Suspension Jack knife
- Target muscle
- Iliopsoas, Rectus Abdominis
- Synergist muscles
- Adductor Brevis, Adductor Longus, Obliques, Pectineous, Sartorius, Tensor Fasciae Latae
- Equipment
- Suspension
- Body part
- Waist
- Type
- Strength
The suspension jack knife is a core strength exercise that primarily targets the iliopsoas and rectus abdominis, with supporting work from the obliques, hip adductors, and hip flexors. Performed with a suspension trainer, it challenges your abs and hip flexors through a full range of motion while demanding shoulder and trunk stability throughout.
How to do the Suspension Jack knife
- 1Set the suspension trainer so the foot cradles hang about 12–18 inches off the ground.
- 2Place both feet in the cradles and walk your hands out into a high plank position with your wrists directly under your shoulders.
- 3Brace your core, squeeze your glutes, and hold a neutral spine so your body forms a straight line from head to heels.
- 4Exhale and draw both knees toward your chest, flexing at the hips and knees while your hips rise only slightly.
- 5Continue until your knees are under your chest and your shins are roughly parallel to the floor, keeping your legs together.
- 6Pause for a moment at the top, maintaining tension through your abs.
- 7Slowly extend your legs to return to the plank position, resisting the swing of the straps.
- 8Complete the desired number of reps, then step out of the cradles to finish.
Form tips
- Stack your wrists directly under your shoulders and push the floor away so your shoulder blades stay spread; a stable upper body keeps the straps from swaying.
- Control the return to plank slowly rather than letting the straps pull your legs back; the eccentric phase builds as much core strength as the pull-in.
- Exhale as the knees come in and finish the rep by drawing your ribs toward your pelvis — the last few inches of the tuck are where the rectus abdominis works hardest.
- Start with a partial tuck and add range as your plank control improves; end the set once you can no longer hold the plank line between reps.
Common mistakes
- Letting the hips sag in the plank before pulling, which shifts the load to the lower back instead of the abs.
- Swinging the legs rather than controlling the movement, which uses momentum and removes tension from the target muscles.
- Bending the elbows during the pull, which recruits the shoulders and chest and reduces core engagement.
- Letting the hips ride up into a pike instead of tucking the knees under the chest, which turns it into a different exercise and takes tension off the hip flexors.
- Rushing through the lowering phase, which sacrifices the eccentric stimulus and destabilizes the suspension straps.
Frequently asked questions
What muscles does the suspension jack knife work?
It primarily targets the iliopsoas and rectus abdominis. The obliques, sartorius, tensor fasciae latae, adductor brevis, adductor longus, and pectineus assist with hip flexion and leg control throughout the movement.
How is the suspension jack knife different from a TRX pike?
Both use a suspension trainer with feet in the cradles, but a jack knife draws the knees toward the chest, while a pike keeps the legs straight and raises the hips overhead. The jack knife places more emphasis on the hip flexors; the pike demands greater hamstring flexibility and shifts more load onto the upper abs.
Is the suspension jack knife suitable for beginners?
It requires a solid plank foundation and good hip flexor strength. If you are new to suspension training, build up with standard planks and mountain climbers first. Once you can hold a suspension plank steadily for 30–45 seconds, you are ready to attempt jack knives.
How do I make the suspension jack knife harder?
Add a pause at the top of each rep, slow the return to plank to a three- to four-second count, or wear a weight vest. Once the tuck feels easy, progressing to a straight-leg pike is the next step up.
How high should the suspension trainer be set?
Position the foot cradles roughly 12–18 inches off the ground so your body starts level in the plank. Set much higher and your hips sit piked before you begin, which shortens the pull; set much lower and your feet drag toward the floor as you extend.
