[Voxx (OP)]

I know that the bones are a support for the veins, tendons, and muscle.  A body builder or athlete will push their muscles and bones to tear and break to strengthen them.  Although, you and I can put a lot of effort into holding a pencil or minimal effort.  So in essence we are only using a portion of our muscle strength, but is the body automatically holding us back so we don't damage ourselves too badly?  Hurt our heart, damage organs, and rip skin?  Adrenaline shuts down normal cellular activity to a minimum to bolster a high amounts of energy through oxygen and glucose right?  But, I have heard extraordinary feats in which a person with their body size, weight and height shouldn't be able to accomplish.  I know the reports are varying, but there has to be some truth in it.  I'm just wondering if there is something more to our bodies than we give credence to?  Any experts want to enlighten me?

I don't know if this is relative, but many animals and insects can lift many times their own body weight.  Is it possible that a human can do the same, but we have just been evolved to the point of not needing to?  Inventing tools and such thousands of years ago.

[RD]

...  So in essence we are only using a portion of our muscle strength, but is the body automatically holding us back so we don't damage ourselves too badly?

Applying electrical impulses to a muscle does generate more force than it's owner could normally produce ...

 In humans and cats, extra forces/torques were larger at short muscle lengths, indicating that a similar regulatory mechanism is involved

http://www.jneurosci.org/content/31/15/5579.long

The function of the "regulatory mechanism" could be to prevent injury as you suggested.

[Voxx (OP)]

Alright, so eletro-stimulation works for producing more energy than we normally can accomplish, but what about those accounts where people lift 2000lb cars off family members and babies?

[RD]

... what about those accounts where people lift 2000lb cars off family members and babies?

Moving, e.g. rolling, a car only requires a few percent of the force required to lift the car clear off the ground.

... a car of 1000 kg on asphalt will need a force of around 100 newtons for rolling

https://en.wikipedia.org/wiki/Rolling_resistance#Rolling_resistance_coefficient_examples

[Voxx (OP)]

Amazing!

But does this mean that humans already use every scrap of muscle fiber in our bodies and the rest is leverage and momentum?

[RD]

But does this mean that humans already use every scrap of muscle fiber in our bodies ...

The electrical stimulation shows you can get an extra 20% force from muscles, maybe in an adrenalized state the "regulatory mechanism" is overridden and the extra 20% force is accessible by the person without electrodes attached, but only in extreme (e.g. life-threatening) situations.

[Voxx (OP)]

So, in theory if you could lift 300lbs and this situation happened, you could lift 360lbs for an instant?

[namaan]

I recall a professor of mine mentioned an old anecdote about a child suffering from leprosy (if I remember right). Due to nerve damage, the child could no longer feel any pain in his hands. A doctor walks by trying to unlock an old rusty lock but is unable to, and the child asks if he could have a go. The doctor, taking it as a joke, gives him the lock and to his surprise, the child unlocks it! Unfortunately, in the process he sheered the skin off his fingers without realizing it. Can't be sure of its authenticity, but it does illustrate the importance of regulation in power output from our muscles.

[Voxx (OP)]

Interesting, so in "theory" you're saying that's our pain receptors that restrain our muscles from exerting their full strength or more specifically our feeling of touch.  Our brain sends information to our hand saying we should stop using more strength our skin is tearing and that could cause infection/trouble down the line.  Is what I'm saying explain your theory?

[RD]

... Our brain sends information to our hand saying we should stop using more strength ...

apparently the restraint mechanism is within the muscle, (not via brain) ...

Anesthetic nerve block experiments in two human subjects also failed to abolish extra torques ... Therefore, these data indicate that extra forces/torques evoked during electrical stimulation of the muscle or nerve are muscle length-dependent and primarily mediated by an intrinsic muscle property. 

http://www.jneurosci.org/content/31/15/5579.long