Why does the shoulder joint have more freedom of movement than any other joint of the body?
The entire process of coordinated, voluntary, muscle contraction is summed up as excitation-contraction coupling. Why are these two things coupled, and what, specifically, is responsible for bring excitation and contraction together in the muscle cell? In other words, what physically connects, or bridges, the two parts?
A joint is the point where two or more bones meet. There are three main types of joints; Fibrous (immoveable), Cartilaginous (partially moveable) and the Synovial (freely moveable) joint. I'll assume that your question deals with synovial joints.
The shoulder joint has a larger range of motion compared to all other joints because it is actually 4 joints working together giving an incredibly wide range of motion with enough stability to make it useful. The only other ball and socket joint in the human body is found in the hip where the femur interfaces with the pelvis. The hip joint is considerably stronger than the shoulder joint, but has significantly smaller range of motion.
Mechanically speaking, the ball and socket joint is already superior to other joints such as hinge, pivot, and saddle joints when considering range of motion.
There are three joints in the shoulder complex. The main joint is the glenohumeral joint. It is a ball and socket (modified ovoid) joint and it is the most mobile joint in the body. The top of the humerus is shaped like a ball and it sits in a socket on the end of the scapula. The ball is called the head of the humerus and the socket is called the glenoid fossa, hence the term "glenohumeral" ...
Answers why the human shoulder joint has the widest range of motion as compared to all other human joints, and has a concise explanation of excitation-coupling in skeletal muscles.