Non-stationarity and power spectral shifts in EMG activity reflect motor unit recruitment in rat diaphragm muscle

… In addition to DIAm EMG recordings, respiratory rate and blood O 2 levels were also
measured using a pulse oximeter (MouseOX, Starr Life Sciences Corp., Oakmont,
PA). A return to baseline respiratory rate and normoxic levels …
We hypothesized that a shift in diaphragm muscle (DIAm) EMG power spectral density (PSD) to higher frequencies reflects recruitment of more fatigable fast-twitch motor units and motor unit recruitment is reflected by EMG non-stationarity. DIAm EMG was recorded in anesthetized rats during eupnea, hypoxia-hypercapnia (10% O2–5% CO2), airway occlusion, and sneezing (maximal DIAm force). Although power in all frequency bands increased progressively across motor behaviors, PSD centroid frequency increased only during sneezing (p < 0.05). The non-stationary period at the onset of EMG activity ranged from ∼80 ms during airway occlusion to ∼150 ms during eupnea. Within the initial non-stationary period of EMG activity 80–95% of motor units were recruited during different motor behaviors. Motor units augmented their discharge frequencies progressively beyond the non-stationary period; yet, EMG signal became stationary. In conclusion, non-stationarity of DIAm EMG reflects the period of motor unit recruitment, while a shift in the PSD towards higher frequencies reflects recruitment of more fatigable fast-twitch motor units.