By day-to-day activities, I mean, for example, using extra-force to open a resistant canned tuna, pushing furniture once in a while.

Is there a correlation between using strength on your muscles with no weights and lifting weights?
Because I could not lift but the force that I am exerting may be equal to the force used, for example, to lift 10kg.

What is the correct interpretation of "do not lift more than 5kg or 11 lbs" ?
Some people may misinterpret it. For example, doing isometrics exercises may produce enough force to lift more than 11 lbs, although you are not lifting. Am I correct?