Former New Orleans Saints football player Steve Gleason, center, battles ALS while trying to make the most of his time with his wife, Michel, left, and son, Rivers, as shown in the documentary “Gleason.” (Suzanne Alford/Open Road Films)

If you were told you had a crippling terminal illness, how would you choose to live out the rest of your life, in the face of that death sentence? In the case of Steve Gleason — the former football player diagnosed with amyotrophic lateral sclerosis in 2011, and the subject of the documentary “Gleason” — the answer is: with a greater appreciation for the time you have left.

As a safety for the New Orleans Saints, Gleason is best known for blocking a punt against the Atlanta Falcons during a 2006 home game at the Superdome, 13 months after Hurricane Katrina. That play felt like rebirth for the team and for the city, and footage of Gleason’s gridiron exploits show seemingly boundless physical energy. That makes the disease’s effects all the more cruel.

Three years after retiring from the NFL, Gleason, then 34, received back-to-back bombshells: Within weeks of a diagnosis of Lou Gehrig’s disease, he learned that his wife Michel was pregnant. Concerned that he may never be able to have a conversation with his son, Gleason began to record a video diary in order to share as much of himself as possible while he was still able to speak. This footage makes up the heart of the film, which director Clay Tweel (“Finders Keepers”) has shaped into a narrative about the strength of the human spirit when confronted with the frailties of the body.

Steve Gleason and his son, Rivers. (Open Road Films)

In such fictional films as “Billy Elliot,” a letter from a dead parent can be a heartbreaking narrative device. In a documentary, it’s even more powerful, so much so that Gleason’s generosity in revealing himself — not only to his son, but to the viewer — occasionally feels like exploitation. The movie essentially allows us to watch its subject slowly die, as Gleason loses more and more control of his muscles.

This uncomfortably intimate film doesn’t spare the viewer from the difficulties experienced by Gleason and his family. As Gleason struggles to maintain his religious faith during this crisis, his father Mike questions his son’s beliefs. In one emotionally grueling episode, Mike takes his reluctant son to a faith healer, with disastrous results. We also see Gleason’s marriage buffeted by the stress of his illness. As the couple’s life together becomes a regimen of around-the-clock care, we’re shown the physical and emotional toll on both husband and wife.

Yet Gleason’s determination to find meaning in his suffering is inspiring. On a trip to Alaska that Gleason and his wife take soon after the diagnosis, his video journal captures a mix of anxiety and hope, against a backdrop of a rugged, seemingly eternal landscape.

In his quest for purpose, Gleason has started a charitable foundation to enable other ALS patients to take trips and to pay for specialized equipment that allows them to communicate, via an eye-activated keyboard, that medical insurers had refused to cover. “Gleason” portrays great strength and great suffering in equal measure, lending vivid credence to tired platitudes about what it means to live life to the fullest.

R. At Landmark’s E Street and Bethesda Row cinemas. Contains strong language, partial nudity and graphic depiction of intimate caregiving. 110 minutes.