Too much static stretching can inhibit the stretch reflex. The stretch reflex is important in the performance of lifting exercises, especially the big lifts like squat and bench press variations. Static stretching might also decrease force production potential in the elongated position (important when using full range of motion exercises) for some time after the stretches have been performed. These two elements can decrease performance, especially during explosive movements, and increase the risk of injuries.
Static stretching can mask pain. It has analgesic effects. Why would that be bad? Because pain is a sign that something is wrong. And when you feel pain during exercise it means you should stay away from that movement until you fix the issue. If you mask pain either with stretching or pain killers you might not feel when a movement is problematic. As a result, you could make things worse.
Stretching doesn't improve active range of motion during training. With traditional static stretching of a relaxed muscle, the parallel elastic component (PEC) of a muscle is stretched. This component is responsible for passive tension. As a result, static stretching of a relaxed muscle will have very little impact, if any, on active mobility and mobility while the muscle is producing tension.
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