Sternum strap at a glance
- Position
- About 1 inch below the collarbones
- Weight it carries
- None
- It sets strap width, it does not hold the load up
- Adjust
- Last, after the load lifters
What does a sternum strap do?
A sternum strap sets the width of your shoulder straps and keeps them there. Without one, the straps on a loaded pack tend to drift outward toward the edges of your shoulders, where they rub and pull. Clipped and snugged, it holds them in a line that leaves your arms free to swing.
The Washington Trails Association describes its job as keeping the shoulder straps from wiggling while you walk. REI adds that connecting the two straps boosts stability on uneven, cross-country ground, where one awkward move could otherwise shift the pack and throw you off balance.
Where should a sternum strap sit?
Roughly an inch below your collarbones, which is the height both REI and the Washington Trails Association give. Most packs mount the buckle on a short rail or a row of attachment points, so you can slide it up or down to find that spot.
Women's packs often place it differently. REI notes that on women-specific packs the sternum strap may fall in a slightly different position to account for chest size, and that the shoulder straps are designed for a narrower frame.
When do you adjust the sternum strap?
Last. REI's fitting order is hipbelt, then shoulder straps, then load lifters, then the sternum strap, because the first three decide where the pack sits and the sternum strap only fine-tunes the width of straps that are already in place. Setting it first tunes the straps to a position the pack is about to leave.
On a running vest or a fastpack the same job is spread across two or more straps or cords down the chest instead of one buckle, because the front of the harness is doing most of the holding.
What this means when you're choosing
- A buckle that slides on a rail, so you can set the height instead of accepting the factory spot
- Enough adjustment range to sit about an inch below your collarbones
- Two or more chest straps on a vest-style harness, which spread the hold across your ribs
The common mistake
Cranking it tight
REI calls overtightening the common mistake: it distorts the fit of the whole harness, constricts your chest muscles and restricts your breathing. Snug enough to stop the shoulder straps drifting outward is the whole job. If it has to be tight to stop the pack moving, the problem is lower down, at the hipbelt or the torso length.
Common questions
How tight should a sternum strap be?
Snug, not tight. It should hold the shoulder straps in place and nothing more. If you can feel it pressing on your chest when you breathe hard on a climb, loosen it a notch.
Does a daypack need a sternum strap?
Most have one, and it earns its place as the load gets heavier or the ground gets rougher. REI's daypack advice is that with heavier loads, a sternum strap and load lifters improve how stable the pack feels and how comfortable it is to carry.
Where this is defined
- Backpacks: Fit & Torso Size Guide, REI Co-op
- How to Choose a Backpack: Sizing & Fit Guide, REI Co-op
- Daypacks: How to Choose, REI Co-op
- How to Choose a Backpack, Washington Trails Association
See it labeled