@ Caufield, this is kind of an old post but I'll respond to your question. It's about improving your cardio vascular endurance. There are a multitude of exercises out there that can do this, TABATA Sprints, HIIT, Crossfit, etc. Swimming and biking are low/no impact but still provide an excellent way to improve cardio while being easy on your joints. If you don't believe that you should try just treading water (in deep water) for 30 mins and you'll see.
Posts: 275 | Location: Djibouti | Registered: 17 May 2010