Effects of paraspinal fatigue on lower extremity motoneuron excitability in individuals with a history of low back pain

Individuals with low back pain (LBP) have been shown to demonstrate decreased quadriceps activation following lumbar paraspinal fatigue. The response of other lower extremity muscles is unknown. The purpose of this study was to determine changes in motoneuron pool excitability of the vastus medialis, fibularis longus, and soleus following lumbar paraspinal fatigue in individuals with and without a history of LBP. A controlled laboratory study was used to compare motoneuron pool excitability before and after a lumbar paraspinal fatiguing exercise. Twenty individuals (10 with history of LBP) performed isometric lumbar paraspinal exercise until a 25% shift in paraspinal muscle surface electromyography median frequency occurred. Motoneuron pool excitability, using H/M ratios of the vastus medialis, fibularis longus, and soleus before and immediately after a lumbar paraspinal fatiguing exercise. There was a significant (P = .03) increase of 15% in soleus H/M ratio in both groups following lumbar paraspinal fatigue. Neither group demonstrated changes in vastus medialis (P = .80) or fibularis longus (P = .86). Soleus motoneuron pool excitability increased following lumbar paraspinal fatigue independent of group allocation and occurred in the absence of changes in vastus medialis or fibularis longus muscles. Increased soleus motoneuron pool excitability may be a postural response to preserve lower extremity function.