Reciprocal inhibition is a neuro-muscular response which prevents a stretch reflex from occurring in an antagonist muscle when the muscle is stretched as a result of the action the muscle opposite.
When you take a drink, for example, your biceps muscle contracts to flex or bend your elbow. The contraction of the biceps and flexion of the elbow causes the opposite muscle, the triceps, to stretch. Normally, when a muscle is stretched a stretch reflex occurs that causes the muscle to contract in order to resist the change in length. If this stretch reflex occurred in the triceps muscle while attempting to bend your elbow, the triceps would try to contract at the same time as the biceps, thus opposing the desired movement. If this happened either no movement would occur or a very jerky and uncontrollable movement.
It is important to realize that reciprocal inhibition does not always occur and that is is quite possible for an agonist and antagonist to contract at the same time. This is called “co-contraction”. This co-contraction is at certain times absolutely necessary in order to carry out daily functions such as carrying objects.