The Press-Up Position with Arm Extensions is a great exercise to work through the arms, shoulders and back muscles while holding the position and work across your core to maintain balance and stability whilst raising one arm at a time.
Set yourself up in a press-up position with your feet slightly wider than usual to give you a better base for balance. In this position we are looking to see a straight line down from the top of your head down your spine and to a place equidistant between your feet.
From here, you should lift one arm and extend it forward. Whilst doing this, your body will naturally rock, try to minimise this rocking by engaging into the core, keeping your shoulders level. Place this hand back down and repeat with the other arm.
To increase the challenge of this exercise you can bring your feet closer together. This will give you a smaller base causing you to engage more into the core to remain stable.