I am generally sceptical of health claims, and physical activity is the area where the evidence is strong enough that scepticism runs out.
The breadth of it
Physical activity has evidence for cardiovascular disease, type two diabetes, several cancers, bone density, falls in older people, cognitive decline, depression and anxiety.
Which is unusual. Most interventions affect one system.
The evidence comes from observational studies, trials, and dose-response relationships, and it is consistent across populations and study designs.
The dose-response shape
The most useful practical finding.
The relationship between activity and mortality risk is steepest at the low end.
Which means the largest benefit comes from moving from doing nothing to doing something, and additional benefit accrues more slowly after that.
Someone entirely sedentary who begins walking regularly gains more than someone already active who trains harder.
That reframes the whole thing, because the message is not about achieving a target. It is about not being at zero.
The guideline numbers
Most national guidelines recommend a weekly amount of moderate activity, with an equivalent lower amount of vigorous activity, plus strength work twice weekly.
Those numbers are population-level targets derived from where the curve flattens, not thresholds below which nothing counts.
Recent guideline revisions have removed the requirement that activity occur in blocks of a minimum duration, since evidence indicated shorter bouts count.
Which was a meaningful change and it removed a barrier that had discouraged people.
Strength training specifically
Historically underemphasised relative to aerobic activity and the evidence has grown considerably.
Muscle mass and strength decline with age, and the decline accelerates, contributing to falls, frailty and loss of independence.
Resistance training slows and partially reverses it at any age, including in people in their eighties and nineties, which has been demonstrated in trials.
It also has metabolic effects independent of aerobic activity.
Mental health
The evidence here is genuine and frequently overstated in both directions.
Trials of exercise for depression find effects comparable to other interventions for mild to moderate presentations.
Methodological quality varies and blinding is impossible, which limits certainty in the same way it does for meditation research.
The mechanisms proposed include neurotrophic factors, inflammation, sleep improvement, self-efficacy and social contact where activity is social.
Which is a lot of candidate mechanisms and no clear answer, and the outcome evidence stands regardless.
Sedentary time
Studied as a separate exposure from activity, on the finding that prolonged sitting is associated with poor outcomes even in people who meet activity guidelines.
The evidence for this is weaker than the coverage suggests, and the association attenuates substantially when activity is properly accounted for.
The practical advice — break up long periods of sitting — is low cost and reasonable regardless of how the evidence settles.
What actually determines whether people do it
The behavioural research is clearer than the physiological research on this point.
Enjoyment predicts adherence better than any belief about benefit.
Convenience matters enormously — activity requiring travel, equipment or scheduling is abandoned faster.
Social commitment predicts adherence strongly.
Which suggests that choosing something tolerable and convenient beats choosing something optimal, since the optimal thing not done has no effect at all.
The caution
Anyone with a cardiac condition, uncontrolled blood pressure, or who is very deconditioned should speak to a doctor before starting anything vigorous.
The risk is low and it is not zero, and the assessment is straightforward.
Steps and where the number came from
The ten thousand figure originated in Japanese pedometer marketing in the nineteen sixties, not from research.
Subsequent research has examined the actual relationship, finding mortality benefit accruing from considerably lower counts and plateauing well before ten thousand in older adults.
Which means the target is arbitrary and directionally reasonable, and treating it as a threshold discourages people who cannot reach it.
Intensity
Moderate intensity is generally defined as being able to talk but not sing, which is a practical field test requiring no equipment.
Vigorous intensity means being unable to say more than a few words without pausing for breath.
Interval approaches alternating hard efforts with recovery have been studied extensively and produce comparable or better cardiorespiratory improvements in less total time.
They are also less pleasant, and adherence data suggests that matters more than the efficiency gain for most people.
Recovery
Adaptation occurs during rest rather than during the session itself.
Which is why progressive overload requires adequate recovery, and why increasing training load without it produces decline rather than improvement.
Sleep is the dominant recovery variable, which links this back to everything else.
Starting from very low activity
The practical point that matters most, given where the dose-response curve is steepest.
Walking counts, housework counts, taking stairs counts, and none of it requires equipment or membership.
The research on accumulating activity in short bouts is favourable enough that the older requirement for sustained sessions has been dropped from guidelines.
Which removes the main structural barrier for people whose obstacle was time in blocks rather than time overall.