Part III: Basic Biomechanics: Why You Move
Chapter 10: Let’s Move, Baby! The Muscles
Let’s Move, Baby! The Muscles
In This Chapter
▶ Grasping the principles of how muscles contract and force is generated
▶ Looking at different muscle fiber types and structures
▶ Recognizing what causes muscle fatigue and soreness
▶ Identifying common muscle injuries
Nearly every aspect of your life involves movement. Even as you sit and read this book, you’re using your muscles: You’re sitting in a chair or lying down, holding the book, turning the pages, moving your eyes across the page, and so on. Movement can be fairly simple, as in the reading example, or it can be quite complex. Not only does your body depend on your bone struc- ture to support movement, but it also depends on the muscles. Without your muscles, you can’t move! Nothing can happen.
The effect muscles have on movement is dictated by the foundations of the muscular makeup. How much force is generated, the amount of joint motion, joint stability, and muscle action differ across muscles, based on their struc- ture. In this chapter, you explore the different types of muscles, how they are organized into different types of fibers, and how you can use these different fibers to run fast, for example, or run for long periods of time. The variety in your muscles helps you perform a wide range of movements.
The Foundations for Muscle Movement:
The Science behind Contraction
If you want to turn on a light, you have to flip the switch to send electricity to the light bulb. If you want to “turn on” a muscle — that is, make the muscle contract — you need to flip a switch in the brain. This “switch” is an action potential that originates in the brain from the motor control centers.
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As we explain in Chapter 3, various parts of the brain send signals down the spinal cord to initiate movement and cause the muscle to contract. These sig- nals are one part of the story of movement. The actual mechanics of muscle contraction are the other part of the story. Any movement at a joint requires that the muscles connected on each side of the joint shorten.
“How is that even possible?” you ask. “And wouldn’t an the entire muscle shortening produce a lot of force? What if you only wanted a little bit of force? How is that controlled?” Read on for the answers. As you’ll see, many things influence how a muscle behaves.
A muscle contraction is a very complex piece of work. It involves nerves, different types of fibers, and different types of attachments, and it can span multiple joints. All these factors are significant contributors to the muscular system.
Uncovering the structure of the muscle
A muscle is really many muscles woven together. It’s made up of many smaller units of muscle fibers, and each unit of muscle fibers is itself made up of components that, when all are working as they should, make movement possible. Figure 10-1 shows the structures of the muscle as you go to the smaller units. Refer to this image as you read the next few sections.
Figure 10-1:
Anatomy of skeletal muscle tissue.
Illustration by Kathryn Born, MA
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Bundling up: Myofibrils
As Figure 10-1 shows, bundles of long fibers, called myofibrils, are grouped together to form a muscle. Each myofibril is made up of smaller, individual units of contracting tissue stacked end to end. The smallest unit that makes up the myofibril (and the one that does the contracting) is called a sarco- mere. You can read more about the sarcomere and its role in contraction in the later section “The sarcomere and its parts: Shortening to produce force.”
Releasing calcium: T-tubules and the sarcoplasmic reticulum
One ion that’s very important for making a muscle contract is calcium.
Although calcium is bound up in bone, it’s also found in a system of storage vesicles, called the sarcoplasmic reticulum, within the muscle. Calcium can be released from this location and spread throughout a muscle via the T-tubules.
All it needs is a stimulus, which it gets by way of the motor unit.
The motor unit: Connecting the nerve and the muscle
In the locations where the nerve actually reaches the muscle, the nerve doesn’t just plug into the entire muscle. Instead, each motor nerve connects to only a certain number of muscle fibers. One nerve may connect to 100 fibers, for example, or it may connect to 1,000. This nerve-fiber connection is called a motor unit. These connections give your brain some control over just how many of those fibers contract.
The sarcomere and its parts: Shortening to produce force
As we note earlier, a myofibril is made up of a series of sarcomeres stacked end to end. Following are the key parts of a sarcomere (see Figure 10-2):
✓ M line: This is the connecting tissue located in the middle of the sarcomere, providing structure and stability to the sarcomere.
✓ Thick filament: Also know as myosin, it is pretty thick.
✓ Thin filament: Also known as actin, this protein filament is much thinner than myosin.
✓ I band: This band appears lighter in the sarcomere because it contains only the thin actin filaments. No myosin overlaps it.
✓ A band: This darker band contains an entire myosin (thick) filament.
✓ Zone of overlap: The action happens here! In this zone, actin and myosin connect and cause shortening of the sarcomere.
✓ Z line: Each sarcomere is connected to another sarcomere by rigid con- nective tissue called Z lines. The Z lines are essentially anchors that connect protein fibers. They provide stability to the sarcomere, and the pulling of myosin on actin moves the Z lines closer together during muscle contraction.
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✓ Actin: Connected to the Z lines are thin protein filaments called actin.
Actin has a twisted appearance, similar to what you’d get if you twisted a pearl necklace. It’s a fiber that, when activated by a strong pull, can pull the Z lines closer together.
✓ Myosin filaments: Between the actin are thicker looking sets of protein filaments that have small “arms” coming out from them. These thick fila- ments, called myosin filaments, do the pulling. Think of the myosin as a rowboat in the water, and the arms as the paddles that do the pulling.
✓ Titin: Holding myosin in place and keeping it connected to everything is a large, springlike protein called titin. Titin, working a bit like a rubber band, helps give muscle an elastic property.
Figure 10-2:
The sarco- mere.
Illustration by Kathyrn Born, MA
Binding sites for muscle contraction
During a contraction, the sarcomere has to shorten, which happens when the myosin heads grab onto the actin at the binding sites and give them a pull.
When a muscle is a rest (that is, not contracting), it can’t grab onto the binding sites because the sites are covered. Uncovering the sites involves two proteins:
✓ Tropomyosin:Tropomyosin is a long filament protein that lays on top of the binding sites on the actin. When tropomyosin covers the binding sites, the muscle is at rest.
✓ Troponin:Troponin is a globlike protein that actually moves tropomyo- sin out of the way, if it has the right incentive, so that the binding site is exposed, a process we outline in the next section.
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Filaments sliding past each other:
Producing muscular force
Because you can’t really see the motions within the sarcomere as it shortens, researchers have come up with a theory that explains what happens when the muscle contracts. Here is the step-by-step sequence of what makes the sarcomere shorten:
1. The brain sends an electrical stimulus down the spinal cord and out to the motor units.
2. The motor units spread the signal to the fibers they’re connected to, activating the release of calcium from the sarcoplasmic reticulum.
3. The calcium binds to the protein troponin, causing troponin to change its shape.
4. Troponin’s shape change moves the tropomyosin out of the way so that the binding sites become available.
Figure 10-3 shows this sequence of tropomyosin movement and calcium binding.
Figure 10-3:
Calcium activates the contraction sequence.
Illustration by Kathryn Born, MA
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5. The myosin heads attach to sites on the actin, connecting the filaments.
In an earlier analogy, we say that the myosin is like a rowboat and the heads are the boat’s oars. This step is akin to the boat putting its oars in the water.
6. The myosin heads rotate and pull the actin on both sides of the sarcomere toward the center.
This action shortens the sarcomere and produces force.
7. As long as there is energy to power the process and stimuli to keep it going, the myosin heads continue to rotate, releasing, grabbing, and pulling the next site and the next.
At this point, the oars of the rowboat are in full swing.
To visualize this process, put both your hands in front of you, palms facing you, with your fingertips just slightly overlapping. This represents your sar- comere at rest. Now move your fingers inward, fingers sliding past each other.
This represents a sarcomere contracting. As the filaments slide past each other and the sarcomere shortens, force is produced.
The Tortoise and the Hare: Fast and Slow Twitch Fibers
Some movement activities require endurance, whereas others require a lot of force over a short period of time. Fortunately, differences in motor units, both in terms of how the nerve functions and the chemistry and action of the muscle fibers, make both types of movement possible.
Muscle fibers are generally divided into two primary groups: slow twitch and fast twitch, although an intermediate category also exists. The twitch is a ref- erence to the speed and frequency of the neural signal passing through the motor unit.
The fiber type you are born with may pick your best sport for you. Fast and slow twitch muscle fibers are not interchangeable, so what you are born with will, to an extent, explain a lot about the type of activity in which you excel.
Born with a lot of slow twitch? Well, the 100-meter sprint may not be the event for you, but you’d probably make a great marathon runner! Lots of fast twitch?
Chances are power sports are great for you. Of course, most of us have a com- bination of slow and fast twitch muscle fibers, enabling us to perform a wide range of sport activities.
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Not too strong, but keeps on keeping on:
The slow twitch muscle fiber
Think of slow twitch muscle fibers like a tortoise: They’re not particularly fast, but they do keep on going. Slow twitch motor units have some common characteristics, outlined in Table 10-1.
Table 10-1 Common Characteristics of Slow Twitch Muscles
Nerve Characteristics Muscle Fiber Characteristics The nerve is small and reaches a
threshold for firing with a small stimulus from the brain.
Slow twitch muscles have large numbers of mitochondria.
The frequency of the nerve twitch is slow, and the magnitude of the twitch is low.
They’re aerobic fibers, capable of making ATP by using aerobic metabo- lism (see Chapter 4). They also use fats, carbohydrates, and lactic acid as a fuel source.
The nerve connects to only a few fibers (maybe 100–500 fibers per nerve), making it handy for fine motor control.
They have large amounts of myoglobin, an iron-containing protein that trans- ports oxygen through the muscle tissue and gives the fibers a darker, red- dish pigment. (Slow twitch fibers are sometimes called red fibers. If you’re a turkey eater, the slow twitch muscles would be the dark meat!)
Slow twitch muscle fibers are relatively smaller and weaker than their fast twitch counterparts.
Slow twitch muscles come in handy for any activity in which endurance is essential. Some muscles have a higher proportion of slow twitch fibers. For example, the soleus muscle, which is in the lower leg and is important for standing, has a high proportion of slow twitch fibers. In addition, because the nerve only connects to a few fibers, slow twitch motor units are usually associated with fine motor skills (like writing, typing, or blinking).
Big, strong, fast . . . and quickly tired:
The fast twitch fiber
Fast twitch fibers, also called fast twitch A fibers, are like the hare: fast! These fibers are built for speed and for generating a lot of force. The downside is that they tend to fatigue quickly. Fast twitch fibers and their associated
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nerves differ from slow twitch in a number of ways. Table 10-2 outlines their characteristics.
Table 10-2 Common Characteristics of Fast Twitch Muscles
Nerve Characteristics Muscle Fiber Characteristics The nerve is large and takes a higher
level of stimulus to reach a threshold for firing. For light activities, the motor unit is generally not active.
Fast twitch fibers produce their energy by using anaerobic metabo- lism (Chapter 4). They don’t use much oxygen.
The frequency of the nerve twitch is
fast, and magnitude is large. Anaerobic fibers produce ATP quickly, so fast twitch fibers can produce more force than slow twitch fibers can.
The nerve connects to many fibers (thousands of fibers per nerve). When the motor does reach a firing thresh- old, a lot of fibers are activated, and a lot of force is generated.
Anaerobic fibers produce more lactic acid, meaning they fatigue quickly.
The fibers are large and respond more to strength training.
Fast twitch X, or intermediate, fibers
One slightly different version of the classic fast twitch fiber is the intermediate fiber (also called fast twitch X or fast twitch B). Although these fibers are still fast twitch, their chemistry is a little different. These fibers have some mito- chondria, so they can get energy both from aerobic and anaerobic metabolism.
Using slow twitch muscles to cool down!
Although slow twitch muscle fibers run on the byproducts of fat and carbohydrate metabolism, they have a unique enzyme that also allows them to use lactic acid as a fuel. The enzyme converts lactic acid back to pyruvic acid, which can then be taken up by the mitochondria. As the slow twitch fibers are activated, they use the lactic acid as a source of fuel, which helps clear away the lactic acid quickly. You can use this behavior to cool down.
Suppose that you’re engaged in an intense activity, like running very hard for three to five minutes. During this activity, you build up a lot of lactic acid. What can you do immedi- ately afterward to recover? Activate your slow twitch fibers by lightly jogging or walking. As you cool down with the light activity, you’re clearing away the lactic acid much more quickly than you would if you just stand there with your hands on your knees!
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Working in Unison: How the Muscle Behaves
Muscles are much more than simple pieces of flesh that merely contract and relax. In this section, we investigate the properties that help define muscle function. How reactive a muscle is, how much it can stretch, and whether or not it returns to its normal length are all considerations that affect or dictate muscle function.
Looking at a muscle’s response
Muscles produce the force needed for any type of movement, whether it be swatting a bug or running a marathon. Obviously, each action is unique and has particular demands. The force that a muscle or group of muscles can pro- duce is dependent on its behavioral properties, outlined in the following list:
✓ Extensibility: Extensibility refers to the muscle’s ability to be stretched or lengthened beyond its normal resting length. A good example of extensibility is how the quadriceps (the muscle in the front of the upper leg) stretch when the hamstrings are contracted to bend the knees.
✓ Elasticity: Whereas extensibility refers to the ability of a muscle to be stretched, elasticity refers to a muscle’s ability to be stretched or elongated and then to return to its normal resting length afterward.
When a muscle is stretched, it returns to its normal length unless an
Fast twitch A versus X: Looking at training’s effect on your muscles’ behavior
Can jogging slow down your sprint speed?
Maybe. Bodies are very sensitive to training and adapt to the type of training they receive.
In the case of muscle fibers, the fast twitch X fibers may adapt to one type of training at the expense of adaptions to another.
Fast twitch X fibers can be trained so that they function more similarly to aerobic fibers or more similarly to anaerobic fibers. If you
do a lot of endurance training, you’ll improve your endurance, but you may lose some sprint speed. If you do a lot of sprint training, you could improve your sprint speed but lose some of your endurance. If you’re a really serious athlete, consider blending your training to include a range of intensity so that all the muscle fibers can adapt and gain performance.
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injury occurs from being stretched too far; in this case, the muscle is strained and the muscle fibers are torn.
✓ Irritability: In this context, irritability doesn’t mean getting angry;
instead, it refers to the muscle’s ability to respond to stimuli. Also known as excitability, irritability describes the reactivity of a muscle and may dictate the timing and amount of stimulus needed for a contraction.
✓ Contractility: In all cases, the muscles must generate tension to create movements. The ability to create tension is referred to as the muscle’s contractility. The amount of tension or how hard a muscle contracts depends on the muscle’s length, its timing, which motor units get innervated, and the position of the joint.
Noting muscles’ organizational structure
You may have heard that form follows function — a maxim that definitely holds true when it comes to your muscles. Muscles have a distinct organi- zational pattern and are connected to the bones in ways that enhance your mobility while simultaneously giving you strength. Read on to discover how muscle organization is key to movement.
The architecture of the muscle fiber
The architecture of your muscles — that is, their size and shape — differs from body part to body part and helps to dictate their function. Some mus- cles enable a large range of motion, and others have less extensive range of motion but provide support and stability.
Muscles are typically broken up into two structural categories: pennate and parallel (see Figure 10-4):
✓ Parallel: Muscles with parallel architecture have fibers that run in a parallel fashion along the length of the muscle. This fiber orientation is more conducive to large ranges of motion. The shapes that possess a parallel arrangement are fusiform, triangular, flat, and strap.
✓ Pennate: In pennate muscle architecture, the fibers are oriented at an angle into their tendons. Because of the larger number of muscle fibers that are attached to the tendon, pennate muscles are able to gener- ate more force. In this category, muscles can be unipennate (attach- ing to one side of the tendon), bipennate (attaching to both sides of a tendon), or multipennate (attaching to the tendon in multiple loca- tions), depending on the number of tendons and how the muscle fibers attach to them.