The Activewear Fabric Guide: GSM, Stretch and the Materials That Actually Matter

Most performance apparel looks the same on a hanger. Two black hoodies, identical cut, identical print. One holds its shape for five years. The other warps at the cuffs after twelve washes. The difference is not in the silhouette, it is in the fabric specification on the back of the tech pack. This guide breaks down the numbers and the materials that separate premium activewear from everything else.

What GSM Actually Means

GSM stands for grams per square metre. It is the weight of one square metre of finished fabric. For activewear, it is the single most important spec you can read on a product page. A hoodie at 240gsm and a hoodie at 400gsm are not the same garment in two weights, they are two different products built for two different purposes.

Most mass-market gym hoodies are produced at 240-280gsm because the math is brutal at scale, every additional 50gsm adds roughly 12-15% to fabric cost. A 400gsm hoodie uses nearly 70% more material than a 240gsm hoodie. That is why heavyweight pieces are rare in fast fashion and standard in premium activewear.

The number is not just about weight on the hanger. Higher GSM means deeper colour saturation (more fibre to hold dye), better structural drape (the garment hangs the way it was designed to hang), and longer wear life (more material to abrade through). A 400gsm hoodie at year five looks closer to its first wash than a 240gsm hoodie at year two.

The Yarn Behind the Number

GSM tells you how much fabric is there. It does not tell you what the fabric is made of. The composition listed on the care label, cotton, polyester, nylon, spandex, modal, viscose, is the second layer of the spec. The blend matters as much as the weight.

Cotton

Pure cotton is breathable, soft against the skin, and absorbent. It is also slow to dry, prone to shrinking, and loses elasticity quickly under load. Cotton hoodies feel luxurious; cotton leggings stretch out and stay stretched. In activewear, pure cotton works for outer layers worn outside the gym. It does not work for performance pieces that need to recover their shape after every session.

Polyester

Polyester is the workhorse of athletic apparel. It is hydrophobic (water rolls off rather than soaking in), dries fast, holds dye exceptionally well, and resists abrasion. It also wicks moisture from skin to outer surface where it can evaporate, the mechanism behind every ‘moisture wicking’ claim on a product page. Pure polyester has one weakness: it does not stretch on its own. That is why almost no performance piece is 100% polyester.

Nylon

Nylon is polyester’s stronger cousin. It is more abrasion-resistant, slightly softer, and holds elastane (spandex) more reliably. Premium leggings are typically nylon-spandex rather than polyester-spandex because nylon retains stretch memory longer, the legging returns to its original shape after 200 wears, not 50.

Spandex / Elastane

Spandex (also called elastane or Lycra) is the stretch fibre. It is rarely used above 20% of total blend because pure spandex is delicate. In a typical legging spec of 80% nylon, 20% spandex, the spandex is what makes the garment recover its shape after squatting, lunging, or sprinting.

Modal and Viscose

Cellulose-based fibres derived from wood pulp. Modal is softer than cotton, drapes better, and resists shrinking. It is often blended into premium tees (e.g., 50% modal, 50% cotton) to add silkiness and breathability without losing the cotton hand-feel. Modal is not used in heavy load-bearing pieces, it lacks the elasticity for leggings or the structure for hoodies.

4-Way Stretch vs 2-Way Stretch

Stretch fabric is not binary. The number of directions it stretches is a separate spec from the GSM and the blend.

2-way stretch means the fabric stretches in one direction, usually across the body (weft), not along the length (warp). This is the standard for most casual athleisure. It allows the garment to move with the body horizontally but not vertically. Acceptable for light cardio and walking. Limiting for squats, deadlifts, or anything that requires range of motion in two planes.

4-way stretch means the fabric stretches in all four directions, across the body, along the length, and diagonally in both axes. This is the spec required for serious training. It is what allows a legging to handle a deep squat, a full clean, or a heavy hip hinge without restriction. It is also what allows the fabric to recover to its original dimensions after extreme load. A 2-way stretch legging permanently deforms at the knees within months of squat training. A 4-way stretch legging at 80% nylon / 20% spandex retains its silhouette for years.

The difference comes from the knit structure, not the fibres alone. A 4-way stretch fabric uses an interlock knit or double-knit construction that allows yarn to flex in multiple planes. This is more expensive to produce, both because it requires more sophisticated machinery and because the fabric uses more yarn per square metre.

The Squat-Proof Test

When premium activewear brands advertise ‘squat-proof leggings,’ they are making a specific technical claim. The fabric must remain opaque under deep flexion. This is a function of three factors:

1. GSM, Heavier fabric is less likely to show skin tone through compression
2. Yarn density (denier), Thicker yarn provides more opacity per area
3. Weave tightness, A denser interlock knit holds opacity at full stretch

A 200gsm budget legging in 75% polyester / 25% spandex will pass the squat test on the rack, and fail it at full hip flexion. A 280gsm premium legging in 80% nylon / 20% spandex at interlock knit construction passes the test at any range of motion. The number that matters is not the price tag. It is the spec sheet.

Moisture Wicking, What It Means and Doesn’t Mean

‘Moisture wicking’ describes how a fabric moves sweat from the skin to the outer surface where it evaporates. The mechanism depends on the fibre’s hydrophobic properties and the fabric’s capillary structure. Polyester, nylon, and modern technical polyester blends wick well. Cotton does not, cotton absorbs moisture into the fibre itself, which is why a cotton t-shirt stays heavy and clammy after a workout.

The performance claim is real, but it is not a feature added to a fabric. It is an inherent property of the fibre selection. When you see ‘moisture wicking technology’ on a product page, the underlying fact is: the fabric is made of synthetic fibres (polyester, nylon, or a blend) rather than absorbent natural fibres. There is no chemical treatment that turns a cotton tee into a wicking tee, only a fibre swap.

Interlock Knit, Jersey Knit, French Terry, What the Construction Tells You

The way yarn is knitted into fabric determines as much about garment behaviour as the yarn itself. Three constructions dominate premium activewear:

Interlock knit. Two layers of jersey knit interlocked together. Smooth on both sides, denser than single jersey, holds shape under repeated wash and wear. Premium leggings, performance tees, and structured hoodies use interlock for its durability and dimensional stability.

Jersey knit (single jersey). The standard t-shirt construction. One smooth side (the knit face) and one textured side (the purl face). Lighter than interlock, more drape, but less structural. Common in standard tees and light layering pieces.

French terry. A looped pile knit with smooth face and looped back. Used in mid-to-heavyweight hoodies and sweatshirts. The looped pile traps air for insulation while remaining breathable. Premium French terry at 320-400gsm produces hoodies with serious heft and warmth without crossing into fleece territory.

Brushed back fleece. French terry with the looped back brushed into a soft pile. Used for the heaviest, warmest hoodies. The brushing process adds another step (and cost) to production, which is why brushed-back heavyweight hoodies are typically the most expensive piece in any activewear range.

Why Premium Costs More, A Material Math Breakdown

The price difference between a £35 hoodie and a £140 hoodie is not arbitrary brand markup. Here is what is actually different in the spec:

The premium hoodie uses roughly 60% more raw material, requires 2-3x more production time, and survives 4-5x more wash cycles before showing degradation. The price difference is the material reality of building a garment to last a decade rather than a season.

How to Read a Product Page Like a Spec Sheet

When you compare two activewear products, the four numbers that actually matter are:

1. GSM, Tells you the weight class and durability tier
2. Composition, Tells you what the fabric is and how it will behave (nylon = stretchy and durable; polyester = wicking and lightweight; cotton = breathable but loses shape; modal = soft and drapey)
3. Stretch type, 4-way for serious training, 2-way is acceptable only for casual wear
4. Construction, Interlock, French terry, or jersey, the knit structure that determines drape and longevity

Brand language varies. Spec sheets do not lie. A premium brand that respects its customers will publish all four numbers on the product page. A brand that hides those numbers behind marketing copy is hiding something.

The Catar Cottega Standard

Every Catar Cottega piece is built to the spec sheet. Our hoodies are 400gsm brushed-back French terry. Our leggings are 280gsm interlock knit at 80% nylon / 20% spandex with full 4-way stretch. Our tees are 250gsm interlock with reinforced flatlock seams. The numbers are on every product page because the numbers are what separates premium activewear from premium-priced activewear.

The fabric is not the marketing. The fabric is the product.

Related reading:

• Best Lululemon Alternatives 2026: The Premium Activewear Comparison Guide
• Fast Fashion vs Premium: The Real Cost-Per-Wear Math
• Catar Cottega vs Fabletics: Honest Pricing or Subscription Trap?

Want the deep dive? Read our complete guide to seamless activewear covering knitting tech, fabric science, sizing and care.

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