Tonic (physiology)

Tonic in physiology refers to a muscle which is predominantly toned. The opposite of tonic is phasic.

Human tonic muscles
It can specifically refer to a muscle which is composed mainly of slow muscle fibers. As these types of muscles (trapezius, calves, jaw) tend to be toned, they tend to shorten and be relatively inflexible and therefore tend to be short in humans.

Typical tonic muscles include muscles which are used to protect and prepare the body under fight or flight situations, a reaction to the hormones epinephrine (adrenaline) and cortisol and other innate autonomic nervous system reactions.