Motor Unit Action Potential Sizes of the First Dorsal Interosseous in Young and Older Individuals

Abstract

Slopes from the motor unit action potential size (MUAPsize) vs. recruitment threshold (RT) relationships provides a non-invasive measurement of MU sizes. As such, the slopes (APslopes) may be able to quantify age-related MU atrophy. PURPOSE: To determine if APslopes differed between young and old individuals. METHODS: Twenty two young (YG, age = 22.5±2.7 yrs) and ten aged (OG, 61.0±2.0 yrs) subjects completed the investigation. Surface electromyography signals were recorded from the first dorsal interosseous (FDI) during an isometric trapezoidal muscle action with the steady force plateau set at 50% maximal voluntary contraction (MVC). The signals were decomposed to yield MUAPsizes and RTs for each MU. The APslope was calculated via the MUAPsizes vs. RT relationships for MUs recruited between 10 and 50% MVC for each subject. Few MUs with RTs < 10% MVC were observed in young or old and, thus, these MUs were excluded from the slope calculation. FDI cross-sectional area (CSA) and echo intensity (EI) were quantified using ultrasonography. Possible differences in APslope, CSA and EI between groups were examined with independent samples t-tests. RESULTS: OG demonstrated significantly reduced APslopes (OG: 0.033 ± 0.010; YG: 0.048 ± 0.020; p = 0.048) and greater EI (OG: 41.3 ± 7.0 AU; YG: 50.6 ± 7.5 AU; p = 0.002), however, CSA was similar between YG and OG (OG: 2.22 ± 0.47 cm2; YG: 2.09 ± 0.31 cm2; p = 0.438). CONCLUSION: The APslopes suggested non-uniform differences in MUAPsizes in relation to RT, likely due to reduced sizes of higher threshold MUs. Higher threshold MUs have been suggested to contain a greater proportion of type II muscle fibers, which demonstrate greater age induced atrophy than type I fibers commonly associated with lower threshold MUs. As such, the difference in APslopes suggested atrophy of higher threshold MUs in OG. In addition, OG’s elevated EI indicates greater infiltration of adipose and connective tissue into the muscle. Greater EI in combination with similar CSA may indicate a reduced contractile tissue volume in the OG, supporting the speculation that the APslope differences were due to MU atrophy.