Age-related differences in the motor unit action potential size in relation to recruitment threshold

Motor unit action potential size (MUAP_size ) versus recruitment threshold (RT) relationship analysis provides a non-invasive measure of motor unit (MU) hypertrophy; however, this method's ability to identify MU atrophy is unknown. This investigation sought to determine if MUAP_size versus RT relationship slope (AP_slope ) comparison could identify evidence of MU atrophy in older individuals. Surface electromyography signals were recorded from the first dorsal interosseous (FDI) of fourteen young (YG, age = 22·29 ± 2·79 years) and ten older (OG, 61·0 ± 2·0 years) subjects during a 50% maximal voluntary contraction (MVC) isometric trapezoidal muscle action. The signals were decomposed to yield a MUAP_size and RT for each MU. For each subject, the MUs recruited between 10% and 50% MVC were linearly regressed as a function of RT to calculate an individual AP_slope . FDI cross-sectional area (CSA) and echo intensity (EI) were quantified via ultrasonography. The mean AP_slope was lower for OG (0·033 ± 0·010 mV %MVC^-1 ) than YG (0·056 ± 0·019 mV %MVC^-1 ). OG and YG possessed similar CSAs (OG: 2·09 ± 0·31 cm² ; YG: 2·08 ± 0·41 cm² ); however, OG (53·25 ± 7·56 AU) had greater EI than YG (43·87 ± 7·59 AU). The lower OG mean AP_slope was due to smaller MUAP_sizes of higher-threshold MUs, likely due to atrophy of muscle fibres that comprise those MUs. In support, similar CSA with greater EI indicated increased adipose and fibrous tissue and reduced contractile tissue in OG. Thus, MUAP_size versus RT relationship may provide a non-invasive measure of MU atrophy.