In the final miles of a long run or triathlon run, your leg muscles become very tired. But your hamstrings, quadriceps and calves are not the only muscles that become fatigued during a hard run.
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...

