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I've read somewhere ( Why is muscle size not proportional to strength?) that under normal circumstances, even when we try our hardest, we can use only 20-30% of our strength.

Under what circumstances can we use a higher percentage of our strength?

(My question is targeted at pull-ups and chin-ups but of course I believe the concept applies everywhere.)

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I heard that we only use 20%-30% of our brain capacity, but not our strength. Could you add the link to the article that you were reading. – Meade Rubenstein Sep 6 '11 at 1:17
I'd love to see a source on that. It's hard to see how someone squatting or snatching isn't using a very high percentage of their strength both in terms of A) muscle activation across the body and B) muscle activation within the affected muscles. I'm not sure how I could be said to only be able to use 30% of my strength--in what way would the other 70% exist? Please clarify. – Dave Liepmann Sep 6 '11 at 1:19
He may be referencing the cases where people are able to lift cars off of other people in extreme emergencies (where it's often credited to an adrenaline dump). That said, I suspect these heros would be quite sore the next day and could do a lot of damage to themselves - our bodies set limits to protect us. – Haphazard Sep 6 '11 at 1:41
@Haphazard I recommend rewriting that as an answer. If you're right maybe we can rephrase the title. – Dave Liepmann Sep 6 '11 at 1:47
up vote 8 down vote accepted

There's a few reasons why that is, but some of the quick ones are:

  • Adrenaline
  • Neurological recruitment
  • We've never demanded more than 20-30% from our muscles

Adrenaline will help us temporarily boost the amount of strength we are using at any given time. The adrenaline response is engaged every time we lift weights, but also in fight or flight situations. It gives us that edge we need to go above our current abilities. Without the adrenaline response our muscle recruitment is very limited.

One aspect that strength oriented weight lifters key in on is neurological recruitment. As you keep increasing weights when you work out, you are requiring that your body recruit more of the muscle into the lift as possible. By continuing to push that envelope, a strength trainer will exceed that 20-30% limit.

Finally, because you've never demanded more than 20-30% of your strength from your muscles, your body has adapted itself to the lower level of muscle recruitment. You have to do work that requires more from your muscles in order to recruit more of them at a time. Someone who can squat 1000 lbs is using pretty near 100% of their strength--even when they are lifting geared (with squat suits).

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The main factor holding one back is the fear response. Our bodies are trying to keep us safe so we should listen. This article describes the effect you speak of:

Under acute stress, the body's sympathetic nervous system prepares the body for sustained, vigorous action. The adrenal gland dumps cortisol and adrenaline into the blood stream. Blood pressure surges and the heart races, delivering oxygen and energy to the muscles. It's the biological equivalent of opening the throttle of an engine.

Vladimir Zatsiorsky, a professor of kinesiology at Penn State who has extensively studied the biomechanics of weightlifting, draws the distinction between the force that our muscles are able to theoretically apply, which he calls "absolute strength," and the maximum force that they can generate through the conscious exertion of will, which he calls "maximal strength." An ordinary person, he has found, can only summon about 65 percent of their absolute power in a training session, while a trained weightlifter can exceed 80 percent.

Under conditions of competition a trained athlete can improve as much as 12 percent above that figure. Zatsiorsky calls this higher level of performance "competitive maximum strength." This parameter is not a fixed number—the more intense the competition, the higher it can go, as the brain's fear centers progressively remove any restraint against performance.

If you are talking about people who get massive adrenaline dumps in extreme emergencies, these folk are able to push past their fear response but will likely damage themselves in the process. But as the article notes, our body can recognize high-importance events (competitions and emergencies) and can operate at much closer levels to its theoretical maximum.

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In a sustained contraction and even in the low tension contraction the muscles continuously undergo to maintain muscle tone some fibres rest whilst others contract. See for further explanation:

It is probably not desirable for an animal to be able to exhaust the majority of its fast twitch muscle fibres in a single exertion, which seems like a plausible explanation for why muscles do not typically operate at maximal tension apart from when the nervous system is persuaded it is necessary.

Of course all the muscle fibres will get used, but they work shifts.

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