Endurance allows people to work out at a certain intensity or for an extended amount of time (hello, marathon!)The effect of endurance training on parameters of aerobic fitness. Jones, A.M. Carter, H. Department of Exercise and Sport Science, Crewe and Alsager Faculty, The Manchester Metropolitan University, England. Sports Medicine, 2000 Jun;29(6):373-86.. There are a number of factors that combine to create an athlete’s “endurance profile,” and two of the most important are VO2 max and lactate threshold. VO2 max, or the maximum rate at which an athlete’s body can consume oxygen during exercise, is the most popular measurement of aerboic capacity (although it’s unclear if it’s necessarily the most accurate)Limiting factors for maximum oxygen uptake and determinants of endurance performance. Bassett, D.R., Howley, E.T. Department of Exercise Science and Sport Management, University of Tennessee, Knoxville TN. Medicine and Science in Sports and Exercise, 2000 Jan;32(1):70-84.Testing for maximum oxygen consumption has produced a brainless model of human exercise performance. Noakes, T.D. Research Unit for Exercise Science and Sports Medicine, Deaprtment of Human Biology, University of Cape Town, Cape Town, South Africa. British Journal of Sports Medicine, 2008 Jul;42(7):551-5. Epub 2008 Apr 18.. Although endurance ability is mostly a matter of genetics, maximal oxygen uptake can be improved with targeted training. High intensity interval training (HIIT) workouts have been shown to to do the trick, increasing athletes’ VO2 maxAerobic high-intensity intervals improve VO2max more than moderate training. Helgerud J, Høydal K, Wang E, et al. Medicine and Science in Sports and Exercise. 2007 Apr;39(4):665-71.. Another piece of the endurance puzzle is the athlete’s lactate threshold, or the level of exertion at which lactate accumulates in the musclesLactate threshold concepts: how valid are they? Faude, O., Kindermann, W., Meyer, T. Institute of Sports Medicine, University Paderborn, Paderborn, Germany. Sports Medicine, 2009;39(6):469-90. Luckily, it’s possible for virtually any athlete to improve both of these measures. To improve lactate threshold—and therefore the ability to workout harder for longer periods of time—Greatist Expert Noam Tamir says tempo runs may do the trick. Endurance athletes often have a higher proportion of slow-twitch muscle fibers, which power steady-state activities like running by using oxygen efficiently to g enerate more energyMaximal oxygen uptake and muscle fiber types in trained and untrained humans. Bergh, U., Thorstensson, A., Sjodin, B. et al. Medicine and Science in Sports, 1978 Fall;10(3):151-4. Running long can train slow-twitch muscles to fuel such workouts more efficiently and to fight fatigue more effectively. A continuous practice of long-distance running can also help convert fast-twitch muscles to slow-twitch muscles, which will enhance endurance.