Biomechanics

Lecture (1)
Properties of the muscle
1. Irritability  the ability of the muscle to respond to stimulus.
2. Contractility  ability of the muscle to produce tension between its ends.
3. Relaxation  the opposite of contractility, ability of the muscle to relief tension.
4. Distensability  ability of the muscle to stretch up to an extent within physiological range.
5. Elasticity  ability of the muscle to recoil back to its original shape after releasing tension from it.
Functions of the skeletal muscles
 Produce movement.
 Gives body segments their shape.
 Stabilizing the joints.
 Maintain posture.
 Forms supporting wall.
1. Classification of muscles according to shape and fascicular architecture
 Longitudinal muscles:
o Fibers are running longitudinally.
o Fibers run along muscle length.
o Force of contraction same direction as muscle length.
o Better suited for greater range of motion but less force.
o Example  Biceps Brachii
 Pennate muscles:
o Fibers arranged in feather like manner.
o Fibers are not arranged in the full length of the muscle.
o Arranged with an angle to the central tendon.
o Better suited for greater force than range of motion.
o Example  Deltoid.
Physiological cross section: perpendicular line to the muscle fibers used to determine potential force of the
muscle.
2. Classification of muscles according to myoglobin content
 White fibers  fast twitch unit.
 Red fibers  slow twitch unit.
3. Classification of muscles according to contractile activity
 Tonic (stabilizers)
o Small.
o Deep.
o Rich in red slow twitch units.
o Contract isometrically.
o Maintain posture.
o Example  Erector spinae muscles.
 Phasic (mobilizers)
o Large.
o Superficial.
o Rich in white fast twitch units.
o Contract concentrically.
o Example  finger flexors.
4. Classification according to muscle attachment
 Spurt muscles:
o Distal end is close to the joint axis and proximal end is far from joint axis.
o Has large rotatory component.
o Example  Biceps brachii
  Shunt muscles:
o Proximal end is close to joint axis and distal end is far away from joint axis.
o Has large translator component.
o Example  Brachioradialis.
5. Classification according to line of pull (function)
 Flexors.
 Extensors.
 Adductors.
 Abductors.
6. Classification according to number of joints crossed
 One joint muscles  vastus medialis.
 Two joint muscle  rectus femoris.
 Three joint muscle  finger flexors.
Muscle contractions
 Isometric contraction:
o No change in agonist length.
o No change in antagonist length.
o No change in joint angle.
o Static motion.
o No work.
o Resistance = force.
o (=)
 Concentric contraction:
o Shortening in agonist length.
o Lengthening of antagonist length.
o Joint angle in direction of force.
o Cause of dynamic motion.
o Positive work.
o Force > resistance.
o (+).
 Eccentric contraction:
o Lengthening in agonist muscle.
o Shortening in antagonist muscle.
o Joint angle in direction of external force.
o Control of dynamic motion.
o Negative work.
o Resistance>force.
o (-).
Role of muscles
1. Agonists: muscles that do the desired action
o Iliopsoas in hip flexion are prime movers.
o Sartorius in hip flexion is secondary mover.
2. Antagonists: muscle group that opposes the action of agonist group of muscles.
o Elbow extensors opposing elbow flexors.
3. Synergists: group of muscles that modify the target action by making agonist stronger or eliminate
undesired action.
o Conjoint muscle group:
- Pair of muscles that perform an action that one muscle cannot do it by its own.
 Dorsiflexion.
o Neutralizers:
- Muscles that cancels the role of other muscle doing the same action as prime or secondary
movers.
 Lateral rotators neutralize the action of hip adductors in hip adduction.
 o Stabilizers:
- Muscles that stop the action of the proximal fixed end of the muscle doing the action.
 Rectus abdominis stabilize proximal end of psoas major preventing pelvic tilting during
raising.
4. Fixators: muscle group that holds another place or segment during motion of other segment.
 Trunk muscle in elbow flexion.
The range of muscle contractility and extensibility
 Absolute amount the muscle can do it.
 Length and arrangement of muscles.
 Structure of the joint.
 Number of joints transverse.
 Resistance of antagonist muscles.
 Presence of opposing load.
Characteristics of two joint muscle
 Economic as it do more actions than one joint muscle.
 Maintain favorable length when shortened on one joint and lengthens on one other joint.
 Capable to be shortened to extent produce full range of motion on the two joints.
Active insufficiency: muscle cannot be more shortened without losing tension.
Passive insufficiency: muscle cannot be more stretched without pain.
Concurrent pattern: movement of flexion or extension in two joints.
Countercurrent pattern: two joint muscle become rapidly shortened allowing antagonist muscle to
lengthen correspondingly and so tension develops at both ends.
Passive movement: done by external force.
Active movement:
 Slow or rapid tension:
o Muscle contraction generates the force.
 Ballistic:
o Muscles initiates the movement and momentum completes.