They found simple measures of physical capability like shaking hands, walking, getting up from a chair and balancing on one leg were related to life span, even after accounting for age, sex and body size.While the phrase "accounting for age, sex and body size" makes this process sound very objective and scientific, there are obviously a lot of other factors that can play a role in life expectancy. Personal differences, such as leading a more active lifestyle, could cause someone to have both more hand strength and also better health in general which contributes to their longevity.
While statistics saying "the death rate over the period of the studies for people with weak handshakes was 67 percent higher than for people with a firm grip" sound very dramatic, it's hard to say if that relationship is reverse-causal; in other words, having a weak grip may signal your lifespan will be short, but will improving your grip really make you live longer? Probably not, which suggests it's far more likely that a common variable - for example, sitting on the coach all day - causes both weak hands and a lower life expectancy.
Common sense says that working with a stress ball or doing forearm exercises to develop a crushing handshake probably won't substantially reduce your chance of death from heart disease, cancer, stroke, or the other leading causes of death for adult Americans. However, this is exactly the impression given by the Reuters article title "Want to live longer? Get a grip!" Heavens forbid someone took this seriously and developed gorilla-like forearms only to find out their fitness investment had been in vain.