What types of interfusal fibers respond the best to a sustained stretch?

The types of intrafusal fibers that respond best to a sustained stretch are the static nuclear bag fibers and the chain fibers. These fibers are sensitive to changes in muscle length and are involved in proprioception. They help maintain muscle tone and provide feedback to the central nervous system about the degree of stretch in the muscle. Dynamic nuclear bag fibers primarily respond to rapid changes in muscle length rather than sustained stretch.