Final answer:
When the resistance of an exercise is being moved in a horizontal plane, the body should be positioned so that the muscles being exercised are moving in the opposite direction of the resistance.
Step-by-step explanation:
Muscles can only contract and cannot actively extend. Therefore, when moving bones in opposite directions, pairs of muscles work in opposition. The muscle that contracts to cause a joint to bend is called the flexor, while the muscle that contracts to cause the joint to straighten is called the extensor.
So, when the resistance of an exercise is being moved in a horizontal plane, the body should be positioned so that the muscles being exercised are moving in the opposite direction of the resistance.