Pennate & Pennation: A crisis of relevance

Off the desk

Off the desk¹

This week the PSU DPT program started the Summer term and welcomed the Class of 2027 to the program. I’m teaching a clinical anatomy lab on osteology and arthrology, and a course on clinical physiology. In clinical physiology we use a book I’m writing - currently in its third draft. The book takes a muscle fiber (cell) centered approach to all of clinical physiology in an attempt to make it more relevant to the aspiring (and I suppose practicing) physical therapist. Chapter 2 is on “Fundamentals” and is a hodgepodge of topics that are fundamental to continuing the muscle centered approach in the book. It’s mostly about the difference between muscle fibers (cells) and the muscles that they build, and how they are connected. The general form of a muscle fiber and its functional unit (the sarcomere) are really identical between all the varied muscles of the body. It’s how they are assembled that gives a muscle its characteristic structure (form), which then influences its function. Like building many different houses with same shape bricks, or many different structures with the same basic set of Lego blocks.

During lecture we had an extended conversation about the functional consequences of pennate muscles and muscle pennation (how that particular structural feature influences function). In particular how pennation influences shortening velocity and force, as well as how a pennate muscle typically can be packed with more muscle fibers. One thing I learned is that it’s important to use the language pennate muscle and muscle pennation since they are separate concepts. And language should help clarify important points. But for some reason it has taken me some time to approach pennation from that angle (pun intended).

All “pennate” muscles have a pennation angle; AND some “non pennate” muscles have a pennation angle.

Muscle pennation is the angle that muscle fibers pull compared to the line of tendon pull of the whole muscle. It’s why when you’re cutting poultry “with the grain” you may have to angle the knife differently for different pieces of poultry (different muscles). Pennate muscles have a very obvious muscle fiber arrangement that includes muscle fibers pulling at an angle to the line of pull of the tendon. But some non pennate muscles also have pennation angles.

Pennate muscles can be further classified into unipennate (one angle); bipennate (two angles) or multipennate (multiple angles). In the human body - the following are muscles falling into each category.

Unipennate Muscles: Flexor Pollicis Longus, Extensor Digitorum Longus, Tibialis Posterior, Semimembranosus, Palmar Interossei, Vastus Lateralis, Vastus Medialis, Vastus Intermedius, Adductor Longus, Adductor Magnus

Bipennate Muscles: Rectus Femoris, Dorsal Interossei, Gastrocnemius, Flexor Hallucis Longus

Multipennate Muscles: Deltoid, Subscapularis

Crisis of relevance

The reflection that led to this post was one of relevance. What’s the point of teaching about this structural characteristic of a muscle built from muscle fibers? Is the point just an academic exercise in thinking? Is it important enough to spend the time we spent? It does show how design has pros and cons. When designing a muscle² there are trade offs. Pros and cons. The arrangement of fibers to have a line of pull that has an angle to the overall line of pull of the muscle allows more muscle fibers to be included for a given volume (a pro)³; even though the shortening velocity of the whole muscle will be less than that of the fibers and the force of the whole muscle will be less than the straight sum of the fiber forces (a con). But I still have a nagging - “so what?” in the back of my head. Each muscle is what it is and does what it does, and if I know that - what’s the point of understanding the pennation angle concept or even the point of being able to classify that one muscle is pennated and another is not pennated? To be clear, I’m not saying there isn’t a point. I’m just admitting that sometimes I struggle to know what it is.

Save the phenomenon

I’m going to try to save the phenomenon⁴ from irrelevance by looking a bit more closely at the hamstrings.

From Cronin et al and discussed below.

The hamstrings are a group of muscles that can flex the knee and extend the hip. They are a collection of three muscles - the Biceps Femoris (long head and short head), Semimembranosus and Semitendinosus. Due to the complex nature of hip and knee movements, combined with the impact of being muscles that cross two joints and thus experience passive or active insufficiency depending on the movement of those two joints and the high velocities and/or forces associated with activation of these muscles both concentrically and eccentrically - - they are mechanically, physiologically and empirically associated with a high risk of being strained. So much so that there is a specific APTA Clinical Practice Guideline on Hamstring Strain Injuries in Athletes. In fact, in the “Pathoanatomical Features” section of the APTA Clinical Practice Guideline (CPG) for Hamstring Strain Injury (HSI) in Athletes, the pennation angle has been identified as significantly different in hamstrings that have been strained and those that have not been strained.

Also – in a systematic review of the Nordic hamstring exercises (NHEs) (which have been shown to decrease the risk of first and recurrent HSI) there is good evidence that NHEs change the pennation angle (PA). Changes in PA in these studies are simply described as in the hamstrings - which include both pennate (Semimembranosus) and non pennate (Biceps femoris and Semitendonosus) muscles.

Finally, the PA can be assessed with muscuolskeletal ultrasound (MSKUS). As a relatively new addition to the PT scope of practice, many of us are working to come up with ways we can get “decision actionable information” from its use. In a study by Cronin et al, hamstring muscle architecture (includine PA) is reliably assessed using MSKUS. And - it is assessed in both the pennate muscle (semimembranosus) and a non-pennate muscle (biceps femoris). Interestingly, the pennation angles of these two muscles are quite similar (see the table below). Meaning - just because the organizational structure of a muscle results in its classification as a “pennate” muscle doesn’t necessarily mean that the pennation angle is larger than the pennation angle in a muscle that is not classified as a pennate muscle.

From Cronin et al. For Zone A and B see the image above.

Conclusion

I’ve deciced to keep teaching about pennation angles. I need to update how I teach it and be sure to include the difference between pennation angles and a pennate muscle. All pennate muscles have pennation angles, and some non pennate muscles have pennation angles. There is relevance to understanding a pennation angle, and its possible that with time we can ascertain decision actionable information from the MSKUS assessment of pennation angles of a muscle. Perhaps as an indicator of injury risk, perhaps as an indicator of intervention effectiveness, perhaps as something I’m not thinking of at the moment!

1. As described in one of my earlier posts - Off the Desk is a column on newer, emerging concepts that I’m teaching, or that I’m considering teaching or that I’m writing for the purpose of teaching (such as my current writing project for clinical physiology). This is where most of my “growing edge” topics will be posted since most of what I do, of what I am, and of what I’m thinking stems from what, why and how I’m teaching.

2. Whether you get to a design via a designer or through an evolutionary algorithm is not the question here. Many worldviews describe the teleological features of living forms as designed.

3. Such as getting more cars (overall) into a parking lot if the spots are angled as opposed to perpendicular or parallel (I’m hoping the student that shared this analogy with me a couple of years ago reads this and contacts me so I can give proper credit ;)

4. Something I learned listening to RC Sproul was that Plato believed that the overall goal of all philosophy and scholarly inquiry was to “save the phenomenon”” to come up with a theoretical system that can explain in a coherent and rationale way the particulars of life as we observe it. “Through millennia, numerous philosophical and mathematical theories have attempted to explain the predictability of the physical universe. What does all of this have to do with salvation? Why would a Christian pay such close attention to the ever-changing world of science? In this message entitled “Saving the Phenomena,” Dr. Sproul helps us understand the need for Christians to understand the world God created.”