Wave properties of action potentials from fast and slow motor units of rats

The purpose of this study was to identify the frequency, conduction velocity, and wavelength of fast and slow motor unit action potentials (MUAPs) from mixed mammalian muscle. Stimulation and blocking pulses to the sciatic nerve produced varying recruitment patterns (confirmed by force measurements) of fast and slow motor units of the medial gastrocnemius of six rats. Myoelectric signals from the muscle were resolved into their intensity in time and frequency space. Slow MUAPs had a mean frequency (+/- SEM) of 183 +/- 8 Hz, conduction velocity of 3.5 +/- 0.6 m s(-1), and wavelength of 19 mm. Fast MUAPs had a mean frequency of 369 +/- 11 Hz, conduction velocity of 6.7 +/- 0.5 m s(-1), and wavelength of 18 mm. Frequency and conduction velocity, but not wavelength, were significantly different between the fast and slow MUAPs. The distinct wave properties of fast and slow MUAPs can thus be used to distinguish action potentials from these motor units, and could be used to determine patterns of motor unit recruitment during locomotion.