This is why your muscles get sore the day after a workout
Why do you get sore the day after you work out?
Scientists call this condition delayed onset muscle soreness (DOMS). It happens when you work your muscles harder than they're used to, or in a different way than they're used to.
DOMS is sometimes attributed to the buildup of lactic acid, but this isn't true, according to the UK's National Health Service.
It is likely caused by several different factors, including microscopic muscle fiber damage, damage to the connective tissue, and inflammation of the injured area. A widely cited review of the scientific literature published in the journal Sports Medicine attributed the muscle pain and soreness to this combination of events, but noted that we don't know for sure what order they occur in.
DOMS is different from pain or soreness you may feel during a workout. The American College of Sports Medicine says symptoms happen 12-24 hours after a workout because part of DOMS is likely a "side effect of the repair
process that develops in response to microscopic muscle damage."
While stretching may seem like a good way to make the pain go away, studies haven't actually been able to show that it helps.
A few studies have reported that painkillers helped with DOMS symptoms, but some others found that they didn't, so whether that intervention works is also unclear.
The review found that light exercise is one of the best treatments to alleviate the soreness. Just make sure not to go too hard.
Otherwise, wait it out. Your muscles should stop being sore in a few days.