What "High Impact" Has to Mean in Centimetres: Sports Bra Support by Cup Size
Low, medium and high impact are marketing categories with no defined threshold behind them. Published breast-biomechanics work puts unrestrained vertical displacement at roughly 2 to 4 cm walking and 8 to 12 cm running, rising with cup size, and a construction has to cut that by 55 to 75 percent to be worth the label. Compression alone stops working around a UK D; above a UK G only wired encapsulation reaches the target.
By the Think Bra Editorial Team
There is no threshold behind the word "high". No governing body defines it, no test protocol sits under it, and a manufacturer can print it on a single-layer compression crop top and on a wired encapsulated bra with a four-hook back on the same product page. The label describes an intention.
What can be measured is displacement. Published breast-biomechanics research consistently finds that breast tissue moves in a figure-of-eight during gait, that the vertical component dominates, and that unrestrained vertical displacement runs to roughly 2 to 4 cm when walking and somewhere in the region of 8 to 12 cm when running, with larger cups sitting toward and above the top of those ranges. A well-built sports bra typically halves that; the best encapsulated designs cut it by around three-quarters.
So the useful question is not which label to buy. It is: how much movement does this activity produce at my cup size, what percentage reduction does that require, and which constructions actually deliver it at that cup size. That is what the matrix below sets out, and the answer changes completely as the letters climb — because compression, encapsulation and wired encapsulation do not scale the same way.
Why the label carries no information
Three specific reasons, all structural.
No defined test. Impact ratings are not measured against a protocol. Where displacement testing exists it is done in-house, at whatever cup size the brand's fit block sits at, and the result is a marketing input rather than a published figure.
No cup-size qualifier. A construction that reduces displacement by seventy percent at a 34B may manage forty percent at a 34G, because the load has roughly tripled while the band width, fabric modulus and strap anchor points scaled by pattern increment. The same garment genuinely is high-support at one coordinate and low-support at another, and the label cannot express that.
No distinction between build types. Compression flattens tissue against the chest wall and controls it as a single mass. Encapsulation holds each breast in its own shaped cup and controls it individually. They fail in different ways and at different points, and both get sold under the same three words.
Everything below therefore ignores the tag and works from displacement, cup band and build.
How much a breast actually moves
The figures worth carrying around are these. Walking produces a few centimetres of vertical travel. Jogging roughly doubles it. Running produces the largest vertical component of common activities, and multi-directional court sport adds substantial side-to-side and front-to-back movement on top of a vertical figure that is already high.
Mass matters more than most people expect. Breast tissue has no muscle in it and is anchored by skin and connective tissue, so the whole load is carried by structures that are not designed to be load-bearing. Roughly speaking, a cup at UK G holds around three times the volume of the same band at B — and the peak force during running is a multiple of static weight, not equal to it.
Two practical consequences follow. First, a construction sized on volume alone under-specifies at large cups. Second, displacement figures quoted without a cup size attached are close to meaningless, which is why the matrix below is indexed on both.
The activity-to-cup matrix
Displacement figures are ranges drawn from published breast-biomechanics work and vary substantially between individuals — treat them as the scale of the problem rather than a personal prediction. The reduction column is the target a construction has to hit to be worth its label at that coordinate. The Hold Index column grades the construction class named in the row, at that cup band.
Activity Impact class Cup band (UK) Unsupported vertical displacement Reduction the build must achieve Minimum build that gets there Hold Index of that build at this cup band Walking Low A – C 1 – 3 cm 40% Compression bralette 74 Walking Low D – DD 2 – 4 cm 45% Compression, reinforced 70 Walking Low E – G 3 – 6 cm 50% Encapsulation, wire-free 74 Walking Low H and above 4 – 7 cm 55% Encapsulation, wire-free 65 Jogging Medium A – C 3 – 5 cm 50% Compression, reinforced 79 Jogging Medium D – DD 5 – 8 cm 55% Encapsulation, wire-free 78 Jogging Medium E – G 6 – 10 cm 60% Combined encapsulation and compression 83 Jogging Medium H and above 7 – 11 cm 65% Combined encapsulation and compression 76 Running High A – C 5 – 8 cm 55% Compression, reinforced 79 Running High D – DD 7 – 11 cm 60% Combined encapsulation and compression 84 Running High E – G 9 – 13 cm 70% Wired encapsulation 88 Running High H and above 10 – 15 cm 75% Wired encapsulation 86 HIIT and plyometrics High A – C 5 – 9 cm 60% Compression, reinforced 79 HIIT and plyometrics High D – DD 8 – 12 cm 65% Combined encapsulation and compression 84 HIIT and plyometrics High E – G 9 – 14 cm 70% Wired encapsulation 88 HIIT and plyometrics High H and above 11 – 15 cm 75% Wired encapsulation 86 Riding Medium to high A – C 4 – 7 cm 55% Compression, reinforced 79 Riding High D – DD 6 – 10 cm 60% Combined encapsulation and compression 84 Riding High E – G 8 – 13 cm 70% Wired encapsulation 88 Riding High H and above 9 – 14 cm 75% Wired encapsulation 86 Court sport Medium to high A – C 4 – 8 cm 55% Compression, reinforced 79 Court sport High D – DD 7 – 11 cm 65% Combined encapsulation and compression 84 Court sport High E – G 8 – 13 cm 70% Wired encapsulation with a high side wing 88 Court sport High H and above 10 – 15 cm 75% Wired encapsulation with a high side wing 86 Find your activity, then your cup band, and buy the build in column six or better. Anything below it will change the reduction figure, not the label on the tag. Read down the running block and the point of the whole page appears. At A to C, a reinforced compression bra does the job and scores well. At E to G the same construction is not on the list at all, because it cannot reach a seventy percent reduction on that load however well it is made. The requirement did not creep upward — it crossed a threshold where one build type stops being a candidate.
The riding rows deserve a note. The vertical figures are not the largest on the table, but the movement is rhythmic and sustained over hours rather than intermittent, which shifts the binding constraint from peak displacement to band comfort across a long wear. That is why the Hold Index component weighting matters as much as the headline number.
The five builds, graded
Construction class is the variable that actually determines the outcome, and each class has a cup letter beyond which it stops meeting its own target.
Build How it is constructed Cup ceiling (UK) Hold Index A – C Hold Index D – DD Hold Index E – G Hold Index H+ What fails first above the ceiling Compression Single-layer stretch panel, no shaped cups, pull-on, narrow band D 74 61 44 29 The band rolls and rides, and tissue moves as one mass rather than being held Compression, reinforced Double-layer front panel, wider elasticated band, racer or cross back F 79 70 55 38 Vertical displacement climbs past target and the upper edge begins to cut Encapsulation, wire-free Separate shaped and seamed cups, moulded or three-part, adjustable straps, hook-and-eye back GG 76 78 74 65 Cup structure loses its shape under peak load; the gore lifts Combined Encapsulated inner cups under a compressive outer layer, wide band, adjustable straps H 80 84 83 76 Band elongation under sustained load; straps start taking share they should not Wired encapsulation Sports-gauge wire, internal sling, high side wing, wide multi-hook band, adjustable straps Graded to K, the deepest coordinate the standard covers 77 85 88 86 Nothing within the graded run; above K there is no published coordinate to grade Pick the row whose ceiling sits above your cup letter, then compare Hold Index in your own column. A build scoring below 50 at your coordinate is not a support garment at that size, whatever the tag says. The compression row is the one that surprises people, because compression bras are cheap, comfortable and everywhere. They are genuinely good at A to C. The score falls off a cliff by E because the mechanism itself has a limit: flattening tissue against the chest wall works while the mass is small enough for fabric tension to hold it, and stops working when it is not, regardless of how many layers get added.
The wired encapsulation row carries the other counterintuitive result — it scores lower at A to C than three of the other builds. Wire, sling and a four-hook back are structure that a small cup does not need, and the comfort cost across an eight-hour wear pulls the score down. Buying more construction than the load requires is its own error.
How the Hold Index is put together
The Index runs 0 to 100 and grades a construction class rather than an individual garment, so a specific bra can sit either side of its class. The weights are published so the score can be argued with.
Component Weight What is assessed Full marks Zero marks Vertical displacement control 40% Reduction in vertical displacement against the unsupported figure for that cup band and activity 75% reduction or better Under 30% reduction Cup-size ceiling 25% The deepest cup letter at which the construction still meets its displacement target Holds to K Falls below target at D Band tension across eight hours 20% Change in band holding force, and pressure marking, at hours one, four and eight No measurable loss of force and no marking Loses more than a third of holding force, or marks the skin by hour four Elastic retention over 100 wears 10% Band length at rest after 100 wear-and-wash cycles against nominal Within 3% of nominal More than 8% longer than nominal Strap load share 5% Proportion of vertical load carried by the straps rather than the band Under 15% Over 40% Use the weights to decide which score matters to you. Somebody riding for four hours should read the band-tension component before the displacement one; somebody sprinting intervals should do the reverse. The band is doing the work here too
Everything on this page routes back through the band, and sports bras get it wrong more often than everyday bras do — partly because so many are pull-on with no adjustment at all.
A pull-on compression bra has one circumference and no hooks. As soon as the elastic gives, there is no way to take it up, so the garment loses holding force continuously across its life with no correction available. That is why the elastic-retention component exists in the Index and why it is worth paying for a hook-and-eye back on anything above a D.
Band width matters more here than in an everyday bra because peak load is higher. A wide band spreads the same tension across more skin, which raises the tension you can tolerate before it marks — and tension is what generates holding force. A narrow band under a large cup either rolls or hurts, and usually both.
Straps should be adjustable and should be adjusted downward, not upward. If tightening the straps is what makes a sports bra feel supportive, the band is too loose and you are loading your shoulders through an entire session.
One test worth running before you commit: put it on, jump ten times, and watch the band at the back in a mirror. If it has climbed, that garment will fail you at minute thirty of anything.
Sizing a sports bra against your everyday size
Start from your measured band and cup rather than from your everyday bra's label, because a sports range is graded from its own block and often a different one from the brand's everyday line.
Take the same band, not a smaller one. A common instinct is to size the band down for extra hold; what that actually produces is a band that cannot be worn for the length of a session and gets loosened mid-way, which is worse than starting correct.
Expect the cup to run differently. Encapsulated sports cups are usually drafted with less projection and more spread than everyday cups at the same letter, because a shape that is flatter against the chest wall moves less. If you sit between letters, take the larger one in an encapsulated build and the smaller one in a compression build.
Do not buy a compression bra a size down to compensate for cup letters it does not offer. A compression crop that stops at alpha sizing has no coordinate for a G, and squeezing into a smaller one converts a support problem into a breathing one.
Replace on the thresholds rather than on the calendar. A sports band takes far more load cycles than an everyday one and reaches its retirement point sooner — often noticeably sooner if it is washed hot or tumble dried, both of which shorten elastane life.
Choosing without the label
- Work out your activity's row in the matrix and read the required reduction. That is the specification.
- Read the build column, not the tag. If the garment does not physically have separate shaped cups, it is compression, whatever the marketing says.
- Above a UK F, treat any pull-on garment with no back closure as a low-support product regardless of its label.
- Check the band width against the cup letter. Narrow bands and deep cups is the combination that rolls.
- Count the hook rows. One row means no adjustment as the elastic gives; three or four means the garment stays serviceable considerably longer.
- Look for an internal sling and a tall side wing on anything E and above — those are the two components that hold tissue at the underarm under lateral load.
- Jump test it in the changing room, watch the back of the band, and buy the one that stays level.
- If the Hold Index of the build you are holding is under 50 at your cup band, put it back. It is not a support garment at your coordinate.