Lacertus Fibrosus: The Biceps’ Hidden Power

The biceps brachii muscle is one of the most well-known muscles in the human body. Ask anyone to flex their biceps, they will probably roll up their sleeves to show off their “guns.” But how well do we know the biceps story? Beyond the muscle belly and tendon, a lesser-known part of the biceps muscle may have more to do with the function of the whole arm than you would think. Allow us to reintroduce you to the lacertus fibrosus—a broad, flat fascial structure that extends across the cubital fossa and is credited with some fairly big capacities, including the ability to improve muscular efficiency, enhance grip strength, and assist with neuromuscular coordination.

The Biceps Brachii “Origin” Story

The biceps brachii muscle is a two-headed fusiform muscle that assists with shoulder flexion and contributes to shoulder stability but is best known for its actions of elbow flexion and supination, as in the action of showing off our “guns.” The two heads find their origin on the scapula, while the single distal tendon inserts at the radial tuberosity. But, if you look carefully in your anatomy textbook, there’s often a second insertion point listed: the deep fascia of the forearm.

Here’s where things get interesting. The biceps brachii tendon inserts deep into the radius bone, but the biceps brachii fascia has its own much broader destination. Fanning out from the tendon, it heads toward the surface, inserting into the highly innervated antebrachial fascia—the membranous deep fascia “sleeve” of the forearm. During contraction, the biceps muscle pulls on the bone while simultaneously stretching the enveloping antebrachial fascia.

Put another way, imagine you have a travel bag for your massage table that allows the handles of your table to pop through the bag. In addition to your table handles, the bag itself has handles. By using both sets of handles simultaneously when you go to pick up your massage table and carry it to your car (biceps contraction), you apply force to both the table (radial bone) and the travel case (antebrachial fascia) via their handles (biceps tendon and lacertus fibrosus, respectively). This kind of fascial connection is called a myofascial expansion—a specific connection that exists between either skeletal muscles or tendons and aponeurotic deep fascia. Myofascial expansions go far beyond simple muscle-tendon-bone connections, and many, if not most, of our muscles have them. The biceps brachii myofascial expansion is so obvious that anatomists have even given it a name: lacertus fibrosus.

Lacertus Fibrosus’s Hidden Powers

The lacertus fibrosus resembles the curl of a lizard’s tail (lacertus comes from the Latin word for lizard) and is easily seen when the skin and subcutis are removed, yet scientists are still exploring its functions.

Looking at whether the lacertus fibrosus is significant biomechanically, in a cadaveric study, one group of researchers measured what happens when the lacertus fibrosus is removed.¹ The effect on muscular efficiency was dramatic:

• 28 percent loss in flexion efficiency
• 50 percent loss in supination efficiency

The lacertus fibrosus appears to increase the biceps lever arm at the elbow. The more extended lever arm saves energy and improves muscle efficiency by creating the same movement with less effort.

But the benefits of the lacertus fibrosus extend beyond simple mechanical advantage. Scientific literature describes it as a “strength strap” that creates functional synergy between the biceps and forearm flexors during grip activities. It also helps elbow flexion and forearm supination work together with what researchers call “rhythmicity”—a coordinated, flowing relationship between movements. Findings from fascia research may further explain this idea of functional synergy and rhythmicity.

A New “Origin” Story for Muscles

We might have to rethink some of our simpler models of how the body moves based on current muscle force transmission studies. It’s not just a story of muscle pulling on bones via the tendons. Studies reveal that up to 37 percent of the force generated by muscle contraction doesn’t go to the bones at all. Instead, it’s transmitted to adjacent connective tissue structures. In myofascial expansions, these connections work in two primary ways: fascia as insertion and fascia as origin.

Fascia as Insertion

Instead of inserting into bone, the muscular fascia expands beyond the tendon and joins with the aponeurotic deep fascia of the next region. The lacertus fibrosus is a perfect example of this in the upper limb—instead of inserting into the radius bone along with the biceps tendon, it expands beyond the tendon like a membranous sail and merges with the antebrachial fascia of the forearm.

Fascia as Origin

Rather than originating from bone, the muscle fibers are firmly anchored into aponeurotic deep fascia. In the forearm, this fascia-as-origin organization is found at the proximal end of the flexors and extensors near the elbow, where they root directly into the inside of their enveloping antebrachial fascia. The connection changes toward the distal end, where the muscles and their long tendons are free to glide beneath the antebrachial fascia.

The lacertus fibrosus tensions the antebrachial fascia during biceps contraction, creating reciprocal feedback that helps coordinate movement.

Because fascia is innervated with sensory nerve endings that perceive stretch, myofascial expansions like the lacertus fibrosus enable distal muscle groups to be informed about proximal muscle contraction states during movement. The lacertus fibrosus tensions the antebrachial fascia during biceps contraction, creating reciprocal feedback that helps coordinate movement. The entire system functions as an integrated unit, rather than isolated parts.

But the lacertus fibrosus isn’t the only place we find this kind of organization. A groundbreaking study provided the first comprehensive evidence that muscles create systematic myofascial continuity throughout the upper limb. The study demonstrated that the fascial insertions were consistently present across all specimens and exhibited constant anatomical patterns of myofascial expansions:

• Pectoralis major brachial fascia (two pathways)
• Latissimus dorsi posterior brachial fascia
• Triceps posterior antebrachial fascia
• Biceps anterior antebrachial fascia
• Palmaris longus palmar and thenar fascia
• Extensor carpi ulnaris hypothenar fascia (in 85 percent of cases)

Myofascial expansions like these create three-dimensional functional architecture that changes how we understand movement, from isolated muscle actions to integrated fascial networks coordinating function across the body.

Why We Care

Myofascial expansions like the lacertus fibrosus are more than just helpful second insertion points; they serve as essential elements for our entire neuromuscular coordination system. Understanding the complete anatomy of myofascial expansions like this one can help us assess and address muscles beyond their borders into their fascial continuities, ultimately leading to better outcomes for our clients, from shoulder to fingertip and head to toe!