The evidence-led answer
NST verdict
A short warm up is worth doing. It reliably improves performance in the session that follows, and the balance of evidence, though not conclusive, points to a modest drop in injury risk, for a cost of only a few minutes. A good warm up raises your body temperature and heart rate with a few minutes of easy movement, then prepares for what you are about to do: for lifting, a few progressively heavier warm up sets; for running, easy jogging and some dynamic movements. It does not need to be long, around 5 to 15 minutes for most training, and it should not be a session of long static stretches before heavy or explosive work, which can briefly reduce strength and power.
Does warming up actually do anything?
Yes, for performance. A pooled analysis of 32 high quality studies found that a warm up improved performance in 79% of the outcomes measured, with little sign that warming up is harmful. Those warm ups involved active movement, not just stretching. So the immediate case is simple: you tend to perform the session better.
Does it prevent injury?
Probably, but the evidence is not conclusive. A review of five high quality randomised trials found that three showed a significant drop in injuries with a warm up and two did not. The reviewers concluded there is not enough evidence to firmly endorse or reject warming up for injury prevention, but that the weight of evidence favours a reduced risk. Given the performance benefit is clear and the downside is a few minutes, that uncertainty is not a reason to skip it.
What should a warm up include?
Two parts. First, raise the temperature: a few minutes of easy whole body movement, a brisk walk, a light jog, an easy row or bike, or the exercise itself at a very light effort, to lift your heart rate and warm the muscles. Second, prepare for the specific work. For lifting, do progressively heavier warm up sets on your first main exercise, working up from a light weight to your working weight over a few sets of a few reps; this grooves the movement and can briefly prime your strength. For running or a field sport, do easy jogging then dynamic movements, leg swings, lunges, skips, gradual strides, through the ranges you are about to use; dynamic movement within a warm up has been linked to a small performance gain.
Should I stretch as my warm up?
Not as the main event. Long, held static stretches before heavy or explosive work can briefly reduce strength and power. A systematic review found that when a warm up includes dynamic activity after any brief stretching, the effect on performance is inconsequential. So a short dynamic warm up is fine; a session of long static holds before you lift is not the tool for the job. The stretching article covers this in full.
How long should it be?
For most training, 5 to 15 minutes is plenty. Spend longer, with a fuller temperature raise, when you are lifting near maximal weights, training in the cold, older or returning after a layoff, or about to do something explosive like jumps, sprints or heavy singles. Spend less on a light, familiar session. The warm up should scale to how hard and how unfamiliar the work is.
Do you need to warm up for a light or short session?
Scale it, do not skip it. A short, easy session still benefits from a couple of minutes of movement and a warm up set or two on the first exercise. For a gentle walk or an easy mobility session, the activity itself is the warm up. The point is to match effort to demand, not to perform a fixed ritual every time.
What this means in practice
A workable template: 3 to 5 minutes of easy movement to raise your heart rate and temperature, then, for lifting, a few progressively heavier warm up sets on your first main exercise before your working sets; for running, some easy jogging and dynamic drills. Keep it to about 5 to 15 minutes, longer for heavy or explosive days and cold conditions. Skip the long static stretches beforehand. It is a small time cost with a clear performance payoff and a likely, if unproven, drop in injury risk.
Practical meaning
A workable template: 3 to 5 minutes of easy movement to raise heart rate and temperature, then a specific preparation, for lifting, a few progressively heavier warm up sets on your first main exercise before the working sets; for running, easy jogging plus dynamic drills. Keep it to about 5 to 15 minutes total, longer for near maximal lifting, explosive work or cold conditions, shorter for a light familiar session. Do not use long static stretches as the warm up before heavy or explosive training.
Limitations and uncertainty
The injury trial review is from 2006 and included only five trials, so 'weight of evidence favours a reduced risk' is not proof. The performance pooled analysis pools varied warm ups, so '79%' is a broad signal. The warm up set advice for lifting is standard practice plus general potentiation research, not a dedicated trial. Some component evidence is from soccer players. This is about performance and injury risk, not long term strength or muscle gain.
No product needed
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Some research pages are deliberately product-free because adding a retailer link would not help the reader make a better decision. When a product is genuinely relevant, it appears after the evidence and is clearly labelled.