Actually it’s been proven that our brain restricts our muscles unconsciously due to the fact that our muscles are powerful enough to rip themselves from our bones by tearing ligaments and the like if they ever contracted at full strength. Sources I’ve seen say roughly only a third of your true muscle strength is usable at any one time no matter how hard you exert yourself.

There have been reports of women lifting cars to save children and people lifting half-ton chunks of rock (it was only an inch or two in the report I saw, but it freed him from being tumbled off of a rock face by letting it slide over him while he lifted).

The human body is more amazing than most people have come to learn yet. 

Well this was shockingly enlightening.

Its the exact reason why we don’t break our own fingers in half or bite our tongues off.