Finns during the Winter War. Wikimedia commons On November 30, 1939, the Soviet Union declared war on Finland.

The odds, you could say, were against the Nordic country. Only independent for 22 years, it had a total population of 4 million people.

On the other hand, the Soviet army was 2.5 million soldiers strong — with 810,000 of those sent Finland's way.

But by March of the next year, what became known as the Winter War was over, as the Soviet Union and Finland signed a peace treaty.

How did such a tiny country hold off one of the greatest superpowers in history?

It wasn't only a matter of mastering wintry terrain.

It was a matter of attitude.

"The Finns have something they call sisu," the New York Times reported in 1940. "It is a compound of bravado and bravery, of ferocity and tenacity, of the ability to keep fighting after most people would have quit, and to fight with the will to win. The Finns translate 'sisu' as 'the Finnish spirit,' but it is a much more gutful word than that."

Emilia Lahti researches that gutful mindset. A graduate from the University of Pennsylvania's Applied Positive Psychology master's program, she's now pursuing doctoral research on sisu at Aalto University School of Science and Technology in Helsinki.

While the workplace outcomes of sisu are yet to be confirmed in lab results, an understanding of the concept is instructive for our understanding of success.

"We've had this concept for 500, 600 years in our culture," says Lahti, who grew up in the small city of Seinäjoki, Finland. "It is not so much about achievement as it is about facing your challenges with valor and determination. It's the psychological strength capacity that enables individuals to power on when they've reached the end of their psychological or physical resources."

Lahti contrasts sisu with other parts of human strength, like resilience, conscientiousness, or grit, which psychologists say is the best predictor of success. But while grit is maintaining passion and performance in the pursuit of a long-term goal, sisu is your ability to take action against long odds.

But there are more examples than Finns and Reds. You're tapping into sisu when you determinedly go after your next job after your company abruptly downsizes, get back into the dating scene after the collapse of a relationship, or literally run an extra mile more than you planned to jog that morning.

When less resilient people run into difficult situations, they can't imagine that opportunities will come afterward.

Sisu is the determination to find those opportunities.

"Sisu isn't just a thought in our head," Lahti says. "It's how we orient toward these situations: Are we able to endure, persevere, and grow, or are we limited by the situation we are in?"

So instead of just thinking that everything will turn out OK, a person with lots of sisu has the conviction that she'll be all right and takes the necessary actions to shape those outcomes.

If you want to cultivate your sisu, Lahti recommends the following:

Run long distances. It gives you a chance to practice pushing yourself.

Reflect on your most sisu-filled times. Finding the evidence of your own ability to overcome long odds will embolden you the next time the Red Army invades.

Think about historic examples of people going beyond their limits. It helps show the real range of human possibility.

Why do all this? Because the more you get comfortable with the discomfort in stretching yourself, the more able you'll be able to bring sisu into your life.

"Give yourself the opportunity to be in a situation to push beyond what you normally can do," Lahti says, because "what you experience in one situation translates into other parts of your life."