1. Why Raising Your Arm Takes More Than Just the Arm Joint
Lifting your arm straight from the front until it reaches beside your ear draws a semicircle. Reference values for the range of motion in the shoulder are 0 to 180 degrees for both lifting forward and lifting to the side [2]. Moving the arm backward is about 50 to 60 degrees. These numbers vary slightly depending on the source.
However, this 180-degree figure is not produced by the joint at the base of the arm alone. If the shoulder blade does not move, the arm can only rise to about 90 to 120 degrees. It is said that when the movement of the shoulder blade combines with the arm, the total reaches 150 to 180 degrees [1]. Some sources write "about 90 degrees," so there is a range. So, in what ratio are these two movements combined?
2. How the Arm Bone and Shoulder Blade Move Together in a 2 to 1 Ratio
While you are raising your arm, the shoulder blade slides across the chest and moves at the same time as the arm bone. The ratio of the movement at the base of the arm to the sliding of the shoulder blade across the chest is said to be about 2 to 1 overall. This coordination is called the scapulohumeral rhythm [1][2].
For example, if the arm rises a total of 150 degrees, applying the 2-to-1 ratio directly suggests that about 100 degrees comes from the arm joint and 50 degrees from the shoulder blade. However, 2 to 1 is an average guide. It is not necessarily constant for everyone or at every angle. Even so, how and where is a shoulder blade that moves this much attached to the body?
3. How the Shoulder Blade Rests on the Chest, Held Only by Muscles
The collarbone is a nearly horizontal, long bone that connects the sternum to the tips of the left and right shoulder blades [3]. The sternum and collarbone are joined by the sternoclavicular joint, and the collarbone and shoulder blade are joined by the acromioclavicular joint [4]. The arm bone attaches at the hollow of the shoulder blade (the glenoid) and swings freely from there.
The interesting part is the space between the shoulder blade and the chest wall. The shoulder blade is attached to the chest wall by muscles only, and it is said that the collarbone also acts as a support for this shoulder blade [3]. If you trace the bones, the order is arm bone, then shoulder blade, then collarbone, then sternum. But between the shoulder blade and the chest wall, there are muscles, not joints. So, can the joint at the base of the arm come out of place?
4. How Muscles Support the Loose, Free-Moving Shoulder Joint
The joint at the base of the arm is a ball-and-socket joint. The round end of the arm bone (humerus) fits into the shallow hollow of the shoulder blade (glenoid) [5][4]. Ball-and-socket joints have a wide range of motion, and the shoulder is a prime example. The same joint is described as unstable and prone to dislocation [5].
Muscles like the supraspinatus, part of the rotator cuff, help keep it from coming out easily. These muscles work to pull the round end of the arm bone into the hollow, functionally supporting shoulder stability [5]. The freedom to move widely and the resistance to dislocation are a trade-off. It is explained that muscles make up for what is lacking in the shape of the bones alone. This free shoulder has been useful to humans in another way: the act of throwing.
5. How Tendons and Ligaments Work Like a Spring in Throwing
The act of throwing is supported not just by shoulder bones, but also by tendons and ligaments. A study published in Nature in 2013 reported that much of the power in human throwing comes from body features that let the shoulder's tendons and ligaments store elastic energy and then release it [6].
While the arm is pulled back in preparation, the tendons and ligaments stretch and store energy. When released, that energy accelerates the arm forward. It is like stretching a rubber band and then letting go. However, what actually stretches is not a rubber band, but the tendons and ligaments of the shoulder. The twist of the upper arm (internal rotation) during throwing can exceed 9,000 degrees per second, which is said to be the fastest movement produced by the human body [6]. This is equivalent to 25 rotations per second, but it is a maximum value that lasts for a very short time. Adult male chimpanzees are said to throw at about 32 km/h (about 20 mph), which is introduced as about one-third of the speed of a 12-year-old baseball pitcher [7]. So, when did these features begin to appear?
6. When Human Ancestors Gained Body Features for Throwing, About 2 Million Years Ago
The study listed three body features that help with throwing [6][7]. First is a torso that can rotate the waist and chest separately. Second is a small twist in the upper arm bone (10 to 20 degrees less than in chimpanzees). Third is that the shoulder joint is oriented toward the side of the body.
These features appeared gradually. The study states that they can be confirmed together starting about 2 million years ago with Homo erectus, an ancient human species [6]. However, this is about when the features aligned, not evidence that hunting by throwing was happening then. The idea that humans hunted by throwing is a hypothesis. So, how is our shoulder moving right now? There is a way to try it with your own body.
7. How to Find Your Shoulder Blade and Feel It Move
Today, you can try this. Sit on a chair and slowly raise one arm while touching the upper back on the same side with your other hand. Can you feel the shoulder blade moving? Stop before your arm hurts, and do not force it; go slowly. Next, press your back against a wall and raise your arm in the same way. Compare how the arm rises and how your back feels using your own senses.
Another comparison: hold a towel with both hands and raise your arms. Touch the shoulder blade area and compare the feeling of movement with when you are not holding a towel. If you feel pain, stop. What kind of movements do the shoulders of other animals make?