Pectoralis major, the bodybuilder’s muscle

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Pectoralis major, the bodybuilder's muscle

We all know the image of a well-muscled boxer or weightlifter doing the “self-clasping handshake” as a gesture of victory; and sometimes politicians have used this gesture to recognize a spirited response from the crowd.  But this position also presents an opportunity for isometric exercise, in which muscles contract but are prevented from shortening by an opposing force — in this case, the opposite arm.  So a bodybuilder might use the occasion of one triumph, as a chance to work out for the next competition.

The pectoralis major might be considered the “bodybuilder’s muscle” because it is the largest of the chest muscles, enhancing the male physique.  It performs several actions, by pulling (at what is known as the insertion point) on the front side of the humerus (arm bone).  The action shown here is horizontal adduction — bringing raised arms toward the midline.  An alternative example would be a musclebound villain strangling his puny foe.

But despite its caricature status, the pectoralis major is one of few muscles with a special ability:  It can undo its own action!  Since all muscles work by shortening (contracting), this is a rare feature indeed.  The key is that the muscle has two divisions (heads) — the origin (attachment point on the body) of one is above the insertion, at the clavicle (collarbone), the other is below the insertion, at the sternum and ribs.  Depending on which head is used, you can move the arm in opposite directions.

When the clavicular head contracts, the arm is brought forward and upward (flexion).  You can test this by facing a wall, with your arms at your sides.  Push against the wall with one arm, and use the other hand to feel the muscle contract, just below your collarbone.  The front wall of your armpit, formed by your entire pectoralis major, hardens as well.

Now try raising one arm up high and in front of you, as if eagerly answering a question in class.  Face the wall again, and push with your raised arm.  Now, you are attempting to extend the arm, bringing it back down to your side.  Using your other hand again, feel that the lower division (sternal head) is contracting, as is the front of the armpit again.  While the two heads can work together in some actions (“two heads are better than one”), they are antagonists in performing flexion vs. extension.

Repeat these exercises a few thousand times, and maybe you too can look like a cartoon!

The kangaroo rat’s adaptable hopping mechanism

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The kangaroo rat's adaptable hopping mechanism

This cartoon summarizes the versatile ways that kangaroo rats deploy their hopping machinery in the desert.  It is based on a presentation by Dr. Craig McGowan of his research.  In studies of animal movement, biomechanicians often describe the muscles and connective structures using a machine metaphor, which helps to identify key adaptations in how animals get around.  Faced with a diverse terrain and hungry rattlesnakes, kangaroo rats use their gastrocnemius or “gastroc” muscle (homologous to our own “calf muscle”) in at least three different ways.  These discoveries may also inform future developments in the design of more versatile lower limb prostheses for humans.

Dr. McGowan directs the Comparative Neuromuscular Biomechanics Lab at the University of Idaho.  His presentation “Built to Hop: Meeting the Mechanical Demands of Locomotion in the Desert”, was a part of the weekly Colloquium at the Integrative Physiology (IPHY) Department at CU Boulder.

Serratus anterior, the boxer’s muscle

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Serratus anterior, the boxer's muscle

Boxers use a lot of muscles, but the quintissential boxing move is a simple forward punch.  Contraction of the triceps brachii straightens (extends) the elbow.  At the same, contraction of the pectoralis major and anterior deltoid muscles bring the arm forward at the shoulder joint, which is considered flexion.

But only one muscle can enjoy the “title” of The Boxer’s Muscle — serratus anterior.  Depicted here, the serratus anterior runs between the ribs and the shoulder blade (scapula).  When it contracts, it pulls the scapula forward (protraction), in the direction of the opponent’s jaw.  It also rotates the scapula upward.  In the illustration, the bottom of the scapula moves left, and the top moves to the right.  This raises the shoulder joint and orients it toward the opponent’s face.  This combination of movements provides a firm base for a forward punch, hence the epithet “boxer’s muscle”.

The name serratus comes from the “saw-like” appearance of the muscle (as in the word serrated), resulting from its several attachments along the ribs.

 

Levator scapulae, the “I don’t know” muscle

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Levator scapulae, the "I don't know" muscle

What happens if you just can’t remember?  Not to worry — there’s a muscle for that!  The levator scapulae does as its name implies — it elevates your scapula, or shoulder blade.  So, next time you’re asked for a muscle action you don’t know, just shrug it off!  Do this a few times and not only will you benefit from the workout, but (if you’ve been paying attention) you’ll never forget the levator scapulae.

But this muscle in fact accommodates a broad range of confidence levels.  If the scapula is held in place by other muscles, then contracting both of your levator scapulae muscles will extend the neck, pulling your head back.  Let your head drop forward again, and repeat a few times — you have just nodded your head in the affirmative!

And finally if you might know, but you’re really not sure?  Then you’ll want to contract just one of your levator scapulae (while fixing the scapula in place) — this will tilt your head to the side, in an expression of quizzical puzzlement.

Biceps brachii, the sommelier’s muscle

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biceps brachii, the sommelier's muscle

Probably you’ve heard of the “biceps”, but you might not have thought of it as the “sommelier’s muscle”!  And yet, the action of opening a wine bottle sums up the two major actions of this muscle.

But first of all, be warned that you have a biceps muscle in your thigh as well — so to be clear, the biceps in your arm is called biceps brachii (“two-headed muscle of the arm”).

The biceps brachii attaches to your forearm on the anterior side, and thus flexes the elbow — pulling the forearm toward the shoulder, and thus folding your upper limb in two.  But it also supinates the forearm — this is a rotational action that twists the forearm (and the hand with it) from a “palm backward” (or downward) position to a “palm forward” (or upward) position.

Supination has many uses, such as turning your cupped hands upward to “drink soup“, begging for mercy as you “supplicate”, or perhaps even expressing a certain attitude with “…’sup bro!” — and these can be helpful mnemonics for remembering this action.

The reason for this lesser-known action of the biceps brachii is that the muscle attaches to the inner surface of the radius (of the two long bones in your forearm, this is the one that sits on the lateral or thumb side).  As the muscle contracts, that surface is pulled toward the shoulder, rotating the radius laterally, which carries the hand with it.

When opening a wine bottle, supination is used to twist the corkscrew clockwise, inserting it into the cork.  This is followed by flexion at the elbow, as you pull the cork out of the bottle.  Be mindful, though, that this only works with your right hand!  Supination with your left hand achieves the opposite, which is helpful at the end — twisting counterclockwise, to get that corkscrew out of the cork.

Latissimus dorsi, the swimmer’s muscle

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Latissimus dorsi, the swimmer's muscle

The latissimus dorsi, or “lat” for short, is often referred to as the “swimmer’s muscle”.  It’s the prime mover of arm extension — meaning it does most of the work when you bring your arm back from a forward position.  Such a movement is especially useful in swimming, because by pushing back against the water, it propels the body forward.  To see a well-developed latissimus dorsi, just visit your local swimming pool and look for someone who just swam some “laps” with their “lats”.  You can also use this muscle for pull-ups, or striking a blow with a hammer, but I’d prefer to let mine carry me across a coral reef.

You have six Achilles tendons

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Cartoon representation of colloquium talk by Jason Franz at Integrative Physiology Department, CU Boulder, November 4, 2019

Your calf muscles are attached to your heel by a tendon — the Achilles tendon.  What you might not know (nor did I) is that each of your major calf muscles — the soleus, and the two heads (divisions) of the gastrocnemius — exerts force through its own subtendon within the Achilles tendon.  These three subtendons (six including both legs) can slide past each other, which allows each muscle to work independently.  That’s good news for walking performance, because each muscle is free to “do its own thing” without having to remain in “lock step” with the others.  Unfortunately, as we age, adhesions form among the subtendons, reducing their independence, and walking performance is reduced.

In his talk, Dr. Franz explained the problem and then introduced his laboratory’s current work on biofeedback techniques (using sensors on the calf muscles and, yes, a futuristic pair of glasses) which holds the promise of restoring some of that youthful gait performance, and thus, a longer period of independent living into advanced age.

The title of his talk was “Mechanics, Energetics, and Stability: Modifiable Factors to Preserve Independent Mobility in Old Age”.  Dr. Franz directs the Applied Biomechanics Laboratory at the Joint Department of Biomedical Engineering, University of North Carolina at Chapel Hill and North Carolina State University.  The talk was presented at the weekly Colloquium at the Integrative Physiology (IPHY) Department at CU Boulder.

Smooth muscle, ace of tubes

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Smooth muscle, ace of tubes

What’s your favorite muscle tissue?  Odds are you’ll say skeletal muscle, the type used in all voluntary movements.  Or, you might be partial to cardiac muscle, the main tissue component of your heart.  But there’s much to appreciate in the third muscle tissue, smooth muscle.  It’s a major component of your tubular organs – those of the digestive, urinary, reproductive, and respiratory systems, as well as your blood vessels – and for good reason.

The name “smooth muscle” refers to the lack of striations – the stripes visible on skeletal and cardiac muscle cells.  Those stripes reflect a highly regular, organized arrangement of protein filaments that give great strength and efficiency to striated muscle tissues.  But it comes at a cost – if you overstretch a skeletal or cardiac muscle cell, it becomes completely unable to contract.  That’s because muscle contraction depends on the sliding of myosin and actin filaments past one another.  Without any overlap to start with, the myosin molecules have nothing to grab onto.

Smooth muscle gets around this problem with a loose, net-like arrangement of myosin and actin.  When the cell is stretched, this network starts to straighten out, which means each group of myosin and actin suffers little tension.  The result is that much more overlap is maintained and these cells remain functional.

Why is this so important for a tubular organ?  Many of your tubes undergo stretching – think of your stomach after a big meal — which in turn, stretches the muscle cells.  But many other tubular organs undergo fluctuations in diameter, and smooth muscle allows them to contract under a wide variety of conditions.

Smooth muscle is also the only type of muscle cell that can divide after birth – a crucial feature in repairing a damaged wall after the passage of a chicken bone or a kidney stone.  Also, blood vessels can grow and change shape in response to changing demands – made possible by the production of new smooth muscle tissue.

Let’s give smooth muscle a little respect.  It may lack obvious “sex appeal” at first.  But considering smooth muscle makes up a big part of your reproductive organs, maybe it’s the “sexiest” muscle tissue of all!

How muscles work

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muscle contraction

Here’s how we move, using the elbow as an example.  A muscle is attached to the bones on either side of your elbow joint.  Inside the muscle, proteins called myosin (red), which are arranged in tiny rows called thick filaments, have little arms that reach out and grab onto proteins called actin (blue), which are arranged in tiny rows called thin filaments.  Alternately grabbing, pulling, and releasing, the myosin, like a tug-of-war team, brings the actin on one side closer to the actin on the other side.  The shortening of the muscle, which results from this sliding filament mechanism, is called muscle contraction.  Because the muscle is attached to each bone by a tendon, the bones are pulled together and the elbow bends.

This arrangement of proteins, like a stack of flattened hexagons, is called a sarcomere.  It’s the “functional unit” of muscle contraction, meaning that in theory, if a muscle had just one sarcomere like in the cartoon, it would still work.

That’s the simple version.

The molecules, of course, are shown greatly enlarged.  To maximize efficiency, the muscle has an intricate structure of repeating units that will make your head spin.  Sarcomeres are attached end-to-end (about 10,000 per inch) to form contractile rods called myofibrils, and myofibrils are stacked side-by-side to fill each muscle cell, which is also known as a muscle fiber.  A unique feature of skeletal muscle tissue (the type that can be voluntarily controlled, as in the example) is that the individual cells are extremely long – almost as long as the muscle itself!  So a muscle in the arm has “only” around 250,000 muscle fibers – far fewer than the billions of cells one normally finds in an organ.  The extreme length of our skeletal muscle cells probably makes them quicker and more efficient.

Now let’s consider the entire muscle again.  Its whole purpose is to move a bone through space.  To do this, it has to have a stable attachment at one end, called the origin.  When the muscle contracts, the other attachment, known as the insertion, is moved closer to the origin, and this is what bends your elbow.  Within the muscle, all of the sarcomeres are shortening at once, but the ones closest to the origin are hardly moving at all.  Meanwhile the sarcomeres near the insertion are moving rapidly, pulled by the cumulative efforts of all the myosin molecules further up the myofibril, allowing the muscle to win the “tug of war” against gravity.

Which tissue do we need the most?

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Which tissue do we need the most?

The entire body is composed of only four basic tissue types.  Muscle tissue, of course, allows you to move around.  But it’s also what moves your internal organs – the beating of the heart, the rumbling of the stomach.  Even your blood vessels have muscle tissue, which controls the distribution of blood in the body.  It’s hard to argue we could live without muscle.

But most of our muscles wouldn’t be much good without nervous tissue, which responds to stimuli and coordinates the activity of your organs.  It’s true that many of the slower, internal processes do not depend on nervous input – they may instead involve hormones, for example.  But what good is a body without a brain to give it meaning?

Epithelium, though, really is essential at the most basic level.  This is the tissue that lines all your external surfaces and your internal spaces.  Every substance that enters the body (food, water, oxygen) must cross an epithelium to do so.  These tissues are therefore the “gatekeepers” to the body, in charge of exchange with the environment – although, under the command of nervous tissue.

So the tissues must work together, and there’s no better example of this than the fourth basic type, connective tissue.  As the name suggests, it is the “putty” that holds the body together, filling in all the spaces between epithelium, muscle and nervous tissue.  But it also provides pathways for the movement of materials within the body.  This is the most diverse tissue type, including blood, the essential medium of transport, but many other types such as bone and cartilage.  The key feature of connective tissue is the presence of a large amount of nonliving “stuff” in between the living cells – the extracellular matrix.  Water is often abundant here, and this interstitial fluid forms another major transport medium for substances to move among all the tissues.

So of course, it’s hard to say any one basic tissue is more important than another.  I don’t know about you, but this “exchange” about the “connections” has been a “moving” subject for me that touches a “nerve”.  Pass the box of tissues!