Plus, how to keep running strong—no matter the cause.
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.
Signs a Fatigued Cardiorespiratory System Is Causing You to Slow Down
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. “You’re not as clear in your thoughts, and sometimes your form will start breaking down because you’re losing a little bit of that focus,” Baldwin tells Runner’s World.
In healthy runners, cardiorespiratory fatigue is not an indication of “weak” lungs or a heart that won’t pump fast enough. “It comes down to your body’s ability to take the oxygen that you’re breathing in, transport it to the muscle, and then use it in the muscle,” says Todd Buckingham, Ph.D. exercise physiologist at PTSportsPRO in Grand Rapids, Michigan.
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.
Signs It’s Tired Legs Making Runs Difficult
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.
“It’s a common story. 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. “That’s happened to me before. My calves got so overworked and tired that they became really rigid and couldn’t function properly, even though, aerobically—breath wise—I wasn’t super overworked. I was somewhat comfortable in that way, but I couldn’t access [my full potential] because there was a roadblock, muscularly,” he recounts.
The dragging sensation associated with tired legs and muscle fatigue can be attributed to a combination of factors, including a buildup of metabolic waste, glycogen depletion, and exercise-induced muscle damage. “When we run, we get micro tears in the muscle fibers, and the longer you run, the more the micro tears become prominent, and it starts causing fatigue,” Buckingham says.
With adequate recovery, nutrition, and hydration, the body eventually repairs these tears, which leads to bigger, stronger muscles. However, before you get to the recovery, “the longer you run, the less your muscles are able to withstand that constant pulling and pounding,” Buckingham says.
How to Avoid Cardio Fatigue
To improve your cardiorespiratory fitness and delay fatigue, Buckingham suggests prioritizing slower, steadier runs. “Zone 1 and zone 2 training in a five-zone [heart rate training] model is going to increase the number of mitochondria and capillaries more than running fast,” he says. Having more of both mitochondria and capillaries will inevitably boost your VO2 max, the metric that represents how efficiently your body utilizes oxygen.
You can use a heart rate monitor to keep your pace in the appropriate range or employ the talk test: If you’re in zone 2, you should be able to carry on a conversation while running. If you’re gasping for air or struggling to finish a sentence, you need to slow down, even if that means switching to a brisk walk or incorporating walk intervals into your workout.
But don’t ditch your faster runs altogether. “You also need to do some higher-end training because zone 4 and zone 5 increase the size of the mitochondria,” Buckingham explains. The ideal ratio depends on your goal and fitness level, but most recreational endurance runners will benefit from an 80:20 ratio, where 80 percent of runs are slow and easy, and 20 percent are faster and more challenging.
How to Ward off Muscle Fatigue
If you need another reason to focus on strength training, this is it. Baldwin explains that sometimes dialing back on running to allow for more time in the gym pays off in increased muscular endurance.
“If [muscular endurance] is your weakest link, it’s not going to matter as much if you increase your mileage, if you can’t get past this one specific muscular issue,” Baldwin says. “Seeking out a professional [personal trainer] and having a real plan where you’re doing some strength work two to three times a week around that specific area can make a difference.”
You can also improve your muscular endurance outside of the gym and your official workouts by clocking more time standing and walking rather than sitting, Buckingham says. “The best way to train muscular endurance is to spend more time on your feet,” he says.
Take walk breaks throughout the day, get a standing or walking desk, do more gardening or yard work—whatever keeps you on your feet will help strengthen your legs and make you more resistant to fatigue.
When Your Fatigue May Actually Be Boredom
Here’s a surprise twist: Fatigue and boredom are inextricably linked. Think about the times you’ve struggled to stay awake during a dull movie or a tedious meeting. If a training run seems harder than it should or your tiredness has undertones of angst or dread, the problem may be in your head.
“One of my favorite ways to tackle boredom is to mix it up,” Baldwin says. “I’ll maybe change a run and get on the trails or hit up a social run.” Or, he’ll jump into a buddy’s workout. Running alongside a friend on their route at their pace forces him to take a mental break from his own training.
Or you could just “suck it up,” Buckingham says, with a laugh. “Honestly, endurance exercise and running marathons is not an exciting thing,” he admits. “You have to be mentally tough to do it.” The best way to build up that toughness—to strengthen that mental muscle—is to keep running even when you don’t want to.
If you can stand it, try using a “bad run” as an opportunity to fine-tune your awareness. “Figure out the cues that your body is giving you because then you can tell, ‘Am I really tired or just bored? Am I checking out because it’s too hard or it’s too easy?’” Buckingham says. “That will allow you to get a better sense of what’s happening and develop that connection with your body.”
