"I was letting her know that even though we may not be there physically together, we share a heartbeat."

Dad with Daughter Opal
Credit: Facebook/Gretchen Trimble

For the past two months, Josh Trimble has watched his youngest child, six-month-old Opal, fight for her life.

Little Opal, of Shawnee, Oklahoma, was just four months old when she was diagnosed acute flaccid myelitis (AFM), a rare condition that affects the nervous system and weakens the body's muscles and reflexes. She's been in the hospital ever since.

Josh, who is also the father of a five-year-old son and three-year-old daughter, unfortunately can't always be by Opal's side, so he developed a way to let her know he is always with her in her heart.

As Trimble was saying goodbye to Opal at the Children's Center Rehabilitation Hospital in Bethany, Oklahoma, last week, mom Gretchen filmed their touching tradition. Standing over her crib, Josh pounded on his chest. Without missing a beat, little Opal pounded her chest, too.

"That moment of me beating on my chest was actually Opal and I sharing a heartbeat," Trimble told Good Morning America. "I was letting her know that even though we may not be there physically together, we share a heartbeat."

Gretchen told GMA that she recorded the moment to show doctors. It was a sign of just how far their daughter has come since her diagnosis. Opal had spent seven weeks nearly immobile.

"That day in particular we were praying about her not only mobility but her intentionality in deciding to move a certain way, and our focus was on her arms," Josh explained.

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"We had just finished some hard days of physical therapy for Opal, and her physical therapist had been trying to get her to reach out for things," added Gretchen. "We were just so encouraged because the fact that she was mimicking him told us she knew what she wanted to do, and she did it."

According to the CDC, AFM is so rare that less than two out of a million children in the U.S. will get it every year. Gretchen told GMA that Opal is the youngest documented case of the disease.

Dr. Michael Johnson, medical director of the hospital and a member of Opal's care team, told GMA that he's encouraged by the video. And she's continued to do the arm movement with her dad—even initiating it sometimes.

"Opal is stronger than us combined, not only mentally but also proving to be very strong physically as well," said Josh. "If you could touch her arms, you could immediately feel her strength. If you could see her smile, you'd immediately be impacted by it."