What does “arm bone in socket” mean?

Let’s admit it: Pilates and movement instructors are known for saying things that seem pretty cryptic to the uninitiated. This is particularly true in a group class setting, where instructors may not have the opportunity to fully elaborate on certain cues or expressions. The exhortation to “get arm bones into shoulder sockets” is a good example of this; a newcomer to Pilates may justifiably wonder where else an arm bone could possibly be besides inside a shoulder socket. But even Pilates veterans may be hard pressed to explain exactly what is meant by this phrase, and indeed, when it’s used as a cue instead of an objective (more on this below), instructors will sometimes see fairly experienced clients move their shoulder blades and ribs in an effort to isolate movement of the arm bone in the shoulder socket. Read on to find out what’s behind this puzzling expression, why it matters, and—most importantly—how “arm bone in shoulder socket” can be achieved.

Basic anatomy of the shoulder joint

The shoulder socket is very shallow (unlike the hip socket, which is very deep), and only about a third of the arm bone sits in the shoulder socket at any given time. The clavicle (collarbone), arm bone (humerus), and shoulder blade (scapula) move together to create all of the motions we make with our arms. Keeping the arm bone in the shoulder socket (which, by the way, is also called the glenoid fossa and is part of the shoulder blade) requires adequate support and timing of muscles firing in the correct sequence. This is all part of something called scapulohumeral rhythm—a 2:1 ratio of movement in which the arm bone moves approximately double the distance of the shoulder blade. The inherent instability of the shoulder joint gives us a wonderfully large range of motion with our arms, but it also makes the shoulder joint prone to dysfunction and injury. Getting the humerus nestled adequately into the glenoid fossa (imagine that as a cue in Pilates class!) optimizes shoulder function, makes muscles pull with more power, and helps to prevent injury.

There are four deep muscles that hold the arm bone in the shoulder socket: the subscapularis, infraspinatus, supraspinatus, and teres minor. Collectively, these are known as the rotator cuff. Rotator cuff muscles become more powerful—which is how we want them—when the shoulder blades (scapulae) are stabilized against the ribs by the trapezius and serratus anterior. It’s important to understand that stable shoulder blades are not fixed or held in one position. What stable means in this context is that the bones stay snug against the ribs and move freely and smoothly without poking out, and without moving too quickly or slowly.

If this sounds complicated and difficult to feel, it can be! Instead of worrying about whether your blades are moving properly while simultaneously trying to access the rotator cuff muscles, the best strategy is to focus on exercises that will train the shoulder blades before addressing function in the rotator cuff.

There is another layer to all of this. The posterior deltoid is also a powerful stabilizer of the arm bone in the shoulder socket, but there’s a caveat: this muscle can be a bit of a bully, overpowering the other muscles and subduing them into not working effectively. Exercises that focus on the posterior deltoid are therefore best performed after exercises for the trapezius, serratus anterior, and rotator cuff.

Working towards optimal shoulder movement

Try these two videos to learn 1) what it looks like and feels like to get your arm bone properly into its socket and 2) a few at home shoulder exercises that can help.

Making “arm bone in socket” happen through action rather than just words

Providing adequate support for the shoulder joint and getting muscles to fire at the right time and in the proper sequence is as complex as it sounds, and isn’t always something that can be achieved spontaneously. The solution? Think of getting your “arm bone in shoulder socket” as an objective to work toward through well-chosen exercises rather than using it as a potentially baffling cue.

Try sensing and feeling the difference in how your shoulders move and feel before and after performing exercises selected from the list below. When the “arm bone in shoulder socket” sensation becomes familiar, it also becomes more accessible—not only because of muscle memory, but also thanks to the conditioning effect of exercises that stabilize the shoulder blades and strengthen the rotator cuff.

Top picks for exercises to stabilize arm bones in shoulder sockets

Classical Mat

Side lift



Control Front
Tendon stretch
Long back stretch



Push thru arms
Standing mid traps
Pull up



Roll down to pedal
Side body twist
Tendon stretch

A shoulder recipe for success

I’ve shared the above list of exercises because of how effective they are at providing support and getting muscles to fire in the correct sequence. Indeed, as you may have gathered from the phrase “scapulohumeral rhythm” and the warning about the posterior deltoid’s bullying tendencies, sequencing is critical when it comes to addressing shoulder function. For a deeper understanding of shoulder biomechanics, check out Body Harmonics’ Simplifying the Shoulder Complex course. You’ll come away with a wealth of knowledge about the shoulder region, as well as a “shoulder recipe” that works magic every time. (Like most recipes, the secret is as much in the sequence of steps as it is in the ingredients used!)

Ready to learn more?

Simplifying the Shoulder Complex Courses