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Muscle spindles are specialized sensory receptors located within the belly of muscles. Their primary function is to detect changes in muscle length (i.e., stretch) as well as the rate of that change—essentially, how quickly the muscle is being stretched. This ability to sense both the magnitude of stretch and the speed at which that stretch occurs is crucial for the body's proprioceptive feedback system, which helps in maintaining posture, coordination, and ensuring balance during movement.

The information collected by muscle spindles is transmitted to the central nervous system, which uses it to facilitate reflexes, adjust muscle contraction, and modulate movement patterns. This feedback is vital for athletes and physical activities that require precision and agility. In summary, muscle spindles play a critical role in understanding and responding to dynamic changes in muscle length and velocity to ensure effective movement and stability.