Background and Purpose: The clinical electromyographer routinely performs an analysis of motor unit potentials (MUPs), consisting of isolating a waveform, recording phases and the peak-to-peak height of the responseor amplitude. Selecting a muscle that will assist in determining whether a chronic condition is present in an anatomical distribution requires analysis of established criteria. Sampling of the first dorsal intereosseous muscle of the foot (FDI) as a distal muscle to survey in chronic neuropathy involvement, has not been address in the literature. The purpose of this study is to determine (1) if there are any age related changes in MUP amplitudes of the FDI of the foot that can be used for clinical analysis, and (2) to determine if there is an appropriate sampling methodology for the FDI muscle of the foot. Methods: The research design was descriptive and normative, reporting data from each foot sample of 90 asymptomatic volunteer subjects, ranging in age from 18 to 84 years old. MUP peak amplitudes were recorded from the FDI muscles of subjects with a monopolar needle electrode and analyzed using manual measurement from the positive to the negative peak. The sampleing method was standardized. Two trials of 6 MUPs were recorded and compared on the same foot. Results: One hundred seventy-five peak MUP amplitude values were recorded from subjects ranging in age from 18 to 84 years old. Five subjects were unable to produce a FDI muscle contraction with one of their feet. The peak amplitude of 175 feet obtained from subjects 18 to 84 years old ranged from 0.8 to 25 mV. The upper limit calculated as the mean plus 2 S.D.=17.7 mV. The peak amplitude of 139 feet obtained from subjects less than 60 years of age ranged from 0.8 to 25 mV. The peak amplitude of 36 feet obtained from subjects greater than 60 years of age ranged from 0.2 to 19.4 mV. Linear regression analysis was performed to assess the ability of age to predict the largest MUP amplitude. No significant relationship was found between age and amplitude. Differences between the mean amplitudes of the first trial of 6 MUPs sampled in comparison to the second trial of 6 MUPs indicated no statistical difference between the two groups:t. The peak MUP amplitude value occurred 58% of the time in the first trial. Discussion and Conclusion: This study found no relationship between age and the ability to predict peak amplitude values. Caution with interpretation of FDI peak amplitude values is recommended due to widespread variability across age groups. Secondly, there is no statistical difference between the mean of the first and second trials of 6 MUPs. The peak MUP occurred in the first set of sampling 58% of the time. Therefore, small muscles such as the FDI may require less motor unit sampling than that recommended in the literature. Reference values for the mean peak MUP amplitude of the FDI are 9.3 mV � 4.2; upper limit 17.7 mV.
