Why might exercise leave us sore?
Your muscles may feel sore after energetic exercise if you are not used to it. Often this occurs a while after you stop and can last for two or three days.
This delayed-onset soreness is more than mere stiffness but less than the major pain from a torn muscle. Although it used to be blamed on a build up of lactic acid from anaerobic energy supply, a more likely explanation is that unaccustomed exercise produces a lot of small tears in muscle fibres, which lead to mild inflammation before they are repaired.
Typically, this soreness occurs after lots of eccentric contractions, where the muscles lengthen under tension (eg lowering a weight in a biceps curl). This is why hillwalking makes your muscles more stiff than flat walking.Lead image:
Jonas Birme/Flickr CC BY NC