You know those aches after a particular strenuous workout, the ones that last for days? These are referred to as delayed-onset muscle soreness (DOMS), and it happens more often than you think.

The reason why you feel sore after a workout is due to micro-tears in muscle fibers and connective tissue. This occurs when a particular muscle isn't accustomed to being moved a certain way (you can blame your sedentary lifestyle for that). Even simply movements like running downhill or walking downstairs can cause soreness as a result of "eccentric muscle contractions," which occur when the muscle must contract even as it is being elongated.

It only after your workout that your body starts working on repairing itself, which results in some low-grade pain in that particular area of muscle. This repairing process is what that actually helps make that muscle stronger, which in turns improves overall fitness.

So as long as you're willing to ache a little, the rewards are more than worth it. And if you need some relief, slow, steady stretching exercises or ice baths may help improve the soreness.