Mechanoreceptors in muscles and tendons detect where our limbs are in space and how much load is being placed on them. As load increases, muscle spindles (stretch receptors) send signals to the CNS to initiate muscle contraction. In this way, a person can hold a mug steady while it is being filled. In contrast, Golgi tendon organs inhibit a contraction that becomes too forceful, protecting the muscle from tearing.

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How does load affect the muscle spindles and Golgi tendon organs?

As you increase the load you stretch the muscle which increases the tension. Both the muscle spindles and Golgi tendon organs increase firing rates with heavier loads.