Reciprocal Inhibition, Successive Inhibition, Autogenic Inhibition, or Stretch Perception Alteration: Why do PNF Stretches Work?

PURPOSE: Proprioceptive neuromuscular facilitation (PNF) stretches are widely used in athletics and rehabilitation. While it has been established that they are superior in range of motion (ROM) increases to the slow or static stretch, the mechanisms responsible remain an enigma. This study was conducted to determine if the neurophysiological mechanisms proposed by Knott and Voss (30), Kabat (28, 29) and Granit (18) are responsible for the success of the PNF stretches or whether the alteration of the stretch perception is a better explanation of ROM gains. METHODS: Eighteen subjects ages 17 to 44 performed 3 stretches: slow stretch and PNF stretches contract-relax (CR) and contract-relax agonist-contract (CRAC) in randomized order. The stretch intensity was controlled to a subjective “4” on a digital pain scale. The stretch force was measured and compared between the slow stretch and CR stretch. EMG data were collected from the semitendinosus and quadriceps muscles via surface and wire electrodes and analyzed for reciprocal inhibition, successive induction and autogenic inhibition. RESULTS: Reciprocal inhibition was not evident; the results indicated an elevated rather than an inhibited EMG during the antagonist contraction, possibly representing co-contraction. Similarly, autogenic inhibition was not evident, the expected inhibited and therefore lower EMG values were higher than baseline. We did confirm the presence of successive induction. Our study also showed that the success of PNF stretches most likely stems from an alteration of the stretch perception by possibly raising the pain threshold. We also established that there exists an order effect in the alteration of stretch perception. During the fourth CR stretch trial a significantly greater amount of stretching force was tolerated than during the first trial. CONCLUSION: Previous neurophysiological explanations for mechanisms of PNF stretching are found to be inadequate. The alteration of stretch perception seems to play a greater role in the success of PNF stretches. At least 4 trials of the CR stretch are recommended to get the greatest ROM gain.