Which stimulus amplitudes are subthreshold or submaximal? Which ones are suprathreshold or supramaximal?

The sub-threshold stimulus are those which does not have an amplitude large enough to open voltage-gated sodium channels and initiate an action potential.

A suprathreshold or supramaximal stimulus is the one which deplorizes the neurons and initiates the action potential resulting into motor unit responses.

However, this nerve depolarization is suprathreshold but not supra maximal when depolarization but is not strong enough to cause action potential and resulting motor response.  

Explanation:

The process of generating an action potential consisted of hyperpolarization, depolarization, overshoot and then repolarization of neurons to open and close voltage-gated sodium channels in muscles. Hence, when signal transduction occurs to initiate depolarization of sodium channel, a signal of strong amplitude causing opening of voltage-gated channels are known as suprathreshold. Later, this depolarization process when results into generation of action potential and resulting motor unit response, it is known as a supramaximal stimulus. However, if the threshold does not produce action potential and is only able to cause depolarization, then the stimulus is suprathreshold but submaximal.

An action potential is caused by either threshold or suprathreshold stimuli upon a neuron. It consists of four phases; hypopolarization, depolarization, overshoot, and repolarization. An action potential propagates along the cell membrane of an axon until it reaches the terminal button.