Knee-In Each Leg
It's a pretty simple exercise to say but trust us, get down and try it and you will be struggling for air in no time. Knee-in each leg works both your shoulders and your abs at the same time and will get your heart racing in just 30 seconds.
Start in a push-up position with your arms right underneath you. Bring one knee up and in towards your opposite shoulder and then back down. Repeat on the other side and feel those abs and shoulders burning.