Strength is hard to define.

I hate when people say that you can't measure intelligence because intelligence is ambiguous and hard to define.

"Strength" is also hard to define. Is endurance a component of strength? Do we care about all muscles equally when measuring strength? How strong do we label someone with strong arms but weak legs? How do we compare the strength of Michael Phelps and LeBron James?

That doesn't mean we can't say or measure anything informative about the differences in physical abilities between professional athletes and elderly grandmothers.

Suppose we had a concept like “SQ” (strength quotient) that was measured by various fitness activities, e.g., how far can you carry a 100-pound cube? People with a higher SQ would, on average, do better in different athletic events. There is no muscle that corresponds to strength. It’s not a thing you can point to. It’s the aggregate of hundreds of different muscles that, in principle, could be completely independent, yet for many reasons do correlate.

An SQ test would not tell you everything about a person's strength, but it would be helpful for making predictions. It would give you information. The measurement of intelligence is a lot like this.