Stimulating more blood flow. A large part of the soreness comes from the crystallization of acids that get stuck in the muscles so any stimulation is going to help work them out. If you have sore muscles it’s safe to assume that they have tightened a bit so stretching combats the tightness more so than the pain and can even cause more pain if the muscles are injured beyond a buildup of acids. Lots if times people will stretch a strained (not sore) muscle and that’s a big no no
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