Key to avoiding training injuries is to have both strong AND flexible joints. It stands to reason that if all of the musculature around your knee is strong then your knee is better supported and protected against injury. The same principle applies to you hip, ankle, elbow and shoulder joints.
However, your joint musculature should be also be flexible, as well as strong, so that everything you do benefits from a full range of motion throughout all of your joints.
Furthermore, both flexibility and strength should be balanced between all of the muscle groups around a joint. For example, too much strength at the front coupled with poor strength at the back is a recipe for injury and the same principle applies to imbalances with joint flexibility.