Your respiratory muscles may also become tired during the workout that you are giving them. As it is these muscles which fatigue first it is the fatigue of these muscles that limit your performance. As these muscles fatigue signals are sent to your nervous system telling it to redirect oxygen from the muscles of your limbs to those of your diaphragm to keep them going. Therefore, during the running the reason your legs may begin to fatigue first is because your respiratory muscles have begun to fatigue first.
Anyone who is a triathlete will be fully aware that during each phase - running, swimming or cycling they will be breathing hard. But few pause to consider that hard breathing requires intense work by the respiratory muscles, which are just as susceptible to fatigue as other muscles. Some scientific evidence does suggest that respiratory muscle fatigue is a limiting factor when it comes to endurance sports performance. However, you may not know this but these muscles can be trained independently of the rest of the body. Take a moment and think you may be doing this right now as you are sitting still and breathing.
Naturally, our everyday breathing is too easy to have a conditioning effect on your respiratory muscles, but when you inhale and/or exhale against resistance with a respiratory muscle training device, these muscles may be taxed even more than they are when you swim, bike and run. As a result, they become stronger and more fatigue-resistant and therefore less limiting in your triathlon performance.
Inspiratory muscle training can benefit all the three disciplines of a triathlon. Where using a breathing training device if you are a runner this is where you will see the benefit of using such device as when running your body is demanding more oxygen