In PNF stretching, what is alternately contracted and relaxed?

In Proprioceptive Neuromuscular Facilitation (PNF) stretching, the primary focus is on the muscles being stretched. This technique involves a combination of alternating contractions and relaxations of the target muscles to enhance flexibility and range of motion.

During a typical PNF stretching session, a muscle is first contracted isometrically against resistance, and then it is relaxed, allowing for a deeper stretch. This process takes advantage of the body's neuromuscular reflexes. When the muscle is contracted, it activates the Golgi tendon organs, which then send signals to the nervous system to facilitate relaxation when stretching is attempted afterward. As a result, the muscle can be stretched further than it could be through static stretching alone, leading to greater gains in flexibility.

In contrast, the other options do not correctly align with the mechanics of PNF stretching. The joints, ligaments, and tendons play supportive roles in mobility and stability but do not engage in the same pattern of contraction and relaxation that the targeted muscles do during this stretching method.