Reduced Neural Excitability and Activation Contribute to Clinically Meaningful Weakness in Older Adults

BACKGROUND: Weakness is a risk factor for physical limitations and death in older adults (OAs). We sought to determine whether OAs with clinically meaningful leg extensor weakness exhibit differences in voluntary inactivation (VIA) and measures of corticospinal excitability when compared to young adults (YAs) and OAs without clinically meaningful weakness. We also sought to estimate the relative contribution of indices of neural excitability and thigh lean mass in explaining the between-subject variability in OAs leg extensor strength. METHODS: In 66 OAs (75.1 +/- 7.0 years) and 20 YAs (22.0 +/- 1.9 years), we quantified leg extensor strength, thigh lean mass, VIA, and motor evoked potential (MEP) amplitude and silent period (SP) duration. OAs were classified into weakness groups based on previously established strength/body weight (BW) cut points (Weak, Modestly Weak, or Not Weak). RESULTS: The OAs had 63% less strength/BW when compared to YAs. Weak OAs exhibited higher levels of leg extensor VIA than Not Weak OAs (14.2 +/- 7.5% vs 6.1 +/- 7.5%). Weak OAs exhibited 24% longer SPs compared to Not Weak OAs, although this difference was insignificant (p = .06). The Weak OAs MEPs were half the amplitude of the Not Weak OAs. Regression analysis indicated that MEP amplitude, SP duration, and thigh lean mass explained  62% of the variance in strength, with the neural excitability variables explaining  33% of the variance and thigh lean mass explaining  29%. CONCLUSION: These findings suggest that neurotherapeutic interventions targeting excitability could be a viable approach to increase muscle strength in order to reduce the risk of physical impairments in late life.