What the research says
Breast support: what a sports bra does to motion
In a laboratory study, 20 women walked at 5 km/h and ran at 7 and 10 km/h on a treadmill while markers on the breasts and trunk were tracked in three dimensions. They were tested bare-breasted, in an everyday bra and in a sports bra. The sports bra conditions showed the lowest movement values. For vertical displacement at 10 km/h, the sports bra gave reductions of 56% against the bare breast and 43% against the everyday bra, while footwear made no difference within each support condition (Leme et al., 2020).
A scoping review of 23 studies adds context. It reports that the breasts are named as the fourth largest barrier to taking part in physical activity, and that although more support reduces breast movement, motion and bra fit remain the most common breast concerns, probably because of limited knowledge when choosing (Haworth et al., 2024).
Asymmetry and adjustability
Breasts do not move identically. Among 167 women running on a treadmill, movement asymmetry was present in 89%, most often (60% to 75%) from greater movement of the left breast. In a sub-group of 12 participants, sports bras reduced the share showing front-to-back asymmetry from 75% to 33%, but vertical asymmetry only from 75% to 67%. The authors recommended that sports bras allow adjustment of the underband and straps to facilitate individual breast support (Exell et al., 2021).
The abstract describes the top-performing bra as having encapsulating cups as well as adjustable straps and underband, so adjustability may not be the only difference between the bras. This finding concerns bra design features in a research setting. It does not tell us how any particular seamless bra performs.
Fit, sizing and education
In a cross-sectional study of 104 women, 85% were wearing ill-fitting bras, and sizes chosen by the women themselves or by measurement systems differed significantly from the size set by professional fitting criteria (p<0.001). Trying on several bras or using sizing measurements did not close the gap (McGhee and Steele, 2010).
In a cluster-randomised trial of 115 adolescent female athletes, a booklet on bra fit improved bra knowledge by 19% more than in the control group at four months. It also raised the share who passed a fit assessment (39% more) and a breast-support test (30% more), though self-rated breast discomfort during exercise did not differ (McGhee et al., 2010).
A smaller pre-clinical study of 24 asymptomatic larger-breasted women found that everyone failed a bra fit assessment in their usual bra and 67% still failed in a professionally fitted one. Fewer fit problems gave immediate changes in measured breast support and spinal posture, but nothing at the 28-day point (Haworth et al., 2022). These studies suggest that fit problems are common and that fitting education and professional fitting changed some measures, but not all, in the groups studied.
A small randomised trial of 18 breast pain patients at a UK hospital found no between-group difference in breast pain or global impression of change after a prescribed bra was worn every day for eight weeks, though quality of life improved within the bra-prescription group (Sharland et al., 2021). This article does not claim that any garment treats or relieves breast pain.
Washing and wear: does support fade?
An intervention study with 22 women compared identical sports bras that were kept as a control, washed, or worn and washed. After 25 washes (19 participants completing), breast motion rose by 20% sideways and 16% vertically in the washed bras, and by 32% sideways and 25% vertically in the worn-and-washed bras. Participants noticed reduced support (Wakefield-Scurr et al., 2024).
Sports bra support measured when new is not the same as support measured after 25 washes in the bras tested. The abstract does not report a comparison of care methods, and it does not predict how long any specific garment keeps its shape.
Seamless knit construction: pressure and heat
The seamless-knitting studies we found concern fabrics and prototypes, not finished sports bras. One 2025 study used seamless knitting to make patterned compression fabrics and measured pressure with sensors on a leg mannequin. Pressure and thermal conductivity were significantly influenced by the number of yarns and by yarn material, but not by the knitted pattern. Adding one end of one yarn type raised pressure by 2.03 mmHg, and one pattern with an extra float-stitch needle had 0.023 kPa·s/m lower resistance to airflow than another (Li et al., 2025).
A separate 2025 study made 18 seamless circular-knitted fabric samples and garment prototypes with a temperature-regulating viscose/cotton yarn. A 1 + 3 mock rib structure showed superior thermal resistance and comfort, and a 50/50 blend with 50 s yarn count performed relatively well across the tests (Cheng et al., 2025). In these two studies, yarn choice and knitted structure affected some measured properties, but neither study tested a finished gym garment worn for exercise.
Stretch, sizing and pressure
Size can affect how much squeeze a garment applies, depending on garment type. In 12 national-level boxers wearing compression garments in three sizes, oversized tights applied significantly less pressure than recommended-size or undersized tights (P<0.001), while leggings showed no significant differences between sizes. Standing produced higher mean pressure than sitting for both garment types (Brophy-Williams et al., 2015). These were dedicated compression garments, so the result should not be read as describing ordinary gym leggings.
Stretch and opacity: what the nearest evidence shows
We found no published study that directly measures see-through behaviour of leggings at full stretch. The closest evidence is laboratory work on ultraviolet protection, which depends on how open a fabric's structure is. In one study, black knitted fabrics lost 53% of their UV protection factor on average at 10% stretch, while fabrics containing miss stitches retained good protection even when stretched by 20% (Wong et al., 2015). A second study found that stretching dry weft-knitted fabrics made from cotton and from cotton blended with another yarn reduced UV protection noticeably, because pores opened up, and that greater stretch brought a greater reduction (Kan et al., 2013).
Two points follow. First, stretching does open knitted fabric, so the knitted structure matters (in one study, fabrics with miss stitches kept good UV protection at 20% stretch). Second, UV transmission is not the same as visual opacity, so these studies explain a mechanism and should not be taken as a rating of any garment.
What this means when choosing
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Treat fit as something to check, not guess. In one study neither self-selection nor sizing measurements matched professional fitting criteria, so a professional fitting is worth considering.
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Look for adjustability. The authors of the asymmetry study recommend adjustable underbands and straps.
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Match support to the activity. The treadmill reductions were reported for running at 10 km/h, so consider the demands of your own activity.
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Fibre blend and measured comfort. In one comparison of four knitted fabrics for cycling wear, fibre blend ratios were associated with differences in measured comfort properties (Teyeme et al., 2020).
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A practical suggestion on see-through behaviour (ours, not a finding). UV protection fell when knitted fabrics were stretched, but no study we found tests see-through behaviour. Because of that gap, one practical option is to check any garment in good light while it is stretched.
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Know that support can change. Sports bra support fell after 25 washes in the bras tested.
Limits of the evidence
- We found no study measuring fabric opacity or see-through behaviour of leggings under stretch. The UV studies explain a mechanism only.
- No study we reviewed evaluates seamless knit construction in sports bras specifically. The seamless evidence concerns fabric pressure and thermal behaviour.
- The fabric studies are laboratory tests on fabric samples, prototypes, a leg mannequin or a thermal manikin, not trials with people wearing garments.
- Several bra studies involve small groups or specific populations, such as adolescent athletes or larger-breasted women, so results may not apply to everyone.
- The pre-clinical and pain studies are small, and none of this evidence supports health claims for clothing.
The Free Muscle product in this context
Free Muscle sells research compounds and gym apparel online. The seamless sportswear item relevant to this article is the Free Muscle Women's Seamless Set, a two-piece matching set of leggings and a sports bra. Everything below is taken from the product page; none of it is backed by the research above. None of the studies above tests this garment.
- It is a two-piece matching set of leggings and a sports bra, described as a seamless knit with four-way stretch.
- The leggings have a high-rise waistband.
- Sizes run from XS to XL. The page suggests sizing down if you are between sizes and want extra compression; the page text includes no size chart.
- Care guidance is to machine wash cold, inside out, and hang to dry.
The page does not state the fibre composition, fabric weight, a size chart with measurements, or details of bra support such as cups, straps or closures. For that reason, none of the research above can be mapped onto this set's materials, and the studies should be read as background.
Frequently asked questions
What does seamless knitting mean in the research?
In the studies reviewed here, seamless knitting refers to fabrics and garment prototypes made with seamless knitting technology or on a seamless circular knitting machine (Li et al., 2025; Cheng et al., 2025).
Does seamless sportswear stop chafing?
The studies we reviewed do not test chafing, so we cannot support a general claim either way. How a garment feels depends on fabric, fit and the person.
Can leggings become see-through when stretched?
In UV-protection studies, stretching opened pores in knitted fabrics (Kan et al., 2013). That is a UV measurement, not an opacity test; how it affects see-through behaviour depends on the knitted structure. One practical option, which is our suggestion and not a study finding, is to check any garment in good light while it is stretched.
How much does a sports bra reduce breast movement?
In a treadmill study of 20 women, a sports bra reduced vertical breast displacement at 10 km/h by 56% against bare breasts and 43% against an everyday bra (Leme et al., 2020). Results in a laboratory study may differ between bras and people.
How do I know if my sports bra fits?
Self-selection is unreliable: in one study 85% of women wore ill-fitting bras (McGhee and Steele, 2010). Trying on several bras or using sizing measurements did not close the gap in that study, so a professional fitting is worth considering.
Does washing change how supportive a sports bra is?
In one study of sports bra support, breast motion rose after 25 washes, more so when the bras were also worn, and participants noticed less support (Wakefield-Scurr et al., 2024). The abstract does not report a comparison of care methods, so it cannot say how long any garment keeps its support.
Should I size down in leggings for a firmer feel?
In a study of 12 boxers wearing dedicated compression garments, oversized tights applied less pressure than other sizes, while leggings showed no significant difference between sizes (Brophy-Williams et al., 2015). That study does not describe ordinary gym leggings. For the Free Muscle set, the page suggests sizing down if you are between sizes and want extra compression; the page text includes no size chart.
How does Free Muscle handle delivery and returns?
UK delivery is free and tracked, in plain packaging, and standard UK delivery takes 2 to 5 working days. Orders placed before 2pm UK time Monday to Friday are processed the same working day (see shipping information). Unopened items in their original packaging can be returned within 30 days (see returns information); for apparel see the refund policy.
This article is published by Free Muscle, which sells the product discussed. It is general information, not medical advice. Free Muscle also sells research compounds for laboratory research use only, to customers aged 18 or over; they are not medicines or food supplements and not for human consumption.
Sources
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- Wakefield-Scurr J, Hamilton C, Reeves K, et al. The effect of washing and wearing on sports bra function. Sports Biomech. 2024;23(11):2420-2430. doi:10.1080/14763141.2022.2046147 PMID 35227159
- McGhee DE, Steele JR, Munro BJ. Education improves bra knowledge and fit, and level of breast support in adolescent female athletes: a cluster-randomised trial. J Physiother. 2010;56(1):19-24. doi:10.1016/s1836-9553(10)70050-3 PMID 20500133
- Sharland E, Burbage J, Noblet M, et al. Multi-study randomised controlled trial to develop, implement and evaluate bra prescription to reduce breast pain and improve quality of life. Complement Ther Clin Pract. 2021;43:101346. doi:10.1016/j.ctcp.2021.101346 PMID 33691268
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- Brophy-Williams N, Driller MW, Shing CM, et al. Confounding compression: the effects of posture, sizing and garment type on measured interface pressure in sports compression clothing. J Sports Sci. 2015;33(13):1403-10. doi:10.1080/02640414.2014.990489 PMID 25530213
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- Haworth L, Aitkenhead R, Grecic D, Chohan A. Understanding experience, knowledge and perceived challenges related to bra fit for sports participation: a scoping review. Res Sports Med. 2024;32(2):302-315. doi:10.1080/15438627.2022.2102922 PMID 35856556
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- Teyeme Y, Malengier B, Tesfaye T, et al. Comparative analysis of thermophysiological comfort-related properties of elastic knitted fabrics for cycling sportswear. Materials (Basel). 2020;13(18). doi:10.3390/ma13184024 PMID 32927922
