One cause of reflux during exercise may be due to improper breathing patterns. When you take a breath in, the diaphragm contracts and pulls down (toward the central tendon) in order to pull air into the lungs. As a result of the diaphragm pulling down against the abdominal contents (guts), the belly should EXPAND all the way around during inhalation. But for far too long, every trainer under the sun told their clients to “pull their belly button toward the spine” to “turn on the abs.” This has created a scenario where many people suck their belly in when they try to breath, essentially reversing the pattern. This sucking-in limits the contraction of the diaphragm and creates upward pressure against the stomach, forcing acid and stomach contents back up into the esophagus.
If you look at the picture, you’ll see there is a section of muscle fibers in the diaphragm that loops around the esophagus. They’ve shown (Eherer 2012, Bittnar 2015) that when you train proper abdominal/diaphragmatic breathing (belly expansion during inhalation), the contraction of this loop of muscle fiber in the diaphragm helps to close the cardiac sphincter (between the esophagus and stomach) during inhalation and prevents the reflux of stomach acid up into the esophagus. Proper abdominal breathing helps to create stability (the diaphragm is part of the core!) which helps us put out more power too.
Here’s a video that shows what abdominal breathing should look like: Diaphragmatic Breathing - YouTube
I realize I’m late to the discussion here but I was catching up on podcasts on a long drive the other day and wanted to add this to the possibilities of why this user was getting reflux during exercises, in addition to the other causes Chad talked about.