For me, muscle weakness is not how strong I am, but how little endurance I have. I can still pick up and move a 40 pound box, like from floor across a room to a table. But, if I have to hold that box while I wait for the dog to get out of my way, or someone to clear a space on that table, my arms start screaming with the effort.

I can reach up into the cabinet to get my preferred cups (due to hand-tendon injuries, we got me special, tall and thin plastic cups.) But, if whoever put them away has stacked them together, or blocked them with other stacks of the regular cups, so that I have to keep my arms reaching up while I move and separate cups to get mine, my arms start screaming that they are too weak to sustain the effort. So, I bring them down, rest half a minute (because I am impatient when all I want is to simply get a stinkin' cup) then reach up and try again until I either get the cup, or have to drop them again and wait to try again.

When a doctor, or my physical therapists, does the standard 'strength tests' of push-pull-resist-raise-lower, I pass easily because they only test for a couple seconds each movement. If they went longer, I'd have to give up. And by the time I leave such an appointment, I am developing pains from overexertion, I have often strained muscles from the efforts, doing more than I am actually capable of.

So, that's some examples of what muscle weakness may be like.