6 Short Stories about the Biomechanics of Movement

By Andrew Biel

Let’s say you want to build a human body capable of movement. Aside from needing the obvious anatomical structures—fascia, bones, muscles, and more­—you’d also need to incorporate some basic physics. That is to say, you’d need to explore the biomechanics of movement.
Now, you might be thinking, “Joints and nerves are one thing, but physics? Come on, Biel!” Yet, before you turn the page, please remember that biomechanics­—the mechanical principles that directly relate to the body—is just another way of talking about everyday stuff.
Washing your hands? Friction. Trying to open a pickle jar? Torque. Beelining across a dance floor to meet that special girl or guy? Vector. Even lifting bags of concrete (Newton’s Second Law of Motion) or raising a water bottle to your mouth after all of that heaving (third-class lever) is biomechanics in action. Whether you are in motion or at rest, these principles illuminate your ordinary actions.
So, let’s keep it fun and simple, and explore how laws of motion and levers relate to mobility and stability in your life and bodywork practice.

Laws of Motion
When Isaac Newton set out to explain (one could say invent) his Three Laws of Motion, he didn’t exactly have muscles and bones in mind. Celestial bodies were his focus, not human bodies. Yet, it turns out that his pronouncements pertain to our tissues just as well as they do to the earth, moon, and sun. As the basis of classical mechanics, these three physical laws describe the relationship between forces which act on an object and the motion resulting from those forces. (Human movement, anyone?)

Story 1
Law of Inertia
Imagine you’re running steadily down a trail. You turn a blind corner and there—standing absolutely still in your pathway—is a massive elk. (Just go with me here.) You “apply the brakes” by clamping your muscles and digging in your heels, but you can’t avoid plowing right into the huge mass of flesh and fur. He wanders off (mumbling about how odd humans can be) and you check yourself over for bruises and broken bones.
This goofy tale illustrates Newton’s First Law of Motion, known as the Law of Inertia. To paraphrase, it states that an object at rest tends to stay at rest, while an object in motion tends to stay in motion. Inertia means that an object will keep on doing what it’s doing. In this case, you were in motion while the elk was at rest.
What’s more, to overcome an object’s inertia, a force is required. This force will cause the object to stop, move, or change direction. In your case, your mobility was altered by the stationary force of the elk. (It stopped you cold.) This highlights how inertia resists a change in motion and explains why you didn’t knock the elk off its hooves upon collision. (Since the elk has more mass and thus more inertia than you, the force of your decelerating body had marginal effect.)
Now, let’s replay the above scenario, but let’s replace you with J. J. Watt (giant American football player) and the elk with a baby goat. With the relative inertias switched, upon contact, Mr. Watt would send the wee goat airborne. In another example of this law, line up two golf balls and gently tap Ball #1 onto the green. While Ball #2 stays in place (an object at rest stays at rest), the force of the club causes Ball #1 to roll toward the hole (an object in motion tends to stay in motion). If we removed resistive forces like gravity, friction, and air resistance, Ball #1 would never stop traveling.
Newton’s First Law may seem so utterly obvious that you might wonder why it even needs to be discussed. Yet, this simple concept pretty much explains mobility and stability. After all, flip the law on its head and imagine the kind of action you would have if an object at rest was inclined toward motion and vice versa. Science fiction chaos.

Story 2
Law of Acceleration, part 1
It’s snowing heavily and your car is stuck in a ditch. Fortunately, another vehicle stops and three big guys jump out to help. While you steer, they slowly push your little Honda back onto the road. You thank the fellows and drive safely to your destination.
You may also want to express your gratitude to Newton’s Second Law of Motion. Known as the Law of Acceleration, it has nothing to do with your gas pedal, but focuses on the relationship between force, mass, and acceleration.
This law states that the acceleration of an object is directly proportional to the force acting on it and is inversely proportional to its mass. The direction of the acceleration is in the direction of the applied force.
What the heck does that mean? Some guys pushed (force) your car (mass) and it moved (acceleration) in the direction that they pushed it.
Let’s go a bit deeper. If we recall, force is any influence that causes an object to undergo a certain change. Thus, the three guys created force and it caused your car to move. Mass—the quantity of matter in an object—pertains to your car. Luckily for the guys, it was small and contained little mass. Acceleration—the rate at which the velocity of a body changes with time—was evident as they shifted your car’s position from ditch to road.
Now, let’s see what happens if we change some of the story’s variables. For instance, let’s say we switch out your little Honda for a 2-ton Hummer. The three guys produce the same amount of force, but now there’s much more mass. For all of their pushing, they barely shift the rig. Thus, the acceleration of the vehicle diminishes. In other words, if the same force is applied (three guys) and the mass increases (the Hummer), acceleration decreases.
But now a UCLA football van pulls up and 10 massive linemen jump out ready to push. Even against the Hummer’s large mass, they are able to produce a great enough force to send the vehicle accelerating back onto the road. Here we see how an increase in applied force increases acceleration.
Under more serious circumstances, such as the sudden stopping (or hurtling) of an automobile, the realities of Newton’s First Law can produce injurious effects on the cervical spine. For instance, if you were rear-ended while stopped in traffic, the flinging motion of your whiplashed head would demonstrate how an object at rest tends to stay at rest. Unfortunately, in those few milliseconds that your head was at rest, your torso was pressed forward by the car seat. These different inertias (torso moving, head stationary) send your cranium flinging into extension and then (as we’ll see in the Third Law of Motion) swing it forward into flexion.
Law of Acceleration, Part 2
The second part of this law tells us that the direction of the acceleration is in the direction of the total applied force. For instance, if you roll a giant snowball to the south, it will travel ... to the south. If the snowball is already moving, a slight nudge to the southwest will alter the snowball’s direction. This portion of the law might evoke you to blurt out, “No, duh.” But consider a parallel universe that offered the opposite: you attempt to flex a client’s shoulder and, shockingly, instead it abducts or extends. Again, science fiction chaos.
Let’s return to the golf course and see this law in action as we replace the golf ball with a bowling ball. This extra mass will require much more force in your stroke to deliver the same acceleration. Return to hitting regular-sized golf balls, but now exert the same force as you did with the bowling ball. Such a large force applied to a small mass will cause greater acceleration (and put you in the rough).
If you’d like to see Newton’s Second Law come to life in your tissues, just wait for a commercial break during a television program. As you rise from the couch to a standing position, your muscles (several forces) pull your bones and fasciae (mass) to generate movement in your limbs (acceleration). The second part of the law is what ensures that the direction of your acceleration will send you where you want to go—the bathroom and not the kitchen.

Story 3
Law of Action-Reaction

You launch a canoe onto a lake. The surface is like smooth glass, and with every stroke of your oar, a ripple of water passes the boat. You glide out to a small, floating dock in the middle of the lake. Since neither the boat nor the floating platform are attached to something steady (like the earth), they shift away from each other as you awkwardly climb from one to the other.
Safely on deck, you hear a loon pass overhead. If that bird were the ghost of Newton, he might have said, “for every action there is an equal and opposite reaction.” And this is exactly the premise of his Third Law of Motion—the Law of Action-Reaction. In other words, whenever one object pushes on a second object, the second one pushes back on the first by the same amount. The strength of the action and reaction are equal and occur in opposite directions.
This law surfaces a few times in the above storyline. First, the action of your paddle against the water is met by the equal, but opposite, reaction of the water on your paddle. (If the water offered no reaction, you’d still be on the shore.) In a sense, the water and you (who generated the force) are pushing against each other, with you gliding forward and the water swirling backward.
This law also came to life when you attempted to climb onto the dock. At a critical moment, you had one foot in the boat and one on the floating island. As you pushed off the boat with your foot, the canoe—much lighter than the dock—floated away.
This situation illustrates how much we take Newton’s Third Law for granted. Whenever we want to take a step—that is, push our body away from one location—we do so by pushing into the ground, trusting implicitly that the ground will push us back and away. But in this case, the “ground” (canoe) has a similar amount of inertia as your body. What’s more, the surface of the water affords very little friction against the boat’s hull to resist your applied action. The result (reaching back to our discussion of Newton’s Second Law) is that both parties accelerate away from each other.
Take a walk in the park and you’ll see this law everywhere. Actually, the simple act of walking in the park illustrates it as your shoes push against the ground and the ground pushes up against your shoes. On the grass is a boy flying a kite; observe the tug-of-war not only between his hands and the string, but also between the kite and the wind. Stop to watch a pickup game of basketball and note the action-reaction dynamic between the player’s dribbling hand and the ball. Finish your walk and step into a Pilates class. Feel the force of your muscles and fascia act and react to the spring-loaded machines.

Imagine a work crew constructing a new building. Bricks, shovels, and crowbars, as well as bones, joints, muscles, and fascia, are all on site. To move supplies—as well as their own bodies—the crew will need many biomechanical elements. One component will be critical—leverage. For that, the workers will need some levers.
A lever is nothing more than a simple machine that can amplify an applied force (effort) by converting it into torque. If that leaves you scratching your head, don’t sweat it. Let’s instead begin by recognizing that levers are used everywhere. For instance, if you cut your friend’s hair, sweep up the trimmings, and then open a can of peaches and poke a slice with a fork, you’ve used four levers. Aside from the scissors, broom, can opener, and fork, you also involved the levers of your musculoskeletal system.
To build an anatomical lever, we’ll need a rigid bar and an axis (fulcrum) for the bar to pivot around. That is to say—a bone and a joint. Virtually all of your movements will occur via the coordinated arrangement of your bony levers. Even your odd-shaped bones like scapulae, vertebrae, and cranium­ will provide leverage, with the long bones of your arms and legs best suited to generate maximum advantage.
If we arrange our bar and axis together, we form a lever that is comprised of three parts:
• Axis.
• Effort.
• Resistance.
The lever pivots around the axis, the effort moves the lever, and the resistance (the load) is moved by the lever. Looking at the can opener again, the point where the opener makes contact with the can is the axis, your hand and muscles provide the effort, and the can’s lid provides the resistance.
The physics of the lever demand that there be
space between these three points. The distance between the effort and axis will be the effort arm, while the resistance arm will be the span between the resistance and axis.
Depending how we arrange these components, three types of levers can be devised. Like superheroes, each will possess one or two special talents in either force, speed, or range of motion. Yet, each of their unique talents will come at the expense of other skills. Thus, some levers will produce great force over a short distance while others produce a weak force over a long distance.

Story 4
First-Class Lever
Back at the construction site, Curly is using a crowbar to lift a boulder. Although he can’t raise the chunker very high, with comparatively little applied effort he can produce a large amount of force, or torque.
The special skill of a first-class lever is force amplification. As we see with Curly, a small exertion can be used to mobilize a large load over a short distance. Thus, this first-class lever gets high marks for force, but scores poorly on range of motion and speed.
The can opener and scissors from our earlier discussion are examples of first-class levers. Of course, the mechanical advantage afforded by this type of lever will depend greatly on the length of its effort arm compared to its resistance arm. For instance, try to raise a boulder with a 2-foot bar versus a 6-foot bar and you’ll see (and feel) right away how the length of the effort arm determines the amount of effort required.
To experience a first-class lever in your body, look up toward the ceiling. The neck and head extensors generate effort, your cervical intervertebral joints provide an axis point, and the weight of your cranium (the anterior portion) serves as the resistance. Your body actually contains few first-class levers, which makes sense considering that you’re a creature designed for maximum mobility, which is not necessarily this lever’s strong suit.
The phrases bargaining chip, sink your teeth in, and get a grip are all linguistic references based in the concept of leverage—the mechanical advantage of a force to move an object. (Mechanical advantage being the benefit gained by moving an object with less effort.) In the body, muscles gain leverage by using bones as levers. But let’s be clear: using leverage doesn’t reduce the amount of work required, it just spreads it out. For example, use an extra-long bar to roll a boulder a short distance; it’s the same amount of work as when using a short bar, just dispersed. You got away with applying less effort, but you had to do it over a longer distance.

Story 5
Second-Class Lever
Lari’s job is to haul cinder blocks to the masons. She fills her wheelbarrow with bricks and—just before she starts to walk—raises the handles of her wheelbarrow. Amazingly, this small action requires little effort, especially when compared to the tremendous weight of the bricks.
By placing the load (bricks) between the axis (wheel) and the effort (Lari’s lift), a second-class lever is incredibly powerful—much more so than a first-class lever, typically. It relinquishes much of its range of motion and speed for an increase in mechanical advantage. As we saw with the first-class lever, changing the relative length of the effort arm to the resistance arm will change the effectiveness of the lever.
Aside from using a nutcracker or bottle opener, you can feel a second-class lever in your body by standing up on your toes. Here, the ball of your foot acts as the axis while your triceps surae (gastrocnemius and soleus muscles) deliver the effort. Situated between these two lever components is the resistance of your entire body weight.
Each of the three lever types requires a rigid bar. What could serve as lever bars in the body? Although taut bands of fascia will definitely provide some leverage, this is clearly the domain of your bones. All bones will provide some mechanical influence, with the long bones of your appendages being particularly effective as levers. (Imagine if your osseous units weren’t hard, but pliable. You wouldn’t move, so much as slither.) Meanwhile, the axes of rotation occur at your joint articulations. (Conversely, picture your skeleton as one, big osseous unit. How much leverage would that afford?)

Story 6
Third-Class Lever
Moe is the “Shovel Master.” He might extract only a small load of dirt with each pitch, but the swing of his tool allows him to remove soil rapidly and plop it in an assortment of locations (Image 22).
A third-class lever is all about range of motion and speed. By placing the effort (Moe’s hand) between the axis (his other hand at the end of the handle) and the load (dirt), this lever type trades away force amplification for maximum range of mobility. As with first- and second-class levers, altering the relative lengths of the lever arms (in this case by moving your grip) will change the mechanical advantage and the effort required (Images 23 and 24).
Brooms, tweezers, baseball bats, tennis rackets, and golf clubs all act as third-class levers. The sporting instruments, in particular, highlight the pros and cons of this lever type. On one hand, a third-class lever affords a big windup and follow-through, and allows the action to occur rapidly. However, it doesn’t provide much force, so the athlete needs to supply the necessary exertion—thus the need for a big stick to hit a tiny ball, followed by a tremendously loud grunt.
Because humans are designed for locomotion, it is not surprising that your body (in the limbs, especially) is comprised mostly of third-class levers. Flex your elbow, wrist or knee (or even clench your teeth) to feel this lever type in action and feel the swiftness and agility it provides.
The functional priorities of your body become clear when you consider the fact that it is teeming with fast, agile, third-class levers and very few strong second-class levers. Mobility trumps force. For instance, the majority of the joints in your upper appendages are constructed as third-class levers. This makes sense since the sole purpose of your shoulder, arm, and wrist is to ensure that your hand is able to mobilize through a broad range of motions.

Andrew Biel, LMP, is the author of Trail Guide to the Body: How to Locate Muscles, Bones and More (Books of Discovery, 2014) and the president of Books of Discovery. This is an excerpt from his forthcoming book Trail Guide to Movement: Building the Body in Motion (Books of Discovery, 2014). He lives outside of Lyons, Colorado, with his wife, Lyn Gregory, and two children, Grace and Elias.