Detraining is a complex process. Some of the effects are touched on in the abstract below. Crossectional muscle area decreases during detraining according to some studies, but not in other studies. I think most bodybuilders would agree that anecdotal observations favor a decrease in area. Other studies have shown that during detraining in strength athletes (heavy resistance trained athletes) type IIa decrease in number relative to type IIb. This mirrors the normal adaption during resistance training where type IIa fibers increase relative to IIb.
If crossectional area stayed the same, the number of myonuclei would too. In the studies where area decreased, there should be a decrease in myonuclei as well. So it is really not clear if nuclear number is part of "muscle memory" or neuromuscular facilitation, as the sports physiologists call it. It is probably more neural than muscular.
Med Sci Sports Exerc 2001 Aug;33(8):1297-303
Muscular characteristics of detraining in humans.
Mujika I, Padilla S.
Departamento de Investigacion y Desarrollo, Servicios Medicos, Athletic Club de Bilbao, Basque Country, Spain.
[email protected]
Skeletal muscle is characterized by its ability to dynamically adapt to variable levels of functional demands. During periods of insufficient training stimulus, muscular detraining occurs. This may be characterized by a decreased capillary density, which could take place within 2--3 wk of inactivity. Arterial-venous oxygen difference declines if training stoppage continues beyond 3--8 wk. Rapid and progressive reductions in oxidative enzyme activities bring about a reduced mitochondrial ATP production. The above changes are related to the reduction in VO(2max) observed during long-term training cessation. These muscular characteristics remain above sedentary values in the detrained athlete but usually return to baseline values in recently trained individuals. Glycolytic enzyme activities show nonsystematic changes during periods of training cessation. Fiber distribution remains unchanged during the initial weeks of inactivity, but oxidative fibers may decrease in endurance athletes and increase in strength-trained athletes within 8 wk of training stoppage. Muscle fiber cross-sectional area declines rapidly in strength and sprint athletes, and in recently endurance-trained subjects, whereas it may increase slightly in endurance athletes. Force production declines slowly and in relation to decreased EMG activity. Strength performance in general is readily maintained for up to 4 wk of inactivity, but highly trained athletes' eccentric force and sport-specific power, and recently acquired isokinetic strength, may decline significantly.
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