Train larger muscle groups first, then smaller ones.
If you want to get the most out of your workouts you need to structure them accordingly. This means beginning with larger bodyparts and working your way down to smaller ones. The reason for doing this is two fold.
For one, larger muscle groups require more energy so doing them first when you’re freshest allows you to give them the effort they require. Secondly, the smaller muscle groups of the upper body often play a role when training the larger ones.
For example, let’s say you’re training your chest. Any pressing or dipping movement will call your triceps into play. Therefore you’ll want to train chest before training triceps. Were you to train triceps first they would likely fail during your chest workout before your larger pectoral muscles ever do therefore not allowing your chest to be worked thoroughly.
You should even consider the role smaller muscle groups play when training larger ones when you structure your week. Just as you shouldn’t train biceps before back, you shouldn’t schedule biceps to be trained the day before back either, assuming you train them on separate days.
Remember, if you want big results from your training, go big first.