What is meant by the arch reflex?

In some situations the movement of the skeletal striated muscles does not depend upon commands of the superior motor neurons, i.e., it is not triggered by volition.

Involuntary movements of those muscles might be happen when sensory fibers that make direct or indirect connection with inferior motor neurons are unexpectedly stimulated in situations that suggest danger to the body. This occurs, for example, in the patellar reflex, or knee jerk reflex, when a sudden percussion on the knee patella (kneecap) triggers an involuntary contraction of the quadriceps (the extension muscle of the thigh). Another instance of the arch reflex happens when someone steps on a sharp object: one leg retracts and the other, by the arch reflex, distends to maintain the equilibrium of the body.