Final answer:
Stretched muscles maintain tension and have sensory neurons firing action potentials at a constant rate, while unstretched muscles do not show this constant activity.
Step-by-step explanation:
When considering the type of muscle in which sensory neurons fire action potentials (APs) at a constant rate, we are discussing a physiological response to muscle stretching. Muscles that are stretched maintain a certain degree of tension, which can be detected by sensory neurons. In this context, stretched muscle would exhibit a consistent firing rate of action potentials by sensory neurons, indicative of maintaining muscle tone or responding to increased muscle length. This can be contrasted with a muscle in an unstretched state, which would not generate the same constant rate of sensory neuron action potential firing.