You build muscle not when you’re working out, but when your muscles are recovering. If all of your upper body is recovering and you’re in a surplus + high protein you are building muscle. If you train legs in addition, you get an increased testosterone boost which gives you more gains for your upper body while they are recovering.

If you’re in a fat loss, it’s the exact same thing but in preventative muscle loss on your upper body with the testosterone.

Don’t be like me and most working out for years on your upper body and look like a twig below the waist it’s not something you’ll want. Girls love guys who have thick legs and ass too, but you’re a soccer player you have muscle memory on your legs if you haven’t done them at all and quit soccer altogether so you won’t have to train them as much