How can a joint extend or flex?

How can a joint extend or flex?

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The idea of a joint flexing or extending doesn't make sense to me, I can see how a leg or an arm might extend or flex but how does a joint extend or flex ?

"The hip joint is most stable when it is extended and the ligaments are taut."

Does that mean in the state that is most right in this picture ?

Short Answer

The words extension and flexion actually mean to increase or decrease the angle of a joint, respectively. In other words, the terms are directly related to the joints themselves. Bones can only move relative to each other at joints, so when the angle between two bones increases or decreases, that action is occurring at the joint. In other words, the joint acts as a vertex of the angle of movement.

Long answer

In general, one should think of extension as "increasing the angle of a joint" and flexion as "decreasing the angle of a joint" (see here, here, here, here, here, here or here for reference). From Saladin's 2015 Anatomy textbook$^1$:

Flexion = a joint movement that , in most cases, decreases the angle between two bones.

Put differently,

Flexion refers to a movement that decreases the angle between two body parts.

I think where you are confused is that you do not understand what a joint is. A joint is anywhere where two bones articulate (or touch). Although various types of joints exist, those that readily move well enough to receive movement terms such as flexion/extension are called synovial joints.

At any given synovial joint, two bones are able to move relative to each other due to a cavity that provides space for movement -- the synovial cavity. Put differently, bones (which are solid, unbending objects) move relative to each other during our body movements, and this relative movement occurs at joints.

Using your example of arms and legs:

When your arm flexes/extends, what's happening is that the head of your humerus is moving inside the glenoid cavity of the scapula. This movement at the "glenohumeral joint" results in the entire arm (humerus and all distal components) either swinging forward or backward due to the angle of the glenohumeral joint increasing or decreasing (though, actually, other movements are possible as well). The same applies to the hip.

Another example is the "elbow" joint -- which actually consists of 2 joints: the humeroradial and humeroulnar joints. Consider when you "flex your arm" as in showing off your bicep. Yes, your lower arm (forearm) moves, but the movement all occurs at the point of the elbow at these two joints. In other words, the elbow joints form the vertex of the angle between your upper and lower arm bones. When you "flex" your arm, the angle of these joints decreases. When you return your elbow to being straight (i.e., "extend" your elbow), your are increasing the angle of the humeroradial and humeroulnar joints.

$^1$ Saladin, K. S. 2015. Anatomy & Physiology: The Unity of Form and Function. Seventh ed., McGraw-Hill, New York, NY. 1248pp.

Joint Function Overview

Grant Hughes, MD, is a board-certified rheumatologist. He is an associate professor at the University of Washington School of Medicine and the head of rheumatology at Seattle&rsquos Harborview Medical Center.

Joint function is an important aspect of a musculoskeletal physical examination. Joint function can be impaired by chronic or acute injuries and by diseases, such as arthritis. What is joint function?

Three Knee Bones

The femur is also known as the thigh bone, and it runs the length of the thigh ending at the knee. The distal portion of the femur interlocks with the kneecap and tibia creating the knee joint. The femur is one of the strongest and longest bones in the entire human body, but it is the only bone found within the upper leg. The upper portion of the femur is near the torso and it contains the neck, head, and trochanters of the femur. Meanwhile, the lower portion of the femur is referred to as the distal extremity, and it is linked to the tibia via the tibial collateral ligament of the knee joint.

The tibia is also referred to as the shank bone or shinbone. It is one of two bones within the lower leg, and it is the larger and stronger of the two bones (the other bone is the fibula). The tibia links the knee ankle bones with the knee. The upper end of the tibia is linked to the knee via a number of ligaments, a bundle of fibrous connective tissue that joins bones together.

The patella Is also referred to as the kneecap, and it is a big piece of bone that fits over the other portions of the knee, protecting the anterior articular portion of the knee joint. The patella is triangular-shaped, and the tendon from the quadriceps femoris muscle is attached to the bottom of the patella. Other muscles, such as the vastus media lists and a vastus lateralis are bonded to the lateral and medial portions of the patella. The vastus intermedius muscle is attached to the patella’s base. The top 75% of the patella is articulated by the femur. The entire articular surface of the patella is covered by cartilage, which helps to protect the patellofemoral joint from the stress of movement.

Do Your Hips Extend? Looking at hip extension and flexion

In a previous blog, I talked about the Psoas, a mysteriously named muscle with many functions including flexing one's hips. Exactly what does that mean?

Hip flexion without bending the knee.

The word flexion actually means to decrease the angle between two bones at joint. Flexing your biceps involves flexing your elbow joint, bringing the hand closer to the shoulder. Thus hip flexion would be bringing the leg closer towards you in the sagittal plane (think plane dividing body into front half back half).

Now the catch with hip flexion is that most of us sit in chairs and end up in a position of passive hip flexion and knee flexion (bent knees) and retain that position for many hours a day. We know now that our bodies process the movement or lack thereof and adapt to the shape that we most frequently inhabit, for better or worse. If you primarily flex the hips and knees and never fully extend them, you may have chronically short or weak hamstrings, limited range of active hip flexion and limited range of active hip extension, for starters!

Extension (as a definition) increases the angle between the bones in a joint. When you extend your knee, you are straightening your knee from the bent position, increasing the angle between the femur and the shin bones. When you are extending your hip, your leg is essentially moving backwards in space, say 10-20 degrees. When you walk, run, or lunge, you have one hip passing through extension. Now why the fuss about these two words?

Standing apanasana can be great to focus on hip flexion (the bent knee) or hip extension (standing leg). Try resting the bent knee on a table and stepping the standing leg back for more extension.

Well, most of us work the hip flexors (including the psoas and iliacus) most of the time- sitting, practicing while seated, cycling, driving. but only in a limited range, i.e. knees and hips bent to 90 degrees. We need to balance out the movements of the hips a bit more- add more extension and more varieties of flexion. For example, sitting cross legged, sitting on the floor, squatting, kneeling, etc. all require more varieties of hip movement. To get more hip extension in your life, you can add some restorative exercises like standing apanasana, lunges (lots of lunges!) and go walk (not on a treadmill). That way, you don't lose your capacity to move those joints to their full capacity, and you will have loaded the tissues in more diverse ways.

Even in great alignment, this double flexion of knees and hips starts to affect our soft tissues and our psoas, since that becomes our most frequent position! Original photo from Back Trouble, by Deborah Caplan PT, which can be purchased here.

Flexors and Extensors: What Make Them Skeletal Muscles

It's important to first understand what defines a skeletal muscle. Skeletal muscles are found on the bone, interact with bones for movement and are voluntarily controlled. When performing a workout, we activate the body's skeletal muscle groups to create movement and burn calories. Flexors and extensors are at the core of this. Together, they bend and straighten the body's joints to create motion and activate other muscle groups, generating muscle activity -- which is another way to say working out.

Shoulder Joints


The glenohumeral joint is the coming together of the upper arm bone, the humerus, and a portion of the shoulder blade called the glenoid. The glenoid is a shallow cup that connects to the humerus. The shoulder has a great deal of motion including bending and straightening, moving away from the side of the body, moving toward the body, and circumduction (a spinning type of motion). Common problems with this joint include stiffness, dislocation, labral tears, bursitis, rotator cuff tears, long head of biceps tendonitis or tears, subacromial impingement, proximal humeral fractures, and arthritis.

Acromioclavicular (AC)

The AC joint is a smaller joint associated with the shoulder. The acromium is part of the scapula (shoulder blade) and the clavicle (also called the collarbone). The AC joint is where the scapula and clavicle come together. There are three major ligaments, the acro-mioclavicular, coracoacromial, coracoclavicular. This joint is involved with raising and lowering the arm and moving the arm forward and backward. An AC separation is a common injury of this joint which occurs from a fall or a direct blow to the shoulder. Many shoulder separations are treated without surgery, but some may require surgery to reconstruct the coracoacromial or coracoclavicular ligaments. Osteoarthritis is also common and can be treated sometimes with surgery.

Sternoclavicular joint (SC)

The sternoclavicular joint is the junction of the sternum (breastbone) and clavicle (collarbone). There is an articular disc of fibrocartilage within the joint. The motion of this joint permits the clavicle to move up and down and front to back. There are no tendons that attach to this joint area. A posterior (back) SC joint dislocation can be a serious injury and puts vital structures at risk such as the heart, aorta, superior vena cava, esophagus, and trachea. Anterior (front) dislocations can also occur and are often somewhat less serious, but can cause pain and clicking.

How can a joint extend or flex? - Biology

Flexibility is defined by Gummerson as "the absolute range of movement in a joint or series of joints that is attainable in a momentary effort with the help of a partner or a piece of equipment." This definition tells us that flexibility is not something general but is specific to a particular joint or set of joints. In other words, it is a myth that some people are innately flexible throughout their entire body. Being flexible in one particular area or joint does not necessarily imply being flexible in another. Being "loose" in the upper body does not mean you will have a "loose" lower body. Furthermore, according to SynerStretch , flexibility in a joint is also "specific to the action performed at the joint (the ability to do front splits doesn't imply the ability to do side splits even though both actions occur at the hip)."

Many people are unaware of the fact that there are different types of flexibility. These different types of flexibility are grouped according to the various types of activities involved in athletic training. The ones which involve motion are called and the ones which do not are called . The different types of flexibility (according to Kurz ) are:

Dynamic flexibility (also called ) is the ability to perform dynamic (or kinetic) movements of the muscles to bring a limb through its full range of motion in the joints.

Static-active flexibility (also called ) is the ability to assume and maintain extended positions using only the tension of the agonists and synergists while the antagonists are being stretched (see section Cooperating Muscle Groups). For example, lifting the leg and keeping it high without any external support (other than from your own leg muscles).

Static-passive flexibility (also called ) is the ability to assume extended positions and then maintain them using only your weight, the support of your limbs, or some other apparatus (such as a chair or a barre). Note that the ability to maintain the position does not come solely from your muscles, as it does with static-active flexibility. Being able to perform the splits is an example of static-passive flexibility.

Research has shown that active flexibility is more closely related to the level of sports achievement than is passive flexibility. Active flexibility is harder to develop than passive flexibility (which is what most people think of as "flexibility") not only does active flexibility require passive flexibility in order to assume an initial extended position, it also requires muscle strength to be able to hold and maintain that position.

According to Gummerson , flexibility (he uses the term ) is affected by the following factors:

  • the type of joint (some joints simply aren't meant to be flexible)
  • the internal resistance within a joint
  • bony structures which limit movement
  • the elasticity of muscle tissue (muscle tissue that is scarred due to a previous injury is not very elastic)
  • the elasticity of tendons and ligaments (ligaments do not stretch much and tendons should not stretch at all)
  • the elasticity of skin (skin actually has some degree of elasticity, but not much)
  • the ability of a muscle to relax and contract to achieve the greatest range of movement
  • the temperature of the joint and associated tissues (joints and muscles offer better flexibility at body temperatures that are 1 to 2 degrees higher than normal)
  • the temperature of the place where one is training (a warmer temperature is more conducive to increased flexibility)
  • the time of day (most people are more flexible in the afternoon than in the morning, peaking from about 2:30pm-4pm)
  • the stage in the recovery process of a joint (or muscle) after injury (injured joints and muscles will usually offer a lesser degree of flexibility than healthy ones)
  • age (pre-adolescents are generally more flexible than adults)
  • gender (females are generally more flexible than males)
  • one's ability to perform a particular exercise (practice makes perfect)
  • one's commitment to achieving flexibility
  • the restrictions of any clothing or equipment

Some sources also the suggest that water is an important dietary element with regard to flexibility. Increased water intake is believed to contribute to increased mobility, as well as increased total body relaxation.

Rather than discuss each of these factors in significant detail as Gummerson does, I will attempt to focus on some of the more common factors which limit one's flexibility. According to SynerStretch , the most common factors are: bone structure, muscle mass, excess fatty tissue, and connective tissue (and, of course, physical injury or disability).

Depending on the type of joint involved and its present condition (is it healthy?), the bone structure of a particular joint places very noticeable limits on flexibility. This is a common way in which age can be a factor limiting flexibility since older joints tend not to be as healthy as younger ones.

Muscle mass can be a factor when the muscle is so heavily developed that it interferes with the ability to take the adjacent joints through their complete range of motion (for example, large hamstrings limit the ability to fully bend the knees). Excess fatty tissue imposes a similar restriction.

The majority of "flexibility" work should involve performing exercises designed to reduce the internal resistance offered by soft connective tissues (see section Connective Tissue). Most stretching exercises attempt to accomplish this goal and can be performed by almost anyone, regardless of age or gender.

The resistance to lengthening that is offered by a muscle is dependent upon its connective tissues: When the muscle elongates, the surrounding connective tissues become more taut (see section Connective Tissue). Also, inactivity of certain muscles or joints can cause chemical changes in connective tissue which restrict flexibility. According to M. Alter , each type of tissue plays a certain role in joint stiffness: "The joint capsule (i.e., the saclike structure that encloses the ends of bones) and ligaments are the most important factors, accounting for 47 percent of the stiffness, followed by the muscle's fascia (41 percent), the tendons (10 percent), and skin (2 percent)".

M. Alter goes on to say that efforts to increase flexibility should be directed at the muscle's fascia however. This is because it has the most elastic tissue, and because ligaments and tendons (since they have less elastic tissue) are not intended to stretched very much at all. Overstretching them may weaken the joint's integrity and cause destabilization (which increases the risk of injury).

When connective tissue is overused, the tissue becomes fatigued and may tear, which also limits flexibility. When connective tissue is unused or under used, it provides significant resistance and limits flexibility. The elastin begins to fray and loses some of its elasticity, and the collagen increases in stiffness and in density. Aging has some of the same effects on connective tissue that lack of use has.

With appropriate training, flexibility can, and should, be developed at all ages. This does not imply, however, that flexibility can be developed at the same rate by everyone. In general, the older you are, the longer it will take to develop the desired level of flexibility. Hopefully, you'll be more patient if you're older.

According to M. Alter , the main reason we become less flexible as we get older is a result of certain changes that take place in our connective tissues. As we age, our bodies gradually dehydrate to some extent. It is believed that "stretching stimulates the production or retention of lubricants between the connective tissue fibers, thus preventing the formation of adhesions". Hence, exercise can delay some of the loss of flexibility that occurs due to the aging process.

M. Alter further states that some of the physical changes attributed to aging are the following:

  • An increased amount of calcium deposits, adhesions, and cross-links in the body
  • An increase in the level of fragmentation and dehydration
  • Changes in the chemical structure of the tissues.
  • Loss of due to the replacement of muscle fibers with fatty, collagenous fibers.

This does not mean that you should give up trying to achieve flexibility if you are old or inflexible. It just means that you need to work harder, and more carefully, for a longer period of time when attempting to increase flexibility. Increases in the ability of muscle tissues and connective tissues to elongate (stretch) can be achieved at any age.

After you have used weights (or other means) to overload and fatigue your muscles, your muscles retain a "pump" and are shortened somewhat. This "shortening" is due mostly to the repetition of intense muscle activity that often only takes the muscle through part of its full range of motion. This "pump" makes the muscle appear bigger. The "pumped" muscle is also full of lactic acid and other by-products from exhaustive exercise. If the muscle is not stretched afterward, it will retain this decreased range of motion (it sort of "forgets" how to make itself as long as it could) and the buildup of lactic acid will cause post-exercise soreness. Static stretching of the "pumped" muscle helps it to become "looser", and to "remember" its full range of movement. It also helps to remove lactic acid and other waste-products from the muscle. While it is true that stretching the "pumped" muscle will make it appear visibly smaller, it does not decrease the muscle's size or inhibit muscle growth. It merely reduces the "tightness" (contraction) of the muscles so that they do not "bulge" as much.

Also, strenuous workouts will often cause damage to the muscle's connective tissue. The tissue heals in 1 to 2 days but it is believed that the tissues heal at a shorter length (decreasing muscular development as well as flexibility). To prevent the tissues from healing at a shorter length, physiologists recommend static stretching after strength workouts.

You should be "tempering" (or balancing) your flexibility training with strength training (and vice versa). Do not perform stretching exercises for a given muscle group without also performing strength exercises for that same group of muscles. Judy Alter, in her book Stretch and Strengthen , recommends stretching muscles after performing strength exercises, and performing strength exercises for every muscle you stretch. In other words: "Strengthen what you stretch, and stretch after you strengthen!"

The reason for this is that flexibility training on a regular basis causes connective tissues to stretch which in turn causes them to loosen (become less taut) and elongate. When the connective tissue of a muscle is weak, it is more likely to become damaged due to overstretching, or sudden, powerful muscular contractions. The likelihood of such injury can be prevented by strengthening the muscles bound by the connective tissue. Kurz suggests dynamic strength training consisting of light dynamic exercises with weights (lots of reps, not too much weight), and isometric tension exercises. If you also lift weights, dynamic strength training for a muscle should occur before subjecting that muscle to an intense weightlifting workout. This helps to pre-exhaust the muscle first, making it easier (and faster) to achieve the desired overload in an intense strength workout. Attempting to perform dynamic strength training after an intense weightlifting workout would be largely ineffective.

If you are working on increasing (or maintaining) flexibility then it is very important that your strength exercises force your muscles to take the joints through their full range of motion. According to Kurz , Repeating movements that do not employ a full range of motion in the joints (like cycling, certain weightlifting techniques, and pushups) can cause of shortening of the muscles surrounding the joints. This is because the nervous control of length and tension in the muscles are set at what is repeated most strongly and/or most frequently.

It is possible for the muscles of a joint to become too flexible. According to SynerStretch , there is a tradeoff between flexibility and stability. As you get "looser" or more limber in a particular joint, less support is given to the joint by its surrounding muscles. Excessive flexibility can be just as bad as not enough because both increase your risk of injury.

Once a muscle has reached its absolute maximum length, attempting to stretch the muscle further only serves to stretch the ligaments and put undue stress upon the tendons (two things that you do not want to stretch). Ligaments will tear when stretched more than 6% of their normal length. Tendons are not even supposed to be able to lengthen. Even when stretched ligaments and tendons do not tear, loose joints and/or a decrease in the joint's stability can occur (thus vastly increasing your risk of injury).

Once you have achieved the desired level of flexibility for a muscle or set of muscles and have maintained that level for a solid week, you should discontinue any isometric or PNF stretching of that muscle until some of its flexibility is lost (see section Isometric Stretching, and see section PNF Stretching).

Trigger Thumb

The bones of the thumb consist of one metacarpal bone and two phalanxes (proximal and distal, respectively). This anatomy varies in comparison to the other fingers which have three phalanxes (proximal, middle, and distal). Other bony constituents of the thumb are sesamoid bones which can be found in the other fingers. The unique function of the thumb is attributed to two movements: opposition and apposition. Additionally, at the metacarpophalangeal (MCP) joint, the thumb can flex, extend, abduct, and adduct.

Trigger thumb is a simple term for stenosing flexor tenosynovitis of the thumb. This is a narrowing of the flexor tendon sheath which causes a clicking or popping sensation on attempted extension of the thumb. Flexion is normally enabled by the extrinsic flexor pollicis longus (FPL) and intrinsic flexor pollicis brevis (FPB). The FPL tendon runs in its tendon sheath through three pulleys (A1, oblique, and A2) located proximal to distal. The A1 pulley is located distally on the metacarpal bone overlapping the MCP joint and the base of the proximal phalanx. Trigger thumb is most commonly due to thickening of the A1 pulley which causes pain and decreased function.


The PIP joint exhibits great lateral stability. Its transverse diameter is greater than its antero-posterior diameter and its thick collateral ligaments are tight in all positions during flexion, contrary to those in the metacarpophalangeal joint. [1]

Dorsal structures Edit

The capsule, extensor tendon, and skin are very thin and lax dorsally, allowing for both phalanx bones to flex more than 100° until the base of the middle phalanx makes contact with the condylar notch of the proximal phalanx. [1]

At the level of the PIP joint the extensor mechanism splits into three bands. The central slip attaches to the dorsal tubercle of the middle phalanx near the PIP joint. The pair of lateral bands, to which contribute the extensor tendons, continue past the PIP joint dorsally to the joint axis. These three bands are united by a transverse retinacular ligament, which runs from the palmar border of the lateral band to the flexor sheath at the level of the joint and which prevents dorsal displacement of that lateral band. On the palmar side of the joint axis of motion, lies the oblique retinacular ligament [of Landsmeer] which stretches from the flexor sheath over the proximal phalanx to the terminal extensor tendon. In extension, the oblique ligament prevents passive DIP flexion and PIP hyperextension as it tightens and pulls the terminal extensor tendon proximally. [2]

Palmar structures Edit

In contrast, on the palmar side, a thick ligament prevents hyperextension. The distal part of the palmar ligament, called the palmar plate, is 2 to 3 millimetres (0.079 to 0.118 in) thick and has a fibrocartilaginous structure. The presence of chondroitin and keratan sulfate in the dorsal and palmar plates is important in resisting compression forces against the condyles of the proximal phalanx. Together these structures protect the tendons passing in front and behind the joint. These tendons can sustain traction forces thanks to their collagen fibers. [1]

Palmar ligament Edit

The palmar ligament is thinner and more flexible in its central-proximal part. On both sides it is reinforced by the so-called check rein ligaments. The accessory collateral ligaments (ACL) originate at the proximal phalanx and are inserted distally at the base of the middle phalanx below the collateral ligaments. [1]

The accessory ligament and the proximal margin of the palmar plate are flexible and fold back upon themselves during flexion. The flexor tendon sheaths are firmly attached to the proximal and middle phalanges by annular pulleys A2 and A4, while the A3 pulley and the proximal fibres of the C1 ligament attach the sheaths to the mobile volar ligament at the PIP joint. During flexion this arrangement produces a space at the neck of the proximal phalanx which is filled by the folding palmar plate. [2]

The palmar plate is supported by a ligament on either side of the joint called the collateral ligaments, which prevent deviation of the joint from side to side. The ligaments can partially or fully tear and can avulse with a small fracture fragment when the finger is forced backwards into hyperextension. This is called a "palmar plate, or volar plate injury". [3]

The palmar plate forms a semi-rigid floor and the collateral ligaments the walls in a mobile box which moves together with the distal part of the joint and provides stability to the joint during its entire range of motion. Because the palmar plate adheres to the flexor digitorum superficialis near the distal attachment of the muscle, it also increases the moment of flexor action. In the PIP joint, extension is more limited because of the two so called check-rein ligaments, which attach the palmar plate to the proximal phalanx. [2]

The only movements permitted in the interphalangeal joints are flexion and extension.

  • Flexion is more extensive, about 100°, in the PIP joints and slightly more restricted, about 80°, in the DIP joints.
  • Extension is limited by the volar and collateral ligaments.

The muscles generating these movements are:

Location Flexion Extension
fingers the flexor digitorum profundus acting on the proximal and distal joints, and the flexor digitorum superficialis acting on the proximal joints mainly by the lumbricals and interossei, the long extensors having little or no action upon these joints
thumb the flexor pollicis longus the extensor pollicis longus

The relative length of the digit varies during motion of the IP joints. The length of the palmar aspect decreases during flexion while the dorsal aspect increases by about 24 mm. The useful range of motion of the PIP joint is 30–70°, increasing from the index finger to the little finger. During maximum flexion the base of the middle phalanx is firmly pressed into the retrocondylar recess of the proximal phalanx, which provides maximum stability to the joint. The stability of the PIP joint is dependent of the tendons passing around it. [2]

Rheumatoid arthritis generally spares the distal interphalangeal joints. [4] Therefore, arthritis of the distal interphalangeal joints strongly suggests the presence of osteoarthritis or psoriatic arthritis. [5]

How can a joint extend or flex? - Biology

Nothing can replace the hands-on experience of isolating muscles. By tracing them from their point of origin to their insertion a better understanding of their function can be gained. This brief tutorial is intended to reinforce what can more easily be learned working with an actual cat. Think of muscles in terms of antagonistic (opposite) actions. When one muscle of an antagonistic pair contracts its antagonist will relax. Also keep in mind that several muscles may have similar actions and that the exact movement of a bone will be the result of a coordinated effort involving many muscles. In these simplified diagrams, arrows indicate the direction in which the force is exerted . Follow the links provided under the different types of joints for illustrations of how the muscles interact with the skeletal elements.

Flex - a motion that decreases the angle between two bones. An example of a flexor is the biceps brachii .

Extend - a motion that increases the angle between two bones. An example of an extensor is the triceps brachii.

At the shoulder joint muscles that extend the forelimb move the limb anteriorly while at the hip joint muscles that extend the hindlimb move it posteriorly.

Flexion of the shoulder results in a movement of the limb posteriorly and flexion of the hip results in movement of the limb anteriorly.

Retract - a motion that moves a bone parallel to the longitudinal axis and in a posterior direction.

Adduct -Moving a skeletal element toward the ventral midline.