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:
OXYGEN TRANSPORT (VO2max) ADAPTATION:
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
LACTATE THRESHOLD (LT) ADAPTATION:
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
RUNNING ECONOMY (RE) ADAPTATION:
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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