Generally speaking, the more fit our bodies are, the less sooner to injury we will be. Now, this depends on a lot of different factors, and given those factors, also effects recovery and rehabilitation after an injury.
Height and Weight Ratio
Depending on your height, your weight can indicate that you are underweight, of average weight, overweight, obese, or morbidly obese. There are many health issues that can compound the matter of weight, but the bottom line from a musculoskeletal standpoint is that overweight body puts extra pressure on our joints and can cause cartilage to wear away much faster as we age. Cartilage is important because it helps cushion our joints (especially in the knees) and the vertebrae in the spine. Extra strain on the joints as well as the muscles can inhibit movement and increase the likelihood of osteoarthritis.
Keeping active is essential to maintaining a strong musculoskeletal structure (including proper bone density).
NOTE: The reason people say that muscle weighs more than fat is because muscle is more dense. If an increase in muscle mass corresponds with a decrease in body fat, you could very well see your weight go up, even if you're in better shape.
We can not stress enough how important flexibility is to the body. You do not have to be a yogi or a contortionist, but keeping your body flexible decrees your chance of injury and can increase speed of recovery. Your muscles, tendons, ligaments, and extremely your skeleton all need to move with you, not fight your movement. This is within reason, of course. If you've never done a backbend in your life, doing one on a whim will likely cause both pain and injury. Baby steps.
Strong ligaments can mitigate the effects of sprains as well.
Muscles and Bone Density
In some cases, swimmers who rarely did workouts outside of the water developed arthritis at a very young age. Swimming is a very low impact (if no impact) physical activity, and while it is truly one of the best forms of exercise you can do, it should not be the only one. In fact, swimming is a great rehabilitation activity because of its low-impact nature.
Bottom line … there needs to be a balance in your physical activity. You do not have the full effects of gravity when swimming. As a result, those who spend a disproportation amount of time in the water do not develop the bone density that will help their joints weather normal wear and tear (or athletic strain).
At the other end, there are high-impact exercises that cause unnecessary strain on your joints. This is why many will tell you not to run downhill, take a high-impact aerobics class, or run stairs. Sometimes, it's just a matter of using proper technique. CrossFit has come under fire because most gyms seem to stress intensity over form. The result? Avoidable injuries. (So if you are a CrossFit aficionado, definitely do your homework on your gym and trainer).
Exercise and Rehabilitation
Exercise in a controlled, gradual, and progressive manner is the only way we can tell our body to heal. Too much too soon will exacerbate the problem and may cause more damage than the original injury.
Injections, medications, and other passive therapies can be important in providing pain relief, but they can not stimulate the healing process. The natural stimulus for the healing process is active exercise, movement. Active exercise means we use our nervous system to tell the muscles what to do, and it requires dedication to an appropriate, comprehensive exercise and rehabilitation program. One can not come without the other.
Injuries can occur no matter how fit we are, but the process of rehabilitation, of healing, follows the same process. We make progress at different rates because we are all different … we have different fitness levels, body types, bone densities, and degrees of flexibility. What we do know is that the body needs to be lead to heal itself.