UNIT II- Introduction to Joints and Muscles  
  Objectives:

At the completion of this unit the student will be able to:

1)  name the appropriate terms for joint action

2)  identify common anatomical movements using appropriate terminology

3)  define common terms used in reference to bone and joint structure

4)  identify types of joints

5)  categorize a joint by its classification

6)  define common terms used in reference to muscles and muscle action

7)  discuss the importance of the kinetic chain to the dancer

 

Anatomical Terms of Joint Action

 Terms are used to describe the different movements of the limbs or other body parts. Movements take place at joints where two or more bones meet or articulate with one another. It is important to be able to describe motion using these terms.  

Flexion   (Decreasing the angle between body parts)- notice fingers curling at right are "flexing" as well as arm and leg moving forward.

Extension (Increasing the angle between body parts) - notice the reverse is true in the lower left figure; the bent arm straightens or "extends".

 

 

Internal Rotation  (turning on an axis medially along the long axis of a bone  versus  External Rotation  (turning outwardly or laterally along the long axis of a bone) see circles to left that show this rotary action. 

Abduction  (moving away from the mid-line) versus Adduction (moving toward the middle) The figure to the left is abducting the arm and leg AWAY from the mid-line of the body and adducting or coming toward the mid-line of the body.  see arrows.  

 

Eversion  (moving the sole of the foot laterally or AWAY from the big toe and medial side of the foot.   

Inversion  (moving the sole of the foot TOWARD the medial border or big toe side of the foot)

Supination      (rotating the forearm & hand laterally)  extend you arm to the front with your palm facing toward the ceiling ...the thumb is on the lateral side of the hand....this is supination 

versus  

Pronation 

(rotating the forearm & hand medially)  extend your arm to the front with your palm facing toward the floor...The thumb is on the medial side of the hand now ....this is pronation

Retraction -drawing the shoulders posteriorly  (or back toward the spine)  versus  

Protraction- drawing the shoulders anteriorly (or forward toward the chest)  

 

 

 

Elevation - raising or moving a part superiorly (lifting the shoulders)

versus

Depression - lowering or moving a part inferiorly (dropping the shoulders)

Circumduction - a circular movement that combines flexion/extension with abduction/adduction (circling the leg in the hip socket)

Terms Used in Reference to Bone and Joint Structure

 Articulation - A place of union or junction between 2 or more bones or parts of bones of the skeleton (ex. ribs articulate with their corresponding vertebrae in the spine)

 Synovial membrane - The synovial membrane consists of vascular (blood) supply connective tissue that produces synovial fluid

 Synovial fluid - The lubricating substance produced by the synovial membrane in a synovial joint. The fluid lies in the joint cavity.(ex. is the knee is a synovial joint with synovial fluid)

 Connective tissue - The layers of soft tissue beneath the skin:  dermis, subcutaneous, and fascia.  These serve to support organs, nerves, and muscles in interconnected layers.

  Cartilage - A resilient, semi-rigid form of connective tissue; It forms parts of the skeleton where motion occurs. You have more cartilage when you are young, but it lessens with age.(ex. the tip of your nose is formed by resilient cartilage)

  Tendon - A fibrous cord or band that connects a muscle to a bone or other structure.

  Ligament - A band or sheet of fibrous tissue connecting 2 or more bones, cartilages, or other structures. Serves as a support or stability for a joint (static stabilizers)

  Bursa - A closed sac with a synovial-like membrane and containing fluid; A bursa  is usually found in areas subject to friction, like over an exposed part or where a tendon passes over a bone. (ex. - trochanteric bursa at the hip.)  

 

Anatomical Terms Describing Joints

 Joint Classifications - Joints are classified in 3 ways according to the type of material by which the bones are united:  

 1. Synovial Joints - This joint has an articular capsule that encloses a joint cavity. Each bone surface is covered with articular cartilage and there is synovial fluid contained inside the joint capsule.  Examples of this are the knee and shoulder joints.

 

2. Fibrous Joints - These joints are united by fibrous tissue. There is usually less motion at these joints. An example is the sutures of the skull or the interrosseus membrane in the forearm.

3. Cartilagenous Joints - These joints are united by hyaline or fibrocartilage. These joints are usually present early in life to allow for bone growth. If the epiphyseal plate of these cartilaginous joints is damaged, growth in children or teens can be stunted.  Some fibrocartilage joints persist in the adult. An example would be the fibrocartilage intervertebral disc in the spine that joins the vertebrae together or the symphysis pubis where the pubic bone joins at the base of the pelvis in the front.

 

 

Relevance to Dance- The synovial joints are crucial to allow movement in the major joints of the body. This movement allowed by the synovial joint design allows the dancer a full range of movement possibilities.

Injuries to the cartilaginous joints can occur in young dancers and result in stunted growth. Dance instructors should be mindful of possible growth plate injuries in young dancers.

 

Photo: Pilobolus Dance Theatre
 Spine injuries can occur in dancers, especially male dancers who lift female dancers. These injuries can damage the disc and result in significant loss of spine function and shorten a dancer's career.

Types of Synovial Joints  

a. Ball & Socket  Joints  -  Multiaxial joints that move in multiple planes and axes

 * Allow a lot of motion

 *One surface is spherical and moves within the socket of the other bone

 *Motion includes flexion/extension, abduction/adduction, medial/lateral rotation and circumduction

            Ex. - the hip joint  

 

b. Hinge Joints -  Uniaxial movement in one plane around one axis.

 * Permits flexion / extension movement only

 * Strong ligaments protect these joints

           Ex. - The elbow joint  

 

c. Pivot Joints - Uniaxial joints allowing rotation

           Ex. - rotation of the radius bone in the arm  

 

d.1 Plane Joints - Permits gliding or sliding movements

 * Usually occurs in small joints

 * Uniaxial - allows movement in only one plane around one axis

          Ex.- acromioclavicular joint - between the scapula (shoulder blade) and the clavicle (collar bone)   

 

d.2 Condyloid Joints - Biaxial movement in 2 planes

 * the axes lie at right angles to each other

           Ex. - knuckle joints

 

  

 

e. Saddle Joints  - Biaxial movement in 2 planes

 * Surfaces that oppose are shaped like a saddle

            Ex. - Carpometacarpal joint at the base of the thumb

 
Relevance to the Dancer - Dancers are most concerned about the ball & socket joints and hinge joints. These joints are responsible for most of the broad body movements performed in dance.

Terms Used to Describe Bony Landmarks  

 Bony landmarks serve as sites for muscle and connective tissue attachments.  Many of these sites are visible on the surface of the body and serve as useful points of reference in dance training to describe body alignment in relation to movement. 

 

 

 

These are some anatomical terms that serve to describe the shapes and contours of a bone.

Depressions or "hollows"

  • Fossa - a cavity or depression in a bone

  • Groove - a long narrow indentation

  • Foramen - a hole

Protrusions or "bumps"

  • Condyle - a large segment of bone usually at the end; a site for articulation of a joint.

  • Crest - a ridge or border of bone

  • Head - a spherical shape usually at the end of a bone

  • Trochanter - a large protrusion of bone

  • Tubercle - a small protrusion of bone

  • Tuberosity - a medium protrusion of bone

  • Process - an "extension" of bone

Terms Used in Reference to Muscles and Muscle Action

 Origin - the site of the attachment of a muscle to a bone that is a fixed point.

 Insertion - the site of the attachment of a muscle to a bone that is a mobile point.

 Isometric contraction - a static contraction where there is no movement between the origin and insertion.  There is force produced in the muscle, but no joint motion.

 Isotonic contraction - a dynamic contraction of the muscle where the relationship between the origin and insertion changes as the joint moves.  

 Concentric contraction - a "shortening" contraction where the insertion is moving toward the origin.  Example:  at the elbow, the biceps insertion moves towards the origin as the muscle contracts to bend the elbow.

 Eccentric contraction -  a "lengthening" contraction where the origin and insertion are moving away from each other.  This type of contraction is usually a "braking" type of movement against gravity.  Example:  At the elbow, the biceps eccentrically contracts when the insertion moves away from the origin as the arm is straightening.  This allows for control of motion into elbow extension, so the arm does not just drop.

Three types of contractions are possible in opposing gravity:

Shortening contraction - as in the figure on left side, the abdominal muscles are shortening their muscle length to pull the torso forward against gravity.

Static contraction - in figure on far right, the muscle is neither lengthening or shortening as it works to hold a position statically. If the positions were "held", they would be considered static.  

Lengthening contraction - as in the middle figure, the back extensors are allowing the descent of the body forward while resisting the pull of gravity.  These muscles are lengthening while they exert control over the forward bend.

Prime movers (Agonists) - are the main muscles that activate a specific movement of the body.  They contract actively to produce the desired movement.

Antagonistic - muscles that oppose the action of prime movers; as a prime mover contracts, the antagonist progressively relaxes, producing a smooth movement.

Synergist - a muscle or group of muscles which cooperate to produce the same action.  They may complement the action of the prime mover. 

Fixator (Postural) - these muscles stabilize the most proximal parts of the body while movement is occurring more distally or laterally.

The same muscle may act as a prime mover, antagonist, synergist, or fixator under different conditions.

The Psoas major is the Agonist or Prime Mover for lifting the leg.

                              The gluteus maximus is the antagonist for the lifted leg action by extending the leg backward as seen in figure on right.

 

Principles of muscle use:

  • Muscles only pull, they do not push

  • Muscles contract in the center and pull equally on both ends

  • The action of a muscle on a joint will be determined by the attachments and path of the muscle.

  • Muscles must oppose gravity to produce a movement or maintain a position

 

Phrasing:

  • Movement happens in phrases.  The preparation and initiation of a muscle action will determine the entire course of action for the phrase.  

Kinetic chains:  

(illustration from Making Connections)
When a series of muscles are activated to product a movement phrase, it is referred to as a movement or "kinetic chain" of events.

Because dancers are constantly working their bodies in a wide variety of movements from extreme stretching to powerful muscle contractions, it is extremely important that a proper alignment between the bones is maintained.

This concludes Unit 2 

You need to return to Blackboard and take Quiz #1 which covers Units I and II in order to proceed to the next section of the course -

or you may want to review the material in this section by returning to Unit I using Blackboard.