What causes muscle pain?

You know that playing tennis all weekend will leave you sore on Monday or that putting in that extra tough workout in the gym is going to make you pay later when going downstairs (oh, your quads!). When you perform activity above and beyond what your body is accustomed to, “a byproduct of energy use is produced, called lactic acid, which is a chemical that causes soreness,” explains physical medicine and rehabilitation specialist, Dr George Kannankeril. Lactic acid usually dissipates in about an hour, but delayed onset muscle soreness can last for one or two days.
But it should go away

Muscle soreness is likely normal if you can pinpoint the reason for the ache (like you did a bunch of bicep curls yesterday) and if it dissipates over time. The pain should clear up after a couple days, or with one to two weeks at most, says Dr Kannankeril.
When to worry

Has muscle pain been lingering longer than two weeks? Is it a sharp pain versus discomfort or aching? Is it limiting your ability to carry out your normal day-to-day tasks? These are all symptoms that should tip you off that there may be something more going on. If this is happening, make an appointment with your doctor, advises Dr Kannankeril.