The Sweet Spot of Suffering: Why a 7-Out-of-10 Effort May Be the Key to Maximum Fitness

A friend of mine ran for the University of Arkansas during its legendary dynasty years in the 1990s, a period when the program was an unstoppable force in collegiate track and cross country. Each fall, a new cohort of wide-eyed recruits would arrive in Fayetteville, ready to test their mettle against returning NCAA champions and Olympians from across the globe. The environment was high-stakes and intimidating. During their first official workout, the coach would typically assign a session—five times a mile, for example—and send the group to the starting line. The nervous rookies, desperate for guidance in this elite atmosphere, would sidle up to the stony-faced veterans. “So, um, how fast are we supposed to run these miles?” they would ask, hoping for a specific pace or a target split. The veterans would rarely offer a number. Instead, they would reply with a cryptic, challenging question: “I don’t know. How fast can you run them?”

This anecdote captures one of the most enduring and fundamental challenges in the world of endurance training: figuring out the right intensity. For decades, the "no pain, no gain" philosophy dominated the tracks of America, suggesting that the only way to improve was to hammer every interval until the lungs burned and the legs turned to lead. However, the pendulum of athletic science has begun to swing in the opposite direction. Today, many of the world’s most successful runners have adopted the so-called “Norwegian Method,” popularized by stars like Jakob Ingebrigtsen. This approach relies heavily on objective data—using portable lactate meters to prick fingers between sets or chest-strap heart rate monitors to ensure that the athlete does not push too hard. The goal is to stay just below the threshold where fatigue becomes exponential.

Yet, despite the rise of high-tech biofeedback, a central debate remains: How difficult should the ideal workout actually feel? Is there a point where pushing harder stops yielding better results and simply increases the risk of burnout? A compelling new study published in Medicine & Science in Sports & Exercise offers a surprisingly simple answer to this complex question. The research suggests that the "sweet spot" for interval training intensity is a 7 on a subjective effort scale of 0 to 10. While this might sound overly simplistic to those used to complex spreadsheets and GPS data, the study provides a deep dive into the physiology and psychology of optimal performance, suggesting that our internal "feel" might be more accurate than we give it credit for.

The research team, led by Daniel Bok of the University of Zagreb in Croatia, sought to quantify the relationship between perceived exertion and physiological adaptation. To do this, they recruited 17 trained runners and put them through a series of three distinct interval workouts. Each session consisted of three-minute intervals repeated three times, with two minutes of passive recovery in between. The variables were not pace or heart rate, but rather the runners’ Rating of Perceived Exertion (RPE). The subjects were instructed to target an effort level of either 6, 7, or 8 on a 0-to-10 scale—a range that typically spans the gap between "hard" and "very hard" in the lexicon of exercise science.

The primary metric the researchers monitored was the time spent at or above 90 percent of VO2 max. In the world of exercise physiology, VO2 max is the gold standard for measuring aerobic fitness. It represents the maximum rate at which an individual can inhale oxygen, transport it through the bloodstream, and utilize it in the muscles to produce energy. For an athlete looking to increase their "engine size," the goal of a hard interval session is to spend as much time as possible in this near-maximal state. If a workout is too easy, the cardiovascular system isn’t stressed enough to trigger significant adaptations, such as increased mitochondrial density or stroke volume. Conversely, if the workout is too intense too early, the athlete will accumulate metabolic byproducts like hydrogen ions too quickly, forcing them to slow down or terminate the workout before they have accumulated enough "time at the top."

The results of the Zagreb study were illuminating. When the runners pushed at an effort level of 6, their time spent above 90 percent of VO2 max and 90 percent of their maximum heart rate was significantly lower than the other two trials. However, when comparing the effort level of 7 to the effort level of 8, the physiological data told a different story. For both heart rate and oxygen consumption, the results were virtually identical. There was no statistically significant benefit to jumping from a 7 to an 8. This led the researchers to conclude that 7 out of 10 represents a physiological "plateau of efficiency." At this level, you are reaping the maximum possible training signal; pushing to an 8 adds more subjective suffering and likely more systemic fatigue, but it does not actually make the heart or lungs work any harder in a way that translates to greater fitness gains.

Beyond the basic numbers, the study revealed a fascinating insight into how runners naturally pace themselves when guided solely by internal cues. The participants performed these workouts on a track without watches, heart rate monitors, or any external feedback regarding their speed. Despite this, they were remarkably consistent. As the researchers instructed them to maintain a constant RPE of, say, 7 throughout the entire three-minute interval, the runners didn’t maintain a steady pace. Instead, they instinctively utilized a "fast-start" strategy. They would begin the interval at a high speed, and as fatigue began to set in and the perceived effort threatened to rise toward an 8 or 9, they would gradually dial back the pace just enough to keep the feeling at a steady 7.

This "constant RPE" protocol is a departure from traditional interval training, where runners usually strive for "even splits"—running every lap at the exact same pace. However, the fast-start method has roots in high-level laboratory testing. Years ago, researchers experimented with VO2 max protocols where subjects were told to adjust their pace to maintain a specific perception of effort. This often resulted in a brutal final stage where the athlete began at an all-out sprint and slowly decelerated as their physiological systems reached their limits. While this method is highly effective at keeping an athlete at their VO2 max for a longer duration, it is notoriously grueling. One anecdote from a veteran sports scientist describes a similar test where he was suspended in a safety harness over a treadmill, gradually slowing the machine down as he gave everything he had to stay upright. The result was a maximal physiological reading, but the experience was so taxing it induced immediate nausea.

The Zagreb study suggests that this intuitive, RPE-based pacing might actually be a superior way to structure intervals for fitness gains. By starting faster, the runners reached the 90-percent VO2 max threshold much sooner than they would have with a conservative, even-paced start. Because they then adjusted their speed based on feel, they were able to "hover" at that high physiological state without blowing up. Bok and his colleagues hypothesize that this could lead to greater adaptations over time compared to traditional pacing. However, this comes with a significant caveat: most competitive races are won by those who can pace themselves evenly or finish with a "negative split" (running the second half faster than the first). Training exclusively with a fast-start, fading-pace model might maximize the aerobic engine, but it could potentially sabotage the psychological and mechanical discipline required for race-day success.

There is also the question of the "cost of doing business." Training is essentially a game of stress and recovery. Every workout sends a signal to the body to get stronger, but it also creates a debt of fatigue that must be repaid. If an effort level of 7 provides the same aerobic signal as an effort level of 8, then the level 7 workout is objectively superior because it likely requires less recovery time. Pushing into that higher "very hard" zone often triggers higher levels of cortisol (the stress hormone) and greater mechanical strain on the muscles and tendons. By staying at a 7, an athlete can potentially train more frequently or with higher total volume, which is the true secret to long-term endurance success.

The takeaway for the everyday runner is both liberating and challenging. It suggests that we don’t necessarily need a $500 GPS watch or a laboratory-grade lactate analyzer to train effectively. Our brains are sophisticated computers that integrate heart rate, breathing rate, muscle temperature, and blood chemistry into a single, actionable metric: how hard it feels. If you head to the track for a session of 800-meter repeats and aim for a 7 out of 10, you are likely hitting the optimal stimulus for your aerobic system.

However, the "7 out of 10" rule isn’t a universal constant for every type of workout. The Zagreb study focused on a relatively short total work duration—only nine minutes of hard running. If a runner were attempting a longer session, such as six repeats of 1,000 meters or a twenty-minute tempo run, the "sweet spot" would likely shift downward toward a 6. The longer the duration, the lower the intensity must be to avoid premature exhaustion.

Ultimately, the study reinforces a message that the Arkansas veterans seemed to understand instinctively thirty years ago: the most important data point in training isn’t on a screen; it’s in your head. Harder isn’t always better. The goal of training isn’t to see how much pain you can endure in a single afternoon, but to provide the body with the specific stimulus it needs to improve. By aiming for that "7" instead of a "10," you might find that you’re not only getting faster but also enjoying the process more—and staying healthy enough to keep showing up at the start line. As the study suggests, if you want to know how fast to run your intervals, "as fast as possible" is rarely the right answer. The right answer is the one that challenges your limits without breaking them.