The action of anterior forearm muscles on the wrist and fingers is primarily:

The anterior forearm muscles primarily act to flex the wrist and fingers. This means they help in bending the wrist and fingers towards the palm. These muscles include the flexor carpi radialis, palmaris longus, flexor carpi ulnaris, and the flexor digitorum superficialis and profundus. These muscles are innervated by the median and ulnar nerves.

Here is a step-by-step breakdown:

1. The flexor carpi radialis and palmaris longus, which are innervated by the median nerve, primarily flex the wrist.

2. The flexor carpi ulnaris, innervated by the ulnar nerve, also helps in wrist flexion.

3. The flexor digitorum superficialis, innervated by the median nerve, primarily flexes the middle phalanges of the fingers.

4. The flexor digitorum profundus, innervated by both the ulnar and median nerves, primarily flexes the distal phalanges of the fingers.

So, the primary action of the anterior forearm muscles is to flex the wrist and fingers.