I Squatted, Sprinted, and Planked to Find the Best Workout Underwear — What Really Works

Table of Contents

1. Key Highlights:
2. Introduction
3. How I tested: motions, conditions, and criteria
4. What matters most in workout underwear (and why)
5. Standouts by category — what I recommend and when
6. How fabric science translates to the gym
7. Practical buying guide: pick by activity, fit, and priorities
8. Real-world examples and how choices played out
9. Sizing and fit: what to watch for and how to adjust
10. Caring for performance underwear to extend life and function
11. When to pick merino, when to pick synthetics, and when to accept cotton
12. Cost versus value: how much to spend and where to save
13. My top picks distilled (quick reference)
14. Final practical checklist before you buy
15. FAQ

Key Highlights:

• Tested a range of thongs, bikinis, briefs, and period options across strength training, sprints, cycling, yoga, and Pilates; the best performers balance stretch, moisture management, and low-profile construction.
• Natural fibers like merino and lyocell can outperform cotton for odor control and breathability, while modal blends and thin synthetic fabrics deliver the most “barely there” feel under leggings.
• Choose underwear by activity: high-absorbency or period styles for long, sweat-heavy sessions; thin modal/Lycra blends or nylon-Lycra thongs for low-profile needs; merino blends for odor control and temperature regulation.

Introduction

Workout underwear should be invisible: not just unseen under tight leggings, but also unobtrusive while you lunge, sprint, or hold a plank. I tested two dozen pairs over weeks of mixed training — heavy lifting, HIIT, cycling, Pilates, and hot yoga — to find which designs and fabrics actually perform. I prioritized pieces that reduce chafing, wick moisture, and resist odor while minimizing visible panty lines (VPL). I also tried to favor garments with higher proportions of natural fibers without sacrificing stretch or function.

The result is a set of concrete recommendations and buying principles based on real movement and sweat, not lab marketing copy. Below you’ll find which underwear rose to the top for particular workouts, why specific fibers matter, and practical sizing and care advice to get the most from each pair.

How I tested: motions, conditions, and criteria

Testing underwear requires movement in three planes: compression and hold (squats and weightlifting), dynamic displacement (sprints and agility), and sustained positions (yoga and Pilates). I used a consistent routine across items:

• Strength sessions: sets of squats, lunges, Romanian deadlifts and core work to test seam durability, front-panel bunching, and hip movement.
• Cardio intervals: repeated sprints and short hill runs to check for slipping, chafing, and sweat saturation.
• Low-impact flow: 45–60 minutes of yoga/Pilates and cycling to observe down-dog fit, fabric sliding, and VPL under tights.
• Hot conditions: a few sessions in warmer environments and a 60-minute treadmill session to test odor and moisture control.
• Daily wear: normal walking and errands to note comfort over longer stretches of time.

Evaluation criteria:

• Fit and comfort: waist and leg openings, whether seams dug in, and tendency to bunch or ride.
• Moisture management: how quickly the fabric moved sweat away from the body, whether dampness pooled.
• Odor resistance: how garments smelled after repeated sweaty sessions.
• Visibility/VPL: how the underwear showed under leggings and running shorts.
• Fabric makeup and construction: proportion of natural fibers, gusset composition, seam type, and presence/absence of elastic.

This method captured how underwear behaves when it matters: during friction, sustained sweat, and frequent stretching.

What matters most in workout underwear (and why)

Performance underwear is about trade-offs. No single fabric or cut wins every category — the right choice depends on the workout and your priorities.

Fabric composition

• Cotton: Comfortable and breathable but retains moisture and dries slowly. Not ideal for high-sweat workouts.
• Modal and Tencel (lyocell): Semisynthetic fibers derived from wood pulp. Soft, breathable, and better at wicking and drying than cotton.
• Merino wool: Natural fiber with excellent odor resistance, breathability, and thermoregulation. Lightweight merino blends wick moisture and dry fast; they also resist odors across multiple wears.
• Nylon and Lycra/Spandex: Provide stretch and shape retention. High nylon content delivers a silky, barely-there feel but is more synthetic.
• Modal/Lycra or modal/elastane blends: Combine softness with stretch; often what gives underwear a skim-silk, skinlike sensation.

Construction and seams

• Flat seams minimize dig-in and VPL. Look for bonded or laser-cut edges on thongs and bikinis.
• Gusset material matters: a cotton gusset helps reduce irritation and promotes breathability where you need it most.
• Elastic-free waistbands feel smoother under high-rise leggings but can reduce hold; conversely, a prominent elastic band can be visible through thin workout layers.

Cut and coverage

• Thongs and Y-back styles reduce VPL but can be less comfortable for long runs or when fabric bunches.
• Bikini and brief cuts offer more coverage and stability but risk VPL under body-hugging bottoms.
• High-waisted briefs give security and coverage for lifting and cross-training but sometimes feel bulky on fast runs.

Odor and absorbency

• Merino and specially-treated synthetics win for odor resistance.
• Period underwear is engineered for absorbency and is excellent for long or unpredictable workouts if you want added protection.

Fit and sizing

• Some brands run true to size; others require sizing down for a snuger fit (especially with merino or elastic-free waists).
• Compression that is too tight causes discomfort; too loose leads to bunching and chafing.

These elements determine whether a pair will stay put during a burpee, wick away sweat on a hot run, or remain invisible under a compression legging.

Standouts by category — what I recommend and when

Below are the top performers from the test, grouped by what they do best. I include specific observations about fabric mix, fit, and real-world performance.

Best overall for workouts: Branwyn Essential Seamless Merino Thong

• Why it stands out: Branwyn’s merino blend — roughly 81% merino, 14% nylon, 5% elastane — balances natural fiber benefits with enough synthetic content for structure and stretch. The thong offered light compression that kept fabric from shifting without feeling restrictive.
• Real-world use: On longer runs and layered under leggings during a chilly morning run, Branwyn performed well: it stayed in place throughout stride cycles and didn’t retain odor after repeated sweaty sessions. Machine washing softened the fabric over time without damaging the shape.
• Trade-offs: Slight texture compared to silky synthetics; if you prefer absolute smoothness against the skin, a modal or nylon blend will feel more “barely there.”

Best for natural-fiber performance (odor control + temperature regulation): Woolly Merino Thong

• Why it stands out: Merino’s natural odor resistance and thermoregulation are valuable for multi-hour training when showering between sessions isn’t an option. Woolly’s thong features an elastic-free waistband and a low percentage of elastane (about 5%), which creates a minimal profile.
• Real-world use: On a sweaty spin class and a later weight session, the Woolly thong didn’t smell as intensely as some synthetics did by the end of the day. It dries quickly and manages temperature—comfortable for both cool mornings and warm studio classes.
• Trade-offs: The elastic-free waistband can make the fit feel less anchored; consider sizing down if you prefer a tighter contour.

Best “undetectable” under leggings: InvisiWear / similar lightweight nylon-Lycra thongs

• Why it stands out: Thin, smooth fabrics with a high nylon/Lycra content and flat seams produce a weightless, second-skin feel. InvisiWear’s design felt more invisible than many — excellent for yoga and casual runs.
• Real-world use: During hot-room yoga and tight leggings, these pooled nearly zero VPL and didn’t cause digging at hip bones. The cotton gusset preserved breathability.
• Trade-offs: High synthetic percentages sacrifice natural fiber content; not ideal if you’re prioritizing sustainability or reduced synthetic exposure.

Best for bikini-style workout underwear (everyday thinness with good fit): Lululemon UnderEase Mid-Rise

• Why it stands out: Lululemon’s modal-Lycra blend is extremely thin and the seams are flat, giving a skinlike feel. The fit is reliable and rarely bunches for me.
• Real-world use: Worn under unlined running shorts and leggings, UnderEase showed minimal VPL and survived multiple high-intensity sessions without slipping.
• Trade-offs: Modal is a semisynthetic (wood pulp processed chemically), so it’s soft but not a fully natural fiber.

Best for higher coverage and security: Uwila Warrior Briefs

• Why it stands out: Uwila’s briefs have generous front and rear coverage with a lower-cut leg opening for more fabric at the hips — good for barbell and cross-training workouts where you want confidence and coverage.
• Real-world use: During a heavy lifting day, the Uwila brief felt secure and silky. Leg movement didn’t cause ride-up. The cotton gusset adds comfort during longer sessions.
• Trade-offs: The extra fabric can feel bulky on fast runs; not the best pick for VPL-sensitive outfits.

Best for period protection / extreme absorbency: Knix High-Absorbency Period Underwear

• Why it stands out: Engineered to trap moisture and fight odors, period underwear doubles as practical workout protection. It handles leaks and heavy flow while keeping moisture away from the body.
• Real-world use: On treadmill intervals when I was concerned about leaks, Knix performed reliably. The absorbent layer prevented embarrassing wet spots and reduced the need for pads.
• Trade-offs: Slight VPL is possible. Period underwear is heavier and less minimal than a thin thong or bikini.

Best for a cotton-forward, flexible option: Organic Basics Y-Back Thong (91% cotton)

• Why it stands out: A 91% cotton Y-back thong offered surprisingly good flexibility without chafing. The fabric provided a chafe-free experience for runs, Pilates, and yoga.
• Real-world use: For low- to medium-intensity runs and studio classes, this was comfortable and breathable. It avoided deep hip indentations and felt more natural than most synthetics.
• Trade-offs: Cotton retains moisture longer, so for intense, sweaty workouts or long-distance runs, it’s not ideal.

Best mid-price everyday performer: Tommy John Air Thong / Tommy John Women’s modal thongs

• Why it stands out: Modal-spandex mixes deliver softness and a smooth fit that’s forgiving across sizes. Tommy John’s construction is comfortable; when the sizing is right it performs well.
• Real-world use: When I sorted sizing correctly, these had excellent softness for long wear and didn’t bunch during low-impact cardio. They could have been a top contender if my size hadn’t felt slacker than expected.
• Trade-offs: Modal blends tend to bunch if sizing is off. Size down if you prefer a tighter hold.

Best budget-conscious cotton blends: Gap Organic Stretch Cotton Hipster; Pact Organic Cotton Lace Thong

• Why it stands out: These options offer natural-fiber comfort at lower price points. Pact’s lace-waist thong uses mostly organic cotton with a touch of elastane for stretch; Gap’s hipster is a reliable everyday choice.
• Real-world use: For errands, walking, and light gym sessions, both brands provided comfort and adequate durability.
• Trade-offs: Not the best for heavy sweat or intense cardio due to cotton’s moisture retention.

How fabric science translates to the gym

Merino wool

• Benefits: Natural antimicrobial properties, excellent breathability, controls odor, and adapts to temperature differences. Quick-drying merino blends are surprisingly light and comfortable for high-intensity efforts.
• Best use: Multi-hour outings, travel, and back-to-back workouts where odor and comfort across temperature ranges matter.
• Caveat: Pure merino can feel textured initially; blends with small amounts of elastane add structure.

Tencel/lyocell (e.g., Organic Basics)

• Benefits: Smooth, soft, moisture-wicking, and ecolabel-friendly processing methods. Dries faster than cotton and is kind to sensitive skin.
• Best use: Yoga, Pilates, moderate cardio when you want a natural-feel fiber that handles sweat without cotton’s sogginess.

Modal

• Benefits: Extremely soft and drapey, modal gives a silky feel and good breathability. It often forms the base of “everyday” performance underwear due to its comfort.
• Best use: Low- to medium-intensity workouts, running when paired with Lycra for stretch.

Nylon + Lycra/spandex

• Benefits: Offers stretch, shape retention, and a smooth, barely-there feel. Brands use this combination to achieve undetectable lines and seamless fits.
• Best use: Tight leggings, hot yoga, or any look where invisibility wins over natural fibers.
• Caveat: More synthetic content may trap odor more quickly and feels less “natural” to some.

Cotton

• Benefits: Soft, breathable initially, and familiar on the skin.
• Best use: Light activity, everyday comfort, or when you prefer natural fibers over performance features.
• Caveat: Absorbs and holds water, increasing friction and weight when wet.

Period/absorbent technologies

• Benefits: Layers designed to absorb and lock away moisture, with antimicrobial finishes to control odor. Many period underwear options function well as backup protection for long runs or intense workouts.
• Best use: Days with active periods, long workouts without easy access to replacements, or as redundancy for endurance events.

Practical buying guide: pick by activity, fit, and priorities

Match fabric and cut to the workout:

• High-intensity interval training and running: choose moisture-wicking, quick-drying fabrics like merino blends, modal-Lycra, or performance synthetics with a cotton gusset. If VPL is a concern, thin nylon-Lycra thongs or bonded-seam thongs work well.
• Hot yoga and Pilates: smooth and thin fabrics with flat seams. Modal/Lycra or nylon blends feel best.
• Weightlifting and CrossFit: more coverage can feel secure; briefs or higher-waisted underwear that provide stability and avoid shifting during hip hinge motions.
• Long runs, travel, or back-to-back sessions: merino blends or period undergarments for odor control and moisture handling.
• Menstrual days: use period underwear or combine a thin performance thong with a tampon/cup depending on flow and comfort.

Choosing a cut

• Thongs (Y-back, string, mini-string): best for minimal VPL and thin workout layers. Good for leggings and ruled by fit — a poor fit can lead to chafing.
• Bikini: a compromise between coverage and invisibility; versatile for varied activities.
• Briefs/high-rise: give security and covered front/rear — useful for heavy lifts and high-movement sessions.

Pay attention to the gusset

• A cotton or breathable gusset reduces irritation and helps with airflow. Look for a cotton-lined gusset even in otherwise synthetic underwear.

Seams and waistband

• Flat seams and bonded edges reduce VPL and abrasion. Elastic-free waistbands feel smooth under clothing, but elasticized waistbands that are low-profile can help keep underwear anchored.

Number of pairs and rotation

• Rotate underwear frequently: wash after heavy sweat sessions and don’t rewear without airing. For long training cycles or travel, pack merino or period options that resist odor longer.

Budget vs. performance

• High-performance pieces cost more, but they often last longer and perform better during intense training. Consider a mixed drawer: a couple of high-quality pairs for sweaty or long workouts, and budget-friendly cotton/modal pieces for lighter activity.

Real-world examples and how choices played out

Scenario 1 — Hot spin class followed by errands:

• Winner: Woolly merino thong or Lululemon UnderEase. The merino handled odor for the rest of the afternoon, while UnderEase felt skinlike for short duration. If you expected another sweaty session later, merino held up better odor-wise.

Scenario 2 — Heavy leg day at the gym:

• Winner: Uwila Warrior briefs. The full coverage and silky fabric prevented shifting during walking lunges and barbell squats. Thong options felt less secure for this workout.

Scenario 3 — Long, layered run and travel day:

• Winner: Branwyn merino thong. The combination of odor resistance and quick drying made it ideal for a run followed by a long day of meetings and travel, with less noticeable odor after several hours.

Scenario 4 — Hot yoga:

• Winner: Thin nylon-Lycra or modal-Lycra thong (e.g., InvisiWear or Lululemon UnderEase). They offered minimal VPL and didn’t bunch in inversions. For those preferring natural fibers, a Tencel option like Organic Basics performed well but showed a slightly more prominent waistband on some designs.

Scenario 5 — Period day and treadmill intervals:

• Winner: Knix period underwear. The absorbency layer prevented leakage and allowed for a high-intensity session without the stress of pad failure.

These examples show how fabric and cut choices translate to different workout needs. No single garment dominated all scenarios, which is why a diversified underwear drawer is the most practical approach.

Sizing and fit: what to watch for and how to adjust

Sizing issues arise more often with thin, elastic-free waists and with merino items that have less built-in stretch. Follow these principles:

• When a garment has an elastic-free waistband or a looser cut (merino often behaves this way), try the next size down if you prefer a snug fit that reduces fabric movement. If you’re between sizes and the brand’s fabric has little elastane, size down for better containment.
• Modal and Lycra blends tend to be forgiving. If a pair has more Lycra, your normal size will likely fit well, and sizing down risks uncomfortable compression.
• Watch the gusset fit. If the gusset seems too roomy, the underwear will shift. If it’s too flat or the seams press, the cut may be wrong for your anatomy.
• Try different cuts for the same brand. Thongs, bikinis, and briefs from the same brand can fit differently because of seam placement and waistband height.

If shopping online, read reviews for fit cues: look for notes on whether a style runs large or small and whether waistband elasticity is strong or weak.

Caring for performance underwear to extend life and function

Proper care preserves wicking ability, stretch, and seam integrity:

• Wash cold on a gentle cycle or hand-wash delicate fabrics like merino and modal blends.
• Use mild detergent; avoid fabric softeners and dryer sheets — they coat fibers and reduce absorbency and breathability.
• Air-dry when possible. Heat from dryers breaks down elastane sooner and can shrink or distort merino and modal.
• For merino, follow manufacturer recommendations; many merino blends tolerate machine washing but benefit from a gentle cycle and low heat or air drying.
• Treat period underwear according to brand instructions; rinse immediately after use when possible and avoid bleach that damages absorbent layers.

Replace timing

• Replace when elastic loses shape, seams begin to fray, or the fabric no longer returns to its intended form. Frequent heavy use reduces longevity; rotating pairs extends usable life.

When to pick merino, when to pick synthetics, and when to accept cotton

Merino

• Choose merino for stink resistance and temperature variation. Ideal for multi-session travel days, longer runs, and situations where you can’t change immediately after sweating.

Synthetics (nylon/Lycra)

• Choose synthetics for a weightless, invisible look and when extreme thinness under tights matters. They provide the least VPL and the smoothest finish.

Modal/Tencel

• Choose modal or Tencel if you want a natural-feeling fabric that still handles sweat better than cotton. These are good for studio classes and casual runs.

Cotton

• Choose cotton for low-intensity, comfort-first days. Avoid for hot weather endurance or multiple sweaty sessions in a day.

Period underwear

• Choose period underwear for protection during workouts when flow is expected or when you want the safety of an absorbent layer.

Cost versus value: how much to spend and where to save

You can find functional workout underwear across a range of prices. Spending more generally gives you better construction, more durable elastane, and higher-quality natural fibers. Here’s a pragmatic approach:

• Spend on the items you use during the most demanding workouts (e.g., long runs, heavy lifting days). A $30–$40 merino or modal performance pair often outlasts multiple budget pairs.
• Save on underwear you use for casual wear or light activity. Gap, Pact, and similar brands offer good cotton/modal options at lower prices.
• Consider period underwear as an investment if you use it frequently; it replaces pads and reduces stress during activity but costs more.

A balanced drawer: two high-quality performance pairs (merino/modals), two undetectable synthetics for leggings/high-intensity studio classes, and a few everyday cotton/modal pairs rounds out most needs.

My top picks distilled (quick reference)

• Best overall for mixed workouts: Branwyn Essential Seamless Merino Thong
• Best for odor resistance and temperature control: Woolly Merino Thong
• Best undetectable under leggings/yoga: InvisiWear or similar high-nylon/Lycra thongs
• Best everyday bikini-style performance: Lululemon UnderEase
• Best high-coverage security: Uwila Warrior Briefs
• Best period/absorbent option: Knix high-absorbency underwear
• Best cotton-forward option: Organic Basics 91% cotton Y-back thong
• Best value everyday: Gap Organic Stretch Cotton Hipster; Pact organic cotton thong

Final practical checklist before you buy

• Identify your primary activity: intense sweat, hot studio work, long runs, heavy lifts, or menstrual protection.
• Choose fabric accordingly: merino for odor and range of temps; modal/Tencel for soft, breathable performance; nylon blends for invisibility.
• Check gusset and seam construction: cotton gusset and flat seams reduce irritation and VPL.
• Read sizing notes and consider trying two sizes if the brand is unfamiliar.
• Buy one performance pair and one backup in a different fabric to compare in your training context.
• Care for them right: cold wash, avoid fabric softeners, air dry.

FAQ

Q: Is merino underwear really worth it for workouts? A: Yes, if odor control and temperature regulation matter. Merino resists odor and manages moisture better than cotton; merino blends that include small percentages of nylon and elastane add durability and stretch. For multi-session days or travel, merino delivers clear advantages over cotton and many synthetics.

Q: Should I wear a thong to run? A: A thong can be an excellent running choice if it fits well and has flat seams and a breathable gusset. Thin, well-constructed thongs minimized VPL and stayed put during sprints and long runs in testing. If you’re prone to chafing or prefer more coverage, a streamlined bikini or brief may be better.

Q: Are period underwear options appropriate for workouts on non-period days? A: Yes. Period underwear can be an effective sweat-management tool because it’s designed to absorb and lock moisture. It’s heavier than a standard thong or bikini, so expect slightly more bulk and possible VPL. Use when you want extra security or for long workouts without easy access to changes.

Q: How do I prevent chafing? A: Choose underwear with a snug but not constrictive fit, soft flat seams, and a breathable gusset. Fabrics that wick moisture away (merino blends, Tencel, modal, and some synthetics) reduce skin-on-skin friction. Apply anti-chafing balm to areas that rub, and avoid cotton for sweat-heavy sessions.

Q: How often should I replace workout underwear? A: Replace when elastic sags, seams fray, fabric thins excessively, or the piece no longer fits as intended. With proper care and rotation, performance underwear can last 12–24 months depending on frequency of use and fabric. Merino and high-quality modal may maintain function longer, while low-cost cotton blends may wear sooner.

Q: Does modal count as a natural fiber? A: Modal is a semisynthetic fiber made from beech wood pulp. It’s processed chemically but originates from natural cellulose. Modal often feels soft and behaves more like a natural fiber in terms of comfort and breathability. It’s not the same as 100% natural fibers like cotton or merino, but it’s a useful middle ground.

Q: Are elastic-free waistbands better? A: Elastic-free waists offer a smooth profile under clothing and can prevent digging, but they may not provide the hold some people need for dynamic movement. If you prefer less compression and a cleaner look under thin leggings, try an elastic-free style but test for secure fit; sizing down may help.

Q: Can I machine dry performance underwear? A: Avoid high heat. Machine drying on low or tumble-dry with care may be acceptable for some synthetics, but air-drying is safest, especially for merino and modal blends. Heat degrades elastane and shortens lifespan.

Q: How many pairs should I own for regular training? A: For regular training (3–5 workouts per week), a minimum of five pairs is practical: three to rotate through workouts and two backups. Add more if you prefer day-specific fabrics (e.g., merino for travel days, period underwear for menstrual days).

Q: What makes a pair of underwear “invisible” under leggings? A: Thin fabrics, minimal seams, laser-cut edges, flat or bonded seams, and minimal rear coverage contribute to invisibility. Nylon-Lycra blends and bonded thongs tend to perform best. A smooth, low-profile waistband helps too.

Deciding on workout underwear comes down to matching materials and cuts to the activity and personal comfort. The best pieces don’t call attention to themselves: they move with you, manage moisture, and let you focus on the session. Build a small, intentional drawer with a couple of high-performance pairs and a selection of versatile, more affordable options to cover every training scenario.