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Type de fibres musculaires : quels impacts pour la musculation ?

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Type de fibres musculaires : quels impacts pour la musculation ?

Messagepar Vincent le 20/09/2003 17h30



"Motor units are recruited by need. The first motor units recruited have slow oxidative (SO) fibers, then come the fast oxidative (FO), then there are fast glycolytic (FG). There are actually two "sets" of the latter. So here's how motor units (and thus fibers) are recruited by need. While force production requirements are low, only the SO fibers are recruited. With higher force production requirements, the FO fibers begin to be recruited. At some threshold, all of the FO fibers will be recruited. And so on.

We can put some actual numbers on these thresholds. These are average numbers, taken from research published in such journals as Spine and Journal of Physiology. The threshold for FO motor units (fibers) is about 40% maximum voluntary contraction (MVC), FG thresholds are at about 70% and 85% MVC.

The researchers have a couple of ways of determining where 100% MVC is. They have found that while there's variation between muscle groups within an individual, and between individuals, most are capable of activating 100% of their motor units. We're sure that over the years you've read things like "depending on the individual, only 10% to 50% of the fibers can be recruited" (garbage) or "50% to 70%" (not much better).

Full recruitment does not equate to 100% MVC. Until the upper threshold is reached, additional force is achieved through recruitment. Beyond that, force increments are achieved via rate coding (the frequency with which the motor units are activated). Now in full disclosure we should tell you that the numbers we quote for thresholds above are for muscles involved in gross limb movement, termed large muscles. Yes, the biceps is considered a large muscle in this schema. Small muscles, called "anatomically constrained" (weird term) recruit to about 50% and then rate code.

Let's apply all this to a single set. If you select a weight above your top threshold, all fibers are recruited from the start. Sometimes you'll read about supposedly needing to push the weight as fast as possible to recruit all fibers. If the load is heavy enough, you've already recruited all of your fibers. In this case, trying to move the bar faster will result in you initially producing additional force through rate coding, followed by lowered force requirements - and hence reduced rate coding and even recruitment - later in the rep. When force requirements are reduced, motor units are progressively "shut off."

If you select a weight below your top threshold, some FG fibers aren't recruited until you start to fatigue. If you push to failure, there will be a spectrum of loading times for the FG fibers. And everybody treats failure in their own way. There's a big difference between getting stuck halfway up and having your partner immediately help you rack the bar, and getting stuck halfway up and then fighting the bar until it descends to the bottom of the movement.

Let's consider a single set of multiple reps. If you're performing your heaviest set so far, you'll probably note that the first rep feels heavy, the second rep better, the third better yet. After a couple of more reps fatigue starts to set in, and the reps get harder. You might notice that if you perform a second set at the same weight, it seems easier at first, though you'll probably feel fatigue sooner.

Alternatively, if you perform a back off set it may feel like you "overshoot" at first, i.e., the weight feels light until a few reps into the set, when fatigue quickly hits. The "excess recruitment" will carry you for a time, obscuring the fatigue that's building up due to waste product accumulation, energy substrate losses, pH changes, etc. There are other factors involved. Repetitions of a movement result in the establishment of a pattern of motor control, and being locked into a good pattern reduces effort. There are also considerations of state of arousal and perception of exertion."

David Maurice

autes infos :

"Originally posted by Faheem Chauhan
Now my next question might be jumping the gun a little but it is relevant, what bearing do the SO, FO and FG fibres have on muscle development?
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1) The higher threshold fibers have a much greater capacity for hypertrophy than do the "slow twitch" fibers.

2) Training SO fibers to fatigue would mean using very low loads and doing sets that you would measure in minutes, not reps. However SO fibers will experience tension in any rep which requires activation of the high threshold fibers.

ergo....?

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I just can't get my head around which fibres have the influence on growth, whether 100% MVC is needed to provide the damage
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MVC is not needed. However training at levels which do not use the high threshold fibers means those fibers would not be stimulated. Re-read PJs post and see what he says about "hypertrophy training."

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Would i be correct in assuming that after the fibres have been recruited they are damaged to a certain extent? And that leads to growth and recovery? Or in any case their recruitment is what leads to their growth
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Tension is the primary stimulus. It sets off a cascade of biochemical processes.

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You said that if weights in excess of your top threshold are selected, all fibers are recruited from the start. In all likelyhood this is what most people would call power training, low reps, singles to triples. Which is what Christy labels reps 1-4, this recruits all fibres and teaches the efficiency of motor recruitment that you mentioned.
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How is that rate coding different from that which occurs at fatigue with a load slightly under the highest threshold - say 75-80%? Doesn't that "teach the efficiency?"

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I cant quite manage to put together how this relates to other rep ranges, any help?
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Well, think about this. 85% is about a 6RM. 70% is about a 12RM. What rep range is often quoted as "best for hypertrophy?" (Not saying it is, just asking if you can explain the connection.)

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But what PJ said about not pyramiding or taking too many wamr ups to get to your max set makes sense. You'd be depleting your stronger fibres reserves, i guess.
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He's right (and the piece the above was extracted from made a similar comment) but how does working with submaximal weights deplete the higher threshold fibers?"

David Maurice

"Slow twitch fibers have much lower abilities to get big and strong than fast twitch fibers. Training slow twitch fibers to fatigue would have you doing much higher reps than you would probably ever consider. (We're talking about sets that would be measured in minutes, and you wouldn't want to count the reps because there would be so many.) So when you're in the gym, don't even think about the slow twitch fibers.

Fast twitch fibers will not be recruited unless load requirements are too high for the slow twitch fibers to do all the work. If the load wasn't too heavy for the slow twitch fibers to handle alone, the fast twitch fibers wouldn't be recruited in the first place; and when the fast twitch fibers fatigue, you won't be able to move the load anymore!... When your highest threshold (and thus last recruited) fibers fail, the set is done. You'll have no choice on that.

The next thing to recognize is that your muscles may "fail" as a result of any one of many different steps in the muscular contraction process becoming slower or even stopping. There is an entire series of biochemical reactions that must take place for a muscle fiber to contract, and any one of them could be the "weak link." It's quite conceivable that the mechanisms responsible for your muscle "failure" differ from one method of training to another. In other words, the biochemical reason you fail after 4 reps with a very heavy weight might be different than the reason you fail after 16 reps with a (merely) heavy weight, and the reason you fail during a very slow rep might be different yet. The bottom line is that for a given fiber, the time to failure will vary with conditions.

Finally, your body adapts. What this means is that with training, you can shift that "time to failure" in either direction. If you do high-rep training for an extended period, you'll increase that time to failure. If you do low-rep training for an extended period, it will decrease your time to failure.

For your first 2-4 years of training, it almost doesn't matter what rep count you use (except for safety considerations related to biomechanical and form factors). If you've been reading hardgainer for any length of time, you'll have noticed that some strength coaches favor the use of moderate to high reps, and that some favor low reps. Doesn't it seem to you that if the trainees that succeeded with high reps had ended up with the strength coach that prefers lower reps instead, they would have succeeded anyway? And that if the trainees who succeeded with low reps had ended up with the strength coach that prefers higher reps instead, don't you think they would have succeeded anyway?"

David Maurice

j'en ajouterais après, ptêtre que ca aidera à comprendre pourquoi il faut pousser lourd (sans pour autant descende en dessous de son 5M), sauf pour certains muscles qui seraient principalement slow fibers comme les mollets, avant-bras chez la pluspart des gens et les abs voire meme les cuisses.
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Vincent
 
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Type de fibres musculaires : quels impacts pour la musculation ?

Messagepar Christian J. le 20/09/2003 18h02

j'en ajouterais après, ptêtre que ca aidera à comprendre pourquoi il faut pousser lourd (sans pour autant descende en dessous de son 5M), sauf pour certains muscles qui seraient principalement slow fibers comme les mollets, avant-bras chez la pluspart des gens et les abs voire meme les cuisses.


Pour les cuisses ca m'étonerai pas trop car je fait mon SQT à 32kg (Ne rigolez pas SVP, je débute depuis 4 semaines sur cet exo) et je vois déjà que mes cuisses on gagné (un peu) en volume.
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Type de fibres musculaires : quels impacts pour la musculation ?

Messagepar Herve Picard le 20/09/2003 18h22

Vincent tu veux pas ns faire 1 resume en francais ?
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Type de fibres musculaires : quels impacts pour la musculation ?

Messagepar Gabriel Z. le 20/09/2003 18h23

C'est pas nouveau ça....
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Type de fibres musculaires : quels impacts pour la musculation ?

Messagepar Fred le 20/09/2003 18h34

Christian J. a écrit:
Pour les cuisses ca m'étonerai pas trop car je fait mon SQT à 32kg (Ne rigolez pas SVP, je débute depuis 4 semaines sur cet exo) et je vois déjà que mes cuisses on gagné (un peu) en volume.



Quand on débute, on progresse toujours!!Même si tu t'entrainerais n'importe comment tu progresserais!!
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Type de fibres musculaires : quels impacts pour la musculation ?

Messagepar Christian J. le 20/09/2003 18h41

Fred a écrit:Quand on débute, on progresse toujours!!Même si tu t'entrainerais n'importe comment tu progresserais!!


Je n'en doute pas mais quand je vois vos perfs au SQT, mes 32kg sont vraiment ridicules.
Bon je me rattrape au curl avec 10*38kg :D
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Type de fibres musculaires : quels impacts pour la musculation ?

Messagepar Sébastien le 21/09/2003 14h03

Attend.. tu prends plus lourd au curl qu'au squat ?
C'est quoi cette embrouille :rolleyes:
Soit tu es as vraiment des jambes qui n'ont aucune force soit tu ne te donnes pas à fond dans le squat.
C'est "hors norme" de prendre + avec ses bras que de pousser avec ses jambes.
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Messagepar Eihwaz le 21/09/2003 16h16

mais non, je te parie qu'il ne force presque pas (ou il croit forcer) c'est juste qu il veut bien faire le mouvement , je pense ! et son SQT doit etre en 20reps
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