Published August 19, 2026 10:36AM
Fatigue is a completely normal part of exercising. When you’re training hard, you simply won’t feel fresh and energetic every workout session.
But athletes throw around the word “fatigue” to mean a whole bunch of things. Maybe your legs feel flat on a run, or maybe you’re struggling with full-body lethargy that sticks around after a cooldown.
Getting specific about exactly what you’re experiencing can offer clues that help you get your pep back—and ultimately improve your running. “Fatigue can be a pretty reliable marker of what’s going on under the surface,” says Seattle-based running coach Laura Norris, who has a master’s in applied exercise science.
Here are seven types of fatigue that might show up, along with expert advice on how to recover from each.
1. Flat, Heavy Legs
Some days, it feels like you have no “pop” in your stride—your legs are heavy from the first step of your run or hike to the last, without getting any better after you warm up and stretch.
This is often thanks to too much mileage without enough recovery. You end up losing some of the leg stiffness that puts the spunk in your step, resulting in lower cadence and increased ground contact time, according to Rich Willy, PT, PhD, associate professor in the School of Health and Rehabilitation Sciences at The Ohio State University.
Other times, in the case of running, your runs might start to feel flat if you’re doing lots of slower running while skipping speed work. “You can begin to experience changes in muscle fiber recruitment where you just aren’t recruiting your fast-twitch muscle fibers well,” Norris says.
How to Fix It: Cut Your Activity Short and Focus on Recovery
Cut your run or other outdoor activity short. But if you’re running, you can try to pick up your cadence. “But if you’re having a hard time sustaining it, cut your run short and focus on recovery,” Willy says. Trying to power through can overload your knees and hips, potentially leading to injury.
If you think too much easy-paced running could be to blame, Norris recommends spicing things up with some strides or hill strides as a low-stress, low-risk way to get your fast-twitch fibers firing.
2. Soreness
When the micro-tears you create in your muscles during a run or hike begin to heal 24 to 48 hours later, you’ll often start to feel what’s called delayed onset muscle soreness, or DOMS. Your muscles might ache just getting out of bed, or you might only feel it once you start moving.
This often happens after you’ve run or hiked longer or harder than you’re used to. However, if you’re chronically sore day after day, it could signal too much volume, too much intensity, or under-fueling, Willy says. A general rule of thumb in training is to only increase one factor at a time; for example, if you’re adding mileage one week, don’t also increase speed.
How to Fix It: Get Sleep and Hydrate
Prioritize sleep, rest, proper nutrition, and hydration, says exercise physiologist Aashaun Khedaru, supervisor of the Sports Performance Center at NYU Langone Orthopedic Center.
If you feel like you’re always sore, Willy suggests switching from a seven-day training cycle to a more flexible ten-day cycle to add in more recovery days. And don’t forget to take a recovery week every three or four weeks, dialing back your volume by 25-30%. “Bone and tendon micro damage take about three weeks to build up to the point where you develop an injury, so if you pull back every third week or so, it gives your physiology a chance to catch up and adapt,” he says.
If you’ve been hiking or climbing, prioritize rest and ensure you’re getting adequate nutrition.
3. Burning Muscles
High-intensity runs like speed work can set your muscles on fire. Khedaru explains that this happens because anaerobic metabolism changes the pH levels in your body, making things more acidic. Eventually, that tiring, burning sensation slows you down so you can reintroduce oxygen to your system. “You start hyperventilating to clear out that waste product until you recover again,” Khedaru says.
How to Fix It: Add Active Recovery into Your Routine
The right training can increase how much lactic acid your body can tolerate. Khedaru tells athletes to incorporate active recovery into their interval sessions by slowly jogging between reps. “That lower-level aerobic work helps your body learn to reconvert that lactate back into fuel,” he says. “Over time, as your body adjusts to that burning feeling, you might be able to push it off for an extra interval or two.”
4. Bonking
Suddenly running out of oomph on a long run or race is super common and super frustrating. “You feel like you want to push, but your muscles can’t do it,” Khedaru says. Your splits get slower (despite putting in the same effort), your cadence tanks, and your form might fall apart.
This is a classic sign of running out of muscle glycogen, which is why run coaches put so much emphasis on proper carb intake. “Your muscles need energy to keep going, and you just can’t provide it anymore because you used all of it,” Khedaru says.
How to Fix It: Consume More Carbs
Norris suggests taking a short walk break to reset while getting in some carbohydrates and drinking some water. Then, as long as you can maintain good form, try to start up again. Just know that you may need to settle for a slower pace.
5. Mental Burnout
Cumulative stress builds up over the course of a training cycle and can leave you too mentally exhausted to run. Knocking out lots of miles on top of family or work obligations becomes overwhelming rather than an escape. “There’s a certain level of mental fatigue that takes form,” Khedaru says.
How to Fix It: Mix Up Your Training and Re-shift Focus
Be kind to yourself. Dial back the volume and find ways to make running fun again. “A missed run here or there is not going to break your entire training cycle,” Khedaru says. That might mean meeting up with friends or a run club, ditching the road for trails, or even swapping some recovery runs for bike rides.
You might need time to journal, read, or talk with someone you trust if your mental overload feels like too much to handle.
6. Form Breakdown
Although rare, in severe situations, heat exhaustion and dehydration can lead to the kind of staggering stride you’ll see in viral videos of marathoners stumbling across the finish line. “They’re losing their balance. It looks like they’re walking sideways. They really have no control,” Khedaru says. He explains that this is a signature of central fatigue, where your brain is trying to protect your body from further damage by literally forcing you to stop running.
How to Fix It: Stop Exercising ASAP
Once your biomechanics fall apart, it’s best to pull the plug on your run or other activity to avoid injury. “Stop, get some fluids in, and eat something,” Khedaru says.
7. Unrelenting Lethargy
If you’re feeling lethargic day after day, dragging not only through runs but the rest of your day, too, something deeper could be going on:
- Overtraining syndrome, in which your body can’t handle the volume or intensity of your training. You’ll notice your performance gets worse over time, you might struggle with constant colds and sinus symptoms, and your heart rate variability will be lower, Khedaru says.
- Chronic low energy availability, which could lead to Relative Energy Deficiency in Sport (RED-S). “A lot of runners unintentionally undereat because they’re not aware of how high their caloric demands are, and long-distance running can cause some appetite suppression,” Norris says. The result is that they’re wiped out throughout the day, not hitting their splits, and not sleeping well, either.
- A nutritional deficiency, like low iron, low vitamin D, or low vitamin B12, can also cause persistent low energy, Norris says.
How to Fix It: Check All of Your Vitals
Norris recommends doing an audit: “What’s my training load? Am I getting enough recovery? Am I eating enough? Have I done blood work recently on these biomarkers?” It can help to check in with a medical provider to talk through the possibilities and what you’ll need to recover. Although some kinds of fatigue you can power through, ongoing low energy is typically a sign that something is off and it needs to be taken seriously.
Want more Outside health stories? Sign up for the Bodywork newsletter. And for tips on how to train, fuel, and recover better, sign up for the Sweat Science 101 newsletter.
