BestsellerShirt · MenX-Bionic
Energy Accumulator 4.0 Shirt LS
All-round- Material
- synthetic
The all-rounder for piste and cross-country trails
X-Bionic advisor
The layer nobody sees. But you feel it.
What you wear under your jacket determines whether you stay warm. Our 20 best-selling baselayers and socks from X-Bionic and X-Socks, explained honestly: what they are made for and where their limits lie. Find your combination in four questions or ask our advisor.
Shirt, 3/4-length trousers, ski sock: three pieces nobody sees.
- 20 Bestsellers explained
- 7 Lines compared
- 4 Questions to your layer
Jacket
JacketMove your mouse or finger through the snow: where there is warmth, vapour rises. The first layer needs to get rid of it.
The Lab
The Bionic Scan: what happens inside the shirt.
First, the scan line moves across the shirt. Then the camera goes into the fabric to show where the warmth travels when you’re sitting in the lift, skiing downhill or climbing uphill. Drag the scan line yourself or choose a layer and lens, then take over.
Lens 1× · On the lift
Warmth stays in circulation
Eight minutes sitting, hardly moving. Arms and legs cool down, the core should stay warm.
According to X-Bionic, Insulating Heatloops® capture body heat in channels and direct it to the zones that need it, without adding bulk. For example in the Radiactor 4.0 shirt and the Energy Accumulator 4.0 pants.Manufacturer information, x-bionic.com
- Warmth circulating in the shirt
1× is the shop photo with the Bionic scan, 10× and 100× are drawn: a schematic based on information from X-Bionic, not a measured thermal image and not a microscope image.
How X-Bionic builds the first layer
Manufacturer information from x-bionic.com, quoted.
3D Bionic Sphere® System with ThermoSyphon®
“The patented, award-winning ThermoSyphon® technology uses complex 3D structures to direct air precisely and maintain the optimum body temperature.”Source: x-bionic.com
Insulating Heatloops®
“Strategically placed heat channels capture and redirect body heat. This creates insulating zones that retain warmth without restricting freedom of movement.”Source: x-bionic.com
Merino wool
“Premium merino wool fibres naturally regulate body temperature and help prevent odour”Source: x-bionic.com
Air-Conditioning Channel®
“… strategically placed ventilation channels that continuously remove warm air and moisture”Source: x-bionic.com
The Comparison
Which line suits which day?
From lightweight for high movement to warm for lifts and breaks. Our classification according to the manufacturer, without temperature figures: how warm you feel also depends on what you wear over it and how much you move.
Lots of movement
Ski touring, cross-country, running: you sweat a lot and want to get rid of it quickly.
All-round
Warming and cooling: for days with movement and breaks.
Cold & breaks
Lifts, breaks, icy days: keeping warm comes first.
Tap a line: manufacturer information, how we classify it and the best-sellers.
X-Bionic · All-round
Energy Accumulator 4.0
According to X-Bionic
“… regulates body temperature through the patented 3D Bionic Sphere® System and ThermoSyphon®. Expansion Ribs® keep the insulation flexible with every movement”Source: x-bionic.com
How we classify it
All-round: according to the shop text, it is designed to keep you warm in the cold and cool during exertion.
According to the shop, made for
Skiing, cross-country skiing
And in your shoes? The socks from X-Socks
In a ski boot
Long socks up over the calf (OTC), padded. Merino if your feet get cold quickly.
In a touring boot
Made for the climb, according to the shop also for cross-country. With its own cut for women.
In a running shoe
Short and light, according to the shop for every season.
The Finder
Find your first layer in 4 questions
A shirt, trousers and sock to match what you’re planning.
Question 1 of 4
Who is the first layer for?
The Top Sellers
Our best-selling X-Bionic and X-Socks products
BestsellerShirt · Men
MerinoShirt · Men
For extreme coldShirt · Men
Retain heatShirt · MenX-Bionic
Heatloop Shirt LS
Cold & breaks- Material
- synthetic
Insulating channels keep warm air against the skin
MultisportShirt · MenX-Bionic
Invent 4.0 Shirt LS
Lots of movement- Material
- synthetic
For running, cross-country skiing and all-year use
BestsellerTrousers · MenX-Bionic
Energy Accumulator 4.0 Pants 3/4
All-round- Material
- synthetic
- Length
- 3/4
- Weight
- 330 g
3/4-length trousers: only the sock sits inside the ski boot
MerinoTrousers · MenX-Bionic
Symbio Merino Pants 3/4
Cold & breaks- Material
- with Merino
- Length
- 3/4
Merino 3/4-length trousers for cold days
For ski touringTrousers · MenX-Bionic
Energy Accumulator Light Pants 3/4
Lots of movement- Material
- synthetic
- Length
- 3/4
Lightweight 3/4-length trousers for uphill sections and cross-country trails
BestsellerShirt · WomenX-Bionic
Energy Accumulator 4.0 Shirt LS
All-round- Material
- synthetic
The all-rounder for piste and cross-country trails
With roll-neckShirt · WomenX-Bionic
Energy Accumulator 4.0 Turtle Neck LS
All-round- Material
- synthetic
- Collar
- Turtleneck
Energy Accumulator 4.0 with roll-neck
MerinoShirt · Women
MultisportShirt · WomenX-Bionic
Invent 4.0 Shirt LS
Lots of movement- Material
- synthetic
For running, cross-country skiing and all-year use
In Comparison
Shirts, trousers and socks at a glance
| Product | Positioning | Material | Length | Collar | Weight | Price from |
|---|---|---|---|---|---|---|
| X-Bionic Energy Accumulator 4.0 Shirt LS | All-round | synthetic | – | – | – | €116.15 |
| X-Bionic Symbio Merino Shirt LS | Cold & breaks | with Merino | – | – | – | €130.65 |
| X-Bionic Radiactor 4.0 Shirt LS | Cold & breaks | synthetic | – | – | – | €119.85 |
| X-Bionic Heatloop Shirt LS | Cold & breaks | synthetic | – | – | – | €104.85 |
| X-Bionic Invent 4.0 Shirt LS | Lots of movement | synthetic | – | – | – | €53.85 |
| X-Bionic Energy Accumulator 4.0 Pants 3/4 | All-round | synthetic | 3/4 | – | 330 g | €108.65 |
| X-Bionic Symbio Merino Pants 3/4 | Cold & breaks | with Merino | 3/4 | – | – | €112.35 |
| X-Bionic Energy Accumulator Light Pants 3/4 | Lots of movement | synthetic | 3/4 | – | – | €103.55 |
| X-Bionic Energy Accumulator 4.0 Shirt LS | All-round | synthetic | – | – | – | €92.35 |
| X-Bionic Energy Accumulator 4.0 Turtle Neck LS | All-round | synthetic | – | Turtleneck | – | €120.45 |
| X-Bionic Symbio Merino Shirt LS | Cold & breaks | with Merino | – | – | – | €135.45 |
| X-Bionic Invent 4.0 Shirt LS | Lots of movement | synthetic | – | – | – | €59.25 |
| X-Bionic Energy Accumulator 4.0 Pants 3/4 | All-round | synthetic | 3/4 | – | – | €108.95 |
| X-Bionic Mightywool Pants 3/4 | All-round | with Merino | 3/4 | – | – | €82.35 |
| X-Socks Ski Perform Merino OTC | Cold & breaks | with Merino | – | – | 110 g | €34.55 |
| X-Socks Ski Discover Merino OTC | Cold & breaks | with Merino | – | – | 130 g | €29.75 |
| X-Socks Helixx Gold 4.0 | All-round | synthetic | – | – | 110 g | €40.15 |
| X-Socks Ski Touring Perform OTC | Lots of movement | synthetic | – | – | 110 g | €34.90 |
| X-Socks Ski Touring Perform OTC Women | Lots of movement | synthetic | – | – | 80 g | €34.90 |
| X-Socks Run Expert Ankle | Lots of movement | synthetic | – | – | 40 g | €25.75 |
Prices at our last check; current prices are in the shop.
Guide
How to put together your first layer
The first layer sits directly against your skin and determines whether you get cold in the lift after a run. Three questions will help you choose.
How much do you move?
If you sweat a lot while ski touring or cross-country skiing, synthetic fibres are a good choice: they absorb very little water and dry quickly. If you spend more time standing and waiting, in the lift or watching others, merino feels more comfortable because the wool still keeps you warm when slightly damp.
For a day of skiing with breaks, either works. Choose whichever feels better against your skin.
Shirt, trousers, sock: what goes together
| Item | What to look for | Why |
|---|---|---|
| Shirt | Close-fitting, with long sleeves for winter | Moisture is only transferred with skin contact |
| Trousers | 3/4 length for skiing | Only the sock should be inside the ski boot, without creases |
| Sock | One sock reaching over the calf | Two socks slip and put pressure on your feet |
What goes over it?
- Second layer for warmth: fleece or a thin down or synthetic-insulated jacket.
- On the outside, wear a jacket that keeps out wind and snow.
- It’s better to have one extra thin layer to take off than one thick layer that makes you sweat.
Last updated:
Good to know
Questions we often hear
Merino or synthetic: which is better?
Neither is better; they simply feel different. Synthetic fits close, absorbs very little water and dries quickly. Good if you move around a lot. Merino wool still feels warm when slightly damp and usually takes longer to smell. Good for long days with breaks. X-Bionic combines wool with its own technology in Symbio Merino and Mightywool.
Why wear 3/4-length trousers for skiing?
So that only the sock sits inside the ski boot. Long trousers bunch up in the shaft, and folds create pressure. 3/4-length trousers end above the boot; underneath, wear a long ski sock that reaches above the calf (OTC stands for over the calf).
Which sock should you wear in a ski boot?
One, not two. Two socks slip and create pressure points. The sock should reach above the calf and fit without folds. If your feet get cold quickly, choose merino, such as the Ski Perform Merino OTC. For ski touring, there are dedicated touring socks such as the Ski Touring Perform OTC.
How close-fitting should a baselayer be?
Close-fitting, but not restrictive. Only with skin contact can the first layer absorb and transfer moisture. If you are between sizes, check the size chart on the product page. X-Socks are available in size ranges such as 39–41 or 42–44.
How do I wash baselayers and socks?
Follow the care label. According to the product page, the merino models and socks in our selection should be washed on a gentle cycle at no more than 30 °C, while some synthetic models can be washed at 40 °C. No tumble drying, ironing or bleach, and preferably no fabric softener.
What should you wear over a baselayer?
The second layer keeps you warm: fleece, a lightweight down jacket or a synthetic insulated jacket. On the outside, wear a jacket that keeps out wind and snow. How much you wear over it depends on the weather and how much you move. The first layer stays the same all day.
Manufacturer information: X-Bionic: technologies · as of 1 October 2026. Product details from the product pages on sportfits.eu.
Product images: X-Bionic / X-Socks via sportfits.de