Part III: Basic Biomechanics: Why You Move
Chapter 9: These Joints Are A-jumping!
These Joints Are A-jumping!
In This Chapter
▶ Identifying types of joints and their functions
▶ Understanding joint stabilization
▶ Investigating flexibility and stretching techniques
Without the joints that make up your body, you wouldn’t be able to move. Certainly, movement is dependent on your muscles’ ability to create the forces that propel you and your bones’ ability to provide the nec- essary structural support. But imagine trying to walk up stairs while keeping your knees straight or trying to write without bending your fingers or wrist.
Life as you know depends on your ability to bend and twist and glide.
Movement is largely dictated by your anatomical makeup, but things like flexibility and prior history of injury, which differ from person to person, can have significant impacts on your activities and ability to avoid injury. The purpose of this chapter is to investigate the types of joints that are in the body, explain how stability is established, and delve into the influence that flexibility has on the tasks you perform daily.
Getting These Old Bones to Move:
Types of Joints
Just as the body has different types of bones (refer to Chapter 8), the body also has different types of joints. You’re aware of these differences just by observing how your body moves or what its limitations are: Have you ever wished that you could spin your knee similarly to your neck or tried to extend your back only to realize you’ve gone too far? Have you ever won- dered why some joints move one way and others go another way? These differences are all based on anatomical design and joint architecture. Joint architecture dictates how bones move in relation to one another and the range of motion that’s created as a result.
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Not all joints allow for the same range of motion. In fact, some joints don’t even move at all, whereas others move freely. Joints can be characterized by how much they move, in what directions, and to what extent. Additionally, they can be classified by structure and function, and by the number of axes of rotation that they allow. The following sections have the details.
A joint is a joint, but in the study of joints, you need to know and be comfort- able with their technical name: arthrosis (singular) and arthroses (plural).
Why? Because the terms used to classify joints by the way in which they move are based on this technical term, not the common one: synarthroses, diarthro- ses, and so on. Remembering these terms and what they mean will just be easier if you latch onto arthroses now. To help you out, we use the technical and common terms interchangeably throughout this section.
Structural classifications: Fibrous, cartilaginous, and synovial
Structural classifications of the arthroses include three types: fibrous, cartilaginous, or synovial:
✓ Fibrous joints: In these joints, fibrous tissue or cartilage connects the bones. Fibrous joints are slightly movable. An example is the lower arm (the radio-ulnar joint).
✓ Cartilaginous joints: These joints contain cartilage, either of the hyaline or fibrocartilage variety. They move a bit more than fibrous joints, and examples include the pubis (a type of bone in the pelvis) and vertebrae (the small bones in your spine).
Hyaline cartilage, the most common kind of cartilage, generally covers the articular surfaces of synovial joints (see the next item in this list), where it reduces friction, protects against shock, and allows the joint range of motion. Fibrocartilage is nice and spongy, which makes it a good shock absorber. You’ll find it between the vertebrae of the spine, for example. For detailed information on cartilage, head to the later section “Enhancing Joint Stability and Longevity: Cartilage and Connective Tissues.”
✓ Synovial joints: These are the most common kind of joint. These joints have a cartilaginous covering (hyaline cartilage) where the articulating bones meet and a synovial cavity (also called a joint capsule), which is essentially a space between the bones of the joint, where a collection of soft tissues provide stability and synovial fluid keeps everything nice and lubricated. Synovial joints possess the greatest range of motion.
Examples include the elbow, wrist, hip, and shoulder joints.
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Functional classifications: Synarthroses, diarthroses, and more
Classifying the joints by function allows you to separate them by how they work.
Immovable joints: Synarthroses
Synarthroses are joints that don’t move. Examples of synarthroses include the following:
✓ Sutures: This kind of joint doesn’t allow for any movement. The joints between the bones of the skull are examples of sutures. The irregularly shaped bones of the skull are joined very tightly and don’t move.
✓ Syndesmosis joints: A syndesmosis is a joint that is connected by a significant amount of dense, fibrous tissue. The dense tissue surrounding this joint allows for only very limited motion. You find syndesmosis joints very tightly connecting the radius and ulna (called the radio-unlar joint in the lower arm) and the tibia and fibula (called the tibio-fibular joint, in the ankle).
Slightly movable joints: Amphiarthroses
Amphiarthroses are slightly movable joints connected by fibrocartilage or hyaline cartilage. Examples include the intervertebral disks, which lie between each vertebra and allow slight movement from one segment to the other. These types of joints help to absorb forces (like compression in the spine) and allow for more motion than the synarthroses.
A type of amphiarthrotic joint is the symphysis. The symphysis is a joint that is separated by a fibrocartilagenous disk or a very strong ligament that links two bones together. Examples of amphiarthrotic joints are the pubic symphysis of the pelvis, the spine, and the joint that connects the scapula to the clavicle in the shoulder. Each allows for only a little movement while providing support and stability for the bones they’re attached to.
Freely movable joints: Diarthroses
Diarthroses, which are joined together by ligaments, are the most common type of joint in the body. They’re also called synovial joints (refer to the ear- lier section “Structural classifications: Fibrous, cartilaginous, and synovial”) because a cavity between the two connecting bones is lined with a synovial membrane and filled with synovial fluid, which helps to lubricate and cushion the joint. The ends of the bones are cushioned by hyaline cartilage.
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Table 9-1 lists the several different types of diarthrotic joints and describes the kinds of movements each type allows. You can also see these joints in Figure 9-1.
Table 9-1 Types of Diarthrotic Joints
Type of
joint Description Movement Example
Ball-and-
socket The ball-shaped head of one bone fits into a depres- sion (socket) in another bone.
Circular. The joints can move in all planes, and rotation is possible.
Shoulder, hip
Ellipsoid
joint An oval-shaped protuberance (called a condyle) of one bone fits into oval-shaped cavity of another bone.
Can move in different planes but can’t rotate.
Knuckles (the joints between metacarpals and phalanges)
Gliding joint Flat or slightly curved surfaces join together.
Sliding or twisting in different planes.
Joints between carpal bones (wrist) and between tarsal bones (ankle) Hinge joint The convex
surface of one bone joins with concave surface of another.
Up and down motion, bending (flexion), and straightening (extension).
Elbow, knee
Pivot joint Cylinder-shaped projection on one bone is surrounded by a ring of another bone and ligament.
Rotation is the only movement possible.
Radio-ulnar joint at the elbow (supination/prona- tion), atlanto-axial joint of the neck under the head (head rotation) Saddle joint Each bone is
saddle shaped and fits into the saddle-shaped region of the opposite bone.
Many movements are possible; can move in different planes but can’t rotate.
Carpo-metacarpal joint of the thumb
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Figure 9-1:
Types of joints.
Illustration by Kathryn Born, MA
By degrees of freedom: Uniaxial, biaxial, and so on
Another way joints can be classified is by the number of axes of rotation that they allow, often referred to as degrees of freedom. Those joints that allow one, two, or three axes of rotation are referred to as uniaxial, biaxial, or tri- axial, respectively. A biaxial joint has two degrees of freedom: an ellipsoidal joint (knuckle), for example, can produce flexion and extension (bending) as well as abduction and adduction (splaying). (Refer to Chapter 7 for a com- plete discussion of the axes of rotation and the different types of movement.)
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Enhancing Joint Stability and Longevity:
Cartilage and Connective Tissues
Although the joints between bones fit together nicely in many cases, a nice fit isn’t enough to make the joint stable. What’s needed is some type of connec- tive tissue that can stabilize the joint and enable it to perform its given func- tion. The most common connective tissues are tendons, ligaments, and joint capsules. In addition, joints often possess a protective covering — articular cartilage — that reduces friction and helps the joint move smoothly.
Smoothing it out: Articular cartilage and fibrocartilage
Several of the joints described in the preceding sections possess a protective covering around them. Called articular cartilage (or hyaline cartilage), this covering reduces friction between the bones and allows for smooth motion.
The articular cartilage is a dense, white connective tissue that is typically very thin (see Figure 9-2). Its purpose is to distribute loads over a wider area, effectively reducing the contact between the two articulating bones, and to reduce the friction that exists during movement. By distributing loads over a larger area and reducing the friction present during motion, the articular cartilage protects the parts of the bones that are needed for movement.
Figure 9-2:
Articular cartilage.
Illustration by Kathryn Born, MA
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Another kind of cartilage that preserves and facilitates movement in the vari- ous joints is articular fibrocartilage. Articular fibrocartilage typically takes the form of a cartilaginous disk (referred to as menisci, or intervertebral disks) between the bones. These fibrocartilagenous structures help with the following:
✓ Distributing loads over a joint’s surface
✓ Improving the fit of articulating surfaces
✓ Limiting bone slip within a joint
✓ Protecting the edges where articulating bones touch
✓ Lubricating the articulating surfaces of bone
✓ Acting as shock absorbers
Think of articular cartilage as more of a sheet covering the surface of a bone.
Articular fibrocartilage, on the other hand, is typically a piece of cartilage located within a joint; it provides a bit more cushion during function.
The articular cartilaginous structures are vital to maintaining the health of the joint throughout a person’s lifespan. Yet the cartilage can be damaged when large amounts of force are delivered or when repetitive stresses are applied to the area over time.
Holding it all together: Articular connective tissue
Although the joints fit together nicely in many cases, the bony connection alone wouldn’t provide enough stability to enable you to perform the many activities that you commonly engage in. Fortunately, you don’t have to rely solely on how well your joints fit together because connective tissues — tendons, ligaments, and joint capsules — provide the extra support required for movement:
✓ Tendon: A tendon is a soft tissue structure that connects a muscle to bone. Often the tendon attaches the muscle to a movable aspect of the joint. The amount of movement created by the tendon depends on the size and length of the tendon, along with the type of joint it attaches to.
A tendon can be overstressed, a condition referred to as tendonitis, the swelling of the tendon.
✓ Ligament: Ligaments connect bones to other bones to keep them orga- nized and in their proper place. The wrist, for example, has a number of gliding bones. The ligaments in the wrist keep these bones from becom- ing disassociated and unorganized, which can result in injury and lack of use. An injured ligament is referred to as a sprain. Commonly people sprain the ligaments in their ankles and knees.
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✓ Joint capsules: In addition to the tendons and ligaments within or around a joint, a joint capsule may also exist. Joint capsules are made up of dense, soft tissues that provide stability and facilitate the function of other structures. For example, a joint capsule provides a connec- tion among ligaments, tendons, and articular fibrocartilage, while also assisting in creating a capsule in which synovial fluid can freely assist in decreasing friction during movement.
These soft tissue structures are considered non-contractile, meaning they’re static, providing support and assistance with movement but not exerting force themselves, which is the muscles’ job. When a muscle contracts, its force is delivered to the affected bone through the muscle tendon. Like tendons, liga- ments also absorb and/or deliver forces to various portions of the joint.
Tendons and ligaments are extensible; that is, they’re able to stretch when force is applied. When the force is relatively low, they can return to their normal length; however, when the force is large enough to stretch these structures beyond their elastic limit, they become damaged and cannot return to their normal length without surgery. When tendons and ligaments are stretched too often and/or too far, they become loose, and their function is compromised.
Getting Physical: Understanding the Functional Basis to Moving
For your body to move the way it’s supposed to, not only do the individual structures — muscles, tendons, cartilage, and bones — have to function as they’re supposed to, but they also must work in an interrelated way to pro- vide stability and normal function. This section investigates the mechanisms responsible for providing stability and explains how such a complex feat as coordinated movement is accomplished.
Perusing the factors that affect stability
Have you ever sprained your ankle, limped around on bad knees, or felt your shoulder pop out? These events all describe what can happen if your joints lack stability. When joints are stable, they allow the bones to articulate, or move, without a lot of displacement. In other words, the bones move the way they’re supposed to; they don’t “pop” out of place.
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Each joint possesses a unique requirement for stability. As we explain in the earlier section “Functional classifications: Synarthroses, diarthroses, and more,” your body contains immovable, slightly movable, and freely movable joints. Factors that play a role in stability involve the bony and soft tissue structures that support the joint: how the bone is shaped, the arrangement of the ligaments, and more.
The shape (and contact points) of things to come
One of the factors that most influences stability is the shape of the articulat- ing bones. Typically, bones that make up a joint are shaped as opposites to their counterparts. Where one bone ends in a socket, for example, its coun- terpart will end in a “ball” (refer to the earlier section “Freely movable joints:
Diarthroses” for a discussion of the different kinds of articulating joints). This arrangement is often referred to as the convex and concave orientation, and it allows for increased stability.
Joints also tend to be more stable at certain points in their range of motion:
✓ Closed-packed position: When the articulating surfaces of a joint are in a position where the most amount of each bone is in contact with the other, they are said to be in the closed-packed position. In this position, stability is increased.
✓ Loose-packed position: When the articulating bone surfaces are in less than maximum contact, the joint is in a loose-packed position (also referred to as an open-packed position).
The more surface area of contact, the more stable a joint will be. The amount of surface area contact between joints varies from person to person. Bony dif- ferences between people and past injury to the bone or soft tissue support structures are potential causes of decreased stability.
It’s articulation time: Do you know where your ligaments are?
Muscles, ligaments, and tendons all connect to the joint and provide for the delivery of or resistance to forces. The arrangement and integrity (condition or strength) of these structures play major roles in maintaining stability.
Ligaments, for example, attach to the bones and resist tension, thus help- ing to keep the bones together. When the bones are kept together within the joint, stability is enhanced. When muscles contract and exert forces on the bones via the tendon, the bones typically move closer to one another, maximizing stability.
184 Part III: Basic Biomechanics: Why You Move the Way You Do How tight or loose are you?
Each of your joints relies heavily on the muscles around the joint to provide movement through contraction. When the muscles have adequate strength and length, the function is good. Yet a muscle imbalance around a joint — when one muscle exerts more force than the other — can actually lead to a destabilizing situation. If, for example, your knee extensors (quads) are a lot stronger than your knee flexors (hamstrings), when you contract your quads, they’ll overpower your hamstrings and either injure the hamstrings or cause your joint to move beyond its normal range of motion.
The muscles around a joint should be strengthened together and in a func- tional way. Doing so ensures that they are all strengthened for that particular function and maintains the structural balance required for stability. So rather than just doing knee extension or flexion, for example, be sure to develop the muscles that are used for all the other motions that are involved with your activity.
Long or short? It matters
Another factor within the musculature that may impact stability is the length of the muscle. Revisiting the hamstring and quadriceps example introduced in the preceding section, if either of these muscles (or muscle groups) has limited flexibility, you won’t be able to achieve the appropriate position that may be required for the activity you’re attempting. If you have tight ham- strings, you may not be able to extend your knee far enough to achieve a normal heel strike while you walk, for example, a situation that has implica- tions with the rest of the activity and, ultimately, the stability of the joint.
Most ligaments and tendons attach to the joint in a way that maximizes stabil- ity, and both adapt to the forces that are applied. Over time, they may atrophy (shrink) or hypertrophy (get bigger), depending on what is required of them.
This situation increases the chances of injury or re-injury. If you have previ- ously sprained your ankle, for example, chances are that that loose ligament will make you prone to another injury. Maybe you’ll injure another structure, or maybe you’ll sprain your ankle again.
The role of other connective tissues
In addition, to the ligament, tendons, and muscles, other connective tissues exist in and around your joints that impact stabilization. For instance, the joint capsule (explained in the section “Holding it all together: Articular con- nective tissue”) and fascia (the connective tissue that surrounds and con- nects the muscles and other soft tissues) may play a role. These soft tissues themselves may help to stabilize the bones by either providing points of attachment for the tendon and/or ligaments or helping to facilitate sensory input from the joint and muscle activity.