Always stretch before exercise to prevent injury
An old model or guideline was retired in favor of a new one.
TaughtLine up and do static stretches before you run. Hold each one for 30 seconds. It prevents injury and improves performance.
NowStatic stretching before activity does not meaningfully prevent injury and temporarily reduces muscle power and strength. The current recommendation is a dynamic warm-up first, and static stretching afterward if at all.
What actually happened
Reviews through the 2000s, including a widely cited CDC-authored analysis and later Cochrane work, kept finding the same thing: pre-exercise static stretching has little to no effect on injury rates. Warming up does help. Stretching and warming up are not the same activity, and P.E. classes routinely conflated them.
Worse, holding a long static stretch before a sprint or a lift measurably reduces force output for a while afterward, an effect sometimes called stretch-induced strength loss. The modern protocol is a dynamic warm-up: leg swings, lunges, gradually intensifying movement that raises tissue temperature and rehearses the motion you are about to perform. Static stretching still has value for flexibility, and that is best done after training or as its own session. If you were made to sit in a hurdler's stretch on a cold field before running laps, that ritual has been retired.
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Who was taught this
Still standard through 2012, so anyone who finished school between 1950 and 2007 learned the earlier version.