Which of the following correctly describes the technique used in PNF (Proprioceptive Neuromuscular Facilitation)?

Passively moving a muscle through dynamic motion describes ballistic or dynamic stretching, which does not involve the voluntary contraction phase that defines PNF.

PNF (Proprioceptive Neuromuscular Facilitation) works by having the subject isometrically contract the target muscle against resistance, which activates the Golgi tendon organ and triggers autogenic inhibition, causing the muscle spindle stretch reflex to be temporarily suppressed. This relaxation phase allows the practitioner to move the limb further into range before the next stretch cycle, producing greater flexibility gains than static stretching alone.

Sequential contraction of extensors then flexors describes reciprocal inhibition training or agonist-antagonist supersets, which is a resistance training concept unrelated to PNF flexibility technique.

The required period of exercise to elicit adaptation describes the training principle of overload and progressive adaptation, which is a foundational exercise science concept unrelated to PNF mechanics.