Golgi tendon organs detect:
Static muscle length
Dynamic muscle length
Muscle tension
Muscle action
STRUCTURE OF GOLGI TENDON ORGAN
Golgi tendon organ  is  situated  in  the  tendon  of skeletal  muscle near the attachment of extrafusal fibers. It is placed in series between the muscle fibers and the  tendon. Golgi tendon organ is formed by a group of nerve endings covered by a connective tissue capsule.
NERVE SUPPLY TO GOLGI TENDON ORGAN
Sensory nerve fiber supplying the Golgi tendon organ belongs to Ib type. The nerve fiber supplying Golgi tendon organ ramifies into many branches. Each branch ends in the form of a knob. 
FUNCTIONS OF GOLGI TENDON ORGAN
Golgi tendon organ gives response to the change in the force or tension developed in the skeletal muscle during contraction. It is also the receptor for inverse stretch reflex and lengthening reaction and thereby prevents damage of muscle due to overstretching.