It's time to see if the insides of Planet Fitness (NYSE:PLNT) can match the head-turning hardbody that its stock has become. The discount gym operator reports its fourth-quarter results after the market close on Tuesday, and expectations are running high again.

Planet Fitness shares have nearly doubled over the past year, and on Friday the stock hit yet another all-time high. The concept seems simple enough, offering casual workout buffs a no-pressure fitness experience for as little as $10 a month and no more than $20 a month. Planet Fitness' popularity is growing quickly with folks that don't typically sign up for pricier and more full-featured gym memberships. The number of active accounts is up to 12.2 million members as of the end of September

Exterior of a Planet Fitness at night.

Image source: Planet Fitness.

Pump you up

Analysts are holding out for another period of heady growth on both ends of the income statement. Wall Street pros see revenue rising 30% to $173.7 million for the fourth quarter. Net income is expected to climb 33% to $0.32 a share, but it wouldn't be a shock if it winds up earning more than that. Planet Fitness has routinely landed ahead of analyst profit targets, including three of the past four quarters. 

A call that is an even safer bet than another market-thumping earnings report is that comps will be positive. Planet Fitness has managed to piece together 47 consecutive quarters of growth. Another year-over-year gain at the store level and we'll be at exactly a dozen years of positive comps. Momentum is strong on that front, as system-wide same-store sales skyrocketed 9.7% last time out. 

Even the 30% targeted increase in revenue may be conservative. Planet Fitness grew its top line by 40% in the third quarter, its strongest percentage gain in more than four years. 

Planet Fitness already has 1,646 of its bargain-priced gyms, but it's not even halfway done with its expansion plan. Planet Fitness is targeting 4,000 units -- and that's before we begin to consider international possibilities. Planet Fitness shares have been hot over the past year, and it's not a fluke. Planet Fitness stock has nearly quadrupled since going public at $16 in the summer of 2015.  

The stock's new highs on Friday were triggered by an analyst upgrade. Randal Konik at Jefferies boosted his rating from hold to buy, bumping his price target from $49 to $75. He sees the growing wellness trend working in its favor, and he feels that Planet Fitness' pricing, expanding affinity network, and savvy digital strategies make it more of a platform than just a gym operator. Konik upgrading the stock just two trading days before its fourth-quarter report isn't likely a coincidence. He wants to make sure he's on the right side of this investment ahead of what is shaping up to be another strong financial update.