Skeletal anatomy of the arm (pictured: right arm, with supinated hand).
Skeletal anatomy of the right hand.
Schematic (overly simplified) drawing of skeletal anatomy of the forearms.
So, because of breaking my left radius two weeks ago, I’ve been trying to educate myself about the treatment, healing, and rehabilitation of this particular injury. To my taste, no better source of information exists than the online Orthopedics textbook at Duke University, Wheeless’ Online. What I read there about the forearm and its osteoanatomy got me thinking about how the radius and the ulna function in various weightlifting, bodybuilding and powerlifting activities.
The radius is the bone that runs from the base of the palm beneath the thumb to the outside (lateral side) of the elbow. The ulna is the bone that runs from the base of the palm beneath the pinky to the inside of the elbow.
In neutral (hand shaking) position, the joints connecting the radius and ulna to the hand (on the distal ends) and the elbow (on the proximal ends) are turned 90 degrees (roughly) to one another causing the radius and ulna to form a triangle with the elbow, with the point at the hand (distal) end.
When the hands are supinated (palms towards the face) the radius and ulna are more or less parallel.
When the hands are pronated (palms away from the face) while the arm is bent, the radius and the ulna are crossed (nearer to the distal end).
When the hands are pronated and the arm is extended, the humerus rotates, and the radius and ulna make a triangle with the point at the elbow and the base at the hand.
The two lines never actually touch; the two pairs of joints at either end of the radius and ulna — either the medial and lateral epicondyles on the proximal head of the humerus or the scaphoid/radius luna/ulna joints of the carpal bones of the wrist — rotate independently and keep the bones apart.
All positions of the radius and ulna provide a truss-like structure to transfer power from the elbow to the wrist. The supinated grip, with its parallel bones forming a sort of rectangular plane, provides the most direct, lever-like use of the contracting force of the bicep. The neutral and pronated grips all form a type of tetrahedron, as the joints at either end of the two bones rotate around each other. These tetrahedral shapes are very strong trusses that can support enormous weights when locked out properly to the wrists and humerus.
I haven’t taken these thoughts much further, but they could influence various decisions about how to train athletes; for example, considering which exercises stimulate which joints, and the manner of stimulation, you could design substitutions for lifters suffering from various injuries or conditions (e.g. medial epicondylitis, aka ‘golfer’s elbow'; or lateral epicondylitis, aka ‘tennis elbow'; or a sprain or injury to either carpal joint).