By Priya Anand, Backpacking Gear Research Writer · Published September 14, 2026, 6:03 PM ET · Updated September 29, 2026, 6:16 PM ET
Direct Answer
A sternum strap is the adjustable horizontal strap connecting a backpack's two shoulder straps across the chest. It keeps the shoulder straps from sliding outward off your shoulders, and it slides up or down a track on each strap so it can sit an inch or two below the collarbone regardless of body size.
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In plain English
The sternum strap is the thin strap that clips across your chest, joining the two shoulder straps. The name comes from the sternum, the flat breastbone in the middle of your chest.
Its only job is to keep the shoulder straps where they belong. Without it, the straps can slowly slide out toward the edges of your shoulders, especially on a bumpy downhill, and start to rub or pinch under your arms. The chest strap works like the clip on a child's car seat harness: it just keeps the two straps from drifting apart.
It is not there to hold the pack up, so it should feel light. If it presses on your chest or makes deep breaths harder, it is too tight. Most sternum straps slide up and down, so you can move it to a comfortable spot a little below the bony ridge at the base of your neck.
A sternum strap does one job well: it keeps the shoulder straps positioned on your shoulders instead of letting them splay outward under a load's weight, especially on descents or across uneven terrain where your body sways from side to side.
How is it adjusted?
Nearly every sternum strap slides along a short track or rail sewn onto the shoulder strap, so its height can move independently of torso length or shoulder width. Positioning it roughly an inch or two below the collarbone is the common starting point; too high and it can press against the throat, too low and it stops doing its job of holding the shoulder straps in place.
Does it carry any weight?
No — a sternum strap isn't a load-bearing connection the way the hipbelt or load-lifter straps are. Cinching it tightly across the chest adds pressure and discomfort without adding any real carrying benefit, so most hikers keep it snug rather than cinched.
Strap
Primary job
Sternum strap
Keeps shoulder straps positioned on the shoulders
Load lifter
Pulls load weight in toward the shoulders/upper back
Hipbelt
Carries the majority of the pack's weight on the hips
Many sternum-strap buckles include a small built-in whistle, a low-cost safety feature that shows up across budget and premium packs alike. A handful of packs also add a short elastic section into the strap itself, which lets the chest expand slightly during hard breathing on a climb without needing to loosen the buckle. This is a minor comfort feature rather than a structural one, and its absence on a given pack isn't a functional problem — a plain webbing sternum strap does the same core job of holding the shoulder straps in place.
What the fit guides actually say about placement
Published guidance on height is close but not identical. Big Agnes says the sternum strap should sit below the collarbone and lie flat with moderate tension,1 while Alpkit is more specific, putting it about an inch below the collarbone.2 This page's range of one to two inches below is a reasonable general guide, but the second inch is not in the verified sources, so start near one inch and adjust for comfort.
Body shape changes the best spot. A strap that sits comfortably on one person may cross breast tissue or press into the throat on another, which is why most packs put it on a sliding track. Deuter's women's SL packs, for example, use narrower S-shaped shoulder straps, which changes where the chest strap naturally lands.3
Why it comes late in the fitting order
Deuter's adjustment guide says sternum-strap height is set after the back length is correct.4 The reason is mechanical: the shoulder straps' path over your chest depends on the torso length setting and on how tight the hip belt and shoulder straps are. Set the chest strap first and every earlier change moves it out of place.
It also should not be used to mask a harness problem. If the shoulder straps only stay on when the sternum strap is pulled hard, the harness may be the wrong width or shape. ULA, for instance, offers J-shaped straps for flat or square shoulders and S-shaped straps for sloping shoulders, because strap shape affects how well they stay put.5
Tension, breathing and discomfort
The page notes that an over-tight chest strap can restrict breathing on a hard climb. No verified source measures that effect directly, but it fits with the wider evidence on strap pressure: a PLoS One study found static strap force is a significant predictor of discomfort during load carriage.6 Keeping the chest strap just snug enough to hold the shoulder straps in place keeps that added force low.
The built-in whistle and elastic strap sections mentioned on this page are common design features, but they vary by model and are not covered by the source library, so check the specific pack rather than assuming either is present. Chest tightness or breathing trouble that does not ease when the strap is loosened is a reason to stop and get medical help, not a fit issue.
