I notice climbing up hills is much easier on the knees than coming down. Why is that?

Runners and hikers have all noticed that stiffness is worse the day after going downhill compared to going up. Going downhill puts much more strain on the muscle fibers and connective tissue compared with moving over a flat route. Downhill running can be damaging because of the greater eccentric muscle contractions that occur. Let me explain.

When your foot hits the ground, the muscles in the thigh contract to support you. But the nature of the downhill action is such that although the muscle is contracting, it's forced to lengthen at the same time. Contracting while lengthening a muscle at the same time is called an eccentric contraction. It can cause trauma and damage to muscle fibers.

Trauma to the muscles and connective tissue around the muscles causes tiny tears of the tissue fibers. Stiffness occurs as a result of muscle damage and breakdown of nearby connective tissue.