Anormal, healthy man is attempting to lift a heavy load, and his muscles "give out." after this experience, there is no apparent problem in neuromuscular function. why would we expect the abrupt cessation of skeletal muscle contraction?

The answer is because of the activation of stretch receptors in the Golgi tendon organs.

Explanation:

Golgi tendon organs (GTOs), also called a neurotendinous organ, are receptors that are activated when the muscle in the body is stretched or there is active (with energy) contraction of a muscle. The receptor then relay information about the muscle strain/tension detected. When these receptors are activated the result is a reflex inhibition in the muscle. The GTO lies where the skeletal muscle fibres join into the tendons of the skeletal muscle. The importance of the stretch receptors is monitoring the condition of muscle in the body. The information is then relayed back to the central nervous system (CNS). The reason why his muscles "give out” is because the stretch receptors are sensitive to the speed of movements as well as the length the muscle is stretched to.

you need to provide the