Planks are designed to increase core muscular strength, endurance, and stability. The primary muscles engaged through planks are the rectus abdominis (abs), external obliques, and the erector spinae (lower back / spinal muscles). However, recent studies have shown that there are a few variations of planks with the instability that result in significantly greater electromyography in these muscles. Electromyography measures the raw muscle activation of a specific muscle or muscle group. Hence, these variations of planks increase muscle activation and will lead to a harder workout and greater increases in strength, stability, and endurance.
Regular plank: Assume a prone, plank position on an exercise mat with your elbows flexed to 90 degrees with only the forearms and toes in contact with the ground. Athletes should maintain a rigid torso, neutral head and spine, and extended leg position throughout the exercise.
Variations with instability: Athletes can place forearms on a swiss ball resulting in an inclined position, or place their feet on the swiss ball and forearms on the ground resulting in a decline position while planking. It can also be performed using a suspension band (TRX), with the handles 6-8 inches off the ground. The athlete can either place forearms or feet in the handles and perform a plank.
Note: Caution should be taken for those individuals with a history of weakness in the lumbar region due to the increases in erector spinae activation during instability planks.