Joint loads for swimming are primarily experienced at the shoulders, while joint loads for cycling are low but can be experienced at the hips, knees, neck and back. Running is the real joint-loader, though, as the peak joint forces experienced at the knee, ankle, and hip joints are approximately 5, 6, and 10 times body weight, respectively.
The good news is that scientific literature doesn’t link repetitive joint loading through repetitive training loads with elevated risk of joint disease, such as osteoarthritis. Rather, it appears it may provide a protective effect against the development of hip and knee osteoarthritis compared with non-runners, thanks to the conditioning effect on the articular cartilage.
The best way to keep our joints healthy is to support the joints through strength and conditioning work around and across the joint. For example, for optimal knee health a triathlete should focus on hamstring, quadriceps, adductor, hip abductors and calf musculature strength. This can be via home exercises or, preferably, some resistance training in a gym.