A robotic joint is connected to two actuators, e.g. air muscles. One flexes the joint, while the other extends it. This arrangement is called 'antagonistic'.
But what if I had an electric motor instead of the air muscles? In that case it can only pull on one tendon at a time, and it's not antagonistic. What it the arrangement called in this case? Untagonistic?
The opposite of an antagonist muscle is an agonist muscle, but that doesn't really answer your question.
In nature, muscles are found in antagonistic pairs because they can only really contract, thus once a muscle is contracted, only an opposing muscle can effectively change the state back again (unless the muscle's natural state is opposite to that which is produced by the muscle).
In your system, by using a single motor to replace both flexor muscle and extensor muscle you are departing from the way nature works and thus the analogy breaks down. As far as I can see, there is unlikely to be a term from nature which can answer your question without compromising the metaphor, but since you have a single motor which is both flexing and extending your joint, you could just call it an agonist.
If you have an electric motor which can drive a linear system in two directions, essentially it is its own antagonist.
Edit:
muscles, as well as single-acting pneumatic cylinders, can only provide force in one direction, so they are arranged in agonist/antagonist pairs.
A motor, perhaps running a belt, is capable of pulling the belt in each direction, thereby providing motive force to what you might consider to be the agonist belt in one direction and the antagonist belt in the other.
