The Biomechanics of Human Skeletal Articulations

Explain the functions of articular, fibro cartilage and articular connective tissue.

Define joint stability and explain factors influencing its.

Define joint flexibility and Explain the advantages and disadvantages of different techniques for increasing or maintaining joint flexibility.

Describe the biomechanical contributions to common joint injuries and pathologies.

Classification of Joints based on Functions

Synchondroses – Sternocostal joint, epiphyseal plates

Symphyses – Vertebral joints and the pubic symphysis

Immovable

(Synarthroses)

Absorb shock but permit little or no movement

Slightly Moveable

(Amphiarthroses)

Freely Movable(Diarthroses or Synovial)

The articulating bone surfaces are covered with articular cartilage, an articular capsule and synovial membrane.

Synovial membrane lining the interior articular capsule secretes a lubricant known as synovial fluid.

Sutures – SkullsSyndesmoses - Tibiofibular joint

SUTURES

The only example in human body, the Skull.Irregular grooved articulating bone sheets mate closely and are tightly connected by fibers.

SYNDESMOSES

Dense fibrous tissue binds the bone together

AMPHIARTHROSES - SYMPHYSIS

Thin plates of hyaline cartilage separate a disc of fibrocartilage from the bones.

SYNCHONDROSES

                        

      

Articulating bones are held together by a thin layer of hyaline cartilage

       

                       

        

                      

DIARTHROSES OR SYNOVIAL

Articular capsule consist of double layered membrane Synovial membrane (stratum) and Fibrous membrane.

Synovial fluid -a clear, slightly yellow liquid that provides lubrication inside the articular capsule.

Associated bursae - small capsules filled with synovial fluid that cushion the structures they separate

DIARTHROSES – CATEGORIZED BASED ON NUMBER OF AXES OF ROTATION PRESENT

                                                             

                

                                                             

                                      

(a)Athrodial;(b)Sellar;(c)Ginglymus;(d)Trochoid;(e)Spherodial (f) Condyloid

Articular or Hyaline Cartilage

Articular cartilage is a white connective tissue

It covers the ends of bones.

FUNCTIONS OF ARTICULAR CARTILAGEWhat are the functions of articular cartilage?

• It provides a protective lubricants that minimizes frictions and mechanical wear.• It spreads loads over a wide area, thereby reducing contact stress.• It has no blood supply and nerves.• Anisotropic, it deforms instantaneously to a low or moderate load. If rapidly loaded, it will become stiffer and deforms over a longer period.

ARTICULAR CARTILAGE

• It is 1 to 5 mm thick, depending on the stress and the incongruity of the joint surfaces.

• The force distributions depend on the cartilage thickness. Ankle and elbow, thin; hip and knee, thick.

• It has a very low coefficient of frictions, range from 0.01 to 0.04, ice at 0º is about 0.1. Almost frictionless, allows gliding.

Functions of Articular Fibrocartilage

What?It is a fibrocartilaginous disc or partial discs known as menisci that intervene between articulating bones.

Lateral meniscus

Posterior cruciate ligament

Transverse ligament

Anterior cruciate ligament

Medial meniscus

Superior view

ARTICULAR FIBROCARTILAGE

Examples are the intervertebral disc and the knee joint.

It is where both tensile strength and the ability to withstand high pressure are necessary.

What are the possible functions of articular fibrocartilage?

• Distributing loads over joint surfaces • Protecting the joint periphery• Lubricating the joint• Absorbing shock at the joint• Improving the fit of articulations• Limiting slip between articulating bones

What are articular connective tissues?

• Tendons - connect muscles to bones

• Ligaments - connect bones to other bones

Do not have ability to contract like muscle tissue but slightly extensible, elastic. Like bone it responds to mechanical stress by hypertrophying and atrophying. Therefore, regular exercises, increase size and strength of both tendon and ligaments.

What is joint stability?

Ability of a joint to resist abnormal displacement (dislocation) of the articulating bones.

What factors effect joint stability?

Orientation or joint position• Stability is maximal when joints are in the close-packed position (maximum contact surface between the articulating bone).• Loosed-packed position - any joint orientation other than the closed packed position.

What factors effect joint stability?

Shape of the articulating surface– Acetabulum has relatively

deep socket for the head of femur and large amount of contact area.

– The small glenoid fossa has lesser contact area and diameter than humeral head, therefore contribute to relatively instable joint.

A strong array of ligaments and muscle tendons crossing the jointStrong ligaments and muscle contributes significantly

to joint stability by holding the articulating bone ends together (E.g. Strengthening the quadriceps and hamstring groups enhance the stability of the knee).

The angle of attachment of most tendons to bones contributes to joint stability. The articulating ends of the bones at the joint are pulled closer together.

Absence of muscle fatigueSince muscle strength effect joint stability, when muscle

fatigues, joint stability also affected.Other Connective tissue

Fascia and the bundles of muscle fiber within provide protections and support.

What factors effect joint stability?

What is Joint Flexibility?

• A description of the relative ranges of motion (ROM) allowed at a joint in different directions

• ROM: the angle through which a joint moves from anatomical position to the extreme limit of segment motion in a particular direction.

• Static flexibility – An indicator of the relative tightness or laxity of a joint. It refers to the ROM present when a body segment is passively moved (by an exercise partner or clinician).

• Dynamic flexibility – It refers to the ROM achieve by virtue of muscle contraction.

Joint Flexibility

Range of motion is measured directionally from anatomical position (zero).

What factors influence joint flexibility?

• Shape of the articulating bone surface and an intervening muscle tissue or fat terminate movement at extreme ROM.

• Tightness in the muscle and collagenous tissue crossing a joint. When these tissues are not stretched, their extensibility usually diminishes (Collagenous tissue - extensibility increase with temperature elevation).

Joint Flexibility and Injury

If the joint flexibility is extremely low or extremely high, the risk of injury is also increased. Too tight, possibility of tearing and rupturing are high. Too loose, it is prone to displacement-related injuries.

People with less physical activity will have less flexibility.

Stretching increase flexibility and avoid injury.

AGONIST AND ANTAGONISTAgonistThe muscle that responsible for moving the body part

to contracts or shortens.AntagonistThe muscle that responsible for moving the body part

back to its original position.

HOW TO INCREASE JOINT FLEXIBILITY?

Stretching the ligaments and muscles that limit the ROM at joint will increase or maintain flexibility.

Certain type of stretching being more effective than the other due to different neuromuscular responses.

The proprioreceptors which transform mechanical distortion in the muscle or joint into nerve impulses that enter the spinal cord and stimulate the motor response.

NEURALMUSCULAR RESPONSE THAT INFLUENCING THE JOINT FLEXIBILITY

Golgi Tendon Organs (GTO)What?It is a sensory receptors that influence extensibility.Where?It is located in the muscle-tendon junction and in the tendons at both ends of muscles.How?It is stimulated by tension either produce by muscle contraction or by passive muscle stretch. It inhibit tension in agonist muscle (promote relaxation) & initiate tension development in antagonists.

GTO MECHANISMContractions of quadriceps muscle increases the tension in the

patellar tendon which stimulates the Golgi tendon organ.  The impulse travel to the spinal cord and some of them head

upward to the higher centers of the nervous system for further interpretation and integration. 

The remaining messages excite either, interneurons that inhibit quadriceps function (extension of the knee), or interneurons that activate hamstring function (flexion of the knee).

NEURALMUSCULAR RESPONSE THAT INFLUENCING THE JOINT FLEXIBILITY

Muscle SpindlesWhat?It is a sensory receptors that are respond to the

amount of muscle (static response) and rate of muscle lengthening (dynamic response).

Where?It is interspered throughout and parallel to the

fibers of muscles.How?Muscle spindles will provoke reflex contraction in

stretched muscle & inhibit tension in antagonists..

KNEE JERK TEST

A tap on the patellar tendon initiates the stretch reflex, resulting in the jerk caused by the immediate tension development in the quadriceps group.

KNEE JERK TEST MECHANISM

Tapping the tendon stretches the quadriceps femoris muscle group.

This activates stretch receptors within the muscle called muscle spindles. Stretching a spindle fiber initiates a volley of impulses in the sensory neuron (called an "I-a" neuron) attached to it. Impulse from muscle spindle travel to spinal cord. Some impulse are carried back to the same muscle causing it to contract. The leg straightens.

KNEE JERK TEST MECHANISM

Some of the branches of the I-a axons synapse with inhibitory interneurons in the spinal cord. These, in turn, synapse with motor neurons leading back to the antagonistic muscle that inhibits tension development in antagonist muscle.

Golgi Tendon Organs and MuscleSpindles: How do they Compare?

OverallEffect

Golgi Tendon Organs

Within tendons near the muscle-tendon junction in series with muscle fibers

Increase in muscle tension

1) inhibit tension development in stretched muscle, 2) initiate tension development in antagonist muscle

Promote stretch in muscle being stretched

Muscle Spindles

Interspersed among muscle fibers in parallel with the fibers

Increase in muscle length

1) initiate rapid contraction of stretched muscle, 2) inhibit tension development in antagonist muscles

Inhibit stretch in muscle being stretched

Location

Stimulus

Response

BETWEEN GTO’S AND MUSCLE SPINDLE

GTO promotes relaxation in muscle developing tension, whereby Muscle Spindles inhibit stretch in muscle being stretch. Therefore, maximizing GTO and minimizing the muscle spindle effect is the general goals in increasing joint flexibility

Techniques for Increasing Joint Flexibility

What are active and passive stretching?

• Active stretching is produced by contraction of the antagonist muscles (Quadriceps contract so that Harmstring is stretched). It provides the advantage of exercising the muscle group used to develop force.

• Passive stretching is produced by a force other than tension (Gravitational force, force applied by another body segment or by another person) in the antagonist muscles. Movement can be carried out father beyond the ROM (compare to active stretching)

Techniques for Increasing Joint Flexibility

What are ballistic and static stretching?

• Ballistic stretching - a series of quick, bouncing-type stretches. The potential for injuries is heightened.

• Static stretching - maintaining a slow, controlled, sustained stretch over time-usually about 30 seconds. This type of stretching is preferred because ballistic activate the muscle spindle respond that inhibits stretching. Why static stretching did not activate muscle spindle?

Techniques for Increasing Joint Flexibility

What is PNF?

Proprioceptive neuromuscular facilitation is a group of stretching procedures involving alternating contraction and relaxation of the muscles being stretched

COMMONT JOINT INJURIES AND PATHOLOGIES

Sprains – abnormal displacement or twisting of the articulating bones results in stretching or tearing of ligaments, tendons, and connective tissues crossing joints.

DISLOCATION

Dislocation - Displacement of the articulating bones at a joint. Result from fall or involving large magnitude of force. Symptoms include pain, swelling and loss of joint movement capability.

BURSITIS

Bursitis – Overuse injury caused by excessive use of a joint that produces frictional irritation and inflammation of one or more bursae.

Pain and some swelling are symptoms of bursitis.

ARTHRITIS

Arthritis - an inflammation of a bone joint accompanied by pain and swelling.

Rheumatoid Arthritis- Type of arthritis where body’s autoimmune system attack healthy tissue. the synovium becomes inflamed. This inflammation causes chemicals to be released that thicken the synovium and damage the cartilage and bone of the affected joint. This leads to inflammation of the synovium causing pain and swelling

Osteoarthritis

What is osteoarthritis?• a common, degenerative disease of

articular cartilage• symptoms include pain, swelling, ROM

restriction, and stiffness• cause is unknown• both too little and too much mechanical

stress seem to promote development

Cartilage that cushions the bones of the hip starts to erode, eventually allowing the bones to grind or

rub together and causing hip pain and stiffness.The exact cause of osteoarthritis is

unknown.