How Much Time to “Get in Shape”?


For distance running there are three main factors that determine racing performance (or fitness): Oxygen transport (VO2max), Lactate Threshold (LT) and Running Economy (RE). The figures below, illustrate how training/de-training affects each of these factors over time:

Training induces improvements in VO2max that maximize on a 2-3 month timescale. The two main factors that contribute to this improvement are: the Stroke Volume (SV) of the heart and Oxygen Extraction (a-vO2) by the working muscle. In the first month of training, a-vO2 is the main driver of VO2max improvements. After 1-2 months, however, SV becomes the main determinant of VO2max: accounting for ~75% of the maximum adaptation.1-6

Unfortunately, these improvements don’t last that long if training stops, at which point the VO2max declines over a 2-3 week timescale. Luckily, fitness loss is never 100% as a small amount of fitness is permanent. This long-term VO2max adaptation is mainly contributed by a-vO2 (as a result of increased capillaries per muscle and elevated myoglobin concentrations in the muscle). 1-6


Training induces improvements in LT that also maximizes on a 2-3 month timescale. The three main factors that contribute to LT are: the concentrations of mitochondrial enzymes (ME) and the concentrations of enzymes involved in Fatty Acid Oxidation (FAO) and Glucose Oxidation (GO). As can be seen in the figure above, endurance training results in an increase in FAO and ME and a decrease in GO. This sparing of glucose reserves in favor of fat reserves results in the production of less lactate because only Glucose Oxidation (GO) can occur anaerobically (without oxygen) and produce lactate. Fatty Acid Oxidation (FAO), on the other hand, can only procede aerobically (with oxygen) and cannot produce lactate.1-3,5,7-8

Just like with VO2max, these improvements are mostly lost after 2-3 week of no training. Again, this loss is never 100% as some of the increase in mitochondrial enzymes(ME) appears permanent.1,5,7


Finally, a large part of the training-induced improvements in RE also occur on a 2-3 month timescale. The two main factors that contribute to RE are neural recruitment (NR) of the muscle fibers and muscular strength (MS). Improvements in NR can be thought of as “gaining muscle memory” as they optimize interactions between your nervous system and your muscles. NR improves very rapidly with significant “muscle memory” improvements occuring in about a week. Muscle Strength (MS) improvements occur more slowly as they require the synthesis and rebuilding of muscle fiber proteins and muscle cells. Synthesis of new muscle proteins begins within a couple hours of a training stimulus,13-14 but assembly of these raw materials into newer and stronger muscle cells takes over a month to fully realize. 9-13

Unfortunately, yet again, the majority of training adaptations in NR and MS are lost within just a few weeks if training stops. Some more permanent changes in RE do occur such as: increased muscle nuclei per muscle cell15 and improvements in biomechanics (which occur and dissipate very slowly).2



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  2. Noakes, T. Lore of Running 4th ed. 2001 Human Kinetics. Champaign, IL.
  3. Saltin, B.; Henricksson, J.; Nygaard, E.; Andersen, P.; FIBER TYPES AND METABOLIC POTENTIALS OF SKELETAL
  4. Coyle, E.F.; Martin III, W.H.; Sinacore, D.R.; Joyner, M.J.; Hagberg, J.M.; Holloszy, J.O. Time course of loss of adaptations after stopping prolonged intense endurance training. J. App. Physiol. 1984, 57, 1857-64.
  5. Henricksson, J. Effect of Physical Training on the Metabolism of Skeletal Muscle. Diabetes Care 1992, 15, 1701-11
  6. Murias, J.M.; Kowalchuk, J.M.; Paterson D.H. Time course and mechanisms of adaptations in cardiorespiratory fitness with
    endurance training in older and young men. J. Appl. Physiol. 2010, 108, 621-7.
  7. Coyle, E.F.; Martin III, W.H.; Bloomfield, S.A.; Lowry, O.H. Holloszy, J.O. Effects of detraining on responses to submaximal exercise. J. App. Physiol. 1985, 59, 853-9.
  8. Phillips, S.M.; Green, H.J.; Tarnopolsky, M.A.; Heigenhauser, G.J.F.; Hill, R.E. and Grant S.M. Effects of training duration on substrate turnover and oxidation during exercise. J. App. Physiol. 1996, 81, 2182-191.
  9. Moritani, T.; DeVries, H. Neural Factors versus Hypertrophy in the Timecourse of Muscle Strength Gain. Am. J. Phys. Med. 1979, 58, 115-30
  10. Hakkinen, K.; Alen, M. Komi, P.V. Changes in isometric force and relaxation-time, electromyographic and muscle fibre characteristics of human skeletal muscle during strength training and detraining. Acta Physiol. Scand. 1985, 125, 573-585.
  11. Narici, M.V.; Roi, G.S.; Landoni, L.; Minetti, A.E.; Cerretelli, P. Changes in force, cross-sectional area and neural activation during strength training and detraining of the human quadriceps. Eur. J. Appl. Physiol. 1989, 59, 310-19.
  12. Sale, D. Neural adaptation to resistance training. Med. Sci. Sports Exerc., 1988, 20, S135-45.
  13. Bickel, C.S.; Slade, J.; Mahoney, E.; Haddad, F.; Dudley, G.A.; Adams, G.R. Time course of molecular responses of human skeletal muscle to acute bouts of resistance exercise. J. Appl. Physiol. 2005 98, 482–488
  14. Seynnes O.R.; deBoer, M.; Narici, M.V. Early skeletal muscle hypertrophy and architectural changes in response to
    high-intensity resistance training. J. Appl. Physiol. 2007, 102, 368–373,
  15. Bruusgaard, J.C.; Johansen, I.B.; Egner, I.M.; Rana, Z.A.; Gundersen, K. Myonuclei acquired by overload exercise precede
    hypertrophy and are not lost on detraining. PNAS 2010, 107, 15111-15116.


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2 responses to “How Much Time to “Get in Shape”?

  1. Thanks for putting this together. This really helps to clarify the many “moving parts” involved in running conditioning. I have a question pertaining to the time frames here: I’m sure when you say that these three main factors of running performance improve over a 2-3 month time frame, you don’t mean to say that an individual runner’s lifetime peak fitness can be attained within the span of three months. As any serious runner knows, benefits can accrue over not just the span of months, but over the span of years and even decades of smart, patient training. So on the graphs with % of fitness on the y-axes, what does 100% fitness really refer to?

    Again, thanks so much for the great work you all are doing. I’m looking forward to future posts.

    • Great Question! In the figures above “100%” just refers to the maximum “sharpening” or “temporary fitness” that can be obtained in a few month timescale (or average season or training cycle). In other words, most of the data indicates that it takes about 2-3 months, of consistent training, to be near Personal Record-shape.

      In addition to that month-scale “sharpening”, there is also a long-term “baseline” or “permanent” adaptation which occurs on a years timescale. You can see evidence of these long-term and permanent adaptations in the data above where the “detraining” or loss of fitness never goes back to 0%. A few sources of these “permanent” changes seem to be in a percentage of: muscle strength, mitochondrial density and muscle capilarization (oxygen extraction).

      In summary the 100% refers to total “sharpening” over over baseline; but, as you correctly point out, the baseline is improving too.

      Thanks for the kinds words and keep the comments coming! We try to be as clear and concise as possible but sometimes being concise means you leave some useful information out!

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