TEST-RETEST RELIABILITY OF MOTOR UNIT FIRING BEHAVIOR DURING MAXIMAL GRIP STRENGTH ASSESSMENTS AMONG OLDER ADULTS

Recent evidence suggests that grip strength is a critical biomarker of brain, muscle, and metabolic aging. Presently, neuromuscular properties related to individual motor unit firing properties have not been explored in the context of maximal grip strength assessments in older adults. The purpose of this investigation was to examine the intra- and intersession reliability of linear and exponential regression model coefficients derived from motor unit (MU) peak firing rates (PFRs) and action potential amplitudes (MUAPs) during maximal grip tests among older adults. Thirty-four participants (mean +/- SD age = 73 +/- 6 years) visited the laboratory on two occasions following a familiarization visit. Two separate maximal grip tests were conducted on participant's dominant hand during the first visit, and participants revisited the laboratory on a separate day to complete another maximum grip strength test. During each grip test, electromyographical (EMG) signals were recorded from the flexor digitorum superficialis (FDS) muscle and decomposed into individual MUAP trains. Test-retest reliability for linear and exponential regression variables derived from MU PFRs and MUAPs was assessed using intraclass correlation coefficients (ICC[2,1]) and paired samples t-test, and validity was determined via calculation of the standard error of the estimate. Y-intercepts (ICC = 0.827) and slopes (ICC = 0.952) from linear models and the decay factor (ICC = 0.866) from the exponential models displayed moderate-to-excellent and good-to-excellent intrasession reliability, respectfully, and poor-to-good intersession reliability (ICCs = 0.646, 0.725, and 0.598, respectively). Paired samples t-tests indicated no group differences in all intra-and inter-session analyses (p > 0.073). Despite historical use of exponential modelling to report MUAP-functioned MU FRs, test-retest reliability of linear regression variables were observed to be more reliable and may be a viable analytic approach to characterizing peak MU firing properties between participants and intervention groups when these methodological procedures are adopted.