The Hunt for Activewear’s Next Big Category

People used to wear old sweats to yoga classes. Then came Lululemon.

When it first emerged on the market two decades ago, the activewear giant gained traction by focussing on yoga, then a niche, untapped sport. Now, it’s a $6 billion empire that’s expanded beyond yoga to create apparel and accessories for sports like running, swimming and tennis, too.

Lululemon’s trajectory has become a model for activewear upstarts that hope to compete…