Reciprocal Inhibition: This topic is both more and less complicated than you might expect.

Simply:

Reciprocal inhibition is a neuromuscular reflex that inhibits opposing muscles during movement. For example, if you contract your elbow flexors (biceps) then your elbow extenors (triceps) are inhibited. This is the idea behind active stretching, and one component of PNF stretching.

Complex:

Reciprical inhibition is a neuromuscular reflex – An increase in neural drive of a muscle, or group of muscles, reduces the neural activity of functional antagonists. This plays a siginificant role in improving the effeciency of the human movement system, and creating ideal arthrokinematics. This more nuanced definition encompasses the role of reciprical inhibition in more complex issues in human movement science. Likely the most important point made in this definition is the terms “increase” and “reduction” implying that reciprical inhibition is not a simple function of “on or off”. For example, postural dysfunction resulting in adaptive shortening and hypertonicity inhibits functional antogaonist (tight psoas-inhibited glutes), but does not decrease the neural drive to the glute complex completely making it possible to move and function (although less than optimally). Or, when looking at complex muscle synergies during multiplaner movement you may note that a muscles may be antagonists in one plane, but not another. For example, the adductor magnus – posterior head is a functional antagonist of the gluteus medius in the frontal plane, but is a synergist during transverse plane external rotation. This is imprortant when considering ideal movement, but must be viewed in conjunction with synergistic dominance, arthrokinematic inhibition, and muscular synergies to understand the resultant effect on exercise…