What does a muscle spindle primarily sense?

A muscle spindle primarily senses the length of the muscle and its rate of change. By providing information about muscle length and the velocity at which this length changes, muscle spindles play a crucial role in proprioception—helping the body understand its position in space and ensuring coordinated movement.

While stretch velocity pertains to the rate at which the muscle is being stretched, muscle spindles are more specifically involved in detecting both static length and dynamic changes. This distinction highlights that while muscle spindles do respond to the speed of stretch, their primary function revolves around the accurate sensing of muscle length itself.

In addition, muscle spindles contribute to reflex actions, such as the stretch reflex, by sending signals to the spinal cord that can quickly initiate a response to prevent injury or maintain posture. Their function supports both immediate motor responses and longer-term adaptations in muscle control.

Understanding that muscle spindles are key sensors for both length and velocity helps clarify their essential roles in movement and stability, emphasizing their importance in neuromuscular control.