Final answer:
Ia spindle afferents exert excitatory effects on agonistic muscles, which are the prime movers during muscle contraction, as part of a stretch reflex to maintain muscle length.
Step-by-step explanation:
Ia spindle afferents exert excitatory effects on agonistic muscles (a.k.a. the prime movers) and synergistic muscles, which help in the motion initiated by the agonists. These afferents are part of a stretch reflex mechanism, where the muscle spindle receptors get activated when a muscle is stretched, leading to direct contraction of the stretched (agonistic) muscle and inhibition of the motor neurons of the antagonistic muscles. This action helps maintain the muscle at a constant length. Therefore, Ia spindle afferents primarily have an excitatory effect on agonistic muscles.