Final answer:
The greatest output effect of Ia spindle afferents is on agonistic muscles, which are the main muscles responsible for movement during stretch reflexes as seen in the knee-je_rk reflex.
Step-by-step explanation:
The greatest output effect of Ia spindle afferents is exerted on agonistic muscles, also known as the prime movers. These afferents are sensory neurons that detect changes in muscle length and are part of the stretch reflex mechanism. When the muscle spindle is activated during a stretch, it results in the contraction of the stretched muscle (agonist) and inhibition of the motor neurons of the antagonistic muscles. This reflex helps to maintain constant muscle length. For example, during the patellar tendon reflex, also known as the knee-je_rk reflex, the Ia spindle afferents from the quadriceps muscle are activated, leading to the contraction of the quadriceps (agonist) and relaxation of the hamstrings (antagonist).