Abstract
Background: The reticulospinal tract (RST) is essential for balance, posture, and strength, all functions which falter with age. We hypothesized that age-related strength reductions might relate to differential changes in corticospinal and reticulospinal connectivity.Methods: We divided 83 participants (age 20–84) into age groups <50 (n = 29) and ≥50 (n = 54) years; five of which had probable sarcopenia. Transcranial Magnetic Stimulation (TMS) was applied to the left cortex, inducing motor evoked potentials (MEPs) in the biceps muscles bilaterally. Contralateral (right, cMEPs) and ipsilateral (left, iMEPs) MEPs are carried by mainly corticospinal and reticulospinal pathways respectively; the iMEP/cMEP amplitude ratio (ICAR) therefore measured the relative importance of the two descending tracts. Grip strength was measured with a dynamometer and normalized for age and sex.Results: We found valid iMEPs in 74 individuals (n = 44 aged ≥50, n = 29 < 50). Younger adults had a significant negative correlation between normalized grip strength and ICAR (r = −0.37, p = 0.045); surprisingly, in older adults, the correlation was also significant, but positive (r = 0.43, p = 0.0037).Discussion: Older individuals who maintain or strengthen their RST are stronger than their peers. We speculate that reduced RST connectivity could predict those at risk of age-related muscle weakness; interventions that reinforce the RST could be a candidate for treatment or prevention of sarcopenia.
Highlights
Muscle mass has previously been the primary factor in defining sarcopenia; with aging, there is a disproportionate loss of muscle strength compared to mass (Metter et al, 1999)
We examined this subgroup closer to check whether Ipsilateral Motor Evoked Potentials (iMEPs) non-responders differed from responders, but the two subgroups were of comparable age, gender, handedness and grip strength, with no statistically significant differences (Table 1B)
We previously showed that strength training led to an increase in cortical excitability and elevated contralateral MEPs (cMEPs) (Glover and Baker, 2020)
Summary
The reticulospinal tract (RST) is essential for balance, posture, and strength, all functions which falter with age. We hypothesized that age-related strength reductions might relate to differential changes in corticospinal and reticulospinal connectivity. Transcranial Magnetic Stimulation (TMS) was applied to the left cortex, inducing motor evoked potentials (MEPs) in the biceps muscles bilaterally. Contralateral (right, cMEPs) and ipsilateral (left, iMEPs) MEPs are carried by mainly corticospinal and reticulospinal pathways respectively; the iMEP/cMEP amplitude ratio (ICAR) measured the relative importance of the two descending tracts. Grip strength was measured with a dynamometer and normalized for age and sex. Younger adults had a significant negative correlation between normalized grip strength and ICAR (r = −0.37, p = 0.045); surprisingly, in older adults, the correlation was significant, but positive (r = 0.43, p = 0.0037)
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