system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why can I scoop and carry around my 120-130 lb sister with relative ease but can hardly pick up a 40 lb barbell in one arm?

As the title says: 

I can scoop and carry people (think man carrying wife into hotel room style) who are 120-140 pounds for up to 5 minutes or so, but when I go to a gym and try to lift a 30-40 pound barbell (in one arm) I can hardly do it for ten reps before I get knocked out. 

ELI5 please.
gpt
Because you're isolating the muscles used when you lift weights. You're using several muscles when you pick up a person, and you can shift your balance and adjust the way you're holding that person to take tension off of tired muscles and redistribute the load to other muscles.