Twenty-seven bones and no finger muscles. Each hand has 27 bones: 8 carpals in the wrist, 5 metacarpals in the palm, and 14 phalanges — three in each finger, two in the thumb. The muscles that curl and straighten the fingers are the extrinsic muscles, and their bellies lie in the forearm; only their tendons continue into the hand. There are intrinsic muscles inside the palm — the lumbricals, interossei and thenar group — which handle spreading, fine positioning and the thumb, but the powerful flexion of a grip comes from the forearm. This is why a clenched fist makes the forearm bulge and the fingers do not.

Why evolution put them there. Mass at the end of a limb is expensive: it must be accelerated and decelerated with every movement, and it raises the moment of inertia about the wrist. Moving the actuators back toward the body and transmitting force through tendons keeps the fingers slim and quick. Robotics arrived at the same answer independently — most dexterous robot hands are cable-driven or tendon-driven for exactly this reason, and the ones that put a motor in each finger joint end up bulky and slow.

Antagonistic pairs are not optional. A tendon can pull and cannot push, so every joint needs at least two opposing actuators. In the hand the flexor digitorum profundus and superficialis curl the finger and the extensor digitorum straightens it. Your elastic strip plays the extensor's part. The same constraint governs every muscle in the body — biceps against triceps, and so on — and it is a genuine engineering limitation, not a quirk of anatomy.

What the model cannot show. The carpometacarpal joint of the thumb is a saddle joint, allowing opposition — the ability to bring the thumb pad against each fingertip, which underpins precision grip. A card hinge cannot reproduce it. The real tendons also run in synovial sheaths lubricated well enough that friction is nearly negligible, whereas cotton cord in a paper straw binds noticeably. Both gaps are worth naming: a model that hides its own limits teaches the wrong lesson.

Anatomy

Blueprints (1)

Articulated Hand Model