Rise time or rise rate: which should be a criterion for analysing motor unit action potentials?

We measured the rise time (RT) and rise rate (RR) of motor unit action potentials (MUAPs) when sharp sounds are heard in concentric EMG in the first dorsal interosseus muscle (FDI) and biceps brachii muscle (BIC) of normal subjects. MUAPs from FDI muscle with an RT of less than 500 microseconds were 85%, and those of more than 500 microseconds were 15%. In contrast, MUAPs from BIC muscle with an RT of less than 500 microseconds were 65%, and those of more than 500 microseconds were 35%. Distributions of the RR for FDI and BIC were also determined. MUAPs from FDI muscle with an RR more than 0.3 mV/ms were 98.3%, and those of less than 0.3 mV/ms were 1.7%. In contrast, MUAPs from BIC muscle with an RR of more than 0.3 mV/ms were 93%, and those of less than 0.3 mV/ms were 7%. We conclude it is better to use RR than RT when accepting MUAPs in clinical EMG, because even when sharp sounds are heard, MUAPs do not always have an RT of less than 500 microseconds. The use of RT and the sharpness of MUAPS therefore need to be reconsidered, or RR should be used in clinical EMG by automatic program.