The Inverse Stretch Reflex
Located within the musculotendinous junctions (MTJs) are sensory nerves called golgi tendon organs (GTOs).
These guys are here to prevent excessive TENSION occuring within a muscle, or tendon of that muscle.
They are activated by the contraction of a muscle and, in contrast to muscle spindles, their stimulation triggers a reflex relaxation of that muscle rather than a reflex contraction (like the myotatic reflex). This is known as the 'INVERSE STRETCH REFLEX'!
This relaxation or inhibition of a muscle is important for certain stretches because it allows muscle fibres to lengthen and stretch further (a relaxed muscle is easier to stretch than a contracted one!)
Relaxation that occurs in the same muscle due to GTO activation is known as autogenic inhibition.
How do we achieve autogenic inhibition? By contracting a muscle immediately before passively stretching it. The contraction will increase GTO activation which increases muscle relaxation!
This is the relaxing effect that occurs in a muscle when the antagonist is contracting.
When an agonist contracts to cause a motion it forces the antagonist to relax (when one builder gets up to do some work, he makes the other sit down to have a cup of tea). The antagonists are INHIBITED from contracting (reciprocal inhibition)