Monday, June 11, 2012

Size Principle / Sequential Recruitment of muscle fibres

I was re-reading Hillfit last night.   It is hard to come back to something that you wrote a few months before -  I keep thinking of ways it could be improved or things that could be expressed better.  I am going to revise it over the next few weeks and maybe also send all those that have bought it a few free bonus chapters and FAQs, but one area in particular that I thought was not expressed well was the idea of the sequential recruitment of muscle fibres / motor units and how the "size principle" leads to the way in which I recommend that you train.


In this post I am going to try out another way of explaining the ideas.  Let me know if this makes sense.
(For background reading on this I'd point towards Body by Science, this article by Carpinelli - THE SIZE PRINCIPLE AND A CRITICAL ANALYSIS OF THE UNSUBSTANTIATED HEAVIER-IS-BETTER RECOMMENDATION FOR RESISTANCE TRAINING or Clarence Bass' thoughts on the paper - Forget Heavy, Think Effort)

Push starting your car....with friends

Imagine that you find that your car battery is flat.  You need to push start the car.  Luckily your friends are there to help.  They are a strange group though - all athletic but with different skills, different aptitudes.  There are lots of them too; whole families of them in fact. 

  • The SLOW clan are there:  a large family, numerous lean and stringy fellas; not particularly powerful but in sports at school they had the reputation of being able to keep going for a long time before getting exhausted; and when they did get tired, they soon recovered.   They were the family that dominated cross country running, and the 5000m on the track.  There are lots of them is a big family.
  • The INTERMEDIATE brothers are around as well:  there are fewer of them than the SLOW clan, but they are more powerful.  More thickset, athletic guys.  They excel at soccer, sprinting, and played on the wings in the rugby team.   They don't last as long as the SLOW guys and they take longer to get back on their feet when they do get exhausted but they have a fair bit more power.
  • ....and then there were the FAST twins :  only the two of them.....but they are a huge pair, muscles hanging off them: solid, strong and powerful.  They are strongmen - competing in powerlifting - but have no stamina.  At school they were the props in rubgy.  Walking round the pitch going from big effort to big effort.  After their big exertions they take a long time to recover...but they are certainly powerful.

Start to push that car.

You jump in the car, ready to turn the key and get the engine going.  Strangely you have parked at the bottom of a hill.  You need some help.  First on the scene are the SLOW brothers.  There are loads of them and they crowd around the car to start pushing.

Gradually the car begins to inch out of its space and move, but it is hard work pushing up that hill.  The SLOW clan have  been working hard but they have their limits and one by one they get exhausted, dropping away as the others keep pushing.

Sequential Recruitment......

You are still not going fast enough to start the car though, so you shout out to the passing INTERMEDIATE brothers who jump in and join in with the pushing.   But it is a tough hill and while you are moving more quickly  it is still not fast enough.  Now the INTERMEDIATE brothers are getting tired and they too drop out.

You still need that push though - you are not going fast enough yet - so as INTERMEDIATEs drop out finally you call over the FAST twins, who just happened to be stood near by.

They bring their bulk and power;  while the crowds of SLOW and INTERMEDIATE have dropped away, the FAST PAIR keep pushing.  Not for long though - they have pushed hard but do not last and are soon spent.  It is OK though.  At last you have enough speed and you manage to turn the engine.

In your mirror your survey a trail of bodies up the road:  SLOW, INTERMEDIATE and FAST are all exhausted...and as athletes they are all grateful for the training effect that pushing your car will have had.  They will ALL get fitter because of this effort


Most of the SLOW guys are just about recovered now and look like they could do the same all over again.  The INTERMEDIATE brothers are starting to get back to normal too and look almost ready to go.  The FAST twins though are puffing away at the kerb - it will take some time before they are ready to go again.

But if it was a smaller car?

What if it was a smaller car, like your sister's mini?  Could you have exhausted all of that crowd?  Well the next day you find out.  There was a frost, damp in the distributor and her mini will not start.  Luckily the same bunch is around and again the SLOW clan crowd around the mini and start pushing.

The car is a lot lighter and the SLOW brothers keep pushing it along.  Some of them tire and drop out, but the car is moving quite fast and they soon recover and join in the push again.  The car keeps moving and gets faster.  Not all the SLOW brothers are pushing at the same time - there are enough of them to keep pushing even while some of them drop out for a breather, recover and then join in again.

Soon the mini starts and you are off.   SLOW were able to get her going, recycling the tired brothers into the push.  She never needed to call on INTERMEDIATE, let alone the FAST twins.  They do not get any work at all!

A heavier car?

It is a bad week for weather and the next day it is not your car, not the light mini or you sister, but a great big SUV that your neighbour has bought.

Time for another push.

Once more everyone gathers around the car.  All of the SLOW clan pushes.....but nothing happens!  The INTERMEDIATE brothers join in, all pushing in tandem.  But once more there is no movement.  It needs more power.  The FAST twins come over. Finally with all pushing in tandem the car starts to move;  but the FAST pair soon get tired.  As soon as they drop out of the push the car stops....the SLOW and INTERMEDIATE were still pushing, not tired, but not strong enough to work to exhaustion.

So lets apply that to exercise!

In an exercise your aim is to exhaust all of the motor units: SLOW, INTERMEDIATE and FAST.  As John Little says:

Retaining in our mind’s eye the purpose of exercise is to recruit and stimulate as many muscle fibers (in all categories – slow twitch, intermediate twitch and fast twitch) as possible, and that for this to occur, a muscle or group of muscles must be given a load sufficient (but not excessive) enough to initiate a sequential recruitment of fibers, and also given a time frame that permits such fibers to be recruited, the question becomes:

I'll always be skinny this isn't bad
The aim is to progress through all 3 motor unit types quickly enough to recruit them all.....but not so quickly that only the fast twitch fibres get the bulk of the stimulation......and not so slowly that the slow / intermediate twitch units can recover and recycle back into the effort, so the fast ones are never called on.

So you do not want steady state easy cardio - that only hits the slow twitch units.  Even when some get tired they recover and come back into play and you never tap into the intermediate or fast units.

And you do not want a really heavy weight where you fail immediately or after one or two reps.  Then you need to call on all of the units - slow, intermediate and fast - to fire in tandem to shift it and as soon as the fast units are exhausted you are finished without really working the others.

You need the "goldilocks" load -  enough to involve the slow units .... and then as they drop out to call in the intermediate ones and then as they fall off the fast ones come in until they are exhausted.

For example the Wall Sit!

Of course you do not actually have to move.  It is the contraction that it important....even if that is a static contraction.  Lets get back to the wall sit, one of the key moves in Hillfit  I say to hold it for between 60 and 90 seconds.

You get into position and get tight.  Your quads are contracting hard to hold you there.  Initially your slow twitch motor units are firing.  Gradually they run ou of fuel and cannot fire anymore.  They can't rest and recover though because you need to keep in that position and to do that you need more units to fire.

So the intermediate units start to work.  Finally they all drop out too - there was not as many of them and they tire faster.

Ultimately you need the fast twitch fibres to get to work.  There are not many of them and they are powerful but they tire fast.  When they are done, you fall to the floor - all of the motor units are exhausted, milked of glycogen, empty of fuel.  All sorts of signals are given metabolically and mechanically to get you growing, so that you are prepared for such an exertion in the future - your body does not like to be incapacitated and vulnerable like that so it will get stronger to protect itself.

That took maybe 90 seconds....but you worked all the units.  Next time you might need to hold some dumbbells and you might fail in 60 seconds.  If you fail in less than that you've gone too heavy - it is like pushing the SUV, the fast twitch units are needed from the start and soon drop out.

Does that make sense?

Let me know in the comments.  I am just trying to explain a key idea in a way that people will understand.

Maybe even try this:


Doug McGuff, MD said...


This is an absolutely awesome way of explaining and demonstrating sequential vs tandem recruitment. Great Job!

Chris said...

Thanks for the encouragement Doug!

JamesSteeleII said...

Chris I agree with Doug, in fact I think I may have to borrow the analogy to use when teaching students.

Great stuff.

Chris G said...


Thats explains this particular aspect of HIT really well. I sort out of "got it" before but you analogy really puts it over well. Thanks for sharing it.

Jason said...

Nice analogy, but.
Given the fact that people are wired differently in the brain some will see what you are saying and some will not. Although I understand and follow your reasoning when I apply it strictly to the different muscle characteristics, when I apply it strictly in terms of pushing a car it seems backwards to me. I would be looking to use the power of the (fast) heavies to get things moving with the inters taking over after the initial momentum has been attained and the heavies begin to fail and so on with the (slow) lightweights taking over when car is rolling, little resistance and just increasing speed.
More to do with the reader that what you have written.

Mark Tyrrell said...

Makes perfect sense to me Chris, wonder if I'll have the image of push starting a car during my next HIT session ; )
Cheers, Mark

Drew Stearns said...

Good stuff!

Anonymous said...

Thanks for goog stuff!
What does say evoltion about superslow? -Tom

George Super BootCamps said...

There seems to me to be something of an assumption here; that people will understand why you'd want to recruit them in that order.

For example, if the muscles get recruited all in 1 go (the suv example), but they give up collectively once the fast bro's give up, how does the extra metabolic demand placed upon the muscles during a sequential activation pattern affect the adaptations?

I think a corollary set of explanations to go with this would be very useful. What do you think Chris?

I've just purchased your Hillfit book BTW, can't wait to dive in and do a review for my blog.


Chris said...


Thanks for buying the book. I hope that you like it.

Possibly there is that assumption - that what we are trying to do is get at all of the fibres. I'll think about that.

Ben said...

wow, phenomenal. Now I just need to know how to actually apply this principle for maximum benefit and what it looks like in real life for me. i.e. how do I know that 60 secs fail on wall sit is too short and I didn't activate all the muscles? or i.e. is maxing out at 4 100 meter hill sprints too few or just right for me? etc.

Anonymous said...

Cool! I can push start an SUV by myself so what does that make me?