Final answer:
A "partial, but sustained, contraction" refers to incomplete tetanus, where muscle contractions occur repeatedly with brief relaxation phases, potentially leading to complete tetanus if relaxation phases are absent.
Step-by-step explanation:
A "partial, but sustained, contraction" describes a scenario typically referred to as incomplete tetanus. During incomplete tetanus, the muscle experiences repetitive cycles of contraction with only a brief relaxation phase in between. If the frequency of motor neuron signaling is sufficiently high, this can progress into complete tetanus, where the relaxation phase is eliminated, and the muscle remains in a state of continuous contraction.
This is different from a twitch, which is a single, quick contraction of the muscle fibers in response to a single stimulus. A twitch does not produce a sustained muscle activity. Furthermore, it is distinct from isometric contraction, where muscle tension changes but muscle length remains constant, and muscle fatigue, which is the muscle's reduced capacity to generate force.