TRX Jack Knife
What Is the TRX Jack Knife?
TRX Jack Knife is a compound exercise that primarily targets the core and also engages the hip flexors, shoulders and serratus anterior. It is an intermediate-level movement that starts in a straight-arm plank with the hands on the floor and the feet hanging in the foot cradles of a suspension trainer, then draws both knees in toward the chest and pushes them back out to the plank.
Every rep asks the abs to do two jobs. In the plank they stop the hips from sagging, and during the tuck they curl the pelvis up under the body while the hip flexors pull the knees forward, all with the straps free to swing so the control has to come from you. In a study of 17 active men new to suspension training, this knee tuck with the feet in TRX straps drove the lower rectus abdominis to about 54% of its maximum and the external obliques to about 42%, while the lower back muscles stayed under 10%. A suspended rollout and body saw tested in the same study worked the lower abs harder, so the knee tuck is an earlier step on the suspension core ladder rather than the top of it.
The rectus abdominis does most of the work, with the obliques helping to keep the pelvis square. The hip flexors, including the rectus femoris, drive the knees in, while the serratus anterior, front of the shoulders and triceps hold the straight-arm position for the whole set. Keeping the legs straight and lifting the hips into an inverted V turns the move into the TRX Pike, a longer lever and a harder rep. Pulling the knees toward one elbow instead of the chest gives the TRX Twisting Jack Knife, which shifts more of the work to the obliques, and driving one knee at a time in a quicker rhythm is the TRX Mountain Climber.
The TRX Jack Knife suits anyone who has outgrown floor core work and wants a dynamic version of the plank, and it usually sits in the core block at the end of a session or in a suspension circuit. Before trying it, build up to a steady 60 second TRX Plank and get comfortable with the Exercise Ball Jack Knife, which teaches the same tuck with the shins on a ball that rolls rather than straps that swing.
Muscles Worked
- Rectus Abdominis
- Primary
- Rectus Abdominis (Lower)
- Primary
- External Oblique
- High
- Rectus Femoris
- High
- Deltoid
- Medium
- Serratus Anterior
- Medium
- Tensor Fasciae Latae
- Medium
- Sartorius
- Low
- Erector Spinae
- Minimal
Instructions
- Adjust the straps so the foot cradles hang at mid-calf height, about 8 to 12 inches off the floor, directly under the anchor.
- Kneel on all fours facing away from the anchor and place one foot, then the other, into the foot cradles so the tops of your feet rest in them and your soles face the ceiling.
- Place your hands flat on the floor directly under your shoulders with your fingers pointing forward.
- Brace your abs, squeeze your glutes and straighten your knees to lift into a straight-arm plank, with your body in one line from head to heels.
- Keeping your arms straight and your feet together, pull both knees in toward your chest, letting your hips rise and your lower back round as the knees travel under your body.
- Stop when your knees are under your chest and your hips are at or slightly above shoulder height, with your shoulders still stacked over your hands.
- Push your feet back out slowly until your legs are straight and you are back in the plank, without letting your hips drop below the line of your body.
- Exhale as you tuck and inhale as you extend, then repeat for the set reps before lowering your knees to the floor and taking your feet out of the cradles.
Common Mistakes
Sagging hips on the return
Letting the hips drop below the line of the body as the legs extend arches the lower back and shifts the strain from the abs onto the lumbar spine.
Swinging the legs
Snapping the knees in and letting them fly back out hands the work to momentum and the straps instead of the abs.
Shoulders rocking off the hands
Letting the shoulders slide back behind the wrists as the knees tuck turns the rep into a body rock and takes tension off both the abs and the shoulders.
Cutting the tuck short
Moving the knees only a few inches skips the part of the rep where the abs curl the pelvis under the body.
Sinking between the shoulders
Letting the chest drop between the shoulder blades leaves the shoulders hanging on their joints instead of pushing the floor away.
Uneven pressure in the straps
Pressing harder into one foot cradle or letting the feet split makes the straps saw and twists the hips off square.
Injuries
TRX Jack Knife is a medium risk exercise when performed with proper technique. The lower back is the main strain point: when the abs tire, the hips sag on the way back to the plank and the lumbar spine arches under the pull of the straps and the hip flexors. The wrists carry part of your body weight in extension for the whole set, and the shoulders have to stay stable while the legs move beneath them.
Set the cradles at mid-calf height with the anchor directly above your feet, keep your shoulders stacked over your hands, and slow the return so the legs never swing the hips into an arch. If you cannot finish a rep with the hips level, the set is over. If your wrists complain, holding on your fists or on push-up handles keeps them straighter.
If the suspended tuck is too hard, go back to the TRX Plank or the Exercise Ball Jack Knife, or bring the knees only halfway in until the full range feels controlled. Once 12 to 15 slow reps feel solid, progress to the TRX Pike or the TRX Twisting Jack Knife, or start with your body farther from the anchor so the straps pull your feet back as you tuck.
Stop if you feel pain in the lower back, sharp pain in the wrists or shoulders, or a pinch at the front of the hips, or if you can no longer return to the plank without the hips sagging.
- Primary Focus
- Core
- Equipment
- Suspension trainer and body weight
- Modality
- Strength
- Difficulty
- Intermediate
Alternative Exercises
Other Names for the TRX Jack Knife
The TRX Jack Knife is also known as:
- TRX Knee Tuck
- TRX Crunch
Frequently Asked Questions
- Q: What is the difference between the TRX Jack Knife and the TRX Pike?
In the jack knife the knees bend and pull in toward the chest, while in the TRX Pike the legs stay straight and the hips lift into an inverted V. The pike works over a longer lever, so most people learn the jack knife first.
- Q: Is the TRX Jack Knife the same as a TRX Mountain Climber?
No. The jack knife brings both knees in together and returns them together, while the TRX Mountain Climber drives one knee in at a time in an alternating rhythm.
- Q: How many reps of the TRX Jack Knife should I do?
Two to three sets of 8 to 15 slow, controlled reps works well as core accessory work. End the set when you can no longer return to a level plank rather than chasing a number.
- Q: Is the TRX Jack Knife safe with lower back pain?
- Q: Where should I set up relative to the anchor?
Start with your feet directly under the anchor so the straps hang straight down. Moving your body farther from the anchor angles the straps so they pull your feet back as you tuck, which makes each rep harder.