(1) This study is correlational, not causal. (2) The summary of the study says that muscle power quartile is correlated with six-year mortality for the members

(1) This study is correlational, not causal. (2) The summary of the study says that muscle power quartile is correlated with six-year mortality for the members of the study. But it doesn't say whether muscle power is any better than other measures like muscle strength. I.e., muscle power isn't necessarily a better predictor; it's just another predictor. Reading the study rather than the summary might be more helpful.