Lately there's a proliferation of papers reproducing a result first found by the geriatrics researchers, that grip strength is strongly correlated with longevity and an excellent measure of overall health. Therefore there's also an army of science-trusting fitness experts admonishing us to train grip strength in order to live a long and healthy life.
And if you're doing this or thinking about it, don't let me stop you - there's a long list of great reasons to do it.
But why do you think they test grip strength in geriatrics? Imagine grandpa goes for his twice-weekly checkup, and doc goes "OK oldie lie down on that bench and let me throw on some plates and let's see whatcha got." Or worse, squats! No, grip strength is safe to test on old people, and easy to test in a clinical setting, and they're looking for a safe and easy test for overall physical strength.
The hypothesis they were looking to test was that muscle strength was important somehow to longevity and health and that strength training was as important if not more so than cardiovascular exercise. Geriatrics was the field to look into it first (and they're still mad everybody else hasn't really started listening), but even if that hadn't been the case it's hard to imagine your GP watching you bang out a few squats yelling "if the bar ain't bendin' you're just pretendin'!" in a bid to predict and/or extend your expration date.
It was only ever going to be grip strength, and the headlines tying that to longevity aren't exactly wrong, and they are technically scientifically accurate, but they are terribly misleading.