Which of the following is NOT a function of the muscle spindle?

The muscle spindle plays a vital role in sensing changes within the muscle. Its main functions include monitoring muscle length and detecting the rate of change in muscle length (or speed of muscle contraction). This information is crucial for the central nervous system to fine-tune muscle contraction and maintain balance and posture during various activities.

While muscle spindles are essential for proprioception and assisting in motor control, they do not directly measure muscle tension. Instead, muscle tension is primarily monitored by another type of sensory receptor known as the Golgi tendon organ, which specifically responds to the force exerted by muscle contractions. Hence, the function of calculating muscle tension directly does not fall under the purview of the muscle spindle, making it the correct choice for this question.