Reciprocal Inhibition - and why I don't cue rectus abdominals (6 pack muscle) in a curl up/chest lift

Reciprocal Inhibition is a term used to describe how muscles “talk to one another”. Muscles work together to move joints, usually with an agonist (the driver), an antagonist (the passenger) and sometimes some synergists (backseat drivers) helping out. When the agonist (let’s say the hamstrings) contracts to bend your knee, this sends neurological messages to the antagonist (the quadriceps) to eccentrically lengthen (we often sloppily say “let go” or “switch off” but it’s more complex than that). This is how muscles communicate to one another to move joints in a coordinated way. It’s important to note that a muscle is never switched off, or deactivated - think instead of it like a coordinated tug-of-war where when one muscle starts winning, the other has to choose to let go, and vice versa.