Triathlon is an aerobic sport, i.e. we use oxygen to produce energy needed to swim, bike and run effectively. Endurance training enhances the aerobic system in a number of ways, but here are some of the key adaptations:
Lungs: Become stronger, allowing bigger breaths and small ‘sacks’ called alveoli, which transition oxygen from the lungs to the blood, to increase in size and number allowing more oxygen to be taken in.
Blood: Red cells that carry oxygen proliferate so that more can be carried to the working muscles.
Heart: If trained, the heart can become bigger and stronger like any other muscle.
Arteries: Oxygen in blood is moved into muscles through capillaries, these also expand and multiply allowing more oxygen to be delivered to the working muscles.
Muscles: Energy production in muscles happens in cells called mitochondria; one of the most fundamental adaptations of all is that the number and size of these important energy factories increases, allowing muscles to work harder.
So how do we train this system? Aerobic training can be defined as any sustained effort of approx. 15mins or more. But much of the really great adaptation happens in lower-intensity training, during which you could hold a conversation, and should make up as much as 80% of your total volume.