Why stretching before a workout can actually reduce your strength
Static stretching — holding a position at maximum muscle length for 20–30 seconds or more — was long considered a mandatory part of warming up. But research over the past two decades shows that when done right before strength work, it can temporarily reduce a muscle's peak force and power output by 5–10% or more.
The mechanism involves viscoelastic relaxation of the muscle-tendon complex and a temporary drop in neuromuscular excitability — put simply, a stretched muscle briefly responds a bit weaker to signals from the nervous system. The effect can last anywhere from a few minutes to half an hour, depending on how long you stretched.
This doesn't mean stretching is bad in general — it's useful for flexibility and joint mobility. Just move it to the end of your workout or a separate session, and warm up before lifting with dynamic movement instead: swings, lunges, joint mobility drills — they warm muscles up without costing you power.