Final answer:
Type Ia afferent fibers are best at signaling muscle position after stretch due to their high sensitivity to dynamic changes, enabling quick feedback about changes in muscle length.
Step-by-step explanation:
The afferent muscle spindle nerve fibers that are better at signaling the new position of a muscle after it has stretched are the type Ia fibers. These fibers have a high sensitivity to the dynamic changes in muscle length, which helps them signal the new muscle position effectively. Muscle spindles are stretch receptors that detect the amount of stretch, or lengthening of muscles. The Ia fibers, also known as primary afferent fibers, are responsible for detecting changes in muscle length. These fibers have high sensitivity to the static position of the muscle, meaning they are better at signaling the new position of the muscle after it has stretched. The Ia fibers, being primary sensory receptors within the muscle spindle, are rapidly adapting and can therefore provide quick and accurate feedback to the central nervous system about muscle length changes. In contrast, type II fibers are better at detecting and signaling the static positions of a muscle, as they are less sensitive to changes in length but maintain a persistent response to a given muscle length, providing the body with information about the position of limbs.