That grinding sensation under the shoulder, the one that shows up right as the elbows swing out level with the shoulders, has a name: subacromial impingement. When you flare your elbows to a full 90° on the bench press, you push the humerus into what specialists call maximum horizontal abduction, and the supraspinatus tendon gets pinched between the humeral head and the acromion, the bony shelf on top of the shoulder. Do that for years, set after set, and you get exactly the kind of grinding, catching sensation that sends people to a physiotherapist’s office with a camera pointed at their side.
The physio in this story wasn’t being dramatic. A side-on video during a bench press is one of the simplest diagnostic tools going, because it reveals the one thing a mirror never shows: how far the upper arm travels behind the frontal plane and how the shoulder blade behaves under load. A 2024 study published in Frontiers in Physiology compared musculoskeletal shoulder loads across bench press variations, including shoulder abduction angles of 45°, 70° and 90°, using motion capture and an instrumented barbell alongside an OpenSim musculoskeletal model to estimate joint reaction forces. The researchers weren’t guessing. They were measuring actual forces travelling through the glenohumeral and acromioclavicular joints under each condition, and the pattern lines up with what physios see on video every week.
Key takeaways
- That grinding under your shoulder has a name, and physics explains exactly why it happens when your elbows flare to 90°
- The most mechanically efficient angle feels best but trades long-term shoulder health for short-term strength—tendons don’t adapt as fast as muscles
- A small change in elbow angle (45-70°) keeps the chest tension while protecting the joint, but retraining the pattern takes patience
Why 90° feels so good, right up until it doesn’t
There’s an uncomfortable truth buried in the biomechanics literature: the most mechanically efficient angle is actually 90 degrees, but it’s avoided because it causes shoulder impingement. That’s precisely why so many lifters gravitate towards it without ever being told to. Wide flare recruits more of the pec fibres in that bottom position, gives a bigger stretch, and honestly, it can move more weight. The 90-degree flare feels powerful in the moment, and you can move weight this way, but you’re trading short-term strength for long-term shoulder damage.
What’s actually happening at the joint is less flattering than the pump you feel. When elbows flare to 90°, the shoulder joint is placed in maximum external rotation under load, which compresses structures in the shoulder including the biceps tendon, bursa and rotator cuff, dramatically increasing injury risk. Repeat that compression rep after rep, session after session, and the tendon tissue doesn’t get a chance to recover between exposures. Tendons are stubborn that way. Tendons and ligaments adapt more slowly than muscles, so your pecs and triceps might be strong enough to press a weight while your rotator cuff and shoulder capsule haven’t caught up. That mismatch is exactly why someone can bench heavy for years before the grinding starts. It’s not the load that catches up with you first. It’s the joint underneath it.
The angle that keeps the chest stretch without the grinding
The fix isn’t tucking the elbows into your ribs either, which starves the pecs of tension and turns the lift into a triceps exercise. The window that most coaches and clinicians now agree on sits well short of 90°. For most people, an elbow angle of somewhere around 45 to 70 degrees will best line up with and activate their chest fibres when pressing, while being safest on the shoulders. Other sources widen that slightly, putting the practical range at 45-75°, with the exact number depending on your individual shoulder anatomy and grip width.
Viewed from above rather than the side, the shape your arms make tells you everything. Tucking the elbows to 45-75 degrees from the body means that, seen from above, your arms should form more of an arrow shape than a T-shape. Grip width matters here too, and it’s not just about comfort. Results from the 2024 Frontiers study showed that narrower grip widths of less than 1.5 bi-acromial widths decreased acromioclavicular compression, which may lower the risk of distal clavicular osteolysis. Translated out of the lab: a slightly narrower grip than the classic wide powerlifting set-up does double duty, protecting the shoulder while keeping the elbows in that safer arc.
Retraining the pattern without losing the chest gains
Changing a motor pattern you’ve grooved for years takes more patience than most lifters want to admit. Expect the first few sessions at the new elbow angle to feel oddly weak, almost apologetic compared to the old flare. This feels weaker at first and your numbers might drop temporarily, but after a few weeks you’ll rebuild strength in the safer position and likely surpass your old numbers. That’s not marketing optimism, it’s how tendon and neuromuscular adaptation actually works once the joint stops fighting the bar path.
A couple of cues make the transition faster. Coaches often tell lifters to “bend the bar” as they lower it, a cue that engages the lats and naturally brings the elbows in, while others prefer imagining someone trying to tickle the armpits, which keeps them partially closed without collapsing the chest. Alongside the cueing, it’s worth building a little direct capacity in the muscles that resist that unwanted flare under fatigue. The infraspinatus, teres minor and rear deltoid are the three external rotators of the shoulder most commonly targeted with light band work between sets, and strengthening them gives the joint something to lean on when the bar gets heavy and technique starts to drift.
One detail that rarely gets mentioned outside physio clinics: the grinding sensation itself, the one that sent our lifter for that side-on video in the first place, often shows up months before pain does. Clicking and catching under load is frequently the joint’s first warning that something in the mechanics has gone wrong, well before inflammation makes itself known as actual soreness. If you can hear or feel that grind on your own bench sessions, filming yourself from the side, even on a phone propped against a water bottle, will tell you more in thirty seconds than another year of guessing ever will. And if the discomfort persists, that’s a conversation for your GP or a physiotherapist, not a forum thread.
Sources : frontiersin.org | nurpurfitnessnews.com