Reverse crunch

Shaun Stafford, personal trainer
'The reverse crunch forces your abs and obliques to work really hard. Start on a flat bench and aim for 8-12 reps. Once you can do three sets of 12 with perfect form, increase the angle to increase the difficulty.'
Visit shaunstaffordtraining.com.

Weighted crunch

Rob Burr, Phoenix Fitness Group
'The abs respond better to being overloaded than they do to performing hundreds of sit-ups. Lie on a gym ball while holding a heavy weight plate on your chest and crunch for three sets of 8-12 reps.'
Visit phoenixfitnessgroup.com.

Woodchop

Neil Odell, Premier Training International
'The woodchop stimulates the entire abdominal region in a more functional way than crunches. Alternate between high and low, horizontal and reverse woodchops in sets of 6-12 reps.'
Visit premierglobal.co.uk.

Gym ball jackknife

Chris Mundle, Destination W1
'Start in the plank position with your back flat and a gym ball under your feet. Roll the ball towards you, tightening your abs into a crunch, and hold for five seconds. Do 15 reps. This is great because it works your shoulders and arms too.'
Visit destinationw1.com.