If the man on the street had to guess the best metric for healthy aging, they might say blood pressure, cholesterol levels, lean muscle mass, blood triglycerides, or even telomere length if they follow aging science closely.

But it turns out that one of the best is grip strength. This relatively underdeveloped area of conditioning is increasingly recognized as one of the most reliable markers of human health, according to Joshua Davis at the University of Derby, UK.

Grip strength is associated with positive health outcomes, whether measuring for diabetes risk or depression, and you don't need any expensive equipment to measure it. Davis recommends you simply pop the top off a can of tennis balls.