The authors of this study suggest that they are measuring the degree to which greater body mass is less harmful when it consists of muscle versus fat tissue - the measures often used in older studies, such as body mass index, are not very discriminating in this sense. It is fairly easy to suggest that the lifestyle choices required in order to have more muscle than your peers, such as greater levels of deliberate exercise, may play a role here:
This study was designed to test the hypothesis that greater muscle mass in older adults will be associated with lower all-cause mortality. All-cause mortality was analyzed by the year 2004 in 3,659 participants from the National Health and Nutrition Examination Survey III, who were 55 years (65 years if women) or older at the time of the survey (1988-94). Individuals who were underweight or died in the first 2 years of follow-up, were excluded so as to remove frail elders from the sample.
Skeletal muscle mass was measured using bioelectrical impedance and muscle mass index was defined as muscle mass divided by height squared. Modified Poisson regression and proportional hazards regression were used to examine the relationship of muscle mass index with all-cause mortality risk and rate respectively, adjusted for central obesity (waist hip ratio) and other significant covariates.
In adjusted analyses, total mortality was significantly lower in the fourth quartile of muscle mass index compared to the first: adjusted risk ratio 0.81 and adjusted hazard ratio 0.80. This study demonstrates the survival predication ability of relative muscle mass and highlights the need to look beyond total body mass in assessing the health of older adults.