There are two foes in the film “Nyad”: nature and age. It is no spoiler to say that Annette Bening and Jodie Foster achieve a conquest over those opponents, not in some trite, scripted or airbrushed way but with the sinews and cartilage of real athletes. Every now and then a cultural moment comes along that exposes how severely and artificially we continue to limit the conceptual range of female ideals, and the cannonball biceps of these actresses in their 60s constitute a significant one. Sun-scorched, straw-haired, scored with tendons, they are glorious.

“Nyad,” directed by husband and wife Jimmy Chin and Elizabeth Chai Vasarhelyi of “Free Solo” fame, is a biopic of marathon swimmer Diana Nyad during her 110-mile open water journey from Cuba to Florida in 2013 at 64 with the help of her closest friend and trainer, Bonnie Stoll. But there is a story within a story in the film, which debuted on Netflix on Friday: the partnership of two women frustrated by a Hollywood culture that permits so few dynamic roles for mature women and who do something about it. When “Nyad” began filming in a 233-by-233-foot tank of water off the coast of the Dominican Republic in April 2022, Bening showed up on set for a “safety” rehearsal with stunt doubles — whom she promptly rendered bystanders. She knifed into the water and glided the length of the tank, swimming 70 yards with elite-quality strokes that barely rippled the water. “We were all jaw-dropped,” Chin says.

Chin, a fabled extreme athlete himself who has scaled peaks, skied Mount Everest and grew up swimming competitively, had not known until that moment how committed Bening was in preparing for the role. “We weren’t sure how far she was willing to take it,” he says. They left Bening alone during preproduction “to respect the actor’s process.” Bening used the time to train every day for a year under the tutelage of a former U.S. Olympian, Rada Owen. “She was incredibly strong. Her stroke was beautiful. She was just flying through the water,” Chin says. “. . . She took it all the way.” Bening was “adamant” that she would swim every stroke in the film — which meant spending four to eight hours each day in the water. “One of the unanticipated consequences was that she was so strong, she had this endurance which allowed us to maximize our shooting schedule,” Vasarhelyi says.

Bening, 65, and Foster, 60, have been unable to promote the film because of the screen actors strike. But it’s plain in conversation with their filmmakers and trainers that both explicitly wanted to shatter age archetypes — really, really break them all to pieces. “It was important to both actors that we not touch their bodies, as in touch up,” Vasarhelyi said. “They were committed to play women of their age.” They were also committed to demonstrating the metamorphosing possibilities of strenuous athleticism. Foster told the directors she wanted to join the film in part because she wanted to show audiences two older women who were “badasses.”

Jodie Foster told the directors of “Nyad” she wanted to join the film in part because she wanted to show audiences two older women who were “badasses.” (Kimberley French/Netflix via AP)

Bening prepared for the role by hanging out with Nyad and even doing an open water swim with her. She began the project with a slender, yoga-fit frame but put on slabs of muscle in her shoulders to cultivate the right heavy endurance build. “When you looked at her, she had a swimmer’s back,” Vasarhelyi says. Bening had little experience in the water other than crewing on a scuba boat as a teenager. But she told the former Olympian Owen that she considered acting an athletic pursuit, saying that “movement is very important to me.” The first time the former Olympian put her through a pool interval workout with just five seconds’ rest between laps, she gasped almost to the point of panic. Owen taught her to manage her breath and energy and elongate her strokes. Most untrained swimmers tend to chop vainly in the water, taking 20 to 30 strokes to swim a 25-yard pool length, “kind of like trying to kayak with a straw,” Owen says. An Olympian such as Owen only needs 11 strokes. By the end of their training sessions, Bening could torque through the water in multiple 300-yard sets without gasping and glide a pool lap in only 15 or so strokes.

Bening added enough inches to her shoulders that she told Stoll, “My jackets fit differently.” She also became a dedicated swimmer who still spends an hour or so daily in the pool. When “Nyad” began to screen at film festivals, Vasarhelyi got a text from a friend’s mother. “Did you airbrush Annette’s legs?” it said. “No, that’s God’s work,” Vasarhelyi replied. But it was also Bening’s.

Foster’s first appearance on the set came with a similar jolt of awe for the directors. On the day she agreed to take the role of Stoll, Foster stood on a New York sidewalk after meeting with Bening and told the directors, “I’m going to have to start training tomorrow for this role.” Stoll, 71, a Los Angeles-based physical trainer and a former world-class racquetball player, is a burnished specimen who does 100 reps of shoulder, biceps and abdominal work a day, power-walks for two hours at a clip and does sets of military style chin-ups and pull-ups.

In her previous film, “The Mauritanian,” Foster played a slack-armed, helmet-haired lawyer in a suit. After that day with Foster on the sidewalk, “We never really heard anything more from her,” Chin says. “And then she showed up just ripped.”

Foster hung out with Stoll regularly, studying her mannerisms and honing an imitative physique. The actress embarked on a regimen of daily workouts that continued through the film, alternating kettlebells with heavy weightlifting. She combined it with a sufferingly strict diet that to the directors seemed to consist of “mostly brown rice, chicken and broccoli.” At the end of the film, she told Stoll, “I hope I never see chicken again.”

“I’m not done,” Benning, right, tells Foster in the movie, “and neither are you.” (Kimberley French/Netflix via AP)

It is arguably the most limber acting job of Foster’s career. She exudes a jubilant physicality, and the audience will be pardoned for doing a double take when she appears on a boat deck wearing cargo shorts and a sports bra and for grabbing their phones to look up Foster’s age. She will turn 61 on Nov. 19. Recently, the directors met her at a social occasion. “I’m being objectified,” she teased them. Vasarhelyi says, “For me, she’s a new body goal, like a new measure that I personally aspire to.”

The theme of Nyad is simply: Aging is an inevitability, but weakness isn’t, not if you’re willing to keep lifting things above your shoulders. “You know, we don’t have to lose our muscle,” Stoll says.

But there is an underlying message, too, a more subtle one delivered by Bening and Foster in their transfigurations from slender actresses into absolute athletes. Not much really separates great athleticism from great acting. Both require rigor, practice, the development of fine sensory perception and a fundamental lack of vanity in search of a mysterious elevation. Bening and Foster give the audience acting and athleticism — and along the way a reminder of the exultation to be found in any physically immersive endeavor, no matter what your age or state of being.

“I’m not done,” Bening tells Foster in the movie, “and neither are you.”

To portray marathon swimmer Diana Nyad, Annette Bening transformed her body. (Kimberley French/Netflix)