A strong core is not what keeps you upright

Ask why someone slumps at a desk and weak core comes back more often than any other answer. It is the explanation offered by trainers, physiotherapists, magazine articles and the person at the next desk, and it has produced two decades of planks performed in the name of posture.

It came from a real piece of research. It just never meant what it was turned into.

Where the idea came from

In the mid nineties a pair of Australian researchers measured when different abdominal muscles switched on as people raised an arm. In those without back pain, a deep sheet called transversus abdominis fired fractionally before the movement. In those with back pain, it fired fractionally later.

A delay of a few hundredths of a second in one muscle. That finding, which was careful and genuinely interesting, became the foundation for an industry: draw in your navel, activate your deep core, stabilise the spine, and the back pain follows.

Two things got lost on the way. The study showed an association rather than a cause, in a small group of people who already had pain. And a difference in timing is not a difference in strength. Nobody had shown the muscle was weak, and nobody had shown that retraining when it fired fixed anything.

Then it got tested properly

Over the following twenty years, core stabilisation training was compared against other kinds of exercise for low back pain, repeatedly, in trials and reviews large enough to settle it.

The consistent result is that stabilisation exercise works about as well as general exercise, and general exercise works about as well as stabilisation exercise. Both beat doing nothing. Neither beats the other by an amount a person would feel.

That is a strange result for a treatment built on a specific mechanism. If the problem were one deep muscle failing to switch on in time, training that muscle specifically should have won comfortably. It did not, which is the usual sign that the mechanism was not the thing doing the work.

The researcher behind the original studies has said much the same in print over the years. The model got compressed into switch on your transversus abdominis and your back is fixed, which was never what the data showed. The overreach happened downstream of the research, which is where it usually happens.

Sitting upright is not a strength problem

Here is the part that settles it for desk posture specifically. Holding a torso upright against gravity is a very low-effort task. Trunk muscle activity in ordinary upright sitting is a few percent of maximum. Nobody, including people with genuinely deconditioned trunks, fails at it for want of force.

You do not fold forward at four o'clock because your abdominals ran out. You fold because holding any single shape for hours is fatiguing in a way that has nothing to do with maximum force, and because you stopped paying attention, which is the variable that actually changed.

Strength training does not fix an attention problem. That is why people who can hold a three-minute plank still finish the day curled over a laptop, and why they assume they need a longer plank.

What is genuinely true

• Exercise helps back pain. This is one of the better supported findings in the whole field. It simply does not matter much which kind.
• Exercise reduces recurrence. People who keep moving have fewer repeat episodes, and again the type is not the deciding factor.
• Trunk endurance is worth having. Being able to hold positions for longer is useful in a general way, which is not the same as it being the cure for slouching.
• Being stronger is good for you. That is a fine reason to train. It is a poor reason to spend your training on one muscle group because of a claim about posture.

So what is the plank for

It is a decent exercise. It builds trunk endurance, it costs nothing and it needs no equipment. It is not a posture intervention, and the version where people work up to holding it for five minutes passed the point of usefulness some way back.

If you enjoy it, keep it. If you have been doing it for two years specifically to stop slumping and you still slump, the exercise is not failing you. The reason you were given for doing it was wrong.

What does change how you sit

When it is not posture at all

Back pain with numbness or weakness in a leg, pain that wakes you at night, any change in bladder or bowel function, unexplained weight loss, or pain that follows a fall belongs with a clinician rather than an exercise plan. Most desk back pain is none of those. It is still worth knowing what the exceptions look like.

The short version

The weak core explanation for slouching traces back to a real study about the timing of one muscle, which was never a study about strength. When core exercise was finally compared with ordinary exercise, the specific version did not win. Holding yourself upright uses a few percent of the strength you already have, so strength was never the limit. Train because it is good for you, and fix your posture with a setup that does not fight you and a reason to move every half hour.