Even if a rat's muscles aren't actually torn or damaged, it can still feel sore after exercise — this soreness seems to come from nerve sensitivity, not from physical damage, especially when the...

When rats do certain muscle exercises, they get sore deep in their muscles—not on the skin—starting a day later, peaking after 2–3 days, and going away by day 4. This soreness is due to increased sensitivity in the muscles themselves.

In rats with muscle soreness after exercise, two brain-chemical pathways team up to make muscles extra sensitive to pain, especially when certain pain-related proteins are present at low levels — and they both work through specific pain sensors in the nerves.