more and more MUs activating as set goes on?

K

Karky

2
I'm reading about muscle contractions and motor units.

When you lift for example a 12 rep max, we all say that at first the smaller MUs are recruited and as they get tired, bigger and bigger MUs are recruited, which means that during the last reps the fast twitch MUs are recruited, which are the ones we want to recruit because of their great potential for growth. However, my book says that this probably only happens when the load is so low that blood circulation is maintained (circulation is severely compromised already at about 25% of maximal isometric force). It says that at around 40% of max it is unlikely that you get more recruitment as you go on. If this holds true, could it mean that the MUs we recruit at first in the set are the ones we will recruit the entire set, which would make concentric speed seem all so much more important since with higher speed we will recruit bigger MUs.

Now, this was done with isometric contractions. What happens when you have concentric followed by eccentric without lockout, and the same with lockout. I think I'm gonna ask both my neurophysiology and anatomy lecturer about this. If there hasn't been much research on this, maybe I'm looking at my master thesis :D

BTW: does anyone know what this tendency is called in English? A term I could use to google or pubmed it to find some more info on the subject?
 
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Is it a bird?
Is it a plane?
No, its Karky's thread passing miles over my head.
Sorry, I wish I could give an answer but I'm not that clever.
Still this reply will serve to bump the thread up the pecking order so hopefully it will get the attention of someone who knows.
 
The recruitment of different size/types of motor units depends greatly on the speed and intensity (required) to move the weight. And I think this has been researched already, I've never read primary literature but seen it referenced as fact in different discussions. I'll see if I can find anything on it.
 
yeah, I've seen a lot about more getting recruited as the movement goes on and you get tired.. but then the book said that about isometric contractions, I just found it odd..
 
Karky said:
BTW: does anyone know what this tendency is called in English? A term I could use to google or pubmed it to find some more info on the subject?
Click to expand...

Is this the Henneman's Size Principle?

Henneman's Size Principle said:
Motor unit recruitment is the principle that the more motor neurons are activated, the stronger the muscle contraction will be. Motor units are generally recruited in order of smallest to largest (fewest fibres to most fibres) as contraction increases. This is known as "Henneman's Size Principle".
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no, that's not it.. that would be the size principle, it's just that the more force you need the higher up in the size of MUs it goes. You'll recruit more big MUs doing 100kg bench press than 60, if 100 is close to your max.
 
My understanding is similar to the already stated, however I would qualify "intensity" as workload. The greater the weight, the more recruitment and I also understand that as the primary movers get more fatigued there is greater recruitment from the surrounding muscles to assist. I would imagine the slower the speed, the greater recruitment since you are not relying on force to drive the weight and using controlled movement is going to add more stress to the MU than uncontrolled drive to end point.

Would the other variables include recovery time and how many sets follow the initial onset of fatigue?
 
more speed (or I should probably say acceleration) = more recruitment. Think about it, to get more speed you need more force! Now if you collect momentum in the first half of the movement and then use that speed to get you through the second part of the movement without you doing anything, then you won't be recruiting a lot through the second part of the movement. However, you'll still be recruiting a lot in the first part of the movement in order to gather all that speed.

and I think the isometric thing just goes for isometrics. I got saw some other research with other types of contractions that confirmed what I said earlier, as you fatigue more MUs will activate.
 
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The better your neural recruitment, the stronger/faster your muscle tissue is per gm.

When you work out, you not only build more muscle. You also build better neural pathways in order to activate the muscle.

Is this what you are talking about?
 
me? no, that's not what I'm talking about. During the lift more and more MUs will activate because the ones that got activated in the start get tired. So if you start a set using 50% of your MUs you might end it using 80% of your MUs. But I'm not sure if the MUs that get tired will stop activating, I think so, but you'll end up with more fast twitch MUs later in the set as the other fibers fatigue.
 
You're talking about fast twitch vs. slow twitch fiber and the percentage used in anaerobic lifts?



Here's another one:
 
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not exactly, I'm talking about fatigue. How the muscle responds to fatigue, what does it do? When you do one repetition, with your 20RM bench press it will be easy and you don't need to recruit all your motor units in order to lift it. But what happnes as the set progresses and you get tired (on the 15th rep for example? or the last rep that you can barely make?)? More and more motor units will become active to deal with it. However, my book said that this does not happen in isometric contractions over 40% of maximal voluntary contraction, I find that to be odd.. so I was wondering if it was the same with "regular" lifting. However, it seems that it is not.
 
Karky, I've tried to answer your questions a few times, and am having a hard time understanding exactly what you are looking for.

When you're talking about MU recruitment, you have to look at the activity being performed. For an example I will use your 20RM scenario, and I'll ask you what you remember about Type I fiber types and the MU associated with those. Low force production, low fatigability (yes, it's a word). No matter what rep you're on, the force doesn't change, and neither does the fiber type recruitment because you don't need it. The type I MU hasn't fatigued, but getting tired like all muscles do.

This is why most endurance athletes, when training in the weight room, used a 15+ RM program...to better train those Type I MUs

If you were to add weight and speed of movement, then yes, you'll recruit a far greater number of type II muscle fibers. Also, I'd like to add that any intensity, you are activating all fiber types, but the ratios are going differ at the two ends of the spectrum.
 
FunctionalTrain said:
Karky, I've tried to answer your questions a few times, and am having a hard time understanding exactly what you are looking for.

When you're talking about MU recruitment, you have to look at the activity being performed. For an example I will use your 20RM scenario, and I'll ask you what you remember about Type I fiber types and the MU associated with those. Low force production, low fatigability (yes, it's a word). No matter what rep you're on, the force doesn't change, and neither does the fiber type recruitment because you don't need it. The type I MU hasn't fatigued, but getting tired like all muscles do.

This is why most endurance athletes, when training in the weight room, used a 15+ RM program...to better train those Type I MUs

If you were to add weight and speed of movement, then yes, you'll recruit a far greater number of type II muscle fibers. Also, I'd like to add that any intensity, you are activating all fiber types, but the ratios are going differ at the two ends of the spectrum.
Click to expand...

As the movement goes on, more and more MUs will be recruited as the MUs that were recruited first starts to fatigue. The new MUs usually have higher recruitment thresholds, meaning that they are larger, according to the size principle.
All
newly recruited motor units had higher recruitment threshold
torques than previously active motor units within each subject.
If two motor units were recruited in an experiment, the
second unit had the larger mean recruitment threshold
torque.
...

Motor-unit recruitment
All nine of the triceps motor units that were newly recruited
into the extension movements had larger mean recruitment
threshold torques than the other motor units within
the subjects that were active from the beginning of the fatigue
task. The recruitment of motor units in ascending order
of recruitment threshold torque is consistent with Henneman’s
size principle ( Henneman et al. 1965 )
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So as you fatigue more MUs with higher thresholds will become active. Though according to my book, this does not happen with isometric contractions above 40% of maximal voluntary contraction. I'm just wondering why and I was wondering if it would be the same with "regular" work, that consists of both concentric, eccentric and isometric contractions. The study I cited used about 17%of MVC if I recall correctly.. I'm trying to find studies done with more than 40% of MVC that are done with concentric and eccentric movements, not only isometric. That way I can find out if the no "de-novo recruitment" (which is what I think this is called) at more than 40% MVC goes for concentric and eccentric contractions as well as isometric.

If it does, then you can't start a set with a light weight and expect your fast twitch MUs to be active in the last reps (which is something a lot of people say will happen, leading to all the talk about the importance of those last reps)

Did that make it clear? damn, this is too complicated to make clear, I'm afraid.. but I hope you see what I mean?
 
Karky said:
As the movement goes on, more and more MUs will be recruited as the MUs that were recruited first starts to fatigue. The new MUs usually have higher recruitment thresholds, meaning that they are larger, according to the size principle.


So as you fatigue more MUs with higher thresholds will become active. Though according to my book, this does not happen with isometric contractions above 40% of maximal voluntary contraction. I'm just wondering why and I was wondering if it would be the same with "regular" work, that consists of both concentric, eccentric and isometric contractions. The study I cited used about 17%of MVC if I recall correctly.. I'm trying to find studies done with more than 40% of MVC that are done with concentric and eccentric movements, not only isometric. That way I can find out if the no "de-novo recruitment" (which is what I think this is called) at more than 40% MVC goes for concentric and eccentric contractions as well as isometric.

If it does, then you can't start a set with a light weight and expect your fast twitch MUs to be active in the last reps (which is something a lot of people say will happen, leading to all the talk about the importance of those last reps)

Did that make it clear? damn, this is too complicated to make clear, I'm afraid.. but I hope you see what I mean?
Click to expand...


It's too bad that most of those studies are done with isometric exercises, maybe it's easier for the study??

That's all I found as well. I've never heard of your body recruiting type II fibers for weight that does not break the type I threshold. If that is true, I've been training endurance athletes completely wrong as I try to develop their type I fibers in regards to resistance training.
 
FunctionalTrain said:
It's too bad that most of those studies are done with isometric exercises, maybe it's easier for the study??

That's all I found as well. I've never heard of your body recruiting type II fibers for weight that does not break the type I threshold. If that is true, I've been training endurance athletes completely wrong as I try to develop their type I fibers in regards to resistance training.
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yeah, apparently isometrics is way easier to study, which kinda sucks since it might be different with regular lifting. My book said the reason "de novo recruitment" didn't happen above 40% probably was because contractions that high restric blood flow or something like that, which is weird, since you'd think that with restricted blood flow you'd have to tap into your fast twitch fibers more quickly since they are better at anaerobic training.. I'm gonna look into research on what happens in muscles that have restricted blood flow.

And I don't know if you're training your endurance athletes wrong. I think with higher reps you'd emphasise slow twitch muscle fibers more, and I think it would be very hard to just work slow twitch and not fast twitch anyways, since you have no idea when the switch occurs.

Also, I don't know if doing a 12RM lift will mean you using slow twitch fibers and very little if any fast twitch, like we are lead to believe by many articles about training. I'm pretty sure it's different from muscle to muscle, I think some muscles will recruit almost all MUs at like 50% MVC and for some you will have to go higher.
For all we know pretty much all our sets in the gym will recruit a lot of fast twitch MUs and that adding more weight or speed to the movement just means getting a higher rate coding. Though this is hard to know, as I don't think researchers can tap into all the MUs, they usually just take a few and then hope that those few are a good representative for the rest of the MUs

I'm reading up on the oxygen thing now, I found this:
kind of interesting with regards to the effects of hypoxia and hyperoxia on muscle
 
Med Sci Sports Exerc. 2008 Apr;40(4):691-8.

Human muscle gene expression following resistance exercise and blood flow restriction.Drummond MJ, Fujita S, Takashi A, Dreyer HC, Volpi E, Rasmussen BB.

INTRODUCTION: Blood flow restriction in combination with low-intensity resistance exercise (REFR) increases skeletal muscle size to a similar extent as compared with traditional high-intensity resistance exercise training. However, there are limited data describing the molecular adaptations that occur after REFR. PURPOSE: To determine whether hypoxia inducible factor-1 alpha (HIF-1alpha) and REDD1 mRNA are expressed differently in REFR compared with low-intensity resistance exercise with no blood flow restriction (CONTROL). Secondly, to determine whether low-intensity resistance exercise is able to induce changes in mRNA expression of several anabolic and catabolic genes as typically seen with high-intensity resistance exercise. METHODS: Six subjects were studied at baseline and 3 h after a bout of leg resistance exercise (20% 1RM) in REFR and CONTROL subjects. Each subject participated in both groups, with 3 wk separating each visit. Muscle biopsy samples were analyzed for mRNA expression, using qRT-PCR. RESULT: Our primary finding was that there were no differences between CONTROL and REFR for any of the selected genes at 3 h after exercise (P > 0.05). However, low-intensity resistance exercise increased HIF-1alpha, p21, MyoD, and muscle RING finger 1 (MuRF1) mRNA expression and decreased REDD1 and myostatin mRNA expression in both groups (P < 0.05). CONCLUSION: Low-intensity resistance exercise can alter skeletal muscle mRNA expression of several genes associated with muscle growth and remodeling, such as REDD1, HIF-1alpha, MyoD, MuRF1, and myostatin. Further, the results from REFR and CONTROL were similar, indicating that the changes in early postexercise gene expression were attributable to the low-intensity resistance exercise bout, and not blood flow restriction.
 
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