or this week’s sessions, you’ll need a timer. You can use your phone, or an app such as Gymboss. Set it up with two intervals: the first for 40 seconds and the second for 10 seconds. Give 100 per cent effort during the working (40 second) interval and use the 10-second interval to switch exercises. Your aim is to increase your heart rate and keep it high for 15 minutes. Before you start, run up the stairs
