AssessmentsMuscles of the forearm
Content Reviewers:Viviana Popa, MD, Scott Caterine, BSc (Hons.), MSc, MB, BCh, BAO (Hons.)
Contributors:Ursula Florjanczyk, MScBMC, Elizabeth Nixon-Shapiro, MSMI, CMI, Evan Debevec-McKenney, Jerry Ferro, Evode Iradufasha, MD
Anatomically speaking, the forearm is the part of the upper limb between the elbow and the wrist joints.
On the anterior part of the elbow, there’s a triangular fat-filled pit, called the cubital fossa, which is where the majority of important nerves and vessels to the forearm and hand can be found.
Okay, before we start, it is important to know that, even though some of the muscles of the forearm attach proximally to the humerus, they still belong to the forearm.
The forearm is divided into two compartments, which are separated by the radius and ulna and the interosseous membrane running between them.
Next, is the posterior compartment, housing the extensors and supinators of the forearm.
Generally, muscles in the same compartment are innervated by the same nerve.
Muscles of the posterior compartment, on the other hand, are innervated by the radial nerve.
Now, the muscles of the anterior compartment are divided into three groups, or layers: superficial, intermediate, and deep.
Now, let's look at these muscles one by one.
On the most medial side lies the flexor carpi ulnaris.
This muscle has two heads: the humeral head that proximally attaches to the medial epicondyle of the humerus and the ulnar head proximally attaching to the olecranon and the posterior border of the ulna.
This muscle proximally attaches to the medial epicondyle of the humerus, and extends distally to form a tendon that attaches to the distal half of the flexor retinaculum and the apex of the palmar aponeurosis.
When it contracts, it flexes the hand at the wrist.
A fun fact about the palmaris longus: it is absent in about 14% of people!
But luckily, its absence doesn’t come with any functional deficits.
It extends distally and laterally to the middle of the forearm, forming a tendon that attaches at the base of the second metacarpal bone.
When it contracts, it flexes and abducts the hand at the wrist joint.
As the muscle extends distally, its two heads fuse into one tendon that attaches to the midshaft of the radius on its lateral aspect.
At the same time, try to remember the muscles that attach to the common flexor origin as well as their distal attachments?
Now, let’s have a look at the intermediate layer of the anterior compartment of the forearm.
It’s a one-muscle compartment, containing only the flexor digitorum superficialis.
This muscle has two heads: a humero-ulnar head that attaches to the common flexor origin and the coronoid process and a radial head that attaches to the superior half of the anterior border of the radius, also called the anterior oblique line.
With a stronger contraction, this muscle also flexes the proximal phalanges at metacarpophalangeal joints.
The flexor digitorum profundus has two proximal parts, a medial and a lateral part, and both of them attach to the proximal three-quarters of the anterior aspect of the ulna and the interosseous membrane.
Distally, the medial part splits into two tendons to attach to the base of the 4th and 5th distal phalanges, and the lateral part splits into two tendons to attach to the bases of the 2nd and 3rd distal phalanges.
An important distinction here is that the lateral part is innervated by the anterior interosseous nerve, a branch of the median nerve, while the medial part is actually innervated by the ulnar nerve along with the flexor carpi ulnaris.
This will remind you that they are innervated by the ulnar nerve as it travels more medially in the forearm as well!
As the flexor pollicis longus extends distally, it transforms into a flat tendon that passes under the flexor retinaculum of the hand through the carpal tunnel to insert to the base of distal phalanx of the thumb.
When it contracts, it is the primary pronator of the forearm while also helping to hold the radius and ulna together.
Now, it is also important to know that the majority of these flexor tendons from the anterior compartment are held in place to prevent bowing by both the palmar carpal ligament and the distal and deeper flexor retinaculum, which is also known as the transverse carpal ligament. Both of these can be found on the anterior surface of the hand and wrist.
Now, it’s okay to take another pause and see if you can remember the only muscle of the intermediate layer and the three muscles in the deep layer of the anterior compartment.
So, let’s now move to the posterior compartment of the forearm.
In a similar fashion to the flexor muscle tendons, the extensor muscle tendons are held in place posteriorly by the extensor retinaculum, which is this thickened part of fascia on the posterior aspect of the wrist to prevent bowing of the tendons.
The posterior compartment muscles are divided into a superficial and deep layer.