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What is Sequential Recruitment?

This is the last explanatory post in this series on exercise. Next post I’ll share with you the actual routine I’m using.

But before I do, I need you to understand one more critical concept. It’s called Sequential Recruitment. This sounds like some kind of military strategy but it’s actual a term used by physiologists to describe the process your body uses to engage muscle fiber in sequence.

First, let me put an end to one of the most commonly stated myths in your local gym. Almost all trainers will tell you that there are two types of muscle fiber, slow-twitch and fast-twitch. This is inaccurate. You actually have 4 different kinds of muscle fiber, slow-twitch, 2 kinds of intermediate-twitch and fast twitch.

Each of these muscle fiber types requires different amounts of energy to engage them and have different recovery times. Slow-twitch muscle fibers require the least energy and recover the fastest. Fast-twitch muscle fibers require the most energy and take the longest tome to recover.

Our bodies are designed to be as efficient as possible with energy. So when we do an activity our body uses the lowest energy muscle fibers first. If they are not sufficient to the task at hand, only then the next level of energy fibers are recruited and so on, up to the high energy fast-twitch fibers. This is called “Sequential Recruitment.”

Let’s take the two extremes to see what this means in terms of trying to exercise all of our muscles.


First, let’s say you use a light weight or no weight This is like aerobics or running for example. Your body recruits the low energy muscle fibers to do this. When they have all fired, the first level of intermediate fibers are recruited. They carry the load until the slow-twitch fibers have recovered (a few seconds) and then the load is returned to the slow- twitch fibers. They can go back and forth like this for hours, certainly for the duration of most exercise routines. This is the most energy-efficient process. But from an exercise standpoint, the result is you have exercised the slow-twitch muscle fibers and some of the intermediate-twitch but NONE of the higher energy fibers.



Now the second scenario, suppose you lift the heaviest weight you can. In this scenario all of your muscle fibers are engaged immediately because you need ALL of them to lift the weight in question. Now what happens? The lowest energy fibers fail first and drop out.

But remember, you needed ALL of your muscle fibers to lift the weight. So, when those weakest fibers fail, you can’t lift the weight anymore. The result of this scenario is again, you worked the weakest fibers but the higher energy fibers got almost no exercise at all.

It sound contradictory, but when you lift very heavy weights your strongest muscle fibers get very little exercise. It is a weakest link scenario. If you lift the heaviest weight you can, your time under load (TUL) is only that of the weakest muscle fibers.

In the previous post I made the case that to truly increase your fitness you need to engage the anaerobic phase of exercise of ALL your muscles and most importantly of the highest energy muscles. This means exercising all your muscles to failure. So, how do we do this?

The key is to use a resistance equal (or nearly so) to the maximum weight your fast-twitch muscle fibers can move by themselves (i.e. when all other muscle fibers have already failed.)


Let’s talk through the sequential recruitment process is this scenario. Let’s presume you’ve already determined the proper resistance (I’ll tell you in a minute how to do that.) When you first start to engage the resistance your body recruits the slow-twitch muscle fibers. But they are not sufficient so you immediately engage the first intermediates. They also are not sufficient and the 2nd level intermediates engage. Again, still more strength is needed and finally some percentage (typically about 50%) of you fast-twitch muscle fibers are recruited and now you can successfully move the resistance.

But very soon the slow-twitch fibers fall out. You engage more high-twitch fibers to compensate. Then the intermediates fall out in sequence and finally you are using only fast-twitch fibers to manage the resistance.

Now, the slow-twitch fibers will already be cycling back in. They will temporarily take some of load. But they fall out again immediately, same thing with the first level intermediates. You can actually feel this pulsing as low energy muscle fibers cycle in and out.

Finally, with enough TUL your fast twitch muscle fibers fail and you are done. You have now fully exhausted and engaged the anaerobic phase of exercise for all your muscle fibers.

In my next post I will explain exactly how to do this as an exercise regimen. For now I just want to explain how to find your ideal resistance. There exist complicated methodologies for this but here is a simple way to get pretty close. Choose a resistance that you find difficult but you know you can manage.

Now hold that resistance under strain for as long as you can. Keep trying different resistances until you find the maximum resistance you can hold in a steady position for 2 minutes, not more nor less. That’s the resistance you will be using for your exercise routine.

That’s it for today. Next time the routine itself. See you then.

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