Imagine you are sitting with your leg at rest and free to move in all directions. An experimenter inserts an electrode into a muscle spindle stretch receptor neuron in one of your quadriceps muscles and excites (depolarizes) the cell, enough that its rate of action potential firing increases substantially. What will happen to that quadriceps muscle

The quadriceps muscles will contract.

Explanation:

Quardriceps are muscles of the thigh located in the front.

It is a group of four vastus muscles named as-the vastus lateralis, vastus medialis, vastus intermedius, and the rectus femoris.

Muscles posses the ability of contraction so an action potential that will excite the muscle cells will obviously result in contraction of the muscles.

So we can conclude that on depolarizing the muscle cell of the quadriceps will result in contraction of quadriceps.