One of the chief joys in watching documentaries is discovering that a subject you cared little about is actually more interesting than you’d thought.

The new documentary “Derby Baby: A Story of Love, Addiction, and Rink Rash,” showing at 7:30 p.m. Saturday at Space Gallery in Portland, will make you think more deeply about the sport of roller derby than you’d ever imagined.

Unless, that is, you’re already a participant or fan of the sport — which, as the documentary reveals, has rebounded from the decline after its 1950s heyday to become “the fastest-growing women’s sport in the world,” with an estimated 30,000 women playing in hundreds of leagues worldwide. Indeed, Saturday’s all-ages screening, which costs $8, is sponsored by our own Maine Roller Derby (mainerollerderby.com).

For the uninitiated, roller derby, played now mostly on a flat track rather than the banked rinks of the ’50s, involves two teams of women skating around in a circle. Each team simultaneously tries to clear the way for its scorer (the jammer) to pass members of the other team while preventing the opposing jammer from doing the same.

This “preventing” is where the action is, as teams use every rugged tactic (save punching, in theory) possible while skating at high speed. As presented in the film, the average roller derby bout is exciting, fast-paced and unrelentingly physical.

And, according to the many players interviewed, uniquely liberating.

For a rapidly growing yet largely self-financed sport, roller derby asks a lot from its participants: Physically, sure (there’s a montage of some jarring injuries), but women players also take on every conceivable role in organizing their leagues.

For most players, that intense participation fosters a sense of ownership commensurate with the self-esteem they get from playing.

“Women are amazed at how strong and how tough they really are,” enthuses one player, and that sense of empowerment is a major theme of the film.

As “Derby Baby” goes on, the film introduces other themes as well, not all of which are as rosy. While most interviewees agree on the physical and psychic benefits of becoming a roller girl, there’s also a lot of interesting discussion of the role that sexuality plays in the sport’s resurgent popularity.

Some argue that, while sex appeal and the spectacle of “hot chicks beating the crap out of each other” might bring audiences in initially, they’ll gradually come to appreciate the athleticism.

Also, participants’ adoption of larger-than-life personae (favorite names: “Annia LateHer,” “Frida BeatHer,” “Shedonist”) and theatrically sexualized costuming (fishnets are a theme) suggests these women are in some sense adopting an oversized approximation of traditional gender stereotypes in order to comment on them.

That theatrical aspect of the sport brings its own set of problems, as one official recounts how difficult it is to get the sports press to take roller derby seriously. Often lumped in with pro wrestling due to the showmanship involved, the very relevant difference (that roller derby is an actual athletic competition and not a staged exhibition) has proven hard to impress upon mainstream audiences.

“Derby Baby” is an insightful, often funny and ultimately inspiring depiction of a uniquely female sports subculture attempting to define itself — the looming prospect of corporate sponsorship threatening the individuality of the sport ends the film.

But the overall image it leaves is one of fun, empowerment and hope for, as one player imagines, “the sort of societal shift necessary for a women’s sport to be taken seriously.”

I’d take it over hockey any day.

Dennis Perkins is a Portland freelance writer.