Relationship between firing rate and recruitment threshold of motoneurons in voluntary 1 isometric contractions

25 We employed a surface electromyographic (EMG) signal decomposition technology to study the 26 control properties of numerous simultaneously active motor units. Six healthy human subjects of 27 comparable age (21 ± 0.63 yr) and physical fitness were recruited to perform isometric 28 contractions of the vastus lateralis (VL), first dorsal interosseous (FDI), and tibialis anterior (TA) 29 muscles at the 20%, 50%, 80% and 100% maximum voluntary contraction force levels. EMG 30 signals were collected with a five pin surface array sensor that provided four channels of data. 31 They were decomposed into the constituent action potentials with a new decomposition 32 algorithm. The firings of a total of 1,273 motor unit action potential trains, 20 to 30 per 33 contraction, were obtained. The recruitment thresholds and mean firing rates of the motor units 34 were calculated and mathematical equations were derived. The results describe a hierarchical 35 inverse relationship between the recruitment thresholds and the firing rates, including the first 36 and second derivatives, i.e., the velocity and the acceleration of the firing rates. This relationship 37 describes an “operating point” for the motoneuron pool that remains consistent at all force levels 38 and is modulated by the excitation. This relationship differs only slightly between subjects and 39 more distinctly across muscles. These results support the “onion skin” property that suggests a 40 basic control scheme encoded in the physical properties of motoneurons which responds 41 consistently to a “common drive” to the motoneuron pool. 42