T Nation

New Muscle Fiber Conundrum ???


Dear Group,

I have had some help from Roger M. Enoka and Per Aagaard, both of their writings are in quotes.

The main part of the debate we are having is on what two members of the forum said; They said in a slow rep set of roughly 4/4 {4 seconds positive, 4 seconds negative} speed for 6 reps, there will be more overall force/tension out/on the slow muscle fibers, than the following faster rep speed, which we think is not so. There would be far more tension on all muscle fibers in a fast rep set, and more overall force output by all muscle fiber, on a .5/.5 speed for 30 reps. Both using, 70% of the persons 1RM. Note the TUT in the faster set is shorter, as you always fail faster in a faster rep speed.

[i]Member wrote;
When you purposely slow down a rep speed you are recruiting less fibers, these are the lower threshold fibers. There is a greater tension on the lower threshold muscle fibers when you purposely rep slower due to their fiber type suiting slower velocities.

When you move a weight fast, you are recruited higher threshold motor units to do so, sharing the load amongst more motor units generates more force yes, but the actual tension placed on each of the muscle fibres is less due to more motor units being recruited.

Think 10 people pushing a car instead of 5

When you use a slow tempo you do 2 things.

1/ Recruit less motor units so less muscle fibres have to lift the same load, more tension is placed on these muscle fibres.

2/ Increase the TUT per recruited muscle fibre increasing fatigue.[/i]

I wrote;

If as you say there is more tension on less muscle fibers when moving slow, why do you not fail faster in the slower rep ??? As if there was more tension placed on these muscle fibers the set should terminate faster, but it does not, as you always fail faster in a faster set using the same weight. That proves that there has been more overall force output by all muscle fibers, thus more tension on the muscle fibers as a whole, thus the fast set terminates faster.

The power/strength overall output is far higher in the faster rep.
Lets calculate how much power/strength I would be used on both rep speeds. Distance weight moved 1.85 M. Since we are using the metric system, we will first need to convert the mass of the barbell into kilograms (200 lb ÷ 2.2 = 91 kg). Secondly, to determine the force we will need to figure out what the weight of the barbell is (W = mg = 91 kg x 9.81 m/s² = 892 kg.m/s² or 892 N). Now, if work is equal to Force x distance then, U = 892 N x 1.85 m = 1650 Nm.
We can calculate that lifting a 200 lb barbell overhead a distance of 1.85 m required 1650 J of work. You will notice that the time it took to lift the barbell was not taken into account.
Let us only add up the positive part of the lift.

The concept of power however, takes time into consideration. If for example, it took .5 seconds to complete the lift, then the power generated is 1650 J divided .5 s = 3300 J/s.

If it took 2 seconds to complete the lift, then the power generated is 1650 J divided 4 s = 412.5 J/s.

Slow set,
412.5 x 6 = 2475.

Fast set,
3300 x 99000

There is a big difference in the poundage moved if the same weight is used, thus more aver all force out put by all fibers, and tension on them.

If your doing 6 reps at 4/4 with 200Ibs = 48 seconds, 200 x 6 = 1200Ibs

If your doing only half the time in the faster reps, 24 reps at .5/.5 with 200Ibs x 24 = 4800Ibs.

Let us say we are using 80% and rep it as fast as possible, this is what in my opinion happens to all muscle fibers ???

My take of how muscle fibers fire.

At repping 70% at a .5/.5 speed, would they all come into play at once, and generate their higher and highest force, as the reps go on, and then their max force, and then some would lose some force, but are still contributing.

If you believe that all muscle fibers have at one point generated their max force. In your scenario above in a slow rep all muscle fibers have not generated max force, untill the very end of the set. In a fast rep all muscle fibers have generated max force at one point, and are still trying to generate as much force as possable.

With far more force, power, strength and overall more accumulated weight moved in the faster rep, I declare that all fibers have generated well over three times as more force and thus tension on all muscle fibers in the faster rep.



Comments on who is right and why please.




read this


Yes, if you're lifting the SAME load for the rest of your life as fast as you can.....

Obviously if you lift a pencil as fast as you can, you still won;t recruit many/any fibers and the tension on each will be miniscule.

The reason you lift as fast as you can is to activate as many as possible and so you can increase the load in subsequient sessions and eventually progress to massive poundages that DON'T MOVE TOO FAST AT ALL (or even quite slowly) even though you're trying to move it as fast as you can.

Since you have learnt to use a many fibers as possible, increasing the total load will be easier and you have a chance of actually getting to fire ALL your fibers to fill potential AND cause them to "fail".


Thx for the link.

Thank you for the above.

But the question or debate is about, which set will produce more overall tension to the slow muscle fibers, also fatigue or work the slow muscle fibers harder. And make them try and produce more force output.

I think it is the faster set, and always lift fast, but was wondering if there are any experts out there for comments ???



The biggest thing you have to ask yourself is are you talking about motor unit recruitment during a single rep of a set OR are you talking about the number of motor units recruited during the entire set? Knowing which one is being discussed makes a big difference.

I wrote an article on this topic that I would be happy to send to you if you like, PM me if you want it.

Also just FYI essentially no one is going to be able to complete 30 reps with 70% of their 1RM no matter what speed they lift.

I also disagree with the idea that you always fail faster when lifting fast vs slow. You agree that intentionally lifting a weight unusually slow is harder than trying to lift it fast, correct? Imagine if I said take 90% of your 1RM on the bench and do as many reps as fast as possible or do it with a 10/10 count, I would think you would be happy to get 2 reps with the latter tempo and should get 3 or more with the fast version.


One more point on this issue while I am thinking about it.

Far too much emphasis has been placed lately on external force output as a measurement of MU recruitment. External force is not that crucial, much more important is intramuscular tension, which is how much tension is in/on the muscle during the activity. This is why anything that builds muscular tension can work to build size be it lifting fast, lifting slow, doing isometrics, doing eccentrics, etc.

But lifting fast is not the end all be all, if it was Olympic lifters would be huge and they are usually smaller than both powerlifters and bodybuilders, and both groups using a slower lifting speed. I also don't think the people that follow the Westside system will tell you their speed days got them huge. I am not saying that lifting fast is necessarily bad, I am saying that it is not the most important variable to focus on.


Out of curiosity TH, any links to your articles (I don't think I've ever come across any of them) ?


Well if you want to compare the two scenarios, make the total TUT for the set be equal.

So something like:

10 reps with 70% of 1rm, 2 sec concentri, 2 sec eccentric

Total time 40 seconds

And then compare that with

20 reps with 50% of 1rm, 1 sec concentric, 1 sec eccentric

total time 40 seconds

Assuming that neither set is going to failure, the faster reps will recruit larger faster MU's. More work is being done in the same time, and even though the load is less, if you are lifting fast you are still going to creat a lot of tension.

The fast lifting set will be much harder as well, because the larger MU's are not fatigue resistant like the smaller ones.

There are plenty of threads on here about this though that will go pretty in depth on this subject. Look into TUT, explosive lifting, and speed. Im sure those searchers will turn up a lot.


If you can successfully do the 10/10 tempo lift, and you're saying that you are doing the fast lift until failure(which with 90% will probably be less than 20 seconds of lifting) I'd have to say the faster lift will be "harder" to your muscle fibers, because you are going to completely fatigue them, and also lasted for a shorter period of TUT.


Tension correlates with acceleration, so a faster rep produces much higher peak tension at the beginning of the movment. So it is not necessarily true that slow movement results in higher tension per fiber, although this may be true of moving at a constant velocity.


If you go to the T-Nation homepage, it should have an author link. If you click on that, all of the authors that have published articles here should be listed, so then you just scroll down to find my name (they are in alphabetical order). Here is the link to save you some time.


I should point out that in the above post when I said I wrote an article about this topic that particular article was not published by T-Nation.


You might be able to last longer although I would have to test it myself with a stop watch, but to answer the question then you really have to start measuring time spent on concentric vs eccentric contractions.

The slow tempo sets do take a long time but that is because the weight is usually light and a lot of time is spent on the eccentric part of the movement which is easier.

To me the simple fact that if I say lift your 1RM fast and you can complete the lift (may not actually move fast but you do a rep) and then say I do the same weight with a slow tempo and you fail pretty quickly, to me that means one way (the fast way) was easier and the slow way was harder.

Even if it takes only 2 seconds to do the concentric part of the 1RM when you are successful, my guess is you fail before 2 seconds concentrically when you try to push it up (although you might last longer isometrically or eccentrically).

But we may be talking about different things. I am focusing more on the completion of the lift, you might be focusing on the total time of the contraction which might be interesting to measure more accurately.


worth baring in mind that whilst you get a very high peak tension due to the initial acceleration against inertia, ones the bar is accelerating you are effectively lifting a lower weight therefore for the rest of the lift the tension is reduced.

You would need to have the profile of the force through the lifts then integrate the best fit line to give an accurate comparison.