Afferent fibers don't stimulate muscles; afferent fibers are sensory fibers that synapse on motor neurons, which are what stimulate the muscles.
"When the muscle experiences a stretch stimulus, sensory impulses are transmitted from the muscle spindle via Ia afferent fibers to the dorsal root of the spinal cord. Once in the dorsal horn of the gray matter of the spinal cord, the fiber synapses on the corresponding alpha motor neuron in the ventral horn of the spinal cord. This alpha efferent fiber then exits through the ventral root and courses back sends an action potential to the neuromuscular junction of the original muscle that initiated the reflex to the synapse to cause contraction."
https://pubmed.ncbi.nlm.nih.gov/31082072/#:~:text=This%20contraction%20allows%20the%20muscle,muscles%20for%20contraction%20or%20inhibition.
Never seen this tested and fairly sure it never will be, no change needed for now
