Quick Answer
Jackknife Sit-Up is a intermediate strength exercise that targets your abdominals and core. It uses only your bodyweight. Lie flat on back, arms extended overhead, legs straight.
Video Tutorial
How to Perform the Jackknife Sit-Up
- 1
Lie flat on back, arms extended overhead, legs straight.
- 2
Engage core. Exhale, simultaneously lift straight legs and upper body.
- 3
Reach hands towards toes.
- 4
Body forms 'V' shape.
- 5
Pause briefly.
- 6
Inhale, slowly lower arms/legs back to start.
- 7
Repeat.
Common Mistakes to Avoid
- !Rushing reps instead of controlling the lowering (eccentric) phase — aim for 2–3 seconds down.
- !Letting form break down as reps add up. Stop the set when technique slips, not when you physically can't do another rep.
- !Using momentum to swing through reps. Pause briefly in the contracted position to keep tension on the muscle.
Tips for Better Form
- ✓Breathe out during the effort (concentric) and breathe in on the return (eccentric).
- ✓Use a mirror or film yourself on your first session — your perceived form and actual form often differ.
- ✓Keep ribs down and lats engaged — this keeps tension on the core, not the hip flexors.
- ✓Start each set with 1–2 warm-up reps at a lighter load to groove the movement.
Alternative Exercises
If the Jackknife Sit-Up isn't right for your body, equipment, or goal, try these similar exercises that hit the same muscle groups:
Pass Between Legs Figure 8
Abdominals, Core • Kettlebells
180s
Obliques, Abdominals, Core • Barbell
High to Low Cable Woodchop
Abdominals, Obliques, Core • Cable
Low to High Cable Woodchop
Abdominals, Obliques, Core • Cable
Suspension Fallout
Abdominals, Core • Suspension
Figure 8
Abdominals, Obliques, Core • Kettlebells
Frequently Asked Questions
What muscles does the Jackknife Sit-Up work?
The Jackknife Sit-Up primarily works your abdominals and core. Secondary muscles include the obliques and hip flexors.
What equipment do I need for the Jackknife Sit-Up?
The Jackknife Sit-Up needs no equipment — just your bodyweight. You can perform it at home or at the gym as long as you have what's listed.
Is the Jackknife Sit-Up suitable for beginners?
The Jackknife Sit-Up is an intermediate exercise. Beginners can try it with a lighter load or an easier variation before progressing to the full movement.
How many sets and reps of Jackknife Sit-Up should I do?
For strength: 3–5 sets of 3–6 reps with longer rest (2–3 min). For hypertrophy (muscle growth): 3–4 sets of 8–12 reps with moderate rest (60–90 sec). For endurance: 2–3 sets of 15+ reps with short rest (30–45 sec). Adjust based on your program and goal.
Can I do the Jackknife Sit-Up every day?
No. Muscles need 48 hours to recover between heavy training sessions. If you want to train abdominals and core more frequently, alternate harder and easier variations and keep overall weekly volume moderate.