H reflex and F wave latencies to soleus normal values and side-to-side differences

Electromyographers must reliably differentiate between H reflexes and F waves when recording from the soleus muscle in the evaluation of S1 radiculopathy. The use of F waves in root-level injuries is questioned, whereas H reflexes have shown value in the evaluation of S1 radiculopathy. We studied the relationship between the tibial H reflex and F wave latencies in the limbs of 40 subjects.

After recording the H wave latency, we changed the gain to 200 microV/cm and increased the stimulation to supramaximal for ten additional responses without moving the recording or stimulating electrodes. We also calculated the predicted H wave latency with the standard formula. Forty subjects, mean age 32 yr, consented and participated.

The mean of the average F wave was 1.76 ms longer than the ipsilateral H reflex latency. The mean side-to-side difference of the average F wave was 0.56 ms. The H reflex latency side-to-side difference was 0.36 ms.

The findings suggest that the average F wave latencies have a predictive value in the clinical context similar to the H reflex.