6) Add a force vector.
Adding a force vector will challenge your muscles to not only hold position, but to resist movement in another direction.
Try the Standing Anti-Rotation Plank.
This is basically a plank performed in the standing position. Stand perpendicular to a cable stack or resistance band. Stand with your feet hip width apart and the handle/band held against your stomach. Stand tall and brace your abs. Press the handle/band straight in front of you. Resist the torque created by the cable/band. Hold the pressed position for 5 seconds. Return the handle/band back to your stomach and repeat.
7) Add a dynamic surface.
Suspension training systems are awesome for increasing the challenge and providing variety to your core training. Dynamic stability training with suspension systems can provide joint stabilizers with a tremendous challenge, normally absent when training on a stable surface.
Try the Suspension System Mountain Climber.
Insert your feet into a suspension system such as a TRX or Jungle Gym roughly 8” from the ground. Go into a pushup plank and pull your toes up toward your hips. Bring one knee toward your stomach. Simultaneously alternate bringing your legs forward and away from your stomach. Keep a flat back during the exercise as it’s common to flex the spine and round the back as the knee drives toward the stomach.
8) Add some weight.
Just as you would progress a bench press by adding weight to the bar, try progressing your plank by adding weight to your torso.
Try the Weighted Front Plank.
Go into a basic front plank on your elbows and toes. Brace your abs and squeeze your glutes. Have a partner place a weight plate on your back/hips. Maintain your neutral position and resist the urge to sag. Maintain for the allotted time.