For endurance runners, the result is determined by three basic physiological parameters: maximum oxygen consumption (VO2max), lactate or anaerobic threshold (LT), and run economy (RE). The figures below show how training and stopping workouts affect these parameters over time.
Temporary adaptation of the IPC (VO2max).
Training contributes to the output of the IPC to the maximum values already in 2-3 months of regular training. The two main factors that contribute to this adaptation are stroke volume of the heart (Stroke Volume, SV) and utilization-oxygen consumption by working muscles (Oxygen Extraction, a-vO2). In the first month of training, the IPC is growing mainly due to an increase in oxygen consumption by muscles. But in 1-2 months, the growth of the shock volume will be the main factor determining the growth of the IPC, it is the growth of the impact volume that contributes about 75% to the overall increase in the IPC.
Unfortunately, if we stop training, the IPC starts to decline rapidly, showing the minimum values already in 2-3 weeks. But the level of the IPC will still be higher than before when we just planned to start training. This is due to the fact that as a result of training in the muscles there are long-term positive shifts in terms of utilization of oxygen by workers and muscles, namely: the density of capillaries increases and the concentration of myoglobin increases.
Temporary adaptation of the lactate threshold (LT).
Тренировка влияет на ПАНО (лактатный порог LT) так же, как и на МПК – его максимальные показатели достигаются через 2-3 месяца регулярных тренировок. ПАНО растет за счет трех основных факторов: концентрации митохондриальных ферментов (Mitochondrial Enzymes, ME) и концентрации ферментов, способствующих окислению жирных кислот (Fatty Acid Oxidation, FAO) и концентрации ферментов, способствующих оксидации глюкозы (Glucose Oxidation, GO).
As you can see in the figure, endurance training leads to an increase in mitochondrial enzymes (ME) and enzymes that oxidize fats (FAO) and reduce the concentration of enzymes that oxidize glycogen (GO).
The shift from the splitting of glucose towards the oxidation of fatty acids is due to the fact that we form a smaller amount of lactate at the same running speed, that is, by training we shift the energy supply from anaerobic cleavage of glucose towards aerobic fission of fats.
By analogy with the IPC, the ANSP falls to the minimum values already in 2-3 weeks, if the training stops. Again, the fall of ANSPs never reaches 100%, as the concentration of some mitochondrial enzymes remains elevated.
Temporary adaptation of economy of run (RE).
And finally, such an important value in the endurance run as the economy of running (RE), also goes to the plateau through the 2-3 month of endurance training. The economy of running depends on two factors: the rate of activation of muscle fibers and the number of involved fibers (Neurological Recruitment, NR), as well as muscle strength (Muscle Strength, MS).
Improving the activation of muscle fibers can be seen as an increase in "muscle memory", since it is she who optimizes the work between the nervous system and muscle fibers. Adaptation in muscle fibers occurs quickly enough with a significant improvement in "muscle memory" and takes about a week.
The growth of muscle strength is slower, because it depends on the synthesis of intracellular protein. Synthesis of new proteins starts a couple of hours after the end of training, and the "building" of new muscle fibers takes more than a month.
But even after several weeks without training, these indicators fall to the initial values, however, remaining slightly higher than the initial values. This is due to the growth of nuclei in muscle cells and improved biomechanics, the quality of which does not decrease so quickly.
Источник информации: Runners’ club (2016), E.Douglass, C.Miller (2015).
Дополнительно про потерю физической формы можно еще почитать в другой заметке: Losing the physical form or detraining the overall view of the problem