Flexion of the the thumb is performed by two major muscles: flexor pollicis longus, that flexes the interphalangeal joint and the flexor pollicis brevis, that flexes the first metacarpophalangeal joint.  Thumb extension and abduction occur due to extensor and abductor pollicis longus, extensor and abductor pollicis brevis.  Opposition occurs due to the opponens pollicis while the abduction is performed by abductor pollicis.