Association of Muscle Strength With All-Cause Mortality in the Oldest Old: Prospective Cohort Study From 28 Countries

Background: Ageing is associated with a gradual loss of muscle strength, which in the end may have consequences for survival. Whether muscle strength and mortality risk associate in a gradual or threshold-specific manner remains unclear. This study investigates the prospective association of muscle strength with all-cause mortality in the oldest old. Methods; We included 1890 adults aged >= 90 years (61.6% women, mean age 91.0 +/- 1.5 years) from 27 European countries and Israel participating in the Survey of Health, Ageing and Retirement in Europe (SHARE) study. Muscle strength was assessed using handgrip dynamometry (unit: kilogram). Using time-varying Cox regression with restricted cubic splines, we determined the prospective association of muscle strength with mortality, controlling for age, sex, smoking, BMI, marital status, education, geographical region and self-perceived health. Results: Over a mean follow-up of 4.2 +/- 2.4 years, more than half of the participants died (n = 971, 51.4%). The mean handgrip strength was 20.4 +/- 8.0 kg for all participants, with men (26.7 +/- 7.5 kg) showing significantly higher strength than women (16.4 +/- 5.4 kg) (p < 0.001). Using the median level of muscle strength as reference (18 kg), lower and higher levels were associated in a gradual and curvilinear fashion with higher and lower mortality risk, respectively. The 10th percentile of muscle strength (10 kg) showed a hazard ratio (HR) of 1.27 (95% CI 1.13-1.43, p < 0.001). The 90th percentile (31 kg) showed an HR of 0.69 (95% CI 0.58-0.82, p < 0.001). Stratified for sex, the median levels of muscle strength were 26 kg for men and 16 kg for women. The 10th percentile of muscle strength showed HRs of 1.33 (95% CI 1.10-1.61, p < 0.001) at 15 kg for men and 1.19 (95% CI 1.05-1.35, p < 0.01) at 10 kg for women. The 90th percentile of muscle strength showed HRs of 0.75 (95% CI 0.59-0.95, p < 0.01) at 35 kg for men and 0.75 (95% CI 0.62-0.90, p < 0.001) at 23 kg for women. Sensitivity analyses, which excluded individuals who died within the first 2 years of follow-up, confirmed the main findings. Conclusion: Rather than a specific threshold, muscle strength is gradually and inversely associated with mortality risk in the oldest old. As muscle strength at all ages is highly adaptive to resistance training, these findings highlight the importance of improving muscle strength in both men and women among the oldest old.