Every legging looks fine standing still in a changing-room mirror. The real test happens under load: does the fabric stay opaque when it stretches, and does the waistband stay put when you bend, squat and move the way you train? Most people find out the answer mid-session, in front of other people, which is exactly the wrong moment to learn it.
The squat test
Do this before you buy, or before you trust a pair for a heavy session. In a full-length mirror, drop into a deep squat, the kind you'd hit under a barbell or in a class. Check the fabric across your thighs, hips and seat for any lightening, thinning or see-through patches. A squat-proof legging holds its opacity through the deepest range of motion, not just standing at rest.
If you're testing in a changing room without a mirror at the right angle, the phone-flashlight trick works: hold a torch or phone light behind the fabric while stretched. If light passes through visibly, the fabric will likely show more than you want under bright gym lighting.
Why some leggings fail this test
It comes down to fabric weight and construction, not just the brand name on the waistband. Thinner, lighter fabrics move more easily and feel great standing still, but they thin out further under stretch, which is exactly when opacity matters most. A heavier, four-way stretch fabric holds its density because there's simply more material doing the work as it stretches, rather than thinning past a point where it stays opaque.
Seam placement matters too. Leggings with a seam running directly up the centre back or through high-stress zones are more prone to visible strain lines under a deep squat, even in decent fabric.
The waistband problem: why leggings fall down
This is a separate issue from squat-proofing, and it's usually a fit problem, not a fabric one. A waistband that rolls, digs or slides during movement almost always means the rise (the waistband's position relative to your natural waist) doesn't match your body, not that the leggings are defective.
Check for: a wide waistband that sits at or above your natural waist rather than your hips, and enough structure in the band itself that it doesn't fold or roll when you bend forward. A high-rise cut generally stays in place better through squats and lunges than a low or mid-rise, simply because there's more surface area holding the fabric to your body.
What to do if your current pair fails
If the squat test reveals thinning, the fix isn't a different size; it's a different fabric weight. Sizing up or down doesn't change how the material behaves under stretch. If the waistband is the problem specifically, check whether you're wearing a rise that suits your torso proportion, which is the same principle covered in our guide to petite gym wear if length and rise have been a recurring issue.
What we build for
Our legging collection, including the grey leggings that are one of our most requested colours, uses four-way stretch fabric specifically weighted to hold opacity through a full squat depth, paired with a high-rise waistband built to stay at your natural waist rather than creep down mid-set.
Test any pair the same way before you trust it for a heavy session: deep squat, full mirror, honest look. It takes thirty seconds and it's the only test that tells you what a changing-room stand and turn won't.