Available on DVD: “The Do-Deca Pentathlon”

Never underestimate the power of residual sibling rivalry to incite crazy behavior. That is the unsettling subtext of The Do-Deca-Pentathlon, Jay and Mark Duplass’s serious farce. The most gripping scene in this near-perfect little sports comedy is a fraternal arm-wrestling contest that reaches apoplectic intensity.

The combatants are Mark (Steve Zissis) — a chubby, out-of-shape pet-food marketing consultant who is married with an adolescent son — and his brother, Jeremy (Mark Kelly), an unmarried professional poker player. They are determined to settle unfinished personal business: In 1990, while in high school, they competed in a self-devised 25-event miniature Olympics that ended in a tie when a breath-holding contest was interrupted.

Each has since secretly harbored a desire to complete the competition, whose outcome would certify "the better brother." When Jeremy, the more insistent on settling it once and for all, crashes Mark’s birthday party at their mother’s home, their long-simmering power struggle boils up.

Mark, who sees a therapist, claims to have outgrown their shared obsession, but he hasn’t really. The moment Jeremy joins Mark in a friendly, noncompetitive mini-marathon run, the brothers break into a race in which they ferociously elbow each other, and Mark vomits from overexertion.

The Do-Deca-Pentathlon was shot in 2008, then put on the shelf while the Duplass brothers went mainstream with Cyrus. Although very funny, this film taps into a primal male competitiveness whose force outweighs reason and common sense. As Mark and Jeremy grasp hands and begin to push, you have the sense of observing two beasts locking horns in a life-or-death struggle on the African veld. For Mark, whose doctor has advised him to avoid stressful situations, there is some risk. As he turns scarlet, and the veins on his head pop out, you half expect him to explode.

Because their mother, Alice (Julie Vorus), and Mark’s wife, Stephanie (Jennifer Lafleur), are dead set against the contest, the brothers try to keep it a secret. Much of the movie’s humor springs from their pathetic attempts to avoid detection by competing late at night or pretending to run errands and rushing out to do battle. Some events, like laser tag, seem more than a little silly. Mark’s inability to let go ultimately threatens his marriage as well as his health.

The film’s casual style works in its favor. It maintains a breezy pace, and much of the dialogue has the spontaneity of expert improvisation. As in the Duplasses’ other movies, the momentum is accelerated by quick zooms, abrupt edits and a restless camera.

If The Do-Deca-Pentathlon refers fleetingly to the brothers’ grown-up discontents, it makes every remark count. When the question "Are you happy?" is posed, it is not as offhand as it sounds, because the questioner is obviously hoping the answer is no.

Each wants something the other has. Mark envies Jeremy’s freewheeling bachelor life. Jeremy looks longingly at Mark’s security and more-or-less stable marriage and makes it a point to bond with his brother’s blasé, longhaired son, Hunter (Reid Williams), who assists the brothers in their deception without becoming emotionally involved.

The movie wouldn’t be as compelling if Zissis and Kelly weren’t also able to suggest an underlying fraternal bond. What passes for love between them is a deep, shared understanding of the importance of the competition.

In the rapidly expanding Duplass output The Do-Deca-Pentathlon is their second-best film, after The Puffy Chair, their mumblecore breakthrough, released in 2006. Aside from its technical crudeness, The Puffy Chair, which was made for about $15,000 and included a few too many "dudes" in the screenplay, was a career-defining film and a scruffy little miracle of truthfulness. So is this, but on a smaller scale.