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[[File:Three Runners 5802443 s.jpg|right|thumb|300px|There are many benefits to endurance training.]]
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>{{Cite book | last1 = Wilmore | first1 = Jack H. | last2 = Costill | first2 = Wilmore, David L. | last3 = Kenney | first3 = Costill, W. Larry. | title = Kenney, Physiology of sport and exercis | , 2008 !!date = 2008 | publisher = !!, Human Kinetics | location = !!publisher!!, Champaign, IL | !!location!!, isbn = 0-7360-5583-5 | pages = }}</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 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">{{Cite journal | last1 = Whelton | first1 = SP. | last2 = Chin | first2 = Whelton, A. | last3 = Xin | first3 = Chin, X. | last4 = He | first4 = Xin, J. | title = He, Effect of aerobic exercise on blood pressure: a meta-analysis of randomized, controlled trials. | journal = , Ann Intern Med | , volume = 136 | , issue = 7 | , pages = 493-503 | month = , Apr | year = 2002 | doi = | , 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.
=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">{{Cite journal | last1 = Trappe | first1 = S. | last2 = Harber | first2 = Trappe, M. | last3 = Creer | first3 = Harber, A. | last4 = Gallagher | first4 = Creer, P. | last5 = Slivka | first5 = Gallagher, D. | last6 = Minchev | first6 = Slivka, K. | last7 = Whitsett | first7 = Minchev, D. | title = Whitsett, Single muscle fiber adaptations with marathon training. | journal = , J Appl Physiol | , volume = 101 | , issue = 3 | , pages = 721-7 | month = , Sep | year = 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%.
* 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=
How long does it take to adapt to endurance training?