During movement, what mechanism primarily protects muscles from excessive contraction?

The correct choice highlights the role of Golgi tendon organs in protecting muscles from excessive contraction during movement. Golgi tendon organs are mechanoreceptors located at the junction of muscles and tendons. Their primary function is to sense the tension or force being applied to the muscle. When tension increases significantly, these organs become activated and send signals to the central nervous system to inhibit muscle contraction. This mechanism helps prevent muscle and tendon damage that could occur with excessive force, thus ensuring the safety and integrity of the musculoskeletal system.

In contrast, muscle spindle fibers primarily detect changes in the length of a muscle and its rate of change during stretching, which plays a critical role in the reflexive responses for maintaining posture and balance, rather than preventing excessive contraction. Intrafusal fibers are the fibers that compose the muscle spindles themselves and are integral to their function, but they are not the primary protective mechanism for excessive contraction. Mitochondrial activation involves energy production within muscle cells and does not relate to the mechanisms of muscle contraction regulation or protection against overcontraction.