Presynaptic action potentials in an axon?

the muscle fiber would not be stimulated and therefore not contract

Always think about the process as a series of events, each one depending on the last: An action potential travels down the length of the neuron (the axon), and when it reaches the synaptic vesicles, acetylcholine is released from the end of that neuron, and attaches all over the end of a different cell (in this case, a muscle cell). This second cell feels that acetylcholine attach to it, and in response, "depolarizes"--which just means that it opens channels that allows it to release negative charges and become overall positively charged. The positive charge of this cell is "strange" and not what the cell is used to, since it's usually negatively charged. The fact that it is positive prompts the cell to send an electrical signal inside the muscle cell, which becomes another action potential being propogated throughout that cell, and eventually results in a muscle contraction.

If you rewind back to the point where acetylcholine was released, and think of the consequences of not enough being generated, you can see that the muscle cell would never respond to it, and never contract--you'd be paralyzed, or at least have your normal muscle functioning be greatly inhibited!