/conditions/fainting
- Coffey, V. G., & Hawley, J. A. (2007). The molecular bases of training adaptation. Sports Medicine, 37(9), 737-763. 'The functional consequences of these adaptations are determined by training volume, intensity and frequency, and the half-life of the protein. Moreover, many features of the training adaptation are specific to the type of stimulus, such as the mode of exercise.' Prolonged endurance training elicits mitochondrial biogenesis, fast-to-slow fibre-type transformation and changes in substrate metabolism; heavy resistance exercise stimulates synthesis of contractile proteins responsible for hypertrophy and increased maximal force; the genetic and molecular mechanisms of the two are distinct. The authors add that it cannot yet be claimed that this molecular work has influenced the training practices of elite athletes (abstract, PMID 17722947). 10.2165/00007256-200737090-00001
- American College of Sports Medicine. (2009). Progression models in resistance training for healthy adults. Medicine & Science in Sports & Exercise, 41(3), 687-708. ACSM position stand: progressive overload through gradual increases in load, volume, or frequency; periodization optimizes long-term adaptation. 10.1249/MSS.0b013e3181915670
- Schoenfeld, B. J., Ogborn, D., & Krieger, J. W. (2017). Dose-response relationship between weekly resistance training volume and increases in muscle mass: A systematic review and meta-analysis. Journal of Sports Sciences, 35(11), 1073-1082. Meta-regression of 34 treatment groups from 15 studies: each additional weekly set was associated with a 0.37% larger gain in muscle size, and higher- vs lower-volume groups differed by 3.9%; as a three-level variable (< 5, 5-9 and 10+ sets per muscle per week) the effect was only a trend (P = 0.074). The authors conclude a graded dose-response; the abstract reports no plateau or upper limit (abstract, PMID 27433992). 10.1080/02640414.2016.1210197
- Heinemeier, K. M., Schjerling, P., Heinemeier, J., Magnusson, S. P., & Kjaer, M. (2013). Lack of tissue renewal in human adult Achilles tendon is revealed by nuclear bomb 14C. The FASEB Journal, 27(5), 2074–2079. Bomb-pulse 14C in 28 forensic Achilles tendon core samples and 4 muscle samples: the tendon core retained 14C levels matching the atmosphere several decades before sampling, showing very limited tissue turnover, while muscle showed continuous turnover (abstract, PMID 23401563). 10.1096/fj.12-225599
- Magnusson, S. P., Langberg, H., & Kjaer, M. (2010). The pathogenesis of tendinopathy: balancing the response to loading. Nature Reviews Rheumatology, 6(5), 262-268. Mechanical loading upregulates collagen expression and synthesis in tendon: 'This increase in collagen formation peaks around 24 h after exercise and remains elevated for about 3 days. The degradation of collagen proteins also rises after exercise, but seems to peak earlier than the synthesis.' 'Despite the ability of tendons to adapt to loading, repetitive use often results in injuries, such as tendinopathy'; microrupture and material fatigue are suggested injury mechanisms, implying one or more weak links in the structure (abstract, PMID 20308995). 10.1038/nrrheum.2010.43
- Soligard, T., Schwellnus, M., Alonso, J.-M., Bahr, R., Clarsen, B., Dijkstra, H. P., et al. (2016). How much is too much? (Part 1) International Olympic Committee consensus statement on load in sport and risk of injury. British Journal of Sports Medicine, 50(17), 1030-1041. Expert consensus on elite sport: 'Emerging evidence indicates that poor load management is a major risk factor for injury.' Load is defined broadly to include rapid changes in training and competition load, competition calendar congestion, psychological load and travel; the statement gives practical guidelines for prescribing and monitoring load and lists research priorities (abstract, PMID 27535989). 10.1136/bjsports-2016-096581
- Gabbett, T. J. (2016). The training-injury prevention paradox: should athletes be training smarter and harder? British Journal of Sports Medicine, 50(5), 273-280. Narrative review proposing the 'Training-Injury Prevention Paradox' model, 'a phenomenon whereby athletes accustomed to high training loads have fewer injuries than athletes training at lower workloads.' 'Excessive and rapid increases in training loads are likely responsible for a large proportion of non-contact, soft-tissue injuries.' The paper proposes the acute:chronic workload ratio as 'a best practice predictor of training-related injuries'; that claim is disputed (see impellizzeri-2020-acwr-pitfalls) and is not used on this site as a number to train by (abstract, PMID 26758673). 10.1136/bjsports-2015-095788
- Impellizzeri, F. M., Tenan, M. S., Kempton, T., Novak, A., & Coutts, A. J. (2020). Acute:chronic workload ratio: conceptual issues and fundamental pitfalls. International Journal of Sports Physiology and Performance, 15(6), 907-913. 'Because no studies have even tried to estimate causal effects properly, manipulating ACWR in practical settings in order to change injury rates remains a conjecture and an overinterpretation of the available data.' Conclusion: 'There is no evidence supporting the use of ACWR in training-load-management systems or for training recommendations aimed at reducing injury risk. The statistical properties of the ratio make the ACWR an inaccurate metric and complicate its interpretation for practical applications' (abstract, PMID 32502973). 10.1123/ijspp.2019-0864
- Albert, C. M., Mittleman, M. A., Chae, C. U., Lee, I. M., Hennekens, C. H., & Manson, J. E. (2000). Triggering of sudden death from cardiac causes by vigorous exertion. The New England Journal of Medicine, 343(19), 1355-1361. Nested case-crossover analysis within the Physicians' Health Study: 122 sudden deaths among 21,481 male physicians who were free of self-reported cardiovascular disease at baseline. Relative risk of sudden death during and up to 30 minutes after vigorous exertion was 16.9 (95% CI 10.5-27.0), but the absolute risk was one sudden death per 1.51 million episodes of exertion, and habitual vigorous exercise attenuated the relative risk (P for trend 0.006). 10.1056/NEJM200011093431902
- Zouhal, H., Jacob, C., Delamarche, P., & Gratas-Delamarche, A. (2008). Catecholamines and the effects of exercise, training and gender. Sports Medicine, 38(5), 401-423. Adrenaline (epinephrine) and noradrenaline (norepinephrine) 'are the main hormones whose concentrations increase markedly during exercise'; studies report '1.5 to >20 times basal concentrations depending on exercise characteristics (e.g. duration and intensity)'. They take part in cardiovascular and respiratory adjustments and in substrate mobilization and use. Endurance-trained men show a higher adrenaline response than untrained men to intense exercise at the same relative intensity (the 'sports adrenal medulla'); in women the evidence is scarce and more conflicting (abstract, PMID 18416594). 10.2165/00007256-200838050-00004
- Siebers, M., Biedermann, S. V., Bindila, L., Lutz, B., & Fuss, J. (2021). Exercise-induced euphoria and anxiolysis do not depend on endogenous opioids in humans. Psychoneuroendocrinology, 126, 105173. Double-blind, randomized, placebo-controlled trial of the opioid antagonist naltrexone: participants (N = 63) showed increased euphoria and decreased anxiety after 45 min of moderate-intensity treadmill running compared with walking, and running raised plasma anandamide and 2-arachidonoylglycerol. 'Opioid blockade did not prevent the development of euphoria and reduced anxiety as well as elevation of eCB levels following exercise.' Conclusion: 'this study indicates that the development of a runner's high does not depend on opioid signaling in humans, but makes eCBs strong candidates in humans, as previously shown in mice' (abstract, PMID 33582575). 10.1016/j.psyneuen.2021.105173
- Raichlen, D. A., Foster, A. D., Seillier, A., Giuffrida, A., & Gerdeman, G. L. (2013). Exercise-induced endocannabinoid signaling is modulated by intensity. European Journal of Applied Physiology, 113(4), 869-875. Recreationally fit runners ran on a treadmill at four intensities: 'eCB signaling is indeed intensity dependent, with significant changes in circulating eCBs observed following moderate intensities only (very high and very low intensity exercises do not significantly alter circulating eCB levels).' The authors conclude the results support the hypothesis that eCB activity is related to the neurobiological effects of exercise, and that future studies must take intensity into account (abstract, PMID 22990628). 10.1007/s00421-012-2495-5
- Ekkekakis, P., Parfitt, G., & Petruzzello, S. J. (2011). The pleasure and displeasure people feel when they exercise at different intensities: decennial update and progress towards a tripartite rationale for exercise intensity prescription. Sports Medicine, 41(8), 641-671. Review of 33 studies published 1999-2009: pleasure falls mainly above the ventilatory or lactate threshold; below it most people report pleasant changes, near it responses vary widely between individuals, and above it they are uniformly negative; a self-selected intensity seems to improve tolerance of higher intensities (abstract, PMID 21780850). 10.2165/11590680-000000000-00000
- dos Santos, M., Ferrari, G., Lee, D. H., Rey-López, J. P., Aune, D., Liao, B., et al. (2022). Association of the 'weekend warrior' and other leisure-time physical activity patterns with all-cause and cause-specific mortality: a nationwide cohort study. JAMA Internal Medicine, 182(8), 840-848. 350,978 US adults (National Health Interview Survey 1997-2013, self-reported activity), median follow-up 10.4 years, 21,898 deaths. Versus inactive participants, all-cause mortality HR 0.92 (95% CI 0.83-1.02) for weekend warriors (1-2 sessions a week) and 0.85 (0.83-0.88) for the regularly active (3 or more sessions). 'Given the same amount of total MVPA, weekend warrior participants had similar all-cause and cause-specific mortality rates as regularly active participants' (HR 1.08, 95% CI 0.97-1.20, for all-cause mortality). Conclusion: people who reach recommended levels 'may experience the same benefit whether the sessions are performed throughout the week or concentrated into fewer days' (abstract, PMID 35788615). 10.1001/jamainternmed.2022.2488
- Khurshid, S., Al-Alusi, M. A., Churchill, T. W., Guseh, J. S., & Ellinor, P. T. (2023). Accelerometer-derived 'weekend warrior' physical activity and incident cardiovascular disease. JAMA, 330(3), 247-252. UK Biobank, 89,573 adults with a week of wrist accelerometry: active weekend warrior (at least 150 min of MVPA with at least 50% achieved in 1-2 days), active regular, or inactive. Both active patterns were associated with similarly lower risks of atrial fibrillation (HR 0.78 and 0.81), myocardial infarction (0.73 and 0.65), heart failure (0.62 and 0.64) and stroke (0.79 and 0.83); at the median threshold of 230.4 min a week the stroke associations were no longer significant. Conclusion: 'Physical activity concentrated within 1 to 2 days was associated with similarly lower risk of cardiovascular outcomes to more evenly distributed activity' (abstract, PMID 37462704). 10.1001/jama.2023.10875
- Meeusen, R., Duclos, M., Foster, C., Fry, A., Gleeson, M., Nieman, D., Raglin, J., Rietjens, G., Steinacker, J., & Urhausen, A. (2013). Prevention, diagnosis, and treatment of the overtraining syndrome: joint consensus statement of the European College of Sport Science (ECSS) and the American College of Sports Medicine (ACSM). European Journal of Sport Science, 13(1), 1-24. Distinguishes functional overreaching, non-functional overreaching, and overtraining syndrome (prolonged maladaptation); recovery balance and monitoring are central. 10.1080/17461391.2012.730061
- Cheung, K., Hume, P. A., & Maxwell, L. (2003). Delayed onset muscle soreness: treatment strategies and performance factors. Sports Medicine, 33(2), 145-164. 10.2165/00007256-200333020-00005
- NHS. (2026). DVT (deep vein thrombosis) (page last reviewed 30 April 2026). DVT is a blood clot in a vein, usually in the leg, and can be dangerous; get medical help as soon as possible. Symptoms: throbbing pain in 1 leg (rarely both), usually in the calf or thigh; swelling in 1 leg; red, blue or darkened skin around the painful area; swollen veins. Ask for an urgent GP appointment or NHS 111 if you think you have DVT. Call 999 or go to A&E if you have DVT symptoms such as pain and swelling and feel short of breath or have chest pain: clots can travel to the lungs (pulmonary embolism), which can be life-threatening; do not drive yourself to A&E. DVT is more likely with varicose veins, age over 60, overweight, smoking, previous DVT, oestrogen-containing contraception or HRT, cancer, pregnancy or a baby in the previous 6 weeks, surgery or hospital stay, and long journeys of more than 4 hours. If a doctor thinks you have DVT you should be referred to hospital within 24 hours for an ultrasound scan. Prevention: stay active, take regular walks, drink plenty of fluids, and do not sit still for long periods. www.nhs.uk/conditions/deep-vein-thrombosis-dvt