Amplitude of M responses. Side to side comparability.

The peak amplitude of the M response from the extensor digitorum longus muscle (EDL) was measured in 26 healthy subjects (12 women and 14 men, aged 19 to 45 years) using conventional peroneal nerve stimulation at the fibular head. The mean amplitude of the EDL was 6.5 mV (+/- 1.3 mV) and 6.1 mV (+/- 1.2 mV) for the right and left sides, respectively. The side to side difference in amplitude of the M response was 0.4 mV (+/- 1.1 mV). Twenty-five of the twenty-six subjects had side to side amplitude differences within two standard deviations of the mean. None of the subjects had an EDL amplitude from the right lower limb over 1.8 mV less than that of the left, nor an EDL amplitude from the left lower limb over 2.8 mV less than that of the right. The technique used in the present study allows an accurate assessment of the M response amplitude from the EDL muscle. These reference data should prove useful in the evaluation of early focal nerve root injury (e.g. after 4 days, within 3 weeks) and in establishing prognosis for recovery in such conditions.