How is this different from a straight jack knife?

The knees travel up and across toward one elbow instead of straight in under the hips, so the pelvis turns and the obliques drive the movement rather than only bracing against it. Everything else, including the plank position and the strap setup, is the same. The supported Exercise Ball Jack Knife is the one to master first.

How many reps should I do, and where does it fit in a session?

Two or three sets of 8 to 12 reps per side works well as accessory or finisher work after the main lifts. Quality falls off quickly once the shoulders tire, so end the set while the plank still looks like a plank rather than chasing a number.

Can I train this pattern without a suspension trainer?

Yes, with a different tool. Shins on a stability ball gives you the Exercise Ball Jack Knife, and a Mountain Climber driven toward the opposite elbow trains the same cross-body tuck on the floor with far less demand on the shoulders.

My wrists hurt in the plank position. What can I do?

Spread your fingers wide and press through the whole hand rather than the heel of the palm, which spreads the pressure instead of concentrating it at the wrist crease. If it still hurts, train the core from a forearm Plank or a Hanging Knee Raise and come back to this one later.

Why do the straps keep swinging?

Usually because the knees are being thrown across rather than carried, or because the anchor is not directly above your feet. Set the cradles so they hang straight down at about mid-calf height, then slow the first half of every rep until the straps stay quiet.

Is it safe with a sensitive lower back?

Not as a starting point. It combines a long plank lever with loaded rotation of the pelvis, which is a lot to ask of a back that already complains. Build up with the Plank and the Cable Pallof Press, and get cleared by a clinician before adding a suspended twist.

TRX Twisting Jack Knife

What Is the TRX Twisting Jack Knife?

TRX Twisting Jack Knife is a compound exercise that primarily targets the core, and also engages the shoulders and hip flexors. It is an advanced-level movement held in a straight-arm plank with both feet resting in the foot cradles of a suspension trainer, drawing the knees up and in toward one elbow instead of straight to the chest.

The twist is what separates it from the straight version. Pulling the knees toward one side asks the obliques on that side to shorten under load and turn the pelvis, rather than simply bracing to stop it turning, so the movement trains the trunk in the plane most abdominal work skips. Because the feet hang in straps instead of sitting on the floor or on a ball, nothing steadies the path of the knees except the trunk itself, and the straps immediately expose any attempt to throw the legs across rather than carry them.

Trainers usually place it as accessory or finisher work at the end of a session, once the pressing and pulling is done and while the shoulders can still hold a clean plank, and two or three controlled sets per side is plenty. Its rotational demand carries over to sports where power travels through the torso, such as throwing, swinging a bat or racket, and striking. A study of seventeen active men comparing four suspension core exercises found that the straight knee tuck reached about 54 percent of maximum voluntary contraction in the lower rectus abdominis and 42 percent in the external oblique, less than the roll-out and the body saw done on the same straps, so the tuck family is better understood as control and coordination work than as a maximal tension drill.

Most of the work lands on the external obliques and the lower portion of the rectus abdominis, with the serratus anterior and both deltoids holding the plank up and the rectus femoris helping draw the knees in. Raising the foot cradles tips more of your weight onto the hands and makes every rep harder, while lowering them toward the floor flattens the line and takes some of that load off the shoulders. Taking the knees only partway across turns the movement back into a straight tuck, so how far the pelvis rotates is the variable that matters most.

It suits lifters who already hold a long, still plank and want rotation added on top of an unstable base, and athletes who need the trunk to transmit force across the body. If the hips drop or the shoulders shake in the plank position, build the base with the Plank and the Side Plank first, then learn the tuck itself with the shins supported in the Exercise Ball Jack Knife before the feet ever go into the straps.

Muscles Worked

MuscleActivation

External Oblique: Primary
Rectus Abdominis (Lower): Primary
Rectus Abdominis: High
Serratus Anterior: High
Deltoid: Medium
Rectus Femoris: Medium
Erector Spinae: Low
Gluteus Maximus: Low
Triceps Brachii (Long Head): Low
Upper Trapezius: Minimal

Instructions

Set the suspension trainer so the foot cradles hang at about mid-calf height, with the anchor point directly above where your feet will sit.
Sit on the floor facing away from the anchor and slide both feet into the cradles, tops of the feet down and toes pointed back.
Walk your hands forward into a straight-arm plank, hands flat on the floor directly under your shoulders.
Straighten your legs behind you so your body forms one line from head to heels, and squeeze your glutes and abs to hold it.
Press the floor away so your upper back stays broad and your shoulder blades do not collapse together.
Exhale and draw both knees up and in toward your right elbow, letting your hips rise and turn as the knees travel across.
Pause briefly with the knees tucked under your torso and off to that side, letting your upper back round naturally.
Inhale and extend your legs back slowly until your body is one straight line again, keeping the straps still.
Repeat toward your left elbow, and keep alternating sides for the set.
End the set once the hips start to sag between reps or the straps begin to swing.

Common Mistakes

Tucking the knees straight in
Bringing the knees to the chest instead of across toward one elbow removes the rotation and turns the movement back into a straight jack knife.
Hips sagging in the plank
Letting the pelvis drop below the line of the shoulders between reps shifts the load onto the lower back and takes the abs out of the position.
Throwing the legs across
Swinging the knees sideways and riding the momentum lets the straps do the turning, so the obliques never have to produce the rotation themselves.
Shoulders drifting ahead of the hands
Creeping forward over the wrists loads them at their end range and drags the shoulder blades out of position.
Pulling with the hip flexors only
Bending the knees up without turning the pelvis keeps the thighs busy and leaves the obliques barely involved.
Dropping back into the start
Letting the legs fall straight rather than extending them slowly skips the half of each rep where the trunk works hardest.

Injuries

TRX Twisting Jack Knife is a medium risk exercise when performed with proper technique. Both legs hang from the straps, so the hands are the only fixed contact with the ground and the wrists, shoulders and lower back carry everything the trunk does not. Nothing about it is violent, but the position leaves very little margin once the core tires.

The wrists sit near the end of their extension range for the whole set while supporting most of your body weight. Spread your fingers, push through the full palm rather than the heel of the hand, and keep the shoulders stacked over the hands instead of drifting forward of them. The shoulders themselves are looked after by keeping the blades flat against the ribs and the chest lifted away from the floor, because sinking between the shoulder blades hands the load to the joint rather than the muscles around it. Suspension straps also add small, unpredictable movements that no floor version produces, which is why this position is worth attempting only once a floor plank is genuinely solid.

The lower back is the third pressure point. If the hips sag on the way out of each tuck, the lumbar spine takes the slack in extension, and because this version also rotates the pelvis under load, anyone with an irritable low back or a history of disc trouble should train the trunk without loaded rotation instead. The hip flexors often cramp before the abs give out, which is usually a sign the knees are being pulled up by the thighs rather than carried across by a turning pelvis.

Regress it by holding a plain Plank until a full minute is easy, adding rotation from the floor with the Side Plank and the Russian Twist, then learning the tuck with the shins supported in the Exercise Ball Jack Knife. Progress it by walking the hands further forward from the anchor, slowing the return, or pausing briefly at the top of each tuck. Stop the set as soon as the hips drop below the line of the shoulders, the straps start to swing, or the wrists, shoulders or lower back begin to hurt.

Overview

Primary Focus: Core
Equipment: Suspension trainer and body weight
Modality: Strength
Difficulty: Advanced

Alternative Exercises

Frequently Asked Questions

Q: How is this different from a straight jack knife?: The knees travel up and across toward one elbow instead of straight in under the hips, so the pelvis turns and the obliques drive the movement rather than only bracing against it. Everything else, including the plank position and the strap setup, is the same. The supported Exercise Ball Jack Knife is the one to master first.
Q: How many reps should I do, and where does it fit in a session?: Two or three sets of 8 to 12 reps per side works well as accessory or finisher work after the main lifts. Quality falls off quickly once the shoulders tire, so end the set while the plank still looks like a plank rather than chasing a number.
Q: Can I train this pattern without a suspension trainer?: Yes, with a different tool. Shins on a stability ball gives you the Exercise Ball Jack Knife, and a Mountain Climber driven toward the opposite elbow trains the same cross-body tuck on the floor with far less demand on the shoulders.
Q: My wrists hurt in the plank position. What can I do?: Spread your fingers wide and press through the whole hand rather than the heel of the palm, which spreads the pressure instead of concentrating it at the wrist crease. If it still hurts, train the core from a forearm Plank or a Hanging Knee Raise and come back to this one later.
Q: Why do the straps keep swinging?: Usually because the knees are being thrown across rather than carried, or because the anchor is not directly above your feet. Set the cradles so they hang straight down at about mid-calf height, then slow the first half of every rep until the straps stay quiet.
Q: Is it safe with a sensitive lower back?: Not as a starting point. It combines a long plank lever with loaded rotation of the pelvis, which is a lot to ask of a back that already complains. Build up with the Plank and the Cable Pallof Press, and get cleared by a clinician before adding a suspended twist.