During the best runs, you hit your stride and cruise at a sustainable pace on fresh legs for as long as you planned. When fatigue hits, it’s the polar opposite: every step forward feels like a battle. It would likely help to know exactly what holds you back.
Do you have tired legs or are you pushing your cardio system too hard? These are two different types of fatigue, and to improve your run performance, it’s helpful to understand the difference. We spoke with a certified run coach and an exercise physiologist who explain how to tell whether it’s your legs or your lungs and heart keeping you from feeling good on your runs.
Plus, how to train so you can fight fatigue overall. The telltale signs of cardiorespiratory fatigue include elevated heart rate and heavy breathing. Besides huffing, puffing, and feeling like your heart is about to beat out of your chest, you may also be a little groggy, says Will Baldwin, USATF and VDOT-certified running coach.
In healthy runners, cardiorespiratory fatigue is not an indication of “weak” lungs or a heart that won’t pump fast enough. Just like running and strength training makes your leg muscles stronger, consistent aerobic training can strengthen the heart, lowering your heart rate and allowing it to pump more blood with each beat. This makes oxygen delivery throughout the body more efficient.
Buckingham also explains that mitochondria, the part of the muscle cell that uses oxygen to create the energy source adenosine triphosphate (ATP), and capillaries, the small blood vessels that deliver oxygen and other nutrients to muscles, tend to be limiting factors of cardiorespiratory fitness. In this case, you’re taking in enough oxygen, but your ability to utilize it is restricted. As research has shown, though, the more consistently you do aerobic exercise, the more efficiently your heart, blood vessels, and mitochondria become at taking in and using oxygen for energy.
While just as unpleasant, muscle fatigue feels different from cardiorespiratory fatigue. Your legs might seem heavy or stiff, as if you’re wearing ankle weights or trudging through mud. You’ll hear people say, ‘I felt fine, but my hamstrings went out, and my quads got really, really tight, and it was hard to move,’” Baldwin says.
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