Strong evidence

Long static stretches immediately before training reduce strength and power output.

In practice

Keep pre-session stretching brief and moving. Save the long holds for after the session or a separate day — and if you stretch first anyway, do some warm-up sets afterwards so the effect washes out before your working sets.

What it doesn't say

The impairment comes from long holds. Short stretches inside a proper warm-up, or mobility work between sets, aren't the same thing.

Source

• Behm DG, Blazevich AJ, Kay AD, McHugh M (2016). Acute effects of muscle stretching on physical performance, range of motion, and injury incidence in healthy active individuals: a systematic review. Applied Physiology, Nutrition, and Metabolism, 41(1):1–11. doi.org/10.1139/apnm-2015-0235