Sling your work bag, gym kit or laptop case over the same shoulder every single day and you’re not just risking neck tension. You’re gradually asking one muscle, tucked underneath your shoulder blade and largely invisible in the mirror, to do a job it was never built to sustain alone. That muscle is the serratus anterior, and when it tires out, your shoulder blade stops sitting flush against your ribcage the way it should.
The serratus anterior is a fan-shaped muscle that originates on the superolateral surfaces of the first to eighth or ninth ribs at the lateral wall of the thorax, with its main part lying deep under the scapula and the pectoral muscles. Because it sits so deep, you can’t see it flex in a mirror the way you can watch your biceps or deltoids. You can feel this muscle by putting your hand just underneath your armpit, but most people never think about it until something goes wrong. It’s sometimes nicknamed the “boxer’s muscle,” as it’s largely responsible for the protraction of the scapula which occurs when throwing a punch, but its everyday role is far less glamorous and far more constant: it acts as a key scapular stabiliser, keeping the shoulder blades against the ribcage when at rest and during movement.
Key takeaways
- One muscle tucked underneath your shoulder blade is responsible for keeping it flush against your ribcage, and most people never train it
- A study of 250 healthy teenagers revealed that 42.8% had serratus anterior weakness directly linked to carrying heavy single-shoulder bags
- The damage accumulates silently with no dramatic warning signs until one day your shoulder blade simply doesn’t sit flat anymore
Why one strap on one shoulder is the problem
The mechanics here are fairly simple once you picture them. Every time you load a bag onto a single shoulder, that side has to work harder to keep the shoulder blade pinned to the ribcage against the extra downward and rotational pull. Do this for years, on the same side, and the muscle can become chronically overloaded and, counterintuitively, weaker rather than stronger. Physiotherapy sources are direct about the link: carrying one-strap bags or uneven weights on one shoulder increases asymmetrical load and causes the shoulder blade to lift or rotate abnormally during movement.
There’s also a nerve-related risk that’s less well known outside clinical circles. Damage to the long thoracic nerve, which controls the serratus anterior muscle, is the most frequent cause of scapular winging, and this nerve can be injured during sports, surgery, or even from carrying a heavy shoulder bag. One rehabilitation resource puts it plainly, noting that long thoracic nerve injury can occur after chest surgery, after carrying a heavy pack on the shoulder for extended periods, or in some contact sports injuries, though it adds that simply never having trained the muscle at all is a far more common driver of weakness than outright nerve damage.
What does this actually look like on a person? The medical term is scapular winging, and it’s more noticeable than most postural issues. The main indication is one shoulder blade sticking out from the back, and the projection can make it difficult to sit on chairs with hard backs, wear certain clothing, and carry bags with shoulder straps. There’s a certain irony there: the very habit that weakened the muscle, the shoulder bag, becomes harder to carry once the damage is done. In more advanced cases, this debilitating condition can affect the ability to lift, pull, and push heavy objects, as well as to perform daily activities of living, such as brushing one’s hair and teeth and carrying grocery bags.
It’s not just adults carrying briefcases
I’ll admit the strongest piece of evidence I found surprised me, and it wasn’t even about adults. A 2025 survey of 250 secondary school pupils with normal BMI, all otherwise healthy, tested serratus anterior strength using manual muscle testing against how heavy a bag they carried. A total of 107 students, or 42.8%, exhibited weakness of the serratus anterior muscle. The pattern by bag weight was striking. Among students carrying bags weighing less than 3 kg, only 15 showed weakness, whereas students carrying bags heavier than 3 kg had significantly higher incidence, with 92 cases showing weakness. The researchers concluded that carrying heavy school bags every day can make the serratus anterior muscle weaker even in healthy kids, with the daily pressure slowly affecting muscle strength without any obvious signs at first.
That last point is the one worth sitting with. There’s no dramatic pain on day one. No single moment where you’d notice anything wrong. The weakness accumulates quietly, rep after rep, satchel strap after satchel strap, until one day the shoulder blade simply doesn’t sit flat anymore.
What actually helps
The encouraging news is that the serratus anterior responds well to targeted, low-effort training, precisely because most people have never asked it to do anything specific before. Clinicians commonly recommend the wall push-up with a “plus”: perform a push-up against the wall, and at the top of the movement, push the hands further into the wall to protract the scapulae, spreading the shoulder blades apart. Resistance band work helps too. Used correctly, resistance exercises activate the serratus anterior, lower trapezius, and rhomboids to improve shoulder alignment, and focused band exercises such as serratus punches, band-assisted wall slides, and rows help build shoulder stability and restore balanced movement gradually.
It’s worth pairing that with a habit change rather than relying on exercise alone: alternate which shoulder carries the bag, or better still, switch to a rucksack style that distributes weight across both sides of the back. For school bags specifically, one study group offered a simple practical rule alongside their findings, advising to keep school bags light, use both shoulder straps, and encourage good posture and simple exercises to keep the shoulder muscles strong and healthy.
Sleeping position Matters More Than people expect too, since sleeping on your stomach forces your shoulder blades into a winging position, compounding whatever imbalance has already built up during the day. And because this muscle sits so deep, strength development work is generally recommended around three times per week with two to three sets rather than daily maximal effort, giving the tissue time to adapt properly.
If you notice one shoulder blade protruding more than the other, particularly alongside aching between the shoulders or difficulty lifting your arm overhead, it’s sensible to get it checked rather than guess. As orthopaedic sources note, persistent winging, pain that does not improve with rest, or weakness that interferes with daily tasks warrants a visit to a physiotherapist or sports medicine doctor, since early intervention improves outcomes significantly. This article is intended as general information rather than a diagnosis, so do speak to your GP or a chartered physiotherapist if you’re concerned about your own shoulder mechanics.
Sources : theaspd.com | sciencedirect.com