Swimming is regarded as the ideal form of exercise because it is so injury-free. Physical problems only really emerge as a result of competitive training, combined with heavy land training. The result? Over-use or repetitive microtrauma injuries such as swimmer's shoulder and breast-stroke knee . These two terms are just generalised names for a variety of injuries that can occur at the shoulder or knee joint because of the heavily repetitive nature of competitive swimming. This stress can be appreciated if you imagine a swimmer training 200-300 lengths per session, x 8+ sessions per week for eight months of the year - those arms certainly circle a lot of times! This is why efficient technique (with regular assessment) and even diet are vital to ensure a swimmer's competitive career is as injury-free as possible.