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There are various adaptations to endurance training, which are discussed in some detail below. Understanding these adaptations can be useful background when looking at the different types of training. (Except where noted, this list is based on "Physiology of sport and exercise"<ref>Jack H. Wilmore, David L. Costill, W. Larry. Kenney, Physiology of sport and exercis, 2008 !!date!!, Human Kinetics !!publisher!!, Champaign, IL !!location!!, isbn 0-7360-5583-5</ref>.)
=Heart Adaptations=
* The heart becomes larger in response to endurance training, with the left side that sends blood to the body having a thicker wall and a larger interior volume.
* The stroke volume (blood pumped with each beat) also increases. The volume is greater at rest and during exercise, with the stroke volume in a highly trained athlete twice that of an untrained person. This increase is due to multiple factors:
** The increased heart size.
** Increased total blood volume.
** A greater force of contraction.
** A lower volume of blood remaining in the heart after contraction. * [[Resting Heart Rate| Resting Heart Rate]] drops with training. A sedentary person with a resting [[Heart Rate]] of 80 BPM may be able to reduce their resting [[Heart Rate]] by 1 BPM per week of training for up to 10 weeks, though some studies have shown much smaller changes (5 BPM over 20 weeks). * The [[Heart Rate| Heart Rate]] for a given exercise intensity is reduced, and sedentary people undergoing 6 months of training can reduce the [[Heart Rate]] for a given intensity by 10-30 BPM. * [[Maximum Heart Rate| Maximum Heart Rate]] does not typically change with endurance training, but several studies have suggested that sedentary people with a Max HR of 180+ BPM may have a slightly reduced value post-training. This reduction in Max HR may be to preserve the time for the heart to refill with blood. Also, older athletes may have less of an age related decline in Max HR than those that are sedentary. * Cardiac output at rest and at moderate intensity exercise remains largely unchanged with training, which is the result of a greater stroke volume at a lower [[Heart Rate]]. (Sometimes the cardiac output will be slightly lower, probably due to greater oxygen extraction by the muscles.) However, at higher intensity exercise the cardiac output is increased, which results in improved performance.
=Circulation Adaptations=
* Endurance training results in an increased number of capillaries in the muscles. The ratio of capillaries to [[Muscle|muscle fibers]] allows for a greater delivery of blood to the working muscles. In addition, the use of existing capillaries is improved with training. * The human body does not have enough blood to fully supply all capillaries in the body, and so it will redistribute the blood flow to the most needed areas. Endurance training improves this redistribution, including redistribution within the muscles themselves, providing more blood to the slow twitch fibers and less to the fast twitch fibers.
* Blood pressure at a given exercise intensity is reduced, but at maximum intensity systolic pressure is increased and diastolic is reduced. (Systolic is the higher pressure when the heart beats and diastolic is the lower pressure when the heart is refilling.)
* Endurance training generally lowers resting blood pressure. Systolic is reduced by ~4 mm Hg and diastolic by 3 mm Hg in people with either high or normal blood pressure<ref name="Whelton-2002">SP. Whelton, A. Chin, X. Xin, J. He, Effect of aerobic exercise on blood pressure: a meta-analysis of randomized, controlled trials., Ann Intern Med, volume 136, issue 7, pages 493-503, Apr 2002, PMID [http://www.ncbi.nlm.nih.gov/pubmed/11926784 11926784]</ref>.
* Blood volume increases with endurance training, and higher intensity produces a greater change. ** Blood plasma increases rapidly, with initial changes seen within an hour of the end of the first bout of training. ** The number of red blood cells, that carry oxygen in the blood, also increases with training. While these extra cells add to the increase in overall blood volume, because the plasma increases disproportionately more than the red blood cells, the net effect is that the blood becomes more dilute and can result in a false indication of anemia (pseudoanemia). Having thinner blood may be an advantage to an athlete, as it will flow more easily through the capillaries.
* The oxygen saturation of blood returning from the exercising body to the heart is lowered after training. This is due to an improved ability of the muscles to utilize the supplied oxygen
=Lung (pulmonary) Adaptations=
* Generally, there is little structural adaptation of the lungs, as their capacity is far greater than is needed in most individuals. * [[Breathing]] at rest is unchanged with training, but for a given sub-maximal intensity, [[Breathing]] is reduced by as much as 20-30%. Maximal [[Breathing]] increases with training, with increases in both the [[Breathing]] rate and volume. * In some highly trained athletes the lungs are unable to keep up at high exercise intensities and the blood oxygen saturation ([[SpO2|SpO<sub>2</sub>]]) can drop.
* Training increases the blood flow to the lungs (perfusion), which combined with greater [[Breathing]] and lower oxygen saturation of the blood entering the lungs results in a greater uptake of oxygen (pulmonary diffusion).
=Muscular Adaptations=
* One of the most important adaptations to endurance training is the increase in the number of capillaries in the muscles.
* Endurance training may increase the size of slow twitch (Type I) muscle fibers by up to 25%, or the training may decrease the size of slow twitch and some fast twitch (Type IIa) fibers<ref name="Trappe-2006">S. Trappe, M. Harber, A. Creer, P. Gallagher, D. Slivka, K. Minchev, D. Whitsett, Single muscle [[Fiber]] adaptations with marathon training., J Appl Physiol, volume 101, issue 3, pages 721-7, Sep 2006, doi [http://dx.doi.org/10.1152/japplphysiol.01595.2005 10.1152/japplphysiol.01595.2005], PMID [http://www.ncbi.nlm.nih.gov/pubmed/16614353 16614353]</ref>.
* There are changes within the fast twitch fibers so that Type IIb/IIx become more like Type IIa with endurance training. There is some evidence of a small change of fast twitch fibers to slow twitch.
* Oxygen is carried within a muscle [[Fiber]] by myoglobin, which is similar to hemoglobin. Endurance training can increase the myoglobin in muscle fibers by up to 80%. * The size and number of mitochondria in the muscles increases with training. In rats, 27 weeks of exercise increased the mitochondria size by 35% and the number by 15%.
* Within the mitochondria, the oxidative enzymes increase with training. Over 7 months of swim training, one oxidative enzyme ([http://en.wikipedia.org/wiki/Succinate_dehydrogenase SDH]) increased by 800%.
=Metabolic Adaptations=
* [[Lactate threshold Threshold]] is the exercise intensity at which lactate [[Lactate]] starts to rapidly build up in the blood and is a key indicator of aerobic performance. Endurance training increases the exercise intensity of this threshold.* Endurance training results in a shift towards fat burning at sub-maximal exercise intensities. * [[Running Economy]] is improved by endurance training. At elite levels this may be a critical part of improving performance. * Maximum oxygen consumption ([[VO2max|V̇O<sub>2</sub>max]]) typically increases by 15-25% is typical for sedentary people who exercise for six months. The lower the initial [[VO2max|V̇O<sub>2</sub>max]], the greater the relative increase is likely to be. Typically an individual will reach their greatest [[VO2max|V̇O<sub>2</sub>max]] possible after 12-18 months. Longer term performance improvements come through being able to sustain higher percentages of [[VO2max|V̇O<sub>2</sub>max]] rather than increases in [[VO2max|V̇O<sub>2</sub>max]] .
* Contrary to popular belief, endurance training does not raise the metabolic rate at rest.
=Time to Adapt=
* Resting HR drops after 3 months with no further improvement thereafter<ref name="Scharhag-Rosenberger-2009"/>
* HR at a given submaximal intensity drops within 3 months<ref name="Scharhag-Rosenberger-2009"/>
= Adaptations and Endurance Running=
[[File:Model of endurance running.jpg|none|thumb|500px|A model of how three adaptations impact endurance performance<ref name="Midgley-2007"/>.]]
This flowchart shows that while [[VO2max|V̇O<sub>2</sub>max]] is a key determinant of performance, it interacts with other factors. Lactate threshold defines what percentage of [[VO2max|V̇O<sub>2</sub>max]] can be sustained, which defines the energy available (ATP re-synthesis). [[Running Economy]] then defines how well this energy can be translated into forward motion.
<ref name="Midgley-2007"> AW. Midgley, LR. McNaughton, AM. Jones, Training to enhance the physiological determinants of long-distance running performance: can valid recommendations be given to runners and coaches based on current scientific knowledge?, Sports Med, volume 37, issue 10, pages 857-80, 2007, PMID [http://www.ncbi.nlm.nih.gov/pubmed/17887811 17887811]</ref>
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[[Category:Science]]