You've probably heard two contradictory pieces of advice about stretching: one says to stretch before you train to "prepare your muscles" and prevent injury; the other says stretching before a workout hurts performance and should wait until the end. Both claims hold a grain of truth, and the confusion comes from a detail almost nobody explains properly: there are different types of stretching, with opposite effects depending on when you do them. Understanding that difference changes how you structure your warm-up and cooldown, and keeps you from losing performance by following advice that's too generic.
Static stretching and dynamic stretching aren't the same thing
Static stretching is what most people picture when they think of "stretching": you take a muscle to a comfortable end range and hold that position still, usually for 15 to 60 seconds, like touching your toes with straight legs and staying there. Dynamic stretching is different: you actively move a joint through its range of motion, in a controlled, repeated way, like swinging a leg forward and back or doing walking lunges. One is a held static position; the other is continuous movement that looks more like a slower version of the exercise itself.
For decades, the standard advice in gyms and on sports teams was to statically stretch before any physical activity, on the logic that a "stretched out" muscle would be a safer muscle. Research over the past two decades has revised that logic: the right type of stretch depends on what you're about to do, and using the wrong type at the wrong time can cost you performance without delivering the benefit you expected.
What happens when you statically stretch before a strength workout
A review pooling multiple studies on the topic found a small but consistent drop in maximum strength following prolonged static stretching done immediately before exercise, averaging around 5%, with smaller effects on power and explosive movements. The mechanism behind this involves two factors working together: a temporary reduction in neural activation of the stretched muscle, and a drop in the stiffness of the muscle-tendon unit, which normally helps transmit force efficiently during a lift.
The good news is that the duration of the stretch matters more than the simple fact of having stretched. Short static stretches, under 20 seconds per muscle, tend not to reduce strength and performance significantly, which leaves room to include some targeted mobility work in your warm-up without paying the price of a weaker session. In practice, the problem was never stretching before you train, it's doing long static holds on exactly the muscle groups you're about to load heavily.