Athletes who are trying to build endurance and stamina often look to running to achieve their goal. The problem is that running can be extremely hard on the knees and ankles. This is partly due to the amount of weight placed on the legs during the run. Another factor is the surface the person is running on. If it is uneven or too hard, the impact on the knees, ankles and feet can be huge. Uneven terrain can lead to sprained ankles and damage to the tendons and ligaments in the knee as the person's body weight is thrown off balance. More and more athletes are looking to swimming sets as viable alternative cardio workouts to running simply because it takes the stress and strain off of the joints in the legs.