Jack Knife (Suspension)

A suspension-based core exercise performed in a plank with feet in TRX straps. Drawing the knees toward the chest — or piking with straight legs — folds the body at the hips, maximally loading the abdominals and hip flexors while demanding serious shoulder stability.

Instructions

1. Set the TRX straps to mid-calf length and face away from the anchor point.
2. Kneel and place both feet (ankles/shins) into the foot cradles, then extend into a high plank with hands directly under shoulders.
3. Engage your core, glutes, and quads to keep your body in a rigid straight line — this is your starting position.
4. Exhale and draw your knees toward your chest by flexing at the hips, allowing your hips to rise naturally as the straps pull your feet forward.
5. Pause briefly when your knees are close to your chest, feeling the abdominal contraction.
6. Slowly reverse the movement, extending back to the plank position under control.
7. Repeat for the prescribed number of reps.

Harder variation: Keep legs straight and pike the hips upward instead of bending the knees — this is the straight-leg jack knife, which increases the lever arm and abdominal demand significantly.

Tips

• Avoid letting your hips sag between reps — a drooping lower back shifts load away from the abs and strains the lumbar spine.
• Keep your shoulders directly over your wrists throughout; do not let them creep forward or backward.
• The movement should be driven by your abs and hip flexors, not by swinging momentum.
• The straight-leg version is substantially harder — master the bent-knee variation first.
• Unlike the TRX Pike (which drives hips straight up into an inverted-V with legs remaining extended), the Jack Knife focuses on the knee-to-chest folding action, making it more hip-flexor intensive and accessible at intermediate level.
• Control the return phase; a slow eccentric extension builds more core strength than a fast reset.
• Keep your head neutral — looking at the floor between your hands helps maintain cervical alignment.