"Your muscles are normally activated in a very certain way that's really efficient," says Zehr. "Why use your whole muscle mass to lift up a cup of coffee?"

When we need to lug that couch up the stairs, we can simply recruit more motor units. Yet even when we feel we are at our limit, we most certainly are not.

Estimates vary, but researchers have pegged the amount of muscle mass recruited during maximal exercise at around 60%; even elite athletes who have trained to get more output from their musculature might only harness around 80% of their theoretical strength.

Why do we keep so much in reserve? Safety, essentially. If we were to exert our muscles to or beyond their absolute maximum, we could tear muscle tissue, ligaments, tendons and break bones, leaving us in dire straits.

"Our brains are always trying to make sure we don't get pushed too far to where we actually damage something," says Zehr. "If you actually used all the possible force or all the possible energy you could to complete exhaustion, you'd wind up getting into a situation where you might die."

Pain and fatigue as semi-illusions

To duly disincentivise us from incapacitating ourselves, we have evolved to feel pain and distress during periods of high exertion. Experiences of those feelings thus dissuade us from trying to move something that we judge to be too heavy, like a car, under ordinary circumstances.

Though our muscles seem to be screaming out "stop!", in many cases, we could do more without incurring injury. Until around 15 years ago, exercise science had long chalked up muscle fatigue solely to physiological factors within the muscles themselves. The 1922 Nobel Prize in Physiology winner, AV Hill, established the dogma that the limiting factor in strenuous exercise was simply the body's ability to take in and disseminate energy-unlocking oxygen to muscles.

This "brainless model," in the words of Timothy Noakes, an emeritus professor in the Division of Exercise Science and Sports Medicine at the University of Cape Town, has withered in recent years. Noakes' work and that of other researchers has reframed the brain not as a bystander, but as a "central governor", with primary responsibility for performance. Per the new thinking, the pain of muscle fatigue is more of an emotion than a reflection of the physical state of exerted muscles in question.