The GHD sit-up as well as the GHD hip extension and back extension (in this workout the back extension is really a hip extension performed with a rigid torso) all develop and demand the same thing: mid-line stabilisation.  This is the primary purpose of the core.  On the GHD, the athlete must recruit their hip flexors to perform the sit-ups, and must use their glutes and hamstrings to perform the back/hip extensions.  The athlete's core muscles act in their main role, maintaining the torso in a rigid state.

Workout

Hold a handstand for a long as you can (supported against a wall is acceptable). The duration of the handstand determines the number of rounds to be performed in the workout.

1-60 seconds: 5 rounds

61-120 seconds: 4 rounds

121+ seconds: 3 rounds

• 25 GHD sit-ups
• 25 GHD back extensions