Striding hard and fast downhill frequently causes bad knees. This common and sometimes serious problem can be prevented or reversed by going downhill slower, not overstriding or jumping, with shorter and smoother steps, using handholds on branches and a hiking stick or ski poles. Foot problems, including various pains and irritations about the foot, plantar fasciitis and Achilles tendonitis are very common and can be diagnosed and successfully treated. They can be prevented by wearing well-fitted boots of good quality, with cushioned inserts, padded socks and sock liners. Soaking feet or other inflamed areas in a cold stream helps reduce inflammation.
Air drying feet and socks during the day helps prevent blisters.
Don't step on tree roots, especially if they are wet or damp. My friend Elmer and others have broken their legs. I diligently avoid stepping on roots, wet or dry, to maintain the habit and vigilance. Avoid falls by watching every foot placement. Trail medicine includes the use of ibuprofen, effective for aches, pains and inflammation.