Changes in soleus motoneuron pool excitability after artificial knee joint effusion

Objective: To compare changes in the magnitude of soleus motoneuron excitability before and over a 4-hour period following artificial knee effusion. Design: Before-after trial. Setting: All measurements were collected in the Sports Injury Research Laboratory, Indiana State University. Participants: Eleven healthy and neurologically sound volunteers (mean age +/- SD, 24 +/- 3yr; height, 173.2 +/- 9.6m; weight, 72.9 +/- 8.7kg) with no history of lower-extremity surgery and no lower extremity pathology in the last year. Interventions: An area superolateral to the patella was cleaned and injected subcutaneously with 2mL of lidocaine for anesthetic purposes. With a second disposable syringe, 25mL of sterile saline was injected through the superolateral knee joint capsule into the joint space to mimic mechanical joint effusion. Main Outcome Measure: Hoffmann's reflex (H-reflex) was elicited by applying a percutaneous stimulus to the tibial nerve in the popliteal fossa. Seven to 12 stimuli were delivered at 20-second intervals with varying intensities to find the maximal H-reflex. The maximal H-reflex was measured five times at the same stimulus intensity with 20-second rest intervals. This measurement was recorded before injection and at 1-hour intervals following the injection for 4 hours. Results: An overall difference between groups was found. Measurements from hours 3 and 4 were significantly higher than the preinjection measurements (p less than or equal to .05). Conclusions: The soleus motoneuron pool was not inhibited as expected. The soleus was facilitated beyond the preinjection level, showing that the quadriceps and soleus do not respond in the same way to artificial knee effusion. Because the quadriceps are normally inhibited during knee effusion, this facilitation could be the result of a compensatory reaction by the soleus in response to inhibited quadriceps. Further studies must be performed to determine the extent and duration of soleus motoneuron pool excitability in relation to quadriceps inhibition elicited by artificial knee effusion.