Starting Position: Place your feet securely into the foot cradles positioned directly under the anchor point. Apply downward pressure with the tops of your feet by pointing your toes. Lie flat on your stomach with your hands placed shoulder width apart, under your shoulders and fingers facing forward. Brace your torso by engaging your core/abdominal muscles. Keep the glutes (butt) and quadriceps (thigh) muscles engaged and strong.
Upward Phase: Exhale and slowly press your body off the floor until your elbows are straight (plank position). Keep your head and spine aligned. Do not allow the low back or ribcage to sag or arch. Do not allow the hips to hike upward. Keep the glutes and quadriceps contracted to maintain hip stability. Your feet should be together throughout the exercise and avoid any back and forth swinging of the TRX straps. Slowly pull your knees toward your chest, bending your knees as they tuck under your hips keeping your feet together and toes pointed. During this movement, your hips will rise upwards and your spine will round into a ball. Continue pulling your knees inwards until they are directly under your hips.
Downward Phase: Inhale and slowly lower your body back to a plank position. Keep the torso rigid, and head and spine aligned; do not allow the low back or ribcage to sag.
Exercise Variation: The intensity of this exercise can be increased by positioning your body further away from the anchor point of the TRX, lengthening the straps, and performing a pike up rather than a knee tuck.
Note: Using the TRX certainly enhances the appeal of many exercises; however, trainers and individuals should not participate in these advanced exercises until they can demonstrate capability to effectively stabilize their lumbar spine (low back) with their core muscles.