Grip strength is measured by squeezing a handheld device as hard as possible. The reading is used far beyond hand function, because it acts as a proxy for the state of the body as a whole.
The measurement is unusually clean
Most physical assessments require equipment, space or trained supervision. A dynamometer is portable, cheap, and produces a number in seconds.
The action is also simple enough that technique contributes little variation, so results are comparable across settings and populations.
That practicality is why grip appears in large studies where more elaborate testing would be impossible, which in turn built the evidence base around it.
What the squeeze is summarising
Producing force requires muscle tissue, functioning motor nerves, adequate nutrition to maintain that tissue, and the capacity to recruit it fully.
Any process that erodes muscle mass or nerve function shows up in the reading, whether the cause is inactivity, chronic illness, poor nutrition or ageing itself.
The number is therefore a summary of several systems at once, which is precisely why it correlates with so many different outcomes.
Weakness is often the visible edge of something else
Age-related loss of muscle mass and function progresses quietly, and grip is one of the few places it becomes measurable early.
Low readings are associated with slower recovery from illness and surgery, because reserve tissue is what a body draws on during a period of poor intake and immobility.
The association describes populations rather than predicting individuals, and a single low measurement is a prompt for assessment rather than a diagnosis.
Reference values depend on the person
Grip strength varies with sex, height, hand size and age, so a raw figure means little without a comparison group.
Clinical thresholds are set against population norms for that group, and cut-offs differ between the standards used in different regions.
Change within one person over time carries more information than a single reading compared to a table.
The number responds to training
Grip is trainable, but improving the reading by squeezing alone does not change what it was standing in for.
Broad resistance training raises grip while also raising the underlying muscle mass the measurement reflects, which is the change that matters.
This is the general problem with proxy measures: improving the proxy directly is not the same as improving the thing it indicates.