[running] a deeply spiritual ceremony that they consider a powerful form of prayer.

"play" as an activity undertaken for no serious practical purpose.


this slow burn [inflammation] steadily and surreptitiously damages tissues in our arteries, muscles, liver, brain, and other organs.
Short bursts of cortisol are natural and normal, but chronic low levels of cortisol are damaging because they promote obesity and chronic inflammation. Consequently, long hours of stressful sitting while commuting or a high-pressure office job can be a double whammy.
Predictably, those who were more sedentary died at faster rates, but these rates were lower in people who rarely sat for long, uninterrupted bouts. In fact, people who rarely sat for more than twelve minutes at a time had lower death rates, and those who tended to sit for half an hour or longer at a stretch without getting up had especially high rates.


Apparently we cannot think while cleansing our brains. We thus must sleep to flush out the cobwebs left behind by the day's experiences.
Then as we become chronically sleep deprived, we produce more cortisol, especially at night, which can then inhibit sleep, keeping the problem going and promoting insomnia.

speed, strength, power

Red fibers are ideal for sustained low-intensity activities like walking or jogging a marathon, pink fibers are best for medium-intensity activities like racing a mile, and white fibers are essential for bursts of extreme power but short duration like sprinting a hundred meters.
If you are curling the weight upward by flexing your elbow, your biceps muscle is generating force while shortening, technically known as a concentric muscle action. Concentric contractions are the primary means by which muscles move us. Muscles, however, don't always shorten. If you hold the weight steady without moving it up or down, your biceps will still try to shorten but won't actually change its length, an isometric muscle action. Isometric muscle actions can be challenging, but it is even harder to lower the weight very slowly by extending your elbow. This sort of eccentric muscle action requires your biceps to fire as it lengthens.
Filaments snap, membranes rip, connective tissues split. This so-called microdamage triggers short-term inflammation, accounting for the swelling and soreness. More important, by intentionally shredding the muscle a little, you stimulate growth because the microdamage stimulates affected muscle cells to turn on a cascade of genes. Among other things, these genes augment the total number and thickness of muscle fibers, thus expanding the muscle's diameter, making the muscle stronger.
Aging does not put an end to muscles' capacity to respond to resistance exercise; instead, modest levels of resistance exercise slow and sometimes reverse sarcopenia regardless of age.
Sports might have evolved as a way to teach impulse control along with skills useful for hunting and controlled proactive fighting.
“Let him be a man in his vigor, and soon he will be one in his reason.” Rousseau


In the eighteenth century, the word “pedestrian” came to denote something dull, commonplace, or uninspired, but I hope you agree that walking is hardly a pedestrian topic.
Nonprofessional runners are no more likely to develop osteoarthritis than non-runners. In fact, running and other forms of physical activity help promote healthy cartilage and may protect against the disease.
Connective tissues like bones, ligaments ,and tendons adapt considerably more slowly than muscles and stamina [so take it slow].
One rarely considered parallel between running and dancing is how both can induce altered states. Long periods of vigorous exercise stimulate mood-enhancing chemicals in the brain including opioids, endorphins, and, best of all, endocannabinoids. The result is a runner's or dancer's high. 
While there might have been selection for genes (as yet unidentified) that help humans live past the age of fifty, there was also selection fir genes that repair and maintain our bodies when we are physically active.
While physical activity initially stimulates inflammation, especially via muscles, it subsequently causes muscles to produce an even stronger, more lasting, and more widespread anti-inflammatory response who long-term effect is less inflammation not just in the affected muscle but elsewhere. As a result, physically active people tend to have lower baseline levels of inflammation. In addition, exercise cause the body to produce more antioxidants than necessary, decreasing overall levels of oxidative stress. Exercise also causes cells to clean out damaged proteins, lengthen telomeres, repair DNA, and more. 
Although these diseases [diabetes, heart disease, osteoporosis, osteoarthritis, Alzheimer's] are commonly classified as diseases of aging because they tend to arise when we are middle-aged, they are not caused by age, nor should they be considered inevitable consequences of aging. Plenty of people live to old age without getting these diseases, which rarely if ever afflict elderly hunter-gatherers and many aged people who live in subsistence societies.
Genes help load the gun, but environment pulls the trigger.

to move or not to move

The resulting paradox is that our bodies never evolved to function optimally without lifelong physical activity but our minds never evolved to get us. moving unless it is necessary, pleasurable, or otherwise rewarding.
As we become more out of shape, our brains become less able to reward us for exercising.
You have only to drive by a serious car accident ... to realize you are better off being forced to wear that belt, but deaths from congestive heart failure or type 2 diabetes usually happen quietly and out of sight in hospitals.

Beyond widespread ignorance about the long-term consequences of childhood inactivity, parents and educators appear to be more worries about test scores, discipline, and safety—all of which would be increased, not decreased by having appropriate amounts of physical activity.
Skipping exercise is sometimes counterproductive for those who are pressed for time.

how much

Because these big, strong hearts at first glance resemble the dilated hearts of individuals suffering from congestive heart failure, worries persist that too much exercise causes pathological expansion of the heart. ... But the superficial similarities in heart size between athletes and those who suffer from heart failure have different causes and consequences.

Cardio increases blood flow to the brain and elevates the production of molecules that stimulate brain cell growth, maintenance, and function. 
cardio + weights is the sweet spot


It is commonly assumed that these conditions (many cancers, osteoporosis, Alzheimer's...), are the inevitable by-products of more of us living to be older. But this is not entirely true. Exercise may not be an elixir, but by stimulating growth, maintenance, and repair, it can reduce our susceptibility to many of these mismatches.

When we exercise, blood pressure rises temporarily, causing the heart's muscular chambers to adapt, mostly by becoming stronger, larger, and more elastic so it can pump more blood with each stroke. Arteries also adapt to exercise to keep blood pressure low, primarily by expanding, multiplying, and staying elastic.

Physical activity is also indispensable because the cardiovascular system never evolved to develop capacity and maintain itself in the absence of demand.

Moderate physical activity has the potential to reduce the risk of contracting certain contagious diseases, including RTIs. In addition, exercise appears to slow the rate at which the immune system deteriorates as we age. ... One possibility is that because heading off to the bush to hunt and gather potentially made our ancestors more likely to encounter pathogens, our immune systems evolved to compensate by ramping up our defenses when we are active. 

[Studies] generally find higher baseline levels of infection-fighting cells among individuals who exercise regularly and moderately, and lower levels immediately following intense, prolonged bouts of vigorous exercise.

Long-term physical inactivity depresses immune competence, moderate levels boost the immune system, and very high doses of physical activity temporarily compromise immune function, thus increasing vulnerability to infection.

Regular physical activity not only increases white blood cell counts but also appears to distribute preferentially certain cells from the bloodstream to the places they are most needed.

When we demand more from our muscles, especially contractions involving resistance, we activate genes that increase the size of fibers as well as repair and maintain muscle cells. 
Physical activity (longer duration, vigorous activities) causes the brain to produce BDNF (brain-derived neurotrophic factor). BDNF first evolved to help mammals get energy during physical activity and at some point took on additional roles in the brain. BDNF ... nourishes and induces new brain cells, especially in regions involved in memory. 
How exercise alleviates depression and anxiety is less clear, and we should remember that some of the benefits of physical activity may arise from our physiology being poorly adapted to excess sedentariness. In this respect, exercise may be medicinal only because persistent inactivity increases our vulnerability to mental health disorders.

Some studies find that self-selected doses of exercise are more effective than specific prescribed doses.