This article is from the Stretching FAQ, by Brad Appleton Brad_Appleton@ivhs.mot.com with numerous contributions by others.
When muscles contract (possibly due to the stretch reflex), they produce
tension at the point where the muscle is connected to the tendon, where the
golgi tendon organ is located. The golgi tendon organ records the change in
tension, and the rate of change of the tension, and sends signals to the
spine to convey this information (See "1.6.1 - Proprioceptors"). When this
tension exceeds a certain threshold, it triggers the "lengthening reaction"
which inhibits the muscles from contracting and causes them to relax.
Other names for this reflex are the "inverse myotatic reflex", "autogenic
inhibition", and the "clasped-knife reflex". This basic function of the
golgi tendon organ helps to protect the muscles, tendons, and ligaments
from injury. The lengthening reaction is possible only because the
signaling of golgi tendon organ to the spinal cord is powerful enough to
overcome the signaling of the muscle spindles telling the muscle to
Another reason for holding a stretch for a prolonged period of time is to
allow this lengthening reaction to occur, thus helping the stretched
muscles to relax. It is easier to stretch, or lengthen, a muscle when it is
not trying to contract.