To explore whether this phrase is valid or not, we need to understand that the use of the words “fire” or “firing” is clearly informal. This is just like when we tell patients we are “releasing” their muscles. When we see a patient in the clinic, we aren’t really looking to show off our physiological vocabulary. We’re trying to make decisions and ascertain how well the body is functioning all whilst keeping the patient interested and more importantly, awake.
When we assess if the glutes are “firing”, we are really looking at the sequence in which the muscles of the leg contract. In a prone position (patient lying facing down), we can assess hip extension (patient lifts leg off the bed slightly). If we cannot feel the glutes contract first, then we usually feel the hamstrings contracting strongly along with the lower back and smaller hip muscles which are compensating for the fact the glutes didn’t accept their invite to the contraction party.
Firing, unless referring to the temperature of the glutes, in which case I’d be incredibly worried for the patient, is usually just a term affiliated with speed. So, by suggesting the “glutes aren’t firing”, we are simply indicating that the glutes are not contracting quickly enough. If that is the case in a relaxed, lying-down position, then you can imagine the urgency to get the glutes contracting quicker or “firing” in runners, cyclists and active patients alike.