We're now just a day away from the next quarterly report out of Planet Fitness (NYSE:PLNT), but the discount gym operator is already starting to bulk up. Shares of the 1,313-unit chain soared 11.6% last week. 

There was no company-specific news fueling last week's surge. On Tuesday, it did confirm that it would be reporting its first-quarter numbers a week later -- after this Tuesday's market close -- but announcing an earnings date is rarely a cause for bullish celebration. Last week's rally was enough to send the stock north of $20 for the first time in more than a month.

The exterior of a Planet Fitness gym.

Image source: Planet Fitness.

Getting pumped up

We know where analysts stand ahead of Tuesday afternoon's report. Wall Street pros are holding out for $91.07 million in revenue through the first three months of 2017, 9.3% more than it served up a year earlier. Single-digit top-line growth may not seem so impressive, but Planet Fitness has already had two slower quarters in its brief publicly traded tenure. 

Analysts are expecting a profit of $0.16 a share, well ahead of the $0.09 a share that it posted during last year's first quarter. 

Planet Fitness already told investors about a meaty milestone that it hit earlier this year. The franchisee-fueled gym business announced in early April that it had topped 10 million active members by the end of the first quarter. Planet Fitness had just 8.9 million members at the end of December and 8.3 million by the end March last year. The spike in accounts -- growing faster than the gym count -- makes it highly likely that Planet Fitness will post another period of positive same-store sales growth. Success on that front would extend that streak to an enviable 41 quarters of positive comps. 

The first quarter is typically a good time to be a gym operator. It's when New Year's resolutions to get into shape translate into new sign-ups. A million of the 1.6 million net additions that Planet Fitness scored a year earlier occurred during the first three months of 2016. 

The seasonality of the registrations doesn't make the first quarter Planet Fitness' strongest period. The first quarter has been the weakest or second weakest quarter in terms of revenue for Planet Fitness over the past couple of years. Folks sign up during the quarter, but the low $10-a-month rate of the basic membership plan keeps folks around long after that.

Planet Fitness is expanding quickly. It expects to go from 1,313 locations at the start of this year to more than 1,500 by year's end. It's making the most of the retail landscape, as superstores and other big-box concepts falter to give Planet Fitness leverage in negotiations with desperate landlords. Membership rates are growing even faster, making it easy to wonder if analysts are being too conservative in expecting single-digit top-line growth. The stock's big gains heading into Tuesday's report may suggest that a lot of the potential upside is already baked into the stock price, but clearly there is room for another positive surprise out of the gym industry disruptor.

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