4. Jackknife

Instructions: You’re well over halfway through now, but the hard work isn’t finished yet. Place a mat on the floor, lie down on your back and extend your arms behind your head. Simultaneously lift your arms and legs toward the ceiling, until your fingertips touch your toes (if you can). Slowly return to the starting position, safe in the knowledge that you’ll soon be more chiselled than a carpenter’s work bag. Do 10 reps.