kinesiology 30

6.1 The Musculoskeletal System

When classifying anatomical parts, scientists define tissue as masses of cells that are similar in function and form (along with the intercellular substances they produce). Muscle tissue refers to a collection of cells that shorten during contraction and, in doing so, create tension that results in bodily movement of one kind or another.

Three Types of Muscle Tissue

In humans (and other mammals) muscle tissue can be classified into three main groups, based on the tissue’s structure and function. Figure 6.1 below shows diagrams and electron micrographs of each type of muscle tissue.

A: Smooth Muscles. Surrounding the body’s internal organs, including the blood vessels, hair follicles, and the urinary, genital, and digestive tracts, are smooth muscles. This type of muscle tissue contracts more slowly than skeletal muscles, but can remain contracted for longer periods of time. Smooth muscles are also involuntary, and their spindle-shaped fibres are usually arranged in dense sheets.
B: Cardiac Muscles. As the name suggests, cardiac muscles are found in only one place in the body—the heart. They are responsible for creating the action that pumps blood from the heart to the rest of the body. Cardiac muscles are involuntary muscles because they are not controlled consciously, and are instead directed to act by the autonomic nervous system. Like skeletal muscle tissue, cardiac tissue is also striated (striped).
C: Skeletal Muscles. Skeletal muscles are those muscles that are attached to the bones (by tendons and other tissue). They are the most prevalent muscle type in the human body—they comprise 30 to 40 percent of human body weight. Skeletal muscles are “voluntary”—humans have conscious control over their skeletal muscles; that is, the brain can tell them what to do. Skeletal muscle tissue is also referred to as striated, or striped, because of its appearance under a microscope as a series of alternating light and dark stripes.

The Components and Functions of the Musculoskeletal System

The musculoskeletal system (also sometimes referred to as the “locomotor system”) is the body system that allows humans to move. The musculoskeletal system is made up of the body’s bones, the skeletal muscles, and the connective tissue that supports and binds bones and muscles together.

As discussed in Chapter 5, skeletal muscle fibre connects generally to bones directly through tough tissue fibres, called tendons. The bones themselves are bound tightly together with other bones through ligaments. The articulation points where bones come together are called joints (the articular system). These joints act as levers in the human body and serve to facilitate human movement. Although subject to wear and tear and eventually deterioration over time, the cartilage tissue at the ends of bones prevents the bones from grinding against one another.

These major elements of the musculoskeletal system fit together, and work together, to allow us to get around, perform physical labour, and engage in sport and physical activity.

The primary functions of the musculoskeletal system are to support the body and keep it upright, to allow movement to occur, and to protect the body’s vital organs. As described in Chapter 5, the skeletal portion of the musculoskeletal system also serves as the main storage system for calcium, phosphorus, and other critical components of the blood.

Agonist and Antagonist Muscle Pairs

Skeletal muscles are typically arranged as opposing pairs (see Table 6.2 below). Since muscles pull on bones, another muscle (on the opposite side) is required to move the bone in the opposite direction. The muscle primarily responsible for movement of a body part is referred to as the agonist muscle. The muscle that counteracts the agonist, lengthening when the agonist muscle contracts, is called the antagonist muscle. Table 6.2 lists some basic movements involving agonist and antagonist muscle pairs.

This antagonistic pairing of muscles can be shown in the case of the human ankle joint. The tibialis anterior muscle (which originates at the upper half of the tibia) dorsiflexes the ankle (i.e., raising the toes) and the gastrocnemius muscle (whose origin is the posterior condyles of the femur) extends the ankle. (In this case, the soleus muscle helps the gastrocnemius muscle.) The ankle joint also exhibits inversion (where the sole of the foot faces the other foot) and eversion (the opposite movement). Such movements are controlled by the tibialis posterior, which inverts the ankle, and the fibularis (peroneal) muscles, which are antagonistic and evert the ankle.

Complex movements such as running involve many muscles acting as agonist and antagonist muscle pairs as well as stabilizers at various points during the movement. In other words, the muscles in our bodies act as functional groups. For example, in running or kicking a ball, the hip and torso muscles act as stabilizers while the quadriceps, hamstrings, and lower leg muscles act as agonist and antagonist muscles during various stages of the movement.

Origin, Insertion, and Function of Major Muscles and Muscle Groups

Skeletal muscle is attached to the bone either indirectly (via tendons) or directly (when the outer membrane of the muscle attaches to the outer membrane of the bone). The most common of the two ways in which muscles attach to bones is the indirect method (i.e., via tendons), as shown in Figure 6.2 below.

When skeletal muscle contracts, it causes movement of the attached bones. The point where the muscle attaches to the more stationary of the bones of the axial skeleton is known as the origin. The other end, the point where the muscle attaches to the bone that is moved most, is known as the insertion.

For example, the short head of biceps brachii originates from the “coracoid process” of the scapula. When you contract your biceps, you pull your forearm towards your shoulder, so you are pulling towards the origin, while the origin stays relatively fixed in its position. The insertion is on one of the bones of the forearm (the radius), called the radial tuberosity, and it is the forearm that moves during contraction.

Figures 6.3 and 6.4 on pages 168-169 show major muscles and muscle groups in the body. The series of illustrations beginning with Figure 6.18 on page 188 (along with tables on the facing pages) also show major muscles and muscle groups, as well as their origin, insertion, and function.

6.2 The Anatomy of Skeletal Muscle

A substantial portion of human body weight is made up of skeletal muscle, and, as noted earlier, these are the muscles that are directly involved in movement and locomotion. We can now look more closely at this type of muscle.

The basic unit of skeletal muscle is the individual skeletal muscle fibre or muscle cell. Looking outward and inward from this basic unit shows how skeletal muscle as a whole is constructed and how it works (see Figure 6.5).

Looking Outward

Looking outward from the surface of the individual muscle fibre, a sheath of connective tissue (the perimysium) binds groups of muscle fibres together. These bundles (fasciculi) in turn are bound together by a larger and stronger sheath of tissue (the epimysium) that envelopes the entire muscle.

The epimysium then extends beyond the muscle and changes its properties as it becomes one with the tendon. The tendon extends itself and becomes one with the bone’s periosteum. This connection happens at both attachment sites (that is, both at the muscle’s origin and at its insertion).

Looking Inward

Now, looking inward, a sheath of connective tissue (the endomysium) also surrounds the muscle fibre itself. Beneath the endomysium lies a plasma membrane or muscle cell membrane—the sarcolemma—which contains the muscle cell’s cytoplasm, referred to as the sarcoplasm. The sarcoplasm of muscle cells is analogous to the cytoplasm of other cells, although it contains larger amounts of stored glycogen and the protein myoglobin, and higher concentrations of calcium and other cellular organelles such as mitochondria.

Running along the muscle fibre’s length are thread-like structures known as myofibrils. Within these are finer “thick” and “thin” filaments (the cellular proteins myosin and actin). Myosin and actin themselves are contained within repeating structural units or compartments called sarcomeres.

Myosin is comprised of a “head” and “tail” and looks similar to a golf club. The myosin head has an attachment site for actin, and actin has a binding site for the myosin head. Actin has two other proteins: troponin, which has a binding site for calcium, and tropomyosin, which is the “stringy looking” cord-like structure that covers the binding site on actin (see Figure 6.5).

Together, these two proteins behave like a swivel-lock mechanism—they will not allow the myosin head to attach until calcium is released by the sarcoplasmic reticulum, a network of web-like channels involved in muscle activation. During muscle contraction, these protein filaments interact at the molecular level, causing them to slide across one another (i.e., the sarcomere shortens). This sliding action is synchronized across the muscle, and what we see and know as muscle contraction occurs.

This interlocking mechanism, commonly referred to as the “sliding filament theory of muscle contraction,” is widely accepted as a description of how muscles work at the molecular level—that is, how muscle contraction occurs. The theory is described in more detail below in the section “Sliding Filament Theory” (see page 178) and in the section “Excitation-Contraction Coupling” (see page 180).

The Neuromuscular System

The neuromuscular system is a general term referring to the complex linkages between the muscular system and the nervous system (the system of nerve impulses originating in the brain and spinal cord). These linkages involve two sophisticated bodily systems “linking up” and working together in a complex interface.

When you prepare to kick a soccer ball, to take a simple example, the messages needed to execute this action are sent from the brain or spinal cord and ultimately cause a chemical reaction in the leg muscle area. All of this happens in an instant, and many nerves and muscles are affected, yet kicking the soccer ball is seemingly automatic.

The Neuromuscular Junction

The nerves that transmit the message directing the muscle to move come into contact with the muscles at points called neuromuscular junctions, as shown in Figure 6.6. The electrical impulse travels along nerve pathways to the contact point between the nerve and a muscle (the junction). There, a chemical “neurotransmitter” is released (the chemical acetylcholine). This chemical is detected by receptors on the surface of the muscle fibre, and the process ultimately results in muscle contraction.

Constant use and regular practice will improve the quality and efficiency of the neuromuscular and nervous systems, and their ability to work together in producing movement. In Unit 4, you will build on the information presented here and learn how to incorporate resistance and aerobic training programs that will enhance your overall level of physical performance.

The Motor Unit

Nerves transmit impulses in “waves” that ensure smooth movements. A
single nervous impulse and the resulting contraction is called a muscle
twitch. (Muscle twitch is discussed more extensively later in this Unit.) One neuron or nerve (called the “motor neuron”) may be responsible for stimulating a number of muscle fibres. As illustrated in Figure 6.7, the motor neuron, its axon (pathway), and the muscle fibres it stimulates are together referred to as the motor unit.

Motor units can be categorized into small or large units. Simply stated, this means that a small motor unit can have a few muscle fibres that it stimulates, which produce fine motor (muscle) movement (such as the motor units of the eye). Other motor units, such as the ones found within the quadriceps group, are larger (they contain lots of muscle fibres) and they produce what are known as gross (or large) motor movements.

A single motor unit within the quadriceps may stimulate 300 to 800
muscle fibres. In order for maximal muscle force to be produced, all motor
units within that muscle or muscle group must be recruited (such as in a
maximal squat lift in the case of the quadriceps). Each motor unit must
fire and contract at the same time. This could include thousands of motor units contracting to their fullest. Generally, slow-twitch muscle motor units are smaller because they have fewer muscle fibres than fast-twitch motor units.

Motor units also comply to a rule known as the all-or-none principle (or law). This principle stipulates that, when a motor unit is stimulated to contract, it will do so to its fullest potential. In other words, if a motor unit consists of 10 muscle fibres (or 800 muscle fibres) and they are “turned on,” either all fibres will contract or none will contract.

6.3 Muscle and Tendon Injuries

The most common injuries sustained to the body during sport and physical activity are to the muscles and tendons—muscle strains and tears, tendonitis, and delayed onset muscle soreness (DOMS).

Muscle Strains and Tears

Muscle strains are caused by excessive twisting or pulling on a muscle or tendon. If strains remain untreated, tears in the muscle or tendon fibres may worsen. Strains can either be acute or chronic. They can occur in contact sports or when improperly lifting heavy objects. Chronic strains are the result of prolonged overuse and repetitive movement.

Strains and tears fall into three categories of severity: first-, second-, and third-degree.

First-degree injuries are the least severe. Often with first-degree injuries there is slight swelling and bruising, and pain is felt only at the end of full range of movement or upon stretching or contraction of the muscle.
Second-degree injuries are moderate but more severe. They require
physiotherapy treatment once diagnosed by a doctor.
Third-degree injuries are the most severe and may require surgery and rehabilitation. They may take from six to twelve months to fully repair.

Delayed Onset Muscle Soreness (DOMS)

Delayed onset muscle soreness (DOMS) is believed to be a result of microscopic tearing deep within the muscle fibres. It is most frequently felt when you begin a new exercise program, change your exercise routine, or dramatically increase the duration or intensity of your exercise routine. It may last from several hours to several days after the exercise session.

Any movement you aren’t used to can lead to DOMS, but movements that cause your muscles to forcefully contract while they lengthen (e.g., running downhill) seem to cause the most soreness. The soreness is usually felt in the first 24 hours, peaks from 24 to 72 hours, then disappears after 5 to 7 days. The degree of soreness depends on the activity performed and the intensity and duration of the activity.

The important things to know about DOMS are:

The pain of DOMS is not the same as the immediate acute pain of a pulled or strained muscle, nor is it the same as the muscle pain or fatigue you experience during exercise.
In addition to tearing, swelling can occur in and around a muscle.
DOMS can be minimized by performing proper warm-up and cool-down exercises, as well as by gradually increasing the intensity of a new
exercise program.
If you experience soreness from DOMS, try icing the area and gently stretching, but most importantly take the time to rest and recover.
If soreness continues for more than a few days, you should consult with your physician.

Tendonitis

Tendonitis is typically an overuse injury, often occurring when a new activity or exercise is begun that causes the tendon to become irritated. It is usually named after the affected tendon or joint. For example, tendonitis of the Achilles tendon is known as “Achilles tendonitis.” Tendonitis in the elbow area is referred to as “tennis elbow” (lateral epicondylitis) or “golfer’s elbow” (medial epicondylitis, similar to tennis elbow, but occurring on the inside, rather than the outside, of the elbow).

Symptoms of tendonitis may include:

Pain or tenderness on the tendon near or around a joint.
Stiffness and pain in the tendon, which restricts movement.
A strong pulling or sharp pain when moving a joint.
Occasionally, mild swelling, numbness, or a tingling sensation at the joint.

Treatment depends on the specific type of tendonitis, but most often involves:

Resting and avoiding movements that aggravate the area.
Protecting the area with splints, slings, or casts.
Applying an ice pack.
Taking prescribed oral medication for inflammation and pain.
Participating in physical therapy.

In most cases, tendonitis can be avoided with proper warm-up exercises, correct technique, and good equipment.

6.4 The Sliding Filament Theory of Muscle Contraction

It is important to keep in mind that muscles “pull,” they never “push.” A limb may push but it is the result of a muscle pulling on the bones. A muscle will shorten (contract) and move the object if the load is light; it will remain the same length if the load equals the muscle strength. But the basic mechanism involved is one of muscle contraction (shortening) so as to move limbs or maintain a certain position or posture.

Myosin Crossbridges

However, whereas the muscle as a whole contracts, the mechanism by which this is achieved is not through a shortening per se but rather an overlapping of the actin and myosin filaments, relative to one another. This causes the sarcomere (and thus the whole muscle fibre) to contract (i.e., to shorten). This is known as the sliding filament theory of muscle contraction.

The sliding filament theory is accepted as a description of how the process of muscular contraction occurs (see Figure 6.11). Its discovery dates back to the 1950s and accurately describes what happens during contraction, but it does not explain why it happens. What causes the filaments to slide in the first place, and what is the energy source and mechanism enabling this to happen?

The explanation for the sliding of the filaments is that a special set of
conditions are created that causes the thick and thin filaments to interact at the molecular level. In fact, at very high levels of magnification, it is possible to detect small bridges on the thick filaments that extend to the thin filaments. Over and over, these myosin crossbridges, as they are called, attach, rotate, detach, and reattach in rapid succession (in a ratchet-like fashion). This process results in the sliding or overlap of the filaments, a shortening of the sarcomere, and what we see and know as “muscle contraction.”

The Role of Calcium Ions and Adenosine Triphosphate (ATP)

At the molecular level, the “trigger mechanism” for the sliding filament process is the release of calcium ions when the nerve impulse is transmitted through the muscle fibre. The release of calcium (in the presence of the proteins troponin and tropomyosin) facilitates (or removes obstacles to) the interaction of myosin and actin molecules.

Muscle relaxation caused by the re-uptake of calcium ions requires adenosine triphosphate (ATP), the energy-carrying molecule that results from food metabolism. ATP is also used to detach myosin from the actin molecule. As the work of the muscle increases, more and more ATP is used up and must be replaced through food metabolism for the process to continue.

A fuller discussion of ATP is provided in Chapter 7, “Energy Systems and Physical Activity.” That chapter describes how the anaerobic and aerobic energy systems allow ATP energy to be created and replenished , which in turn allows muscle contraction and body movement to continue. The sliding filament theory is explained further in the next feature spread, which describes the process from the initial electrical impulse through to the final muscle contraction, a process sometimes referred to as “excitation-contraction coupling.”

6.5 Reflexes, Proprioception, and Movement

Reflexes are an important part of all physical movement. They are an automatic and rapid response to a particular stimulus. If the command centre for the reflex is located in the brain, it is commonly referred to as a cerebral reflex; if the control is located in the spinal cord, it is called a spinal reflex.

Autonomic reflexes are mediated by the autonomic division of the nervous system and usually involve the activation of smooth muscle, cardiac muscle, and glands. These reflexes regulate such bodily functions as digestion, elimination, blood pressure, salivation, and sweating.

Somatic reflexes involve stimulation of skeletal muscles by the somatic
division of the nervous system, and include such reflexes as the stretch reflex and the withdrawal reflex (discussed on pages 186-187). Reflex contraction of skeletal muscle is not dependent on conscious intervention by higher centres of the brain but are a way in which the body responds to an unexpected stimulus.

The Reflex Arc

There are three types of neurons in the human body: sensory neurons, motor neurons, and interneurons. Sensory neurons detect or sense information from the outside world, such as light, sound, touch, and heat. Motor neurons send signals away from the CNS and elicit a response, for example, movement of a leg or an arm. Interneurons form interconnections between other neurons in the CNS. Neurons transmit information to each other through a series of connections that form a circuit.

An example of a simple circuit is the reflex arc, which allows an organism to respond rapidly to inputs from sensory neurons and consists of only a few neurons. The stimulus from sensory neurons is sent to the CNS, but there is little or no interpretation of the signal. Few, if any, interneurons are involved. The signal is transmitted to motor neurons, which elicit a response, for example, a knee jerk.

The basic arrangement of the reflex arc involves a receptor, an adjustor (usually), and an effector (see Figure 6.13 on the facing page). The afferent (incoming) impulse from the receptor is passed along the sensory nerve axon to the adjustor, which then interprets the message and sends an efferent (outgoing) impulse along the motor nerve axon to the effector organ or muscle.

There are five parts to a reflex arc:

the receptor, which receives the initial stimulus (say, a pinprick to the skin or a loud noise);
the sensory (or afferent) nerve, which carries the impulse to the spinal column or brain;
the intermediate nerve fibre (the adjustor or interneuron), which interprets the signal and issues an appropriate response;
the motor (or efferent) nerve, which then carries the response message from the spinal cord to the muscle or organ; and
the effector organ itself (e.g., a skeletal muscle), which carries out the response (such as removing the hand or leg away from danger).

Proprioceptors and the Control of Movement

The process (known as excitation-contraction coupling) and the mechanism (sliding filaments) by which a single nerve impulse or series of such impulses is translated into muscle contraction was outlined earlier in this chapter. But what exactly determines the extent to which a muscle contracts, the moment when a muscle relaxes, and how muscles coordinate with other muscles and with other muscle groups in a particular area of the body?

The answers to these questions lie in specialized receptors located within tendons, muscles, and joints. These receptors are called proprioceptors, and they provide sensory information about the state of muscle contraction, the position of body limbs, and body posture and balance. Balance, or equilibrium, is part of a broader sense called proprioception, which is a person's ability to sense the position, orientation, and movement of the body. Being able to sense body
position is vital for the survival of all animals. This all-important feedback, along with control over muscles, is provided primarily by the afferent (sensory) input from two sensory receptors: tendon organs and muscle spindles.

The proprioceptor system plays an indispensable role in physical movement. Tendon organs and muscle spindles continuously monitor muscle actions and are essential components of the neuromuscular system. They “tell” the nervous system about the state of muscle contraction, act as a kind of safety device, and allow the nervous system to respond accordingly. Let us examine these two sensory receptors in greater detail, by looking first at their general anatomy, and then at their function.

Muscle Spindles and the Stretch Reflex

Muscle spindles play an essential role in all physical movement. They are the means by which muscles constantly and automatically adjust to the demands placed on them. They lie parallel to the main muscle fibre and send constant signals to the spinal cord. The spindle (so-called because it resembles the spindle of a spinning wheel) consists of specialized muscle fibres, known as intrafusal muscle fibres, that run the length of the muscle (see Figure 6.14 below).

The spindle itself is several millimetres long, and about five intrafusal muscle fibres run through it. The spindle fibres are thinner and shorter compared to the ordinary skeletal muscle fibres, although they behave in much the same way and look more or less the same. The swelling of the spindle is produced by fluid contained in a capsule surrounding the central area of the intrafusal fibres.

Muscle spindles help to maintain muscle tension and are sensitive to changes in muscle length (rather than tension). The muscle spindle contains two afferent and one efferent nerve fibres. The spindle detects changes in the muscle fibre length and responds to it by sending a message to the spinal cord, leading to the appropriate motor responses. The resulting contraction allows the muscle to maintain proper muscle tension or tone (e.g., an erect posture).

Muscle spindles are involved in the reflex contraction of muscles (the so-called stretch reflex). The stretch reflex action is present in all muscles and plays an especially important role in the major extensor muscles of the limbs. The usual example of this action is the knee-jerk reflex (the patella reflex) but it is also responsible for overcompensation responses when additional weight is suddenly placed on a weight-bearing muscle.

Golgi Tendon Organs and the Tension Reflex

Golgi tendon organs (GTOs) are named after their discoverer, Camillo Golgi (1844-1926). Golgi was an Italian physician, pathologist, scientist, and Nobel laureate. Several structures and phenomena in anatomy and physiology are named for him, including the Golgi tendon organ and the Golgi tendon reflex.

Golgi tendon organs are sensory receptors that terminate where tendons join to muscle fibre. GTOs are aligned in series with the muscle, such that any muscle stretching also stretches the GTO receptor. Golgi tendon organs are thus ideally positioned to detect increased tension exerted on the tendon.

The Golgi tendon organ, illustrated in Figure 6.15 below, projects to the motor neurons located within the spinal cord. When the change in tension is detected, an impulse is sent along afferent neurons to the central nervous system (CNS), where they synapse with motor neurons of that same muscle.

The efferent neurons instantly transmit an impulse, causing the muscle to relax, thereby preventing injury. The sequence of steps is the same as in the muscle spindle (stretch reflex) described on the facing page.

Essentially, GTOs serve as a kind of tension detection device for the muscle system. They help protect the muscle from excessive tension that would otherwise result in damage to the muscle or the joint or both.

Golgi tendon organs provide feedback to the central nervous system (CNS) regardless of magnitude of the tension. For this reason, it is likely that they play an important role in the development of strength and power, since, in order to be able to exert greater force, it is necessary to overcome obstacles presented by the Golgi tendon organ itself.