# Fiber Tension, Recruitment, Rate Coding

Discussion in 'Hypertrophy-Specific Training (HST)' started by NWlifter, Nov 26, 2018.

1. ### NWlifterActive Member

Thought I'd just pop this up, just for reference, and typing this out helps me organize my thoughts.

For a long time, I had a foggy understanding of tension (whole muscle, individual fiber), with respect to rate coding and tetany etc. Just in case someone else hasn't seen it clear, I'm just going to post some info. about it.

Some physiology information states something that can be mistaken in context, it's that when a fiber is in tetany, it's producing it's maximum force. Which yes is true, but it's maximum 'momentary' force. Ie. the most force it can produce in it's current state. If fresh, it's truly it's maximum force, if fatigued, it's the maximum it can produce in that state of fatigue. Total crossbridge number does equal total force, but in fatigued state, crossbridging is hampered, therefore a maximum neural drive can still produce tetany yet that tetanic force is less than if the fiber was un-fatigued.

Recruitment is said to operate on the rule of 5's. A neural drive is received at the soma, as this 'builds' (to put it simply), eventually it summates enough to produce an 'action potential'. Almost like a balloon blowing up till it pops, but without damaging the balloon. Then it rebuilds again, etc. If the first motor unit is running at 5HZ (5 pops a second, so five action potentials a second) that MU is twitching with minimal force, if the neural drive increases to where that MU reaches 10Hz, the larger soma of the next larger motor unit is building now at the rate of 5Hz. If the neural drive increases more to where the first is building more rapidly at 15HZ, the 2nd MU is now building at the rate of 10HZ and the drive is now enough to get the 3RD motor unit running at 5HZ., etc.
This can continue until all motor units are active. Many are of the first recruited at this point are in 'tetany' they are solid on, producing their maximum force, but many later units are not yet in tetany so are twitching with less than maximum force. If the set continues until very close to 'failure', then activation is maximum, and all possible MUs are in tetany.

Now, the key thing, if one uses a lighter load, yes at this point all fibers are basically in tetany, but from the earlier reps causing fatigue before this point, they are not producing actual maximum fresh tension, they are producing maximum tension in that state of fatigue. So if a large motor unit when fresh and in tetany could produce say 5 lbs of tension, now when fatigued and in tetany, it might produce say 3lbs of tension. All the lower frequency twitching before reaching tetany, did produce fatigue and lower it's force. We know this as it's the reason recruitment and activation increases during a set with an unchanging load. Fatigue is lowering force of the first recruited, which requires an increase in the drive to 'maintain' that same level of actual force. (technically also as MU's fatigue, their threshold for recruitment lowers too, so even the same level of drive is relatively 'more drive' as they are more sensitive to that drive).

So the take away, is that a 5RM does actually increase the tension per fiber over say a 10RM. Somehow though, 'time' (mechanical fatigue?) does also factor in, making it some type of 'equation' (something like tension x time = stimulation) (see TTI studies). Which is why if we factor out tissue condition, various loads can cause equal hypertrophy. Perhaps as long as tissue condition is exceeded, anything above that level is 'enough' to trigger hypertrophy, but that line changes once adaption has occurred. This means one or both of the variables must be increased in the future.

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2. ### NWlifterActive Member

From: Roger M Enoka
Sent: Tuesday, September 22, 2020 5:28 AM
Subject: Hi, have a question that I've wondered about for so many years (fiber tension)

Good morning Ron,

It is important to realize that motor units are probably never achieve a fused tetanus during a voluntary contractions. The rate at which action potentials are generated is only ever sufficient to produce an unfused tetanus, which is less than the maximal force a motor unit can produce. Of course, the force produced by a motor unit during an unfused tetanus can range from low to high, but it is never maximal. We do not know why rate coding is never enough to achieve a fused tetanus.

I hope this helps.

-------------------------

Hello Ron,

A higher load will increase the actual tension in single muscle fiber, but mostly in those that belong to high-threshold motor units.

Roger M. Enoka, Ph.D.

Professor

Room W205B, Ramaley

Department of Integrative Physiology

354 UCB

Boulder, CO 80309, USA

303-492-7232

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3. ### NWlifterActive Member

Hey O&G
Saw a post on this via email but don't see it? you said
Simple terms..
Voluntary we never max out rate coding enough to cause a fiber to create it's true max tension (for that I'm thinking times when a person is trapped under a car maybe it might happen?)
When a fiber 'comes online' (is recruited) it's at a lower than even it's usable max force, but it does increase it's force as rate coding (firing frequency) increases.
Fatigue limits force so if you activate a fiber as much as you can when it's 'fresh) (think heavier load over lighter load) it will put out more force than if its fully activate later in a set when fatigued.

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4. ### _Simon_Active Member

Hmm that last paragraph is interesting! So heavier load when fresh (unfatigued) works better at stimulating muscle fibers than lighter loads taken closer to failure? OR is it just that it is able to put out more force?

Whether that means its 'better stimulated' (in terms of hypertrophy) I'm not sure..

5. ### NWlifterActive Member

You can create more tension with a fresh fiber (unfatigued fiber) at voluntary max activation, over a fatigued fiber yes.
Whether that is more stimulation or not, nothing I see addresses that.
Oddly, max activation with super light loads (so very low actual fiber tension) still shows similar hypertrophy.

If you perform a set with say a 20RM, activation increases to maintain tension, so the tension is similar during the whole set, (low) but activation increases to compensate for force loss from fatigue.

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6. ### Bryan HaycockAdministratorStaff Member

I'm not sure if this adds anything or not but anabolic signaling is triggered by both active (longitudinal tension) and passive (membrane distortion) tension on the sarcomere. So whether or not crossbridging is occurring (i.e. activation) in the specific sarcomere does not preclude the cell from reacting anabolically to the tension.