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Age‐related differences in the motor unit action potential size in relation to recruitment threshold
Author(s) -
Sterczala Adam J.,
Herda Trent J.,
Miller Jonathan D.,
Ciccone Anthony B.,
Trevino Michael A.
Publication year - 2018
Publication title -
clinical physiology and functional imaging
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.608
H-Index - 67
eISSN - 1475-097X
pISSN - 1475-0961
DOI - 10.1111/cpf.12453
Subject(s) - isometric exercise , medicine , motor unit , atrophy , muscle hypertrophy , dorsum , electromyography , ultrasonography , anatomy , cardiology , nuclear medicine , surgery , physical medicine and rehabilitation
Summary 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 cm 2 ; YG: 2·08 ± 0·41 cm 2 ); 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.