Sit-ups and how to improve yours
Sit-ups can tell you a lot about your core strength
To do a good sit up a lot of muscle groups have to be in balance:
- strong, activated, abdominals
- activated gluteals
- stable hip flexors/iliopsoas
- stable hamstrings
Should I have my knees bent or legs straight to do sit ups?
The short answer is: Keep your knees bent.
Knees Bent:
Primary muscle worked = Abdominals
Back Safety = Protects the lower spine
Effectiveness = Better isolation of the 'six-pack'
Why Straight Legs Can Be Problematic
When your legs are flat on the floor, your hip flexors do the heavy lifting. Because these muscles are attached to your lumbar spine (lower back), as they contract to pull your torso up, they tug on your spine, often causing it to arch aggressively. This is a common culprit for lower back pain after a workout.
However, learning to do a straight leg sit-up without over-activating your hip flexors is a great test for balancing your abdominals, hip flexors and glutes.
Tips for Proper Form
To get the most out of your sit-ups:
- The 90-Degree Rule: Bend your knees at roughly a 90-degree angle with your feet flat on the floor.
- Don't Pull the Neck: Avoid interlocking your fingers behind your head; instead, graze your temples or cross your arms over your chest. This prevents you from 'yanking' your neck upward.
- The "C" Curve: Think about curling your spine one vertebra at a time rather than lifting your back like a flat board.
I can only go half way with my knees bent, but can go all the way to sitting with them straight. Why is that?
The reason you can sit all the way up with straight legs—but not with bent knees—comes down to biomechanics and muscle recruitment.
1. The Lever Effect
When your legs are straight, they act as a heavy counterweight. Think of your body like a see-saw. Having that long, heavy weight extended out makes it much easier to pivot your upper body upward. When you bend your knees, you shorten that lever and bring the weight closer to your center, removing the "counterweight" advantage.
2. Hip Flexor Hijacking
This is the main reason. Your hip flexors (the muscles that connect your thighs to your lower back and pelvis) are much stronger than your abdominal muscles:
Straight leg sit-up: Your hip flexors are at an optimal length to fire with maximum power. They essentially 'yank' your torso up, allowing your abs to just enjoy the ride.
vs.
Bent knees sit-up: This position puts the hip flexors in a shortened state, which mechanically puts them at a disadvantage. It effectively 'turns them off' or inhibits them (or at least turns them down).
The Result: Without the hip flexors doing the heavy lifting, your abs are forced to do the work alone. If you can only go halfway, it simply means your abdominal strength isn't quite ready to lift your full torso weight without help yet.
Is the "Halfway" Point Bad?
Actually, no! In the fitness world, that 'halfway' point where you’re shaking and can’t go further is often called the point of maximum contraction. Staying in that range is actually better for building a strong core than using momentum and hip strength to sit all the way up. If you keep working in that 'halfway' zone (often called a 'crunch'), your strength will eventually increase until you can do the full movement.
How to Bridge the Gap
If you really want to master the full sit-up with bent knees, try these:
The "Butterfly" Sit-up: Lay with your knees bent but dropped open to the sides (soles of feet touching). This further disables the hip flexors and is a favorite in CrossFit.
Eccentric Lowers: Sit up straight (use your hands to help), then try to lower yourself back down to the floor as slowly as possible—count to five!
The Weighted Counterbalance: Hold a small weight (like a milk jug or a 5lb plate) out in front of you. It sounds weird, but the weight acts as a "reach" that helps pull your torso up.
DO NOT:
- hook your feet under something or get someone to hold your feet down
- all this does is allow your hip flexors to activate and pull you up
- this is because your hip flexors are also trunk flexors when your feet are anchored.
The Frog-Legs Sit Up
The Frog-legs Sit-up is a classic 'hack' used in sports like CrossFit specifically to isolate the abdominals by taking the 'cheating' muscles out of the equation.
Here is why it works and how it helps you get stronger:
1. It shuts down the hip flexors
When your knees are bent and feet are flat (the standard way), your hip flexors are in a prime position to pull.
By opening your knees out like a butterfly, you put the hip flexors in a shortened, mechanically disadvantaged state.
- Because the hip flexors can’t 'grip' as well, your brain is forced to recruit the abdominals to do the heavy lifting.
2. It Improves Range of Motion
Standard sit-ups are often stopped short by the hip flexors taking over.
The frog legs version allows your spine to move through a larger arc. This helps you build strength at the bottom of the movement, which is usually where people are weakest.
3. It Prevents Back Arching
By 'turning off' the hip flexors, you reduce the forward pull on your lower spine. This makes it much easier to keep your back slightly rounded—which is exactly what you want for a sit-up.
A flat or arched back is usually what causes that 'stuck' feeling.
- Exhale on the way up: Start your exhale just before you move. This "pre-tenses" the abs so they are ready for the load.
Note:
- Because the Frog-Legs Sit-up forces the abs to work harder, you might find you can actually do fewer reps at first than you could with your feet anchored. Don't let that discourage you! That just means you've successfully stopped 'cheating' and are finally hitting the target muscle.
Why can’t I do a sit up in the morning, but often can later in the day?
This is actually a very common problem, and it’s usually caused by a mix of spinal biomechanics and nervous system 'sleep inertia'.
1. Spinal Disc Hydration (The "Stiff Board" Effect)
While you sleep, your spinal discs (the "cushions" between your vertebrae) soak up fluid and expand. This is why you are actually slightly taller in the morning.
- Because your discs are plump and pressurised, your spine is naturally stiffer and more resistant to bending.
- Since a sit-up requires your spine to curl (flexion), your brain senses that extra pressure in your discs and inhibits the abdominals to prevent you from forcing a bend that might hurt your back. It’s your body's built-in safety brake.
2. The "Shortened" Hip Flexors
Most people sleep in a 'fetal' position or with knees slightly bent.
- After 7–8 hours in that position, your hip flexors (psoas) become physically tight and short.
- When you wake up and try a sit-up, those tight hip flexors immediately yank on your lower spine, pulling it into an arch. Once your back arches, your abdominals are mechanically 'turned off'.
3. Neural "Sleep Inertia"
Your nervous system operates at a lower voltage when you first wake up.
- Motor Unit Recruitment: It takes time for the brain to start efficiently recruiting high-threshold motor units (the ones needed for a 'fast-twitch' power move like an unassisted sit-up).
4. Body Temperature
Core body temperature is at its lowest right when you wake up. Cold muscles and tendons are less elastic. If the tissue doesn't want to stretch (like the muscles along your back), the opposing muscles (your abs) will be inhibited to avoid a 'tug-of-war' that could cause a strain.