Source notes

A shoulder muscle does not pull with the same mechanical advantage in every position. Evidence from 20 experimental studies describes how the muscles' moment arms, their leverage around the joint, change as the arm rises or rotates. The front and middle parts of the deltoid generally had strong leverage for lifting the upper arm. The rotator cuff muscles contributed to rotation and, to differing degrees, elevation. The findings also show that a muscle's contribution can change with the plane and angle of movement, making a single fixed label an incomplete description of its function.

Leverage is not the same as actual muscle force or total strength. It describes the turning effect a given force could produce in a particular position. Activation, muscle properties and the contributions of other muscles still matter. Some broad muscles also contain regions with different lines of pull, so an average value for the whole muscle can hide meaningful differences within it.

How Does This Show Up in Real Life?

Lifting the arm forwards and lifting it out to the side can involve different mechanical contributions even when the same muscles are active. The paper helps explain why changing position changes the task. It is particularly useful for building shoulder models and understanding what a tendon transfer or other anatomical change might do to leverage. It does not, however, establish the best exercise for a painful shoulder or identify a universally safest lifting angle. The measured movements were selected anatomical motions, while reaching, pushing and sporting actions combine movements in more complex ways. A diagram showing a favourable moment arm therefore cannot by itself tell someone how much force their muscle is producing, how much load a tendon experiences, or whether a training exercise will improve their symptoms.

Studies differed substantially in methods and reported values. Some used older cadaver specimens, and body size was not always accounted for. Locating the joint's rotation centre and averaging muscle regions introduced further uncertainty. Evidence was sparse for several everyday positions, including elevation beyond 120 degrees. No training intervention or follow-up established that applying these measurements improves function or prevents injury.

Use the measurements as a foundation for understanding shoulder mechanics, with attention to the exact position tested. They are most informative when combined with the other determinants of movement. They cannot turn a complex joint into a simple ranking of muscles or exercises.