This Training Method Helped UTMB Winners Build World-Class Fatigue Resistance. It Can Boost Yours, Too.

Most distance-running plans are built to make the aerobic engine bigger: boosting metrics like VO₂ max and lactate threshold.Coach Scott Johnston looks beyond these traditional numbers. When preparing runners for ultramarathons—including Tom Evans and Ruth Croft, the respective winners of the 2025 men’s and women’s Ultra-Trail du Mont-Blanc (UTMB) race—he cares less about how fast they look early on than what they can produce once the race starts collecting its toll.His preferred way to train that staying power? Muscular endurance work.Johnston defines muscular endurance as a muscle’s ability to repeatedly contract at a relatively high percentage of its maximal force output for an extended period of time. His philosophy on training muscular endurance centers around the idea that if the muscles you use when you run can resist fatigue longer, you’ll sustain fast paces longer into runs. This concept matters as much to 5K hobbyists as it does to professional ultrarunners.“The goal of any endurance athlete is to improve fatigue resistance—because [fatigue] is what makes you slow down,” Johnston says.While threshold training and VO₂ max workouts, among other training tactics, are some of the most popular ways to build resistance to fatigue, Johnston found that improving muscular endurance is the most effective method.Ahead, you’ll discover how muscular endurance training works, why Johnston swears by it, and how you can implement it in your own training.The Benefits of Muscular Endurance TrainingTo understand why muscular endurance matters, it helps to know a bit about muscle fiber types, which Johnston describes on a spectrum.At one end are slow-twitch fibers (type I), built for long-lasting, aerobic work like walking or slow jogging. At the other end are fast-twitch fibers (type IIx), used for powerful, anaerobic, short bursts of work like weightlifting and sprinting.Between the type I and type IIx are type IIa fibers, which Johnston calls “frontier fibers” because of their versatility when it comes to endurance-based exercise. They are also known as intermediate fast-twitch fibers because they fatigue quicker than type I fibers, but, unlike type IIx fibers, you can train them to take on endurance characteristics.Johnston says lactate threshold workouts and high-intensity interval training (HIIT) recruit these frontier fibers in an effort to help you run faster for longer, but they come at a cost not present in muscular endurance training: whole-body fatigue.Early on, Johnston noticed his athletes often slowed dramatically midway through high-intensity interval sessions designed to tap into fast-twitch muscle fibers. Upon further inspection, Johnston saw athletes’ heart rates lowering proportionally to their paces—in other words, once they slowed back down, their heart rates dropped too.With this trend in mind, he concluded that the athletes were experiencing whole-body fatigue (as shown by high heart rates) due to excessive lactate production, which happens when training over the anaerobic threshold at a high intensity.With the cardiovascular system tapped out, the athletes were unable to receive any more muscular benefits of high-intensity training because their bodies simply did not have the capacity to do any more hard efforts. That led Johnston to a simple question: What if we could load up these frontier fibers at a much lower intensity?Avoiding whole-body fatigue by purposefully focusing only on the muscles you use when you run—and not so much of the cardio system—is why muscular endurance training is so effective.Johnston developed a strategy to increase training load on frontier fibers directly while keeping heart rate well below lactate threshold, allowing athletes to build fatigue resistance without overwhelming the cardiovascular system.Later, when athletes return to traditional intervals, their muscles are equipped to handle the workload, allowing them to maximize the cardiovascular aspect of the training.For runners comfortable with intense interval training and long mileage, the slower, controlled muscular endurance workouts may seem moderate at first, but the lingering soreness that comes from this training is no joke, Johnston says. “It feels pretty easy while you’re doing it,” he adds, “but then the next day you can hardly go down the stairs.”To benefit from muscular endurance sessions, you need both restraint and care for your body, Johnston says. Once a week is plenty, and the workouts should always take place between one or two recovery days, creating a buffer from other quality workouts.You'll begin to see benefits of muscular endurance training within a few weeks, but the rapid results come with the need for heightened attention to recovery.How to Apply Muscular Endurance Training to Your RunningEvans and Croft completed weighted uphill sessions (think fast hiking) once or twice a week in preparation for UTMB. These workouts involve filling water jugs with more than 25 pounds of H₂O and carrying them in backpacks for sustained climbs that net more than 1,000 meters of elevation gain, before dumping out the water and running back down at threshold pace.This grueling combination mimics the environment and running motion of ultramarathons, building fatigue resistance both with a loaded uphill effort and downhill threshold run.Luckily, that level of training is not necessary (or even recommended!) for the average runner. But even if you only run three or four times a week, adding one muscular endurance session can help you feel stronger late in long runs.“Running up a hill with a relatively light weight—something in the range of 10 to 15 percent of your bodyweight—can have a very pronounced effect on training your muscular endurance,” Johnston says. For first-timers, he recommends about 30 minutes of effort, which you can split into intervals (see below).For true flat-ground road runners, a gym-based routine may work best because it better mimics their running motion (see below on that too).Regardless of which workout you choose to take on, “remember, the load you carry must be heavy enough that the local fatigue in your legs is the limitation, not your breathing,” Johnston says. It’s about how long you can go, not how fast you get it done.In the broader context of a training plan, Johnston says these workouts should start early in the cycle. “That way when you shift gears into the more conventional interval training, you’re going to have established this muscular endurance that allows you to get more out of it,” says Johnston.Although you want to build muscular endurance early in a plan, Johnston stresses that it only works if you have a strong aerobic foundation. Easy miles may not be glamorous, but they’re what give your slow-twitch fibers the capacity to clear the lactate your frontier fibers produce.Your Beginner Uphill Muscular Endurance WorkoutWhy it works: Johnston says that weighted uphill climbs benefit trail runners the most because they mimic the environment and running motion required to traverse it.It’s best to do this workout on a StairMaster machine at the gym so you have a steep climb, but you can also find the steepest hill in your area. Just remember the goal is to work until you feel fatigue in your legs—that’s how you know you’re going hard enough.How to do it: 10-minute warmup jog6 x 5-minute StairMaster (or steep hill) intervals at 3-to-4 RPE pace carrying 10-15% bodyweight; rest for 60 seconds in betweenRemember, weighted uphills are basically fast hiking, so brisk walking at a significant incline with weight on your back could be your 3-to-4 RPE pace for this workout10-minute cooldown jogYour Gym-Based Muscular Endurance WorkoutWhy it works: Johnston says the goal of this workout isn’t to crush yourself. It’s to give your working muscles—and those all-important frontier fibers—just enough added stress to become more fatigue resistant. The moves you do will mimic running form, helping build endurance in the same muscles you use on the road.How to do it:This workout is geared toward beginners, or those trying muscular endurance training for the first time. For the advanced, Johnston has a 14-week progression, starting with this workout, then adding more exercises, weight, and repetitions.Matt Rudisill is an Associate Service Editor who has been with Runner's World since 2025. A Nittany Lion through-and-through, Matt graduated from Penn State in 2022 with a degree in journalism and worked in communications for the university's athletic department for three years as the main contact and photographer for its nationally-ranked cross country and track & field teams. Matt was also heavily involved in communications efforts for Penn State football, men's basketball, and women's gymnastics. In his role with Runner's World, Matt has interviewed Olympians, world champions, and countless experts in the field to create service content that helps runners of all ages and experience levels train smarter and race faster. When he’s not out jogging, Matt can be found tweeting bad takes about the Phillies or watching movies.