Fundamental Endurance: Pace, Heart Rate and Zone 2, According to Research

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Iron in bisglycinate form, chelated to glycine and combined with vitamin C, in a single capsule per day. Each capsule provides 70 mg of iron bisglycinate, or 14 mg of iron (100% of recommended daily intake), 80 mg of vitamin C (100%) and 200 µg of vitamin B9 (100%). Take 1 hour before a meal.

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Physiologists divide intensity based on two thresholds measured during an exercise test with respiratory gas analysis. Below the first ventilatory threshold, zone 1, blood lactate remains low; between this threshold and the second, called the respiratory compensation point, extends zone 2 from researchers, the threshold work zone; beyond that, zone 3 groups intense interval training. Fundamental endurance is zone 1.

Without laboratory equipment, the most reliable marker is speech. A 2022 review concludes that the moment when speaking becomes difficult corresponds to the ventilatory threshold, just as a perceived exertion of 10 to 11 on the Borg scale, graduated from 6 to 20. As long as you can speak full sentences without gasping for breath, you are in fundamental endurance.

Running slowly also counts for health. In a cohort of 55,137 adults followed for an average of 15 years, runners had, after adjustment, a 30% lower risk of all-cause mortality and a 45% lower risk of cardiovascular mortality, starting from less than 51 minutes of running per week and below 9.7 km/h. These are associations, not proof of cause and effect.

<<<22>>> Mitochondria mitochondries produce energy from oxygen. According to a 2018 review, training volume could be a determining factor for their quantity in muscle, and relative intensity an important determinant of their function. The review dedicated to world-class distance runners similarly describes low-intensity volume as an important stimulus for muscle adaptations, and a large volume of easy running appears to characterize the most economical runners, those who expend the least energy at the same pace.

<<<25>>> Capillaries capillaires, which deliver oxygen to fibers, multiply: an additional third of capillary per fiber on average (0.33) in a meta-analysis of 57 trials. In sedentary individuals, the relative gain was 21% higher with moderate continuous effort, between 50 and 80% of VO2max, and 54% higher with intense intervals, than with effort below 50% of VO2max; in trained individuals, no training type added any. For a beginner, fundamental endurance therefore benefits from remaining a true moderate run, not a slow walk.

Fat oxidation <<<29>>> peaks at moderate intensity. In 300 adults tested on a treadmill, it reached its maximum at 48% of VO2max on average, or 61.5% of maximum heart rate, slightly higher in women (52%) than in men (45%), with large variations between individuals. culmine à intensité modérée. Chez 300 adultes testés sur tapis, elle atteignait son maximum à 48 % de la VO2max en moyenne, soit 61,5 % de la fréquence cardiaque maximale, un peu plus haut chez les femmes (52 %) que chez les hommes (45 %), avec de grands écarts d'une personne à l'autre.

<<<31>>> The heart cœur becomes more economical. In the HERITAGE study, 507 sedentary adults followed 20 weeks of cycling, 3 times per week, from 30 minutes at 55% of VO2max to 50 minutes at 75%: resting heart rate decreased by 2.7 to 4.6 beats per minute, and the decrease at equal workload during exercise was considered clinically significant. Finally, a meta-analysis of 28 studies measures a VO2max gain of 4.9 mL/kg/min with continuous endurance, compared to untrained groups.

The 80/20 rule comes from observing the best. In world-class distance runners, who do 11 to 14 sessions per week, at least 80% of running volume is done at low intensity throughout the year. American coach Bill Bowerman summed up the recipe: 2 to 3 interval sessions per week, one long run, and the rest at easy pace.

In 30 trained recreational runners trained for 10 weeks, a polarized distribution (77/3/20) improved 10 km time by 5.0%, compared to 3.6% with a threshold-centered distribution (46/35/19): a difference of approximately 41 seconds, not statistically significant, which became clear (effect size of 1.29) in runners who adhered best to their program.

Meta-analyses provide nuance. In 2019, a synthesis of 3 trials found an average effect of the polarized model compared to the threshold-centered model in time trials (0.66), based on studies of modest quality. In 2025, a network meta-analysis of 13 studies and 348 athletes found no difference between the polarized model and the pyramidal model, neither for VO2max nor in time trials. For VO2max, competitors seemed to benefit slightly more from the polarized approach, recreational athletes from the pyramidal approach, which places slightly more work at threshold and slightly less high-intensity work.

For a recreational runner, the issue is therefore not to choose a model, but to avoid running easy runs too fast. Our guides for preparing a marathon, a trail or a hyrox race detail how to organize the week according to your goal.

The 220 − age formula is the most widespread, but its validity had never been established in enough older people. In 2001, a meta-analysis of 351 studies comprising 18,712 people proposed 208 − 0.7 × age, confirmed in the laboratory with 514 healthy adults: maximum heart rate, or max HR, depended mainly on age, with no difference between men and women or according to physical activity level, and 220 − age underestimated it in older individuals.

In 2013, the Norwegian HUNT study measured max HR in 3,320 healthy adults: the best equation was 211 − 0.64 × age, with a standard error of 10.8 beats per minute. In 2020, 9 equations, including these three, were compared against 99 treadmill stress tests: all showed poor individual accuracy, with the 220 − age formula even being the least biased in this sample. The authors recommend measuring max HR during a stress test, which can also identify your two thresholds using respiratory gas analysis.

Values in beats per minute, calculated by us: at 60 years old, the difference between the classic formula and the HUNT study formula reaches nearly 13 beats.

Watch zones are based on a percentage of max HR, yet the first threshold does not occur at the same level for everyone. In 100 adults tested in the laboratory, the first lactate threshold was located between 60 and 90% of max HR and between 45 and 74% of VO2max depending on the individual. The authors conclude that fixed percentages poorly control actual intensity: at 75% of their max HR, one runner may be well below their threshold, while another is already above it.

VMA, or maximal aerobic speed, serves as a pace reference. In amateur marathoners tested before the Athens marathon, the first lactate threshold was at 11.6 km/h for a VMA of 16.5 km/h in the fastest runners (under 4 hours), approximately 70%, and at 9.2 km/h for a VMA of 14.4 km/h in the slowest, approximately 64% (calculations by us based on published averages). Base endurance is run at speeds below these thresholds.

The talk test overcomes these individual differences: in complete sentences, you are below the first threshold; choppy and uncertain speech, you are at the boundary; only a few words, you are beyond it. And as the threshold rises with training, the pace at which speech remains easy rises with it.

Zone 2 is presented by many as the ideal intensity for mitochondria and metabolic health. The panel of 14 scientists and coaches convened in 2025 first noted the absence of a shared definition, then established it just below the first lactate or ventilatory threshold, whether continuous or interval-based, while acknowledging that the expected adaptations might not be unique to it: sessions slightly above or slightly below could also produce them.

What is proven : continuous moderate-intensity training improves VO2max, multiplies capillaries in sedentary individuals, and lowers heart rate during effort. It is also the intensity that allows accumulating the most volume without exhaustion, and volume could be a determining factor in the quantity of mitochondria.

What is not proven : that zone 2 is the optimal intensity. A 2025 review notes that this idea rests mainly on observation of elite athletes who train extensively. It concludes that the data do not designate zone 2 as the best intensity for mitochondrial capacity or fat oxidation, and suggest favoring higher intensities to maximize cardiometabolic benefits, especially when training volume is low.

At the meta-analysis level, the difference remains modest: compared to continuous endurance, intense intervals provide 1.2 mL/kg/min more VO2max, a small effect. Zone 2 thus serves as a volume base, supplemented by some intensity: over 2 to 3 outings per week, one harder session has its place; beyond that, additional kilometers are run at base endurance pace.

A common pitfall is running easy runs too fast. A German team recalled this in 2023: low-intensity training is often performed faster than intended and drifts toward the threshold. In their trial, even the requirement to breathe only through the nose did not prevent this drift over 60 minutes of cycling at free pace.

Among amateur marathoners in Athens who completed the race in 4 h 20 to 5 h 28, the first threshold was on average at 9.2 km/h, or 6 min 31 s per kilometer: their base endurance, which is run below this threshold, therefore had to be even slower. With a VMA of 11 km/h, 64% yields 7 km/h. Running at 7 or 8 km/h is not unusual for a beginner, and the cohort mentioned above already associated running below 9.7 km/h with lower mortality.

The easy run is also not simply a recovery session: the review dedicated to top distance runners emphasizes that no study shows it accelerates recovery, its value lying in the volume it allows to accumulate. Above all, it reminds us that no single indicator is sufficient: heart rate, pace, and perceived effort vary from day to day and from person to person, and benefit from being cross-referenced. If your usual pace requires an unusually elevated heart rate for several days, our guide fatigue or overtraining helps make the distinction.

There is no mandatory breathing rhythm. A 2022 synthesis notes that many runners spontaneously synchronize breathing and stride, at ratios of 3 to 8 footstrikes per breath, and that breathing frequency closely follows perceived effort.

Nose or mouth? The switch generally occurs toward the mouth beyond approximately 40 liters of air per minute, but athletes accustomed to it have maintained nasal breathing up to 85% of their VO2max. In the German trial already cited, 60 minutes of exclusively nasal breathing at low intensity reduced ventilation, oxygen consumption, and breathing rate, for the same perceived effort. At high intensity, on the other hand, breathing through the nose raises heart rate without a power gain.

In base endurance, nasal breathing is thus an option, not an obligation. The synthesis authors estimate that breathing techniques likely bring nothing in the short term, and perhaps 1 to 5% after long-term practice, a hypothesis that trials must confirm. Better established: specifically training respiratory muscles improves endurance performance according to a systematic review of 46 studies, including 8 controlled trials pooled in meta-analysis, with benefits all the clearer the lower the initial fitness level.

Side stitch affects up to 70% of runners each year. Its cause remains debated, and the idea that an odd rhythm, such as 5 footstrikes per breath, would prevent it by alternating the expiration foot is only a hypothesis.

The 10% rule, which limits weekly mileage increase to 10% per week, was tested in 532 beginners preparing for a 6.7 km race: a progressive 13-week program built on it performed no better than a standard 8-week program, with 20.8% injured versus 20.3%.

Among top distance runners, return to training begins at approximately 40 to 60% of the target maximum volume, then volume increases by 5 to 15 km per week over 8 to 12 weeks, first by adding sessions, then by extending them. Alternating running and walking also helps: in 42 non-elite runners, the marathon run this way was completed in a comparable time to running without breaks, 4 h 14 versus 4 h 08, with less muscle soreness and fatigue at the finish.

Iron is a component of hemoglobin, which transports oxygen to the muscles, and deficiencies are common in runners. In 54 university runners followed over two seasons, 57% of women and 35% of men presented with iron deficiency at least once, defined in particular by ferritin below 20 µg/L, despite supplements provided by their team.

In endurance athletes with deficiency but not anemia, a meta-analysis of 17 studies found a moderate effect of iron on VO2max (0.61). The Cochrane review of 67 trials conducted in 8,506 menstruating women confirms that daily iron reduces anemia and fatigue, with approximately twice as much constipation or loose stools compared to no iron.

Iron supplementation should therefore be discussed after ferritin testing. Our guides on low ferritin and iron deficiency explain how to read this test. If supplementation is useful, our iron bisglycinate provides 14 mg of iron per capsule with 80 mg of vitamin C; our articles when to take iron and iron bisglycinate or other forms expand on this point. When reserves are very low, the doctor may also recommend intravenous iron: in 27 highly trained distance runners, it raised ferritin faster than oral iron, and in the group with the lowest reserves, VO2max increased by 3.3%.

This plan assembles the parameters from the cited studies. It suits a healthy runner who already runs somewhat; a complete beginner can alternate running and walking in the first weeks.

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