Fibre recruitment

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Leuk artikel over fibre recruitement.

Fiber Recruitment in Simple Terms

By Ron Sowers

Posted on NaturalStrength.com on October 18, 2004
Fiber recruitment can be a tricky thing to understand. There is one piece in the puzzle that is almost always mis-understood. Most understand the concept of increased force requiring an increase in the amount of contracting fibers, but most, I'm finding are stuck in the same place I was, thinking that by adding weight to the bar, you are increasing the 'tension per fiber'. Before you think I'm going against a logical and obvious fact, please read on.

Muscle fibers are recruited by the nervous system. A group of fibers that are controlled by a single nerve are called a 'motor unit' When a 'signal' from the CNS demands a contraction, all these fibers in a common motor unit or "MU", are called upon at the same time. All fibers in a motor unit are of the same 'type' ie. Type I, II, IIb or IIx. The intervating nerve determines the type of fibers in that motor unit.

Muscle fibers contract on the 'All or None' principle. In simple terms, what this means, is that when the nervous system sends a signal to contract, IF that signal is strong enough, an 'action potential' is reached. It's like a switch, all fibers in that MU are now on for that quick point in time. The fibers then contract as hard as they are capable of for that short point in time. They cannot contract with 50% or 80%, but only with as much force as they are capable of. The force they exhert is equal to the tension they experience. This is the part that throws many people. Don't gloss over this. Think about it. If you are capable of supporting 200 pounds, then the tension you feel is also 200 pounds.

As a muscle repeatedly contracts, fatigue sets in. Both to the nervous system and to the fiber itself (mechanical and metabolic). It can experience damage, loss of available fuel and a "backing up" of metabolic waste products. These things will lower the force a fiber is capable of exhibiting. The fiber is still contracting as hard as possible, even though the force displayed is lessening.

If a muscle is contracting against a less than maximal weight, for example, 50% of 1RM. It will call upon the exact amount of fibers required to lift that weight. This does not mean that all the fibers called upon are being used in an 'always on' state. The CNS will use a pool of fibers and cycle between them. Think of a 'tug of war', you might use 50 of your 100 available people, but all 50 might still take turns tugging. As those fibers experience fatigue and show less force, more fibers are called upon to 'help'. Once all fibers are contracting, there are, obviously, no more to call upon. Force will continue to drop, 'rate coding' and 'synchonization' are then employed for added force. What this means, is that the MU's receive the 'contraction' pulses at a faster rate and are called upon in a more 'coordinated' fashion. This optimizes the force they are applying. Incidently, if you start a set with 80% or more of your 1RM, all voluntarilly contractable fibers are called upon right from the first
rep, rate coding and synchronization would begin immediately for added force.

Now, back to the one key point I want to emphasize with this article. The point that you cannot increase the load on a particular fiber by increasing the weight on the bar.

Read the following statement. (Read it mutliple times if you have to, until the idea 'clicks'. )

"The tension a fiber feels must be equal to the tension it creates"

What this means: If a fiber can generate (arbitrary number here for illistrative purpose) .0001 lbs of force, and that is all the tension it can generate, then this is also the most tension it can 'feel'. You cannot 'put more tension' on that fiber, all you can do is cause more fibers to contract, or contract more often. This all stems from the fact that fibers contract on the 'all or none' basis. Now of course, as a fiber fatigues, it will contract less forcefully and/or less often. But
it still will only have 2 states, on or off. (And also remember, the 'on'states are not a solid 'on' like a winch, it's a quick tug. For a fiber to move from stretch to full contraction, it takes string of many 'on'
pulses)

Example for clarification: Let's say you are just like a muscle fiber. You can pull with your full force, or, be at total rest. Your max fresh pulling force is 500 pounds. Now, you and 9 other people are each holding a rope hooked to a large object. You each have a wire hooked to you that sends a quick shock. When you feel that shock, you tug one time as hard as you can, then rest. Now if you receive a shock and no one else does, you will pull with that 500 pounds of force, the tension your rope will feel will be 500 pounds of tension. The object may or may not move. Let's say it doesn't move. More people start receiving shocks, once 7 of you are receiving them, the object starts to move. Your combined force of 3500 lbs is enough to move the object.

To save on fatigue, as much as possible, all ten of you are being used, but only 7 are ever pulling together at the same time. Shocks are applied to 7, then a second later, 3 of those 7 do not receive a shock but the other 3 do, this pattern keeps rotating. Eventually, everyone starts to tire and weaken.

Now that all 10 of you are fatigued and weaker, it takes 8 people at once, then 9, then finally, all 10 just to inch that large object ahead. The next stage then occurs. Since all 10 of you are having trouble moving it, the pulses come faster now, forcing you to repeatedly tug, bang bang bang... then failure.

Now let's say you and the others are allowed to rest and refresh to your full starting strength of 500 pounds. But now someone increases the weight of the object. You can still only pull with 500 pounds of force, and indeed you are, but the 7 of you cannot budge this object, now it takes 8 just to induce movement. Do you feel anymore than 500 pounds? No, you still only can take 500 pounds of the load, it's all the strength you have. No matter how heavy that object becomes, you can only pull with 500 pounds of force, thus your rope can never have more than 500 pounds of tension across it. The only change that is made by increasing the weight of that object, is how often your rope feels that 500 pound tug. This is the way things are with the neuro-muscular system.

I hope this can help others to understand also.