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Post Info TOPIC: Stored energy?


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Stored energy?
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Eric, what do you mean stored energy?  In what form? I can only assume that you are talking about the stretch reflex similair to what we get in the squat.



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And what about the effect from the CNS being 'primed' from the previous rep?  Has such been tested?

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I'll have a crack at it as I think I'm starting to see something.

Energy cannot be destroyed or created, only converted from one for to another, now when we deadlift we are tranfering potential energy in us, into kinetic energy as we move it into potential energy again as we hold it. Some energy will be "wasted" in heat, sound etc but this is the gist.

Now we drop the bar, potential energy is once again converted into kinetic energy and a whole lot of sound (a less useful form of kinetic energy), however what eric would seem to suggest is that some how, we have a larger portion of potential energy in our system this time round. Now, I'm not doubting it must be true but the basic physics of it actually doesn't make sense: without getting into formulas etc, we seem to have added some energy into the system from somewhere. We had to initially have the potential stored energy in us on the first rep equivalent to the force gravity would have acting on the bar at the height of lockout. Otherwise we wouldn't complete the lift, so the energy system goes us-kinetic-gravitational potential-kinetic-us/the bar. Yet somehow we are saying in that final state there's now more energy in the system... Where from?




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I THINK what Eric is getting to is that when you pull initially, all the potential energy that is applied to the bar is chemical energy that is created in your body.

However, when you descend with the bar after the inital rep, you also store some mechanical energy (like compressing a spring) in your body which can then be subjected to the bar along with your chemical energy on the second rep.

I'm probably completely wrong and should have my engineering degree removed from me LOL.

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I see what you mean Joe but there still can't be anymore energy added to the system as the whole thing kicked off with the chemical energy which must have been sufficient to transfer to gravitational potential at the top of the lift.

All I can think is that the increased mechanical energy after the initial rep is more accessible to the body, hence the faster rep.

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lol... this is getting complicated even for me!

Mark, you're partly right about the mechanical energy aspect of things, but you're just not looking deep enough. In this situation, it doesn't act like a closed system. The energy to move the barbell initially is chemical energy in the form of ATP. Once ATP is depleted, it is repleneshed from Creatine Phosphate (used ATP -- ADP -- takes a phosphate from CP to become ATP again). Then each subsequent energy system restores the next until you get to the aerobic system, which is why you still breath heavy after a single. Now, that's an oversimplification to some degree, but you get the idea.

The stretch reflex is from stored *mechanical* energy from lowering a weight. Eric is saying that there is stored mechanical energy on the subsequent reps of the deadlift. Which may be true. We talked about it on the phone the other day and Eric feels that the stretch reflex can be stored for up to 4 seconds. However, I have only seen one study that claims 4 seconds. All attempts to reproduce that study have shown it to last more in the neighborhood of 2 seconds. I kind of think it doesn't even last that long due to release of the hips between sets. To this, Eric basically says what else could it be? In experiments he and Dave have done (where they would stand up and re-set on the bar), bar speed performed as expected.

What's the answer? I guess it depends on who you're asking right now. I don't really know if it's inefficiency or stored energy, but Eric's experiment sounds interesting, for sure.

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Ok, its clicked. Nice sports science lesson, so on the second rep we have our replenished (almost) energy systems plus a potential energy left in the tank from our last effort=faster pull.

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That's the theory.

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After some thought, I am going to agree partially but make the hypothesis that the majority of the speed increase is not due to stored mechanical energy but due to the unique conservative nature of the human nervous system.

The stretch reflex I beleive to be a good form of stored energy but once the lifter stands back up the reflex is lost since the muscles are somewhat lengthened in the standing position. With the argument that this reflex last for several seconds I will sound like a idiot for disagreeing with a an actual lab test but I have seen many poor conclusions from many tests. That being said consider a simple stretch like a sitting hamstring stretch, when you sit and reach and feel the stretch, the moment you relax do you bounce back, now stretch again, you don't bounce back noticably harder the next time.
As for energy being stored in the muscle other than the in the form of tension, it is not possible, the krebs cycle cannot work in the reverse. Meaning that lengthing a muscle does not create ATP, which is the only source of energy that can cause a muscle contraction. The potential energy that is in the rasied bar is converted to noise and heat in the muscles as the opposing muscles resist the decent.

I hypothesis that the real reason for the faster pull is due to the ramp up of the human nervous system. Like me, the nervous system is lazy. It does not instantly activate the maximum number of muscle fibers at its disposal. Instead it adds recruitment until the bar begins to move then adds more as we get to our sticking point, this enables us to "grind it out" a process which is always slow. I have no evidence to back this up but it makes a great deal of sense since your likely not getting stronger as you hit your sticking poin,, and the nervous system always fires at the same strength,meaning it doesn't fire harder just fires more neurons. What does this have to do with it getting easier on the second rep, well my theory is that the first rep the nervous system has to run all the way from its usual number of fibers up to the number of fibers it requires to lift the weight. On the second rep the nervous system starts at a higher number of fibers since it knows what is required now, and the process to get to the required number is quicker, thus the rep is quicker. The key to my argument is that the nervous system is very much like me. Not only lazy but also forgetful. It resets to its usual number of fibers in only a few seconds, since using excess muscle fibers is inefficent and takes alot of calories that the body does not want to give up. So after several seconds you return to the bar and the blast you got from the second rep is gone.

Also this argument does not apply after several sets, but this is mostly due to fatigue and should not be considered since the body is low on fuel and this point and regardless of recruitment it does not have the calories to use, thus the nervous system won't jump in a car without gas.(meaning it won't recruit muscles it can't afford to contract)

Finally, the extra recruitment of fibers during a grind is borrowed from a physiology study I read and not my hypothesis.

Ok sorry for the length, I hope this at least leads to some good thinking

-- Edited by Nick U on Tuesday 31st of March 2009 09:09:33 PM

-- Edited by Nick U on Tuesday 31st of March 2009 09:10:16 PM

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Are you guys talking about the deadlift and stretch reflex? If so, it definitely exists even if the eccentric isn't necessarily like a squat or bench.  This is why some coaches advocates singles-only training for deads so you train like you compete. 

I think some Russian research alluded to the stretch reflex lasting up to 6 seconds. I think Dave Tate said that himself and Louie Simmons(don't quote me on this) figured it could last up to 10 seconds based on some stuff they had experimented with.  Mike, I think this is what you referred to or was I reading wrong??

Applied to training, I'm a big advocate of NOT doing touch and go deadlifts and I think most benches should have at least a short pause.  I can not get my training partners to stop doing touch and go reps in the dead, especially when geared. Gear+slight bounce=less work. 

People like to refer to the "CNS" A LOT nowadays, but I think its misquoted most of the time. 

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Dano, yes that's what I was referring to. From what I have read, one study said the stretch reflex lasts up to 4 seconds. But subsequent studies failed to reproduce this result (most suggest around 2 seconds).

It doesn't matter anyway, because the stretch reflex has to be a mechanical stretch under load. When you pause on a bench press, you are still under load (muscles still holding tension). In the deadlift, if you let the bar fully rest on the floor, then you are not holding the muscular tension required to maintain a stretch reflex, thus it should dissapate even faster.

So, to me, it could last 2 seconds under ideal circumstances, but in a deadlift, it will be significantly less than that.

And I agree with you on the amount of times "CNS" is misrepresented.

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Here is more scientific research to support my orginal view on this topic. In this study the same result was observed but in this case the resitance was changed. The key here however is that there is no stretch relex to speak of and the sprints were conducted minutes apart. Again while not said conclusively in the study, nervous system priming is got to be the answer, since there is the same level of muscle recruitment in the second sprint as there was at the peak of the first

"A part-finding from a study [13] based on 8 females performing a series of cycle sprint tests showed that when two 30 seconds maximal cycle sprints were performed - the first against a greater resistance and therefore slower pedal rate (by ca. 26%) than the second - the peak power output in the second sprint was statistically greater (442W vs 402W) even though the muscle metabolite response profile was similar. The metabolite data and additional findings reported in the paper seem to suggest that recovery of power is not exclusively determined by muscle metabolites. We all know and understand that warm-up leads to elevation of muscle temperature and enhanced muscle performance. This result, however, appears to suggest that in order to attain true maximal output a maximal 'rehearsal' effort performed within a very close timeframe (i.e. within minutes), possibly against an increased resistance relative to the actual attempt, may yield a subsequent competition performance benefit for some power athletes"

(http://www.brianmac.co.uk/articles/scni13a6.htm#i)



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