There are numerous ways that the body may be injured. Maybe you had an acute injury like our friend in the picture. Sports and the acute injuries that come from them are the most common type that people think of when they think of sustaining an acute injury.
Maybe however, you have an injury that built up over time and is now really starting to bother you (repetitive strain). Repetitive strain can happen from something as simple as sitting at your desk all day. This loads the same tissues constantly and doesn't give them the break they need to recover.
Pre-hab involves training muscles that are prone to injury in order to reduce the likelihood of injury during sport.
Part of my role as a Chiropractor is to make sure that if you are doing an activity that can lead to injury (yes even sitting at a desk!) that you take all the measures you can in order to reduce the chances of sustaining one. This can be as easy as performing a few simple exercises throughout the day to reduce the effect of a seated posture. Or it may involve full programs designed to strengthen the athlete in identified areas of weakness.