Grist For The Mill

by M. Doug McGuff, M.D.

It has been some time since my last article, particularly anything to do with training theory. This has been for two reasons. First, my jobs have been keeping me busy; I have been working more ER shifts than I have in the past, and UE has remained very busy as well. More importantly, I have just not come across anything important enough to warrant writing about. What I offer in this article is not earth-shattering and perhaps not even original. However, I have had some thoughts that might be germane to High Intensity/SuperSlow training.

Rest-Pause at Failure-A Useful Intensity-Extender?

If you refer to some of the other articles on this website (particularly the dynamism vs. stasis series), you will note that, since 1998, we at UE have called into question the utility of the deep inroad technique in advanced subjects. We still use the deep inroad technique in most of our clients as they learn to push themselves harder and harder. But as they become more adept at reaching what we judge to be true muscular failure, we shorten the time a client spends in deep inroad. The clients probably don’t notice, because we still count down from ten to zero, we just count quicker. In truly advanced subjects we stop immediately at failure and sometimes short of failure. In an advanced subject, continuing to push for 10 seconds after failure represents an intensity level that is very difficult to recover from, and (in my opinion) produces no further growth stimulus. In fact, it may actually retard growth. Stopping short of this, and sometimes short of failure allows the advanced subject to recover in an adequate time frame, and also allows an adequate volume of exercise to maintain metabolic condition. If intensity is so high that only 2 or 3 movements can be performed every 10th or 12th day, many in the field have noted that general metabolic condition will suffer…and they are correct.

Let us define deep inroad technique. In the simplest terms, it is the act of continuing to exert against the resistance after you have reached momentary muscle failure. Even though the weight is not moving, you continue to exert against the movement arm for an additional 10 or 15 seconds. This produces a much deeper level of momentary fatigue and a higher percentage of demonstrable strength is lost. As a subject lifts and lowers a resistance, his muscles progressively fatigue. Slow twitch motor units are recruited first, then intermediate, and then the fast twitch units are brought into play. If all three types of motor units are fatigued quickly enough (before the slow twitch fibers can recover) then failure under that load will ensue. By the time you are about 85% of the way to failure, you have probably recruited 100% of the motor units that you are capable of recruiting. What happens between 85% and 100% failure? Summation is what happens. The involved motor units are fired at a faster rate, sort of like revving an engine to get the pistons to fire faster. What happens at failure and during deep inroad? Tetany is what happens. Tetany means that the motor units are receiving nerve impulses so quickly that there is no opportunity for a relaxation phase…all the motor units are essentially stuck in the “on” position. If tetany occurs for a long enough period, all of the acetylcholine (the neurotransmitter at the junction between the nerve and the motor unit) will be exhausted. When this occurs, the muscle cannot respond to a nerve impulse. The muscle essentially does not know what to do. It just lies there and quivers in response to its own electrolyte fluxes and membrane instabilities. This is why Ken Hutchins describes an alpha subject who could inroad very deeply as “quivering like a frog injected with strychnine”. Interestingly, strychnine acts by blocking the action of acetylcholine at the neuromotor junction; that is why the quivering of our frog looks like the quivering of an alpha subject.

Having described the mechanism of extreme deep inroading, I would like to offer some observations. (Please note that these observations are my own theoretical musings and are not scientifically proven fact.) Such a practice, if done infrequently is a great mechanism for improving strength in a given movement. By recruiting as many motor units as possible, and forcing them to fire as rapidly as you can you are “beating a neuromotor dog-trail” that will result in better performance in that movement over time. By exhausting neurotransmitter, I think you might produce a situation where the motor end-plate would act by upregulating its acetylcholine receptors. With more receptors, each motor unit could be recruited more efficiently. This improvement could come at a cost, however. It seems logical that the motor units that would be most sensitive to this exhaustion of neurotransmitter would be the fast twitch units. Since these fast twitch units have been shown to be the slowest to recover, they probably are the slowest to recover functional levels of neurotransmitter at their nerve terminal. This process is regulated by an enzyme called cholinesterase which inhibits the destruction of acetylcholine in the synapse. I suspect that this enzyme is in shorter supply at fast twitch nerve terminals, and may have some contribution to their fatigue rate irrespective of the metabolic differences in the muscle fiber itself…but I digress. This relative lack of neurotransmitter creates a situation where the motor end plate might decide to make more receptors. Later, when normal levels of acetylcholine re-accumulate, the extra receptors bind the neurotransmitter more aggressively and more quickly. This results in an earlier recruitment of fast twitch motor units, more demonstrable strength, but a more rapid rate of fatigue (shorter TUL). Slower twitch units recover functional strength very quickly, and are would not be subject to such a phenomenon. In fact, research done by Arthur Jones in the 1980’s and 90’s showed that slow twitch motor units respond better with multiple exposures to exertional stress.

If this theory were correct, there would necessarily be a dark side to all of this. The million dollar question is this….How long does it take to recover a functional level of neurotransmitter in the fast twitch units? My guess is that in the fast twitch motor units, it may be several days of even weeks. If this is true, that is long enough for those particular motor units to suffering from what I call “functional dennervation”. Essentially, when a motor unit is not receiving any neurotransmitter from its designated motor nerve, it will behave as if its motor nerve has been transected. As anyone familiar with physiology knows, a motor unit cut off from its nerve supply will atrophy. This phenomenon is most pronounced in the large, fast-twitch motor units. So a technique which might potentially increase demonstrable strength could actually produce atrophy in the very motor units which have the most potential for producing hypertrophy. However, when neurotransmitter returns, these atrophied motor units might still perform well because of an upregulated number of acetylcholine receptors. I have seen enough 150 pound subjects use the entire stack on a machine to wonder if this is exactly what is happening.

So what might be a better intensity extender if one’s goal were not simply demonstrable strength increases? To answer this question we need to again review the types of muscle fibers/motor units and their characteristics. For simplicity sake, we will refer only to the general categories of slow twitch, intermediate twitch, and fast twitch. Slow twitch motor units produce modest contractile force, fatigue slowly, and recover quickly. Because of their fast recovery profile, these are the motor units that might stand to benefit from repeated exposure to stress and fatigue (this has been borne out in data collected by Arthur Jones that showed subjects with a predominance of slow twitch fibers actually perform better on a second set after a first set to failure). Fast twitch motor units produce high contractile force, fatigue quickly, and recover slowly. Subjects who have predominantly fast twitch fibers show marked weakening after a single set to failure and a long respite is required before strength returns to its baseline. Intermediate twitch motor units fall between these two extremes. During a set to failure, slow twitch units are recruited first. If fatigue occurs more rapidly than the time it takes for these slow twitch units to recover, then intermediate and fast twitch units will be recruited. If the fast twitch units become fatigued before any of the slower twitch units can recover, force output will eventually fall below the weight being used to fatigue that muscle and failure (the inability to move that weight) will occur.

The problem with most intensity extenders is that they involve ongoing effort after failure has occurred. This is true for forced reps (partner helps to lift weight, bleeding off resistance so that movement can continue), cheat reps (momentum or extraneous muscle groups are used so that movement can continue), drop sets (weight is quickly reduced so that movement can continue), or deep inroad (effort is continued even though movement is not occurring). In all of these scenarios, all 3 types of motor units are still activated. The fast twitch motor units are thus exposed to repeated firing of the neuromotor end-plate. This can result in exhaustion of neurotransmitter and exposes the motor units that are most intolerant to fatigue to repeated bouts of stress.

A more ideal intensity extender would be one that exposed only the motor units that would benefit from repeated bouts of stress, while sparing those that are stress sensitive.
Well, consider what happens if you rest just long enough after failure to allow yourself to get just one more repetition (say 10-15 seconds). During this 10 second respite, the slowest twitch motor units have enough time to recover. As a result, they can now contribute to the lifting effort and you are able to get one more repetition. The faster twitch motor units have not yet recovered, and since some newly recovered motor units are now available, these faster twitch motor are spared from recruitment. One could do another rest/pause, and possibly another. In each case, the slowest twitch (quickest recovering) motor units would be available to complete the movement. By using such a protocol, one could selectively expose only those motor units which would benefit from repeated bouts of exertion (the slow twitch units), while sparing those motor units that suffer when exposed to multiple bouts of exertion (the fast twitch units).
Note that this is different from traditional rest/pause training where heavier than usual weights are used. Minimal repetitions are achieved, necessitating a pause to complete another bout of minimal repetitions. While the concept of motor unit recovery is still responsible for the ability to continue after the rest/pause, the heavier weight allows the recruitment of the three types of motor units in tandem, rather than sequential recruitment (as is seen with more modest weights). With recruitment of all three types of motor units in tandem, it only takes the dropping out of the extremely fast motor units for failure to occur. As a result, motor units that are still significantly fast twitch will be exposed to multiple recruitments and the slower twitch units will never really get the opportunity to fatigue. So while traditional rest/pause might seem similar on first inspection, I believe it might produce just the opposite result of rest/pause at failure.

For the past year, we have experimented with this protocol at UE. We have only used it on ourselves, and a very few clients. We have seen some encouraging responses (minimal increases in measurements of subjects who were not growing). Some might suggest that it was simply the change in stimulus that produced the results, and this may certainly true (as Brian Johnston has argued with his theory of blitzing, chaos training, and cycle blasting ™). We have taken a group of six long standing clients and subjected them to blitz routines as suggested by Johnston and have shown minimal, but positive results in clients who had otherwise stopped growing. However, when rest/pause at failure is used as the primary intensity extender, the results seem a little better and the protracted fatigue that results from other intensity variables seems less. Understand that these are completely empirical (and frankly subjective) observations. I am not making a claim that this is THE protocol for intensity extension. This is simply something we have tried with positive results, and you may want to give it a try in your own training.

I would like to offer a few precautions to those who do try it. Use it sparingly. If your usual TUL is less than 80-90 seconds, only do one rest-pause rep. If your TUL’s run longer, you can try as many as three. For those with shorter TUL’s at failure, rest/pause 15-30 seconds before you attempt another rep. For those with longer TUL’s at failure, 8-10 seconds will be about right. Once you get the hang of this protocol, you can judge the rest/pause duration almost entirely by feel. If you do experience positive results, you must be cautious not to continue the protocol for too long (as Brian Johnston has cautioned with blitz training). We have found that if this is continued for longer than 4-6 sessions, then overtraining (or perhaps overadaptation) will ensue and further response stalls. In general 12 or more weeks of standard training followed by 2 weeks of intensity extension seems to be a good ballpark guide.

For many in the field, this may be old hat. I certainly don’t think it is original. However, I think there are some theoretical reasons why it should be considered. Perhaps it will put a spark in your training. At the very least, I hope it provides some “grist for the mill”.