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.