Quick Answer: Most running vests have two sternum straps that slide vertically on a rail. Where you place them determines whether the vest is held against your torso as a unit or pinched at one point while the rest moves. Straps too high leave the bottom of the vest free to swing; too low and the top gapes. The reliable setup is one strap just below the collarbone and one just above the bottom of the ribcage, with the vest snug rather than tight.

Position first, tension second. Reversing that order is why the vest still bounces.

Why does tightening not fix bounce?

Because bounce is usually a hinge problem, not a compression problem.

A vest held at one point behaves like something pinned to a wall — it stays put where it is pinned and swings everywhere else. If both your sternum straps are clustered in the middle of your chest, you have effectively pinned the vest at its center. The top and bottom are free to move.

Tightening that center pin harder does two things: it increases the pressure at that point, and it does almost nothing about the parts that are moving. The result is a vest that is uncomfortable to breathe in and still bouncing.

What actually controls bounce is holding the vest against your torso at two well-separated points, so it moves as a unit with you. That requires vertical separation between the straps, not more tension on them.

There is a second reason tightening fails. Over-tightening restricts ribcage expansion, which matters more than people expect during hard effort. A vest that limits your breathing makes you feel like you are working harder, and runners commonly interpret that as the vest being too heavy rather than too tight.

The geometry also explains a symptom that confuses people: a vest that is fine walking and bounces running. Walking produces low vertical accelerations, so even a poorly located pin holds. Running produces much higher peak forces, and the unsupported sections start to move. The setup that passes the walking test frequently fails the running one.

Where exactly should the straps sit?

Two positions, and the separation between them matters more than either individually.

Upper strap: just below the collarbone. High enough to control the top of the vest and keep the shoulder straps from spreading outward, low enough that it is not against your throat. If the top of the vest gapes away from your chest when you run, this strap is too low.

Lower strap: just above the bottom of your ribcage. This is the one people set too high. It controls the bottom of the vest, where the flasks and the bulk of the weight sit. If your vest bounces at the bottom, this strap is almost certainly too high.

The separation should be as wide as the rail allows while keeping both straps in comfortable positions. Two straps three inches apart give you a single pin. Two straps six or seven inches apart give you a held unit.

On tension: snug enough that you can slide a flat hand under the strap with mild resistance. You should be able to take a full deep breath without the strap restricting it. If a deep breath is limited, loosen it — you will need that capacity later in the run.

The order of operations matters. Set the strap positions first with the vest loaded, then adjust tension, then run. Setting tension on an empty vest and then loading it is how most people end up with a setup that only works for the first mile.

A note for women specifically: the lower strap position often needs to be set around a sports bra band, and a strap sitting directly on top of the band is a common chafe point. Moving it an inch either way usually solves a problem people assume is unavoidable.

What Nobody Tells You About Vest Fit

The vest must be loaded when you adjust it. An empty vest sits differently, and the whole adjustment is invalid. Fill the flasks and pack it the way you actually run before you touch a single strap.

Side cinch straps do a job the sternum straps cannot. Most vests have them, most people never touch them. They control how closely the sides hug your ribcage, which is what stops lateral sway. If your vest moves side to side rather than up and down, that is what these are for.

Half-empty flasks bounce worse than full or empty ones. Sloshing liquid produces forces the vest never sees when the flask is full. This is not a fit problem and it will make a well-fitted vest feel wrong at exactly the midpoint of your run.

Front pocket loading is a stability variable. Weight distributed evenly across both front pockets stabilizes; weight concentrated on one side introduces rotation. Two half-full flasks beat one full one.

The setup changes with layers. A vest set up over a singlet is set up wrong for the same run over a long sleeve. As mornings cool through September, expect to re-adjust — and expect it to feel wrong the first cool morning you forget.

Chafe on one side and not the other usually means rotation, not a defect. The vest is being pulled slightly to one side, typically by uneven loading or one shoulder strap set shorter than the other. Check strap symmetry before you blame the fabric.

How do you test a setup properly?

A short protocol, done once, that saves a lot of mid-run frustration.

Load the vest exactly as you will run it. Full flasks, food, phone, shell — everything.

Put it on and set strap positions. Upper strap below the collarbone, lower strap above the bottom of the ribcage, maximum comfortable separation.

Set tension to snug, then take three deep breaths. If any of them is restricted, loosen. Breathing capacity is not negotiable.

Jog on the spot for thirty seconds. Not walk. The forces only appear at running cadence. Feel where the movement is: top, bottom, or side to side.

Adjust based on what moved. Top moving means the upper strap goes higher or tighter. Bottom moving means the lower strap goes lower. Side to side means the side cinches need attention.

Then run ten minutes and reassess. Vests settle, straps creep, and the load shifts. A ten minute check catches what thirty seconds cannot.

One seasonal note worth planning around. A vest dialed in over summer kit will need resetting as you add autumn layers, and the change is not trivial — a long sleeve and a shell together add enough bulk to alter both strap length and the ideal position. Rather than discovering that on a cold morning, spend five minutes now setting it up over the layers you will be wearing in a month, and note the strap positions.

This is general information, not medical advice. Persistent numbness or tingling in the arms or hands while wearing a loaded vest means the shoulder straps are compressing something they should not — loosen and reassess rather than pushing through.

FAQ: Running Vest Questions, Answered

How tight should the vest be overall?

Snug enough that it does not move independently of you, loose enough that a full deep breath is unrestricted. If those two feel incompatible, your strap positions are wrong rather than your tension.

Why does the bounce start halfway through the run?

Almost always half-empty flasks. Liquid sloshing generates forces a full flask does not. Some runners solve it by drinking one flask fully before starting the second rather than alternating.

Should the sternum straps be parallel?

Roughly. They do not need to be perfectly level, and a large discrepancy usually means the shoulder straps are set at different lengths, which introduces rotation and one-sided chafe.

Is a bigger or smaller vest better for bounce?

Smaller and well-filled beats larger and half-filled. Empty space lets contents move. If you routinely carry little, a smaller vest will be more stable than a large one used at partial capacity.

TL;DR:

• Bounce is a hinge problem: straps clustered together pin the vest at one point and leave the rest free to swing.
• Set the upper strap just below the collarbone, the lower just above the bottom of the ribcage, with maximum comfortable separation.
• Adjust with the vest fully loaded, and keep tension snug enough to allow a full deep breath.
• Side cinch straps control lateral sway; half-empty flasks cause mid-run bounce no fit can fix.

Before your next long run, load the vest properly and jog on the spot for thirty seconds. Where it moves tells you which strap to change.