A lot of lifters talk about the mysterious “muscle memory” that allows them to rapidly recover strength after a layoff from the gym.  However, it has been really difficult to pinpoint exactly what might account for this phenomenon.  The information below may finally shed some light on this elusive subject.

In a 1991 study by a Staron and colleagues 20 women underwent a 20 week resistance training protocol, took 2 weeks off, and then trained again for 6 more weeks.

As would be expected, there was an increase in cross-sectional area of the fibers (i.e., muscle growth) during the first 20 weeks of training.  There was also an expected fiber type shift from Type IIB to Type IIA muscle fibers.  That is, the percentage of Type IIB decreased and Type IIA increased.

At this point it isn’t necessary that you understand the exact science of fiber type changes.  All that is really important for our purposes is that you understand that there was a shift from one type to another and that this is expected with resistance training.

During the two weeks of detraining the fiber types immediately starting to shift back to their pre-training status with the percentage of Type IIB fibers going up and Type IIA fibers going down.

Here’s the interesting part:
When the women started training again after the two week layoff their fiber type shift back to where it was when they were training happened much quicker than it did when they first started training.  In essence, the body adapted more quickly when the person had previously trained which may be a plausible explanation for “muscle memory”.

Of course, there is a big theoretical assumption here.  Truthfully, it is not known whether or not a fiber type shift is responsible for strength gains in early training or not.  We do know that strength does go up during the first eight weeks of training when there is usually very little actual muscle growth to account for it. 

We also know that fiber type changes do happen during this time period and that they often coincide with strength increases.  However, there are certainly nervous system changes contributing to strength increases during those first eight weeks as well. 

So although it isn’t 100% conclusive, fiber type shifting can’t be ruled out as a possibility for at least part of strength gains associated with early training and a possible explanation for muscle memory.