Quantitative estimation of active motor-unit subpopulations in contracting muscles of children and adults, corroborated by independent findings

The study proposes a quantitative i.e., motor-unit subpopulation identification approach with contextual validation against compatible, independently-derived findings. Twenty men and 20 boys performed ramped isometric knee extensions to 70%MVC. Vastus-lateralis surface EMG signals were decomposed (Delsys’ Neuromap) and derived identification-ordered MU properties were converted to size-ordered sequences. Residual-sum-of-squares analysis showed segmentation as superior to continuous polynomial regression in describing the MU data. This formed the basis for differentiating 2‒3 MU-subpopulation via segmental slope changes, defined as boundaries and quantified by their relative locations along the MU-yield sequence (%MUY). A first/single boundary, detected in all participants, was 25.6% higher in the boys than in the men (70.85 vs. 56.42%MUY, respectively; p = 0.0005). A second boundary was identified in 19/20 men but only 12/20 boys (80.98 vs. 87.10%MUY, respectively; p = 0.022). The boundary-demarcated MU-subpopulations closely resembled previously-observed child‒adult differences in electromyographic and other thresholds, as well as known inter-relationships among the three classical muscle-fibre/MU types (I, II A , II X/B ). A definite identification of the MU-subpopulations as those MU/fibre types could not be confirmed or refuted by the available data. The findings’ congruence with independent threshold data, however, constitutes contextual validation of the proposed method. Elucidating the nature of the identified subpopulations warrants further research.