Does Increased Neural Tension During Hamstring Stretching Affect Stretch-Induced Strength Loss?

PURPOSE: The purpose of this study was to examine the effect of neural tension during hamstring stretching on stretch-induced strength loss. METHODS: 11 healthy subjects (10 men, 1 woman; age 34±12 yr) performed maximal isometric hamstring contractions at 100°, 80°, 60° and 40° knee flexion pre and post 6x1 min hamstring stretches performed in either a spinal neutral position or a neural tension position (slump test position). One leg was stretched in the neutral position and the other in the neural tension position. Surface hamstring EMG activity was recorded during all contractions and stretches. Strength data were fitted to 2nd order polynomials to provide a representation of the length-tension relationship. Effect of stretch technique (neutral vs. neural tension) on passive resistance to stretch, strength-induced strength loss, and EMG activity was assessed with repeated measures anova. RESULTS: Passive resistance to stretch was reduced by 19% after stretching (P=0.001; no difference between neutral or neural tension stretches P=0.41). Stretch-induced strength loss was greater (P=0.043) after neural tension stretches (13%, P<0.01) versus neutral stretches (5%, P=0.17). There was a rightward shift in the length-tension (angle-torque) curve after neutral stretches (15% strength loss on ascending limb, 10% strength gain on descending limb, P<0.001). This effect was not apparent after neural tension stretches (P=0.43; 21% strength loss on ascending limb, 9% loss on descending limb). Stretching did not affect EMG activity during isometric contractions (<2% decline P=0.58; no difference between neutral and neural tension, P=0.86). CONCLUSIONS: The addition of neural tension to hamstring stretching increased stretch-induced strength loss but this effect was not associated with observable neural inhibition. Strength loss after neutral stretching was due to a shift in the length-tension curve. The rightward shift in the length-tension curve after neutral stretches indicates greater muscle fiber shortening during contractions due to increased tendon-aponeurosis elongation. The absence of this effect after neural tension stretches indicates that the tendon-aponeurosis was not effectively stretched due to parallel neural tissues bearing tension.