Running Underwear

Running Underwear: A Physiological and Functional Analysis

Choosing running gear usually focuses on shoes, but the base layer against the skin has a decisive effect on a runner’s performance, comfort and skin health. Dedicated running underwear (running briefs/boxers) is designed to meet the mechanical and thermal challenges of physical exertion.

Benefits

• Moisture management and sweat wicking: Unlike cotton, which absorbs liquid and stays wet, technical underwear is made of synthetic fibers (such as polyester and nylon) that wick sweat away from the body. This keeps the garment from getting heavier and maintains a stable body temperature.
• Preventing chafing: This underwear is designed with flat seams (flatlock seams) or no seams at all in sensitive friction areas. The anatomical construction reduces the risk of blisters and skin irritation in the groin and inner thighs.
• Mechanical support: Running involves multi-axis movement and oscillation. Dedicated underwear provides structural support for the organs, which reduces discomfort and excess movement that can lead to pain over the long term.
• Antibacterial properties: Most sports models incorporate silver fibers or chemical treatments that prevent the growth of odor-causing bacteria, maintaining better hygiene during long runs.

Downsides

• Pressure and restricted blood flow: If the underwear is too tight or has a rigid waistband, it can restrict capillary blood flow or press on lymph nodes in the groin area, which hurts recovery and comfort.
• The ventilation issue: With fabric that is too thick or a lack of ventilation panels (mesh), an overly hot and humid “microclimate” can form. This environment can encourage fungal infections (such as tinea cruris) if the gear doesn’t wick well enough.
• Reliance on chemicals: Some of the odor-repelling agents or softeners used in making synthetic fabrics can trigger an allergic reaction (contact dermatitis) in runners with especially sensitive skin.

Choosing running underwear is no less critical than choosing shoes. The right underwear will prevent the runner’s number one enemy — chafing — and keep the area dry and cool.

Below is a comprehensive comparison of the different fabric types, ranked from the most recommended for running to the one you should avoid entirely.

1. Synthetic fabrics (polyester and nylon) — the performance standard

Most professional sports underwear is made from a blend of these fabrics together with elastane (Lycra) for stretch.

Polyester

The most common material in sportswear.

• Pros: Excels at wicking sweat (moving moisture away from the body to the outside), lightweight, very durable in the wash and holds its shape.
• Cons: Tends to build up bad odors (bacteria) faster than natural fabrics, unless it has had an antibacterial treatment.
• Bottom line: A workhorse. The safe, common choice for most runners.

Nylon

Another synthetic material, usually smoother and softer to the touch than polyester.

• Pros: Very soft to the touch (a “buttery” feel), stronger and more tear-resistant than polyester, glides over the skin (excellent for preventing friction).
• Cons: Wicks sweat slightly less well than polyester (absorbs a little more water), but still excellent for running.
• Bottom line: The best choice for anyone with sensitive skin or looking for an especially soft feel.

2. Advanced natural materials (merino wool and bamboo) — comfort and premium

Merino Wool

The luxury choice. Very fine wool from New Zealand sheep.

• Pros: Temperature regulation (warms when it’s cold, cools when it’s hot), exceptional sweat wicking (it absorbs moisture into the fiber but leaves the skin dry), and above all — it doesn’t stink even after long runs (naturally antibacterial).
• Cons: Very expensive, less resistant to mechanical wear (holes) over time, dries more slowly than synthetics.
• Bottom line: Perfect for ultra runs, winter runs, or anyone who wants the very best and is willing to pay.

Bamboo / Modal

Fibers produced from wood cellulose (such as bamboo or beech). Sometimes called viscose or rayon.

• Pros: Extremely soft, hypoallergenic, feels cool on the body.
• Cons: Tends to lose its shape when wet, dries more slowly than synthetics. If the run is long and you sweat a lot, it can feel “heavy”.
• Bottom line: Great for easy runs, the gym or everyday wear, less so for marathon running.

3. Cotton — the big danger

• The situation: Cotton is an excellent fabric for everyday wear but a poor one for intense aerobic activity.
• The problem: Cotton is hydrophilic (water-loving). It absorbs sweat and doesn’t release it.
• The result: The underwear becomes heavy and wet, loses its shape and starts rubbing against the wet skin. This is a sure recipe for severe chafing in the groin.
• Bottom line: Avoid completely for running.

Summary comparison table

The Verdict

1. For everyday runs / training in the Israeli summer: Go with polyester or a polyester-nylon blend (such as snug compression underwear). Look for a “snug boxer” cut (boxer briefs) with at least a 6-inch inseam to prevent friction between the thighs.
2. For winter runs or very long runs: Consider merino wool. The comfort and lack of odor are worth the investment.
3. For sensitive skin: Try underwear based on modal/bamboo (for short runs) or quality nylon.
4. For very long runs: The longer the run — and in Zone 2 training it is usually the longest of the week — the more the accumulated friction adds up, and the quality of the seams matters more than the fabric itself. Also pay attention to the loss of sodium in sweat, which worsens skin irritation.

Part 1: Recommended models (from the top end to the best value)

I’ve divided the recommendations into categories, so you can choose what suits your budget and needs:

1. The undisputed king: Saxx (Kinetic HD model)

Saxx revolutionized the field with its patented “BallPark Pouch” — a kind of internal “hammock” that separates your anatomy from your thighs.

• Why it’s good: Completely prevents skin-to-skin contact in sensitive areas.
• Fabric: Nylon and spandex (semi-compression). Fits snugly but doesn’t squeeze.
• Suited to: Long runs, marathons, and people who suffer from chronic chafing.

2. The workhorse: Under Armour (Tech Mesh model)

Under Armour were the pioneers of wicking apparel, and their underwear is still excellent.

• Why it’s good: Made of a very thin, highly breathable mesh throughout. Its wicking is outstanding. No seams on the sides and back for maximum comfort.
• Fabric: Polyester and elastane.
• Suited to: Hot summer runs and people who sweat a lot.

3. The budget solution: Decathlon (Kalenji / Kiprun brand)

Decathlon offers dedicated running underwear at prices significantly lower than the big brands.

• Why it’s good: Excellent value for money. They do the job — they wick, fit well (“snug boxer”) and are comfortable.
• Fabric: Usually a polyester blend.
• Suited to: Beginner runners, everyday training, or anyone who needs to buy a lot of pairs without going broke.

4. Natural luxury: Smartwool (Merino 150 model)

For those who want the benefits of merino wool.

• Why it’s good: Amazing temperature regulation and a very soft feel.
• Suited to: Winter runs, hikes and ultra runs.

Part 2: Tips for care and preventing chafing (beyond the underwear)

Even with the best underwear in the world, a 20 km run in the Israeli summer can cause problems. Here are the iron rules:

1. The Long Leg Rule: If your thighs touch each other when you walk or run, don’t buy underwear in a “brief” (triangle) cut or short boxers (trunks).

• The solution: Look for snug boxers with a 6-inch (about 15 cm) or 9-inch inseam. The fabric must cover the area where your thighs rub.

2. Lubrication is your best friend: For long runs (over 10 km), it is recommended to apply a lubricant to friction areas (groin, inner thigh).

• Recommended: Body Glide — a dedicated stick that doesn’t stain clothes and is water/sweat resistant.
• Less recommended: Vaseline (because it’s greasy, clogs the fabric pores of wicking underwear, and is hard to wash out).

3. The “no underwear” option (lined shorts): Many running shorts come with a built-in inner brief (liner).

• If you buy shorts like that — don’t wear underwear underneath! It creates two layers of fabric that move against each other and cause chafing and unnecessary heat. Trust the built-in liner.

4. The right size: Running underwear must be snug. If it’s loose, the fabric will fold and rub the skin. If it’s too tight, it will cut off blood flow. It should feel like a “second skin”.