Butt Formation takes a simple premise and runs with it: girls working on their asses with the help of a private trainer, until the trainer decides the best exercise is his cock. The site is part of the Gasm network, the European group behind Harmony Vision and Magma Film, and it is built for fans of fit bodies, tight leggings and anal. We list it among our public porn sites because so much of the action starts outside, with jogging, squats and stretching in parks and on quiet streets before the pair heads somewhere private, or not so private, to finish. The name is a pun on boot camp, and the tone is playful rather than rough.

Every scene follows the training format. A coach puts a girl through a sweaty routine, lingers on her ass in tight shorts, slaps it, oils it and then moves on to fingering, rimming and full anal in positions that show off the glutes she has been working. The performer list is strong European talent, with names like Amirah Adara, Alexa Tomas, Claudia Bavel and Enna on the tour page, and the directors clearly know how to shoot bodies in motion. Butt Formation promises three updates per week across its channels, and the membership opens the rest of the Gasm catalogue, which the network describes as tens of thousands of HD videos from sixteen independent directors. That turns a niche fitness site into a very large library. Scenes are tagged by performer, position and setting, and the member area adds the usual favourites, ratings and comments.

Production is where Gasm earns its reputation: full HD streaming, clean camera work, proper sound, and a player that runs on phones, tablets and desktop without an app. The site stresses easy browsing and easy cancellation, with a recurring membership you can end from the account page. Prices move with promotions, so check the join page when you land. Billing is processed by a third party under a discreet name, performers are documented adults, and the outdoor scenes are staged productions with a consenting cast, as you would expect from an established European studio. The whole network has a single login and one bill, which is simpler than juggling several niche memberships.

Our verdict: Butt Formation is one of the more creative public porn sites we have reviewed, because the fitness angle gives every scene a natural reason to be outside and sweaty before the anal starts. Add the Gasm network access and the price-to-content ratio is excellent. Open Butt Formation through our link to preview the latest training sessions and the current membership offer.