The human body is an incredible “machine”. It will get rid of anything it doesn’t need in a minuscule amount of time and muscle is no exception. If you don’t work out for a couple of months and happen to lose 6 pounds of muscle, then start training again it will come back in a shorter amount of time than it took when your first started. This theory has been controversial in the past, but we are now starting to discover why this phenomenon occurs.
This function most likely has a significant amount to do with myosin heavy chain (MHC) isoforms which are a type of contractile protein in muscle fiber. They determine whether muscle fibers function as fast or slow twitch. Certain MHCs are known to change in response to resistance training.
There are several types of MHC that we’re concerned with. Fibers that contain MHC IIX are an undetermined type of fiber until they’re used. Once in action they become MHC IIAs. Muscle fibers that contain MHC IIX proteins serve as a source for creating MHC IIA fibers, but can also generate more fibers containing MHC IIX which eases muscle redevelopment.
A study was conducted with adult sedentary men that analyzed distribution of MHC isoforms, fiber type composition, and fiber size of the vastus lateralis (section of the quadricep) before and after three months of training, and again after three months of detraining. After three months of training, their MHC IIX content went from 9% to 2% and a corresponding increase of MHC IIA content from 42% to 49%. After the 3 months of detraining the MHC IIX content reached 17% which is higher than both before and during training. Hypertrophy was observed after the three months after training and remained higher than baseline even after 3 months of detraining.
This would indicate that detraining following heavy load resistance training seems to stimulate an increase in the amount of MHC IIX to values far higher than those observed before resistance training. Based on the evidence that has been observed it would appear that muscle memory is indeed a real phenomenon at least in the short term.