It's not just the members working up a sweat that are pumped up for Planet Fitness (NYSE:PLNT) these days.

Shares of the discount gym operator surged 12.3% after the company posted another period of blowout financial results last week. It went on to boost its full-year revenue, comps, and adjusted earnings guidance. Planet Fitness stock hit all-time highs last week on the news, and set new high-water marks earlier this week

The quarter was undeniably strong. Revenue rose 12% to $97.5 million, a combination of brisk expansion -- 31 new fitness centers opened during the quarter and 190 net new locations have sprouted over the past year -- and a hearty 9.5% spike in systemwide same-store sales. If you didn't view classic gyms as viable turf for growth investing, you probably haven't taken the Planet Fitness model for a spin.

Exterior of a Planet Fitness at night.

Image source: Planet Fitness.

Keeping up the pace

Planet Fitness is growing quickly on the strength of its value proposition. Monthly rates start at $10 and top out at $20, a lot cheaper than membership fees at full-service rivals. Planet Fitness is able to offer a low price as its centers lack gym classes, day care, swimming pools, and other old-school luxuries. It also makes it less cumbersome for members to bow out. 

Retention isn't the game here. There is seasonality to this model, and this is the third year in a row in which revenue experiences a sequential dip between the second and third quarters. Year-over-year growth is the real measuring stick in this case, and Planet Fitness has now posted 43 quarters of positive comps. It topped 10 million active members earlier this year.

The model works. It's scalable. If revenue is growing fast, it's a safe bet that the bottom line is growing even faster. Indeed, adjusted earnings rose 19% for the quarter. 

The 1,432-unit chain is at the right place at the right time. It's a growing retail concept that needs ample size -- a dinner bell for hungry landlords looking to fill the vacancies left behind by failing department stores, book superstores, and other strip-mall anchors. 

Planet Fitness revealed last week that it's testing the price elasticity of its high-end Black Card membership, but for now it's sticking to its $19.99 price point for existing members. Lifting the price ceiling is one way to increase revenue, but Planet Fitness continues to do a good job of growing its empire by getting more franchisees to open new stores. 

No streak lasts forever, and eventually Planet Fitness won't be able to get enough new members to offset those taking a break. However, until we see signs of that happening, it would be hard to bet against the market darling.

Rick Munarriz has no position in any of the stocks mentioned. The Motley Fool recommends Planet Fitness. The Motley Fool has a disclosure policy.