When the wrist is in flexion it is difficult to fully flex the fingers. Please explain why.

The wrist muscles involved with flexion are the palmaris longus, flexor carpi radialis (FCR), and the flexor carpi ulnaris (FCU). The muscles involved with the flexion of the fingers are the flexor digitorum superficialis (FDS) and the flexor digitorum profundus (FDP). For the antagonistic muscles, the muscles are the extensor carpi radialis longus and brevis, extensor carpi ulnaris, and the extensor digitorum. Difficulty with flexing fingers while the wrist is flexed is due to the passive resistance created by the tendons of extensor digitorum. When the wrist is flexed (contraction by FCR, FCU, FDS, and FDP) the long extensor tendons of the fingers (primarily extensor digitorum) are positioned almost at their maximum stretch. Since the extensors are already lengthened, it is very difficult and painful to make a tight fist with the wrist full flexion. The reverse holds true when the wrist is fully extended. Martial arts and self defense techniques make use of this principle of passive resistance to force an attacker to release a weapon held tightly in a clenched fist.