Why do my clients yawn in between their sets although they have had plenty of sleep?
Although people often yawn when they are getting tired, the function of the yawn is to receive more oxygen to the brain. Many individuals who are or have suffered from breathing problems such as asthma, a collapsed lung or any number of things will be constantly yawning to force more oxygen in the system. I would check how their VO2 max compares to the average. It is a long shot, but possibly under the stress of exercise they are not receiving adequate oxygen and their bodies force them to yawn to make up for the deficiency.