You can do exercises sitting down.
You can do exercises standing up.
If overall functional ability is a goal and you have a choice, stand up. Here’s why.
When you exercise sitting or lying down, you engage fewer muscles. Think about it. You’ve got a machine or bench to brace yourself against. For example, do a standard bench press and your body is braced by the bench. The primary muscles of the chest do the work, but the rest of your body isn’t engaged.
Stand up and do a chest press using a cable machine. Your lower body has to engage to keep you standing. Your core has to engage to keep you upright. The stabilizing muscles of the body are firing as you’re performing the exercise and managing the independent movement of each cable and the constant tension of the cable machine.
Engage more muscle and you use more calories. As an added bonus, you’re simulating normal daily activity where the body moves as a system rather than isolating a specific muscle group.
Go ahead, stand up! Here’s a video to get you started.