Thermoregulatory activity of motor units during human development

The presented investigation was designed to elucidate whether age impacts the basic neurophysiological characteristics of motor unit (MU) thermoregulatory activity in human subjects. Human newborns and infants (aged from 7 days to 3 years, n = 81) were exposed to ambient temperatures of 26-27 degrees C for 5-7 minutes. The adults and elderly (18-76 years old, n = 90) were exposed to 10 degrees C for 30-60 minutes. MU action potentials were followed by surface electromyography in the brachial triceps muscles, bilaterally. The firing rate of MUs (n = 245) during thermoregulatory muscle tonus was 7-35 Hz. Thirty six per cent of MUs (periodical MUs) of the newborns were firing by short trains of high-frequency (20-35 Hz) discharges, with uniform interspike intervals. The other part of MUs in the newborns was stationary firing at frequencies from 12 to 20 Hz (stable MUs). No periodical MUs were found in infants and adults and elderly humans. MU firing rate at 18-60 years of age was 8-9 Hz, and was statistically significantly lower than in the ages over 60 years (9-11 Hz). These data allowed us to compute a parabolic dependence of MU firing rate on the age during thermoregulatory tonus: Fmu = 12.60-0.22A + 0.03A2 (r = 0.91; Fmu-MU mean firing rate, Hz; A-age, years).