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, nor are they necessarily the first muscles to bonk.
Your respiratory muscles may also become tired. And to the
degree that these muscles fatigue first, it is their fatigue—not
that of your legs—that limits your performance. In fact, as your
respiratory muscles begin to fatigue, your nervous system will
redirect oxygen from the muscles of your limbs to those of your
diaphragm to keep them going. Thus, during running your legs may
fatigue because your respiratory muscles have begun to fatigue
first—and to prevent these muscles from fatiguing to a dangerous
extent. Every triathlete is aware he or she breathes hard when
running (or swimming or cycling) 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. There is sc