They used electrical stimulation applied to the muscle to cause it to contract and measured the frequency of action potentials from the Golgi tendon organ to assess its response.
It is the name given to the spinal reflex response that occurs when the Golgi tendon organ receptor is activated, then producing an inhibitory response in the motor neurons that return to the same muscle and its synergists.
This article has attempted to correct some misperceptions about the function of the Golgi tendon organ (GTO):
Binder MD: Further evidence that the Golgi tendon organ monitors the activity of a discrete set of motor units within a muscle.
Schoultz TW, Swett JE: The fine structure of the Golgi tendon organ.
Eccles JC, Eccles RM, Lundberg A: Synaptic actions on motoneurons caused by impulses in Golgi tendon organ afferents.
Gregory JE, Brockett CL, Morgan L, et al: Effect of eccentric muscle contractions on Golgi tendon organ responses to passive and active tension in the cat.