It may seem like Americans have always exercised in the same way that they do now, but you might be surprised how new many of our pastimes are. Jogging, for example, wasn't a thing until Nike (NYSE:NKE) co-founder Bill Bowerman visited New Zealand in the early 60s, and saw the locals running around in packs. Now, you can't push a pram on a nice day without running over an aspiring marathon runner.

In a similarly quick-paced transition, women athletes went from representing 7.4% of high school athletes in the mid-70s to over 40% today. With the institution of Title IX in 1972, the size of the female athlete population has boomed at both the amateur and professional levels. Now, athletes and sponsors are free to focus less on getting women involved, and more on how to increase the fan base and drive income from the boost in popularity.

Pushing for more
Under Armour
(NYSE:UAA) has been particularly vocal about evening out its gendered income, with a goal of getting to a more even spilt in the coming years. The company now earns 44% of its revenue from men's apparel, and 23% from women's. Its three-year plan is to cut men's slice of the pie down to 39%, while slightly increasing women's sales to account for 24% of total revenue by 2016.

While that may not seem like a big deal, it represents a 95% increase in women's revenue, while men's is only planned to grow 56% over the same period. Part of the reason for the rapid growth in the post-2000 era can be attributed to Lululemon's (NASDAQ:LULU) influence on the segment.

Lululemon did almost precisely what Nike did years ago by making a novelty -- pants for Lululemon, fancy shoes for Nike -- seem like a requirement. Estimates of the yoga market now put total revenue at more than $25 billion annually in the U.S., and almost 75% of participants are women. That's helped Lululemon leverage stretchy, thin pants -- sometimes too thin -- into a $1.4 billion business.

The field narrows
The massive market for yogawear and other women's apparel has spurred a race to the top. Nike launched a campaign last year set to coincide with, and celebrate, the 40th anniversary of Title IX. Later in 2012, Under Armour kicked off its biggest ever advertising push aimed at female athletes.

Even with all the growth, there's still a long way to go, and companies investing in women's sports are doing so with the hope that there is even more growth to come. In 2011, a Bloomberg study found that every single women's college basketball program actually lost money, due largely to the high cost of coaching. While that's all too common -- accounting for male sports, only 23 of the 228 NCAA athletic departments are self-funding -- it still points to a problem for future growth.

The success stories are likely to come, not from organized sports, but from casual runners and the like. Lululemon has built its whole empire on the back of revenue from individuals, not teams. And while Nike and Under Armour use organized sports and the celebrities those sports generate to advertise, the companies still rely heavily on you and me to buy shoes to go running, or baseball gloves for kids, or hoodies to form massive social networks that we take public. The shortfall at the college level isn't holding these companies back.

The bottom line
For now, Under Armour seems to be out in front. While Nike is pushing the running angle, Under Armour has generated a well-rounded line of products, with its ArmourBra taking up a lead role in getting traffic into its stores. Lululemon's current transition, and the holes in its executive lineup, mean that the company probably isn't moving as fast as it would like to grab more pie in the expansion of women's spending. Luckily, the company didn't seem to lose too much market share after its recent sheer-pants debacle, so it may still have a bright future.

The size of the opportunity and its growth means that the land grab is under way. Companies are free to roam around and get what they can, with very little butting of heads required, due to the size of the market. As things mature, though, it's going to get heated, and not everyone is going to come out a winner. This is a competitive environment, and an early lead could make all the difference.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.