By Bryan Haycock, MS

Anyone who has lifted weights, on and off, for several years is familiar with the concept of "muscle memory". Muscle memory in this context refers to the observation that when a person begins lifting weights after a prolonged lay off, it is much easier to return to their previous levels of size and strength than it was to get there the first time around. Even when significant atrophy (muscle shrinking) has taken place during the layoff, previously hypertrophied muscle returns to its previous size more quickly than usual.

A recent study looking at fiber type conversions during muscle hypertrophy may have uncovered a possible mechanism for this phenomenon. For those of you not crazy about scientific lingo bear with me. Towards the end you will see what Iím getting at with this study. In this study the distribution of myosin heavy chain (MHC) isoforms, fiber type composition, and fiber size of the vastus lateralis muscle were analyzed in a group of adult sedentary men before and after 3 months of resistance training and then again, after 3 months of detraining. Following the period of resistance training, MHC IIX content decreased from just over 9% to 2.0%, with a corresponding increase in MHC IIA (42% to 49%). Following detraining the amount of MHC IIX reached values that were higher than before and during resistance training, over 17%! As expected, significant hypertrophy was observed for the type II fibers after resistance training, and even remained larger than baseline after 3 months of detraining.

Myosin heavy chain isoforms, or MHCs, refer to the types of contractile protein you see in a given muscle fiber. MHCs determine how the muscle fiber functions. MHCs are what make a fiber "fast twitch", "slow twitch", or something in-between. Certain MHCs are known to undergo a change in response to resistance exercise. In this case, fibers that contain MHC IIX are fibers that arenít really sure what kind of fiber they are until they are called to action. Once recruited, they become MHC IIAs. So, fibers containg MHC IIX proteins serve as a reservoir of sorts for muscle hypertrophy because the can transform themselves into fibers containing MHC IIX which grow easily in response to training.

Like any great study, these researchers found what they expected as well as a little extra that they didnít. I think this study caught my attention because it showed a long-term alteration in skeletal muscle following resistance training. It has been this long-term change that has been the focus of my own training philosophy, which incorporates what I call "strategic deconditioning". This study showed that resistance training decreases the amount of MHC IIX while reciprocally increasing MHC IIA content. This was expected and has previously observed with changes in fiber type after resistance training. What they didnít expect was that detraining following heavy-load resistance training seems to cause what they refer to as an "overshoot" or doubling in the percentage of MHC IIX isoforms, significantly higher than that measured at baseline. What does this mean? It could mean that there are more fibers available for hypertrophy (growth) after a lay off from training than there are before you start training. This could very well explain the "muscle memory" effect many of us have experienced ourselves. It may also have implications for natural bodybuilders looking to overcome long-standing plateaus.

There are a few questions that this study did not answer. For instance, they waited until 3 months after they stopped training before they took final measurements. It would have been nice if they had taken measurements regularly so that the optimal period of detraining could be identified corresponding to peak MHC IIX levels. Because it takes 3-4 weeks for these contractile muscle proteins to turn over, it would take longer than one month and probably less time than 6 months (previous research). Still the optimal time remains to be elucidated.

Also, how would these guys respond to the same training regimen after the detraining period? Would their quads grow to their previous trained size, or even further? How long would it take? These questions, if answered, may add a new twist to typical training regimens. It may very well be that extended breaks from training may actually allow greater growth over a 12-month period than if training is uninterrupted. For serious athletes and bodybuilders, this would be important information and could significantly extend their competitive careers.