OBJECTIVE: To determine whether exercise-induced endogenous opioid release can play a role in the analgesic and treatment outcomes for patients with LBP and to determine what level of exercise might be beneficial in the suppression of MNA and possibly LBP.
METHODS: To test this hypothesis, male (n = 3) and female (n = 3) healthy volunteers were tested 6 times over a 4-week period. The 6 trials included high-intensity treadmill exercise at 75% O(2max) with placebo or naloxone, low-intensity exercise at 40% O(2max) (placebo or naloxone) and no exercise control (placebo or naloxone). The evoked spinal Hoffmann H-reflex (soleus muscle) was measured as the criterion for MNA before and after exercise and expressed with the maximal M-wave as the maximal H(max)/M(max) percent ratio. Naloxone (10 mg) or isovolumic saline solution was administered double-blind (1 mL bolus) after recovery from exercise and before H-reflex measurement.
RESULTS: The results show a significant reduction in the H(max)/M(max) percent ratio for both exercise conditions (40.0 +/- 7.1 to 33.9 +/- 9.1% for 75% O(2max) and 37.4 +/- 4.8 to 33.0 +/- 5.3% for 40% O(2max); P <.01). Naloxone treatment did not attenuate the exercise-induced H(max)/M(max) percent ratio suppression.
CONCLUSION: Endogenous opioids do not appear to modulate motoneuron responses to exercise under these experimental conditions.
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