Changes in alpha-motoneuron excitability with positions that tension neural tissue

The slump test assesses the contribution of neural tissue to the referred symptoms associated with spinal pain and musculo-skeletal injuries of the lower limb. The limitation to full range of movement in performing this test has, in the past, been attributed to a mechanical restriction in mobility of neural tissue. Recent literature suggests that the limitation may be caused by protective reflex muscle action. The purpose of this study was to establish whether the slump test was associated with an increase or a decrease in excitability of alpha-motoneurons and, therefore, an alteration in muscle activity at the end of the range of movement of the test. Forty-three normal subjects and eight subjects with abnormal neural tension participated in this study. Changes in alpha-motoneuron excitability in neck flexion, moderate slump, and maximum slump positions were assessed by observing changes in H-reflex recruitment curves. Linear regression analysis on the rising portion of the H-reflex recruitment curve enabled calculation of the dependent variable Hslp for statistical analysis. Normal subjects in the moderate and maximum slump positions demonstrated a significant decrease (p < 0.05) in the slope of the H-reflex recruitment curve. Subjects with abnormal neural tension showed a non-significant increase in slope when in these positions. Subject flexibility had a significant influence on motoneuron excitability in the moderate neural tension position with inflexible subjects demonstrating a significant inhibition of motoneurons. The difference between the flexible or moderately flexible subjects and inflexible subjects was not significant in the maximum neural tension position. These findings have important implications for the rationale for treatment selection and success of treatment outcomes in the clinical setting.