You've been sitting at your desk all day, and your body's aching. The biggest mistake that people do at work? Bad posture, says fitness expert Sarah Robichaud.

When you're seated on a chair and not thinking about posture, your core is completely disengaged, according to Robichaud.

"We're not activating our stomach and back muscles. And because you're not moving for long periods of time, your hip flexors get really tight. That could also put strain on your back which end ups as back pain."

And if you have back pain and a weak core, you try to compensate by hunching over the computer. This could result in kyphosis, rounding of the back. As this develops, you end up putting a lot of pressure and strain on your neck and shoulder, resulting in pain in those areas.

The way you're sitting is causing your body injury.

Sitting too long creates imbalance and takes away from your productivity, says Robichaud.

Want to give your body relief? Move. Robichaud recommends two easy exercises you can do at your desk.

1.Stand up and sit down as if you're doing the wave at a baseball game. You're completely stretching your whole body outwards, opening up your chest.

"You'll also get your heart rate up a bit because I'll ask you to do it 15 times every hour. Every hour, stand up and sit down 15 times," says Robichaud.

2. Do a stretch with this spinal rotation to help activate core muscles. Place your hands outside of the opposite leg. Lift up through the spine, engage your stomach and carefully, gently, see what's behind you. Carefully, gently return to the other side. Repeat.

More: Get back in shape