Motoneuron excitability and the F wave

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Abstract
For three motoneuron pools that differ in excitability to Ia inputs [tibialis anterior (TA), abductor pollicis brevis (APB), and soleus], F‐wave parameters were measured at rest, during voluntary contraction, and following prolonged vibration. There was an inverse relationship between F‐wave chronodispersion and F‐wave persistence at rest, and this appeared to be related to the ease of recording the H reflex for the motoneuron pool. During a steady voluntary contraction, overall F‐wave activity increased in amplitude but decreased in duration for TA and APB. Following vibration of the test muscle at 50 HZ for 10 min there was a long‐lasting depression of the H reflexes of TA and APB, but no significant change in F‐wave measurements. These findings are consistent with the view that reflex discharges can prevent F waves in low‐threshold motor units, and that chronodispersion is affected by the extent of reflex activity; that is, chronodispersion and related F‐wave measurements do not measure motor properties exclusively. The findings also suggest that F waves provide a flawed measure of the excitability of the motoneuron pool. Muscle Nerve 27: 720–727, 2003