The muscles in the forearm and palm (thenar muscles) all work together to keep the wrist and hand moving, stable, and well-aligned. The image below shows the bones of the hand from the back side. The red lines show where the tendons attach the muscles to the bones.
Many of the muscles that move the fingers and thumb originate in the forearm. Long flexor tendons extend from the forearm muscles through the wrist and attach to the small bones of the fingers and thumb. When you bend or straighten your fingers, these flexor tendons slide through snug tunnels, called tendon sheaths, that keep the tendons in place next to their respective bones. Within this sheath, a slippery coating called tenosynovium surrounds the tendons, and keeps the tendons moving smoothly under the ligaments when the hand is in motion.
Tendons are white, flexible rope-like cords at the ends of muscles that attach muscles to bone. When muscles contract, they pull on the tendons to move the bones. The tendons that run down our fingers are held in place by a series of ligaments, called pulleys, that form stable arches over tendons, forming a “tunnel-like” sheath. Normally, the tendons glide easily through the tunnel.