Don't let your brain's capacity be taken over by fear. REUTERS/Michaela Rehle Capacity is the sort of flat, familiar word we don't typically give a second thought. It's also the key to a high-performance work culture and a sustainably productive life.

Capacity refers to the maximum amount that something can hold or contain, but it's also the power to produce or perform. It's how much fuel you have available to get stuff done, but it's also the quality of that fuel.

There are no synonyms that fully capture the richness of capacity, but several antonyms hint in reverse at its power: limitation, inability, inadequacy, impotence. In practical terms, capacity is the fundamental ingredient necessary to bring your skill and talent fully to life in the face of relentless demand. You may be a great athlete, but you can't excel if you're running on empty. You'll also fall short if you can't summon the right focus, commitment or conviction — all capacities — when you need to perform at your best.

Capacity tells you how many people can fit in a stadium, but also how many thoughts we can hold in our working memory or how large a reservoir of patience or persistence we can draw on in challenging circumstances. At the same time, it refers to how capable we are of lifting a heavy weight, thinking creatively or caring about others.

For all that, capacity is something most of us have long taken for granted, mostly because we sensed we had enough and assumed there was always more where that came from. It's the same way we've long treated the resources of the planet, which as a consequence are now imperiled.

"More, bigger, faster" has long been capitalism's rallying cry. From the start of the Industrial Revolution some 200 years ago, human beings began to pattern their own work practices after machines. The dawn of the digital age promised some relief, in the form of efficiency and convenience, but just the opposite has occurred.

Computers operate at even higher speeds than machines, and they can run multiple programs at the same time. Today, we race to keep up with our technology, but increasingly fall short. More, bigger, faster works only so long as resources are infinite. Many of us have hit our internal limits.

Allostasis is the process by which we maintain internal physiological stability in the face of demand and change. Allostatic overload occurs when demand exceeds our capacity. The consequences include breakdown, burnout, illness and even death. In Japan, the ultimate consequence of excessive physical, emotional or mental stress in the workplace has been given a name — karoshi — which literally translates as "death by overwork."

The inescapable fact is that human beings aren't designed to work continuously, for long hours, or to do multiple things at the same time. We're at our best when we pulse between spending and renewing our energy. Just think about breathing. It's not enough to be good at inhaling.

Flickr / Fabrizio Salvetti The antidote to allostatic overload is to align our work practices with our unique needs as human beings. Unlike computers, which require a single source of energy, human beings require four different kinds of energy — physical, emotional, mental and spiritual. Energy is defined in physics as "the capacity to do work." To be at our best, we need to feel physically energized, emotionally secure, mentally focused and spiritually connected to a purpose beyond ourselves. We're more needy than computers, but no computer comes close to matching the range of capacities we can call on at our best.

As demand rises, an increasing number of employers talk about the need to build a more resilient workforce. Resilience is the ability to recover quickly from difficulties — to spring back into shape after being pummeled, much like a boxer who can take multiple hits and somehow stay in the ring, bloodied but not obliterated.

Sufficient capacity, regularly renewed, is the secret sauce in resilience. Having more fuel in our tanks won't influence how much stress we must endure, but it will make us able to manage it more skillfully, gracefully and at less cost. Rather than trying to get more out of their employees, employers are far better served to systematically invest in building and sustaining their capacity by better meeting their four energy needs.

Capacity — cultivated and regularly renewed — is what makes it possible for both individuals and organizations to get more done in less time, at a higher level of quality and more consistently. We're way past due to enter the Human Age at work.