Concepts of Biomechanics

Biomechanics and Kinesiology

• Biomechanics: The study of the mechanical laws governing the movement of living organisms.
• Kinesiology: The study of the mechanics of human movement.
• Both fields are crucial for exercise selection and training execution to achieve fitness goals and prevent injuries.

Anatomical Reference Terms

• Used to reference anatomical position or location.
• Refers to the body in anatomical position: facing forward with arms at sides, palms and toes pointing forward.

Anatomical Location Terms

• Anterior (Ventral): Front of the body or toward the front relative to a reference point.
• Posterior (Dorsal): Back of the body or toward the back relative to a reference point.
• Superior (Cephalic): Above a reference point.
• Inferior (Caudal): Below a reference point.
• Proximal: Closer to the center of the body relative to a reference point.
• Distal: Farther from the center of the body relative to a reference point.
• Medial: Closer to the midline of the body.
• Lateral: Farther from the midline of the body.
• Prone: Lying facedown.
• Supine: Lying on one's backside.
• Deep: Further beneath the surface relative to another reference point.
• Superficial: Closer to the surface relative to another reference point.
• Unilateral: Refers to only one side.
• Bilateral: Refers to both sides.
• Ipsilateral: On the same side.
• Contralateral: On the opposite side.
• Volar: Relating to the palm of the hand or sole of the foot.

Anatomical Movement

• Describes how muscles act on the skeleton to generate movement.

Terms for Anatomical Movement

• Abduction: Movement away from the midline.
• Adduction: Movement toward the midline.
• Flexion: Movement decreasing the angle between two body parts.
• Extension: Movement increasing the angle between two body parts.
• Lateral Flexion: Flexion in the frontal plane.
• Protraction: Abduction of the scapula.
• Retraction: Adduction of the scapula.
• Elevation: Movement in a superior direction.
• Depression: Movement in an inferior direction.
• Plantar Flexion: Extension of the foot downward (inferiorly).
• Dorsiflexion: Flexion of the foot upward (superiorly).
• External Rotation: Rotational movement away from the midline.
• Internal Rotation: Rotational movement toward the midline.
• Circumduction: Circular movement of a limb extending from the joint where the movement is controlled.
• Inversion: Movement of the sole of the foot toward the median plane.
• Eversion: Movement of the sole of the foot away from the median plane.
• Pronation: Turning the palm or arch of the foot down.
• Supination: Turning the palm or arch of the foot up.
• Hyperextension: Position that extends beyond anatomical neutral.
• Ipsilateral: Same-side movement.
• Contralateral: Opposite-side movement.
• Lateral: Situated away from the midline.
• Medial: Situated toward or closer to the midline.

Planes of Motion

• Used to describe the direction of movement.
• Frontal Plane: Divides the body into anterior and posterior halves; lateral movements like hip and shoulder abduction occur here.
• Sagittal Plane: Divides the body into left and right halves; flexion and extension occur here (e.g., squat).
• Transverse Plane: Divides the body into inferior and superior halves; anything requiring rotation occurs here (e.g., golf swing).
• Everyday activity involves movement in all three planes.
• Reduced range of motion (ROM) may occur in the frontal and transverse planes, as well as spinal extension.
• Joints are healthiest when able to move through a normal ROM without restriction.
• Range of Motion (ROM): The measurement of movement around a specific joint or body part.

Balance, Equilibrium, and Stability

• Constantly challenged in human movement.
• Maintaining optimal position reduces effort to hold position and produce/accept force.
• Balance: Ability to maintain the center of gravity within the base of support.
• Stability: Body's resistance to change in joint or body position.
• Gravity: Attraction between objects and the Earth.
• Muscular Force: Generated to move the skeleton, creating imbalance/instability.
• Dynamic Balance: Ability to remain upright over a moving base of support.
• Static Balance: Ability to remain upright and balanced when the body is at rest.
• Center of Gravity: Hypothetical position where combined mass appears concentrated.
• Base of Support: Area beneath an object or person that includes every point of contact with the supporting surface.
• Mass: Amount of matter in an object.
• Weight: Gravitational force of attraction on an object.
• Line of Gravity: A vertical line straight through the center of gravity.
• Balance and stability are maximized when the center of gravity can be determined and shifted.
• Stability is easier with a larger base of support.
• Training enhances awareness of the limit of stability (kinesthetic awareness).
• Altered joint motions due to tight muscles can cause faulty alignment.
• Kinesthesis: Awareness of the position of the joints.

Joint Mobility and Stability

• Healthy joints should move through the proper ROM with control.
• Collaboration between nervous system and muscles.
• Joint Mobility: Degree of movement around a joint before restriction by surrounding tissues.
• Joint Stability: Ability of muscles around a joint to control movement or hold it in a fixed position.
• Movement dysfunctions arise from overactive (tight) and underactive (weak) musculature.
  Joints typically needing greater mobility: foot/ankle, hip, thoracic spine, shoulder, and wrist
  Joints typically needing greater stability: knee, lumbar spine, cervical spine, and elbow

Laws Of Motion

• Laws of Motion: The laws of physics describing movement.
• Explain observed phenomena in fitness and human movement.
• Relate an object's motion (e.g., dumbbell) to forces acting on it (muscle actions) and gravity.

Newton's First Law: Inertia

• A body in motion stays in motion, and a body at rest stays at rest unless acted on by an outside force.
• Muscular contraction produces force and can change the status of movement.
• Inertia: Resistance to action or change; describes acceleration and deceleration of the human body.
• Acceleration: The rate of change of velocity.
• Deceleration: A special type of acceleration where a person or object is slowing down.

Newton's Second Law: Acceleration

• A change in acceleration occurs in the direction of the force causing it.
• Change of acceleration is directly proportional to the force and inversely proportional to the mass.
• $a = \frac{\text{change in } v}{\text{change in } t}$, where:
  • $a$ = acceleration
  • $v$ = velocity
  • $t$ = time
• Acceleration is measured in meters per second squared ($m/s^2$)
• Velocity: The speed of an object and the direction it takes while moving.
  • $F = m \times a$, where:
    • $F$ = force
    • $m$ = mass
    • $a$ = acceleration
• Force-Velocity Curve: Representation of the inverse relationship between force and velocity in muscle contraction.
  • $F \times \text{velocity} = P$, where:
    • $F$ = force
    • $P$ = power
• Momentum: The quantity of motion of a moving body, measured as a product of its mass and velocity.

Newton's Third Law: Action and Reaction

• For every action, there is an opposite and equal reaction.
• The body provides the force to move, and the surface provides a reactionary force.
• Speed: The ability to move the body in one direction as fast as possible.
• Ground Reaction Force (GRF): The force the ground exerts on a body in contact with it.

Force

• Interaction that creates work or physical change.
• Components: magnitude, direction, point of application, and line of action.
• Types:
  • Compression: Two surfaces pressing toward one another.
  • Tensile: Two surfaces pulling apart from one another.
  • Shear: Two surfaces moving across one another.
  • Compression Force: The force of two surfaces pressing toward one another.
  • Tensile Force: The force when two surfaces pull apart from one another.
  • Shear Force: The force of two surfaces moving across one another

Muscular Contraction and Motion

• Muscular Contraction: The shortening or resistance to lengthening of a muscle fiber.
• Muscles can only pull to create movement.
• Types of motion:
  • Linear: Straight or curved line.
  • Angular: Rotation around an axis.
  • Linear Motion: Movement along a line, straight or curved.
  • Angular Motion: Rotation around an axis.
  • Axis: Point of rotation around which a lever moves.
• Joint movement is almost always angular motion.
• Displacement: The distance an object is displaced from a starting point.
• Distance: The total or sum of the length an object travels.
• Angular Displacement: The change of location of an object that is rotating about an axis.
• Linear Displacement: The distance an object moves in a straight line.

Friction

• The force created by the resistance between two surfaces of two objects moving across one another.
• It is the force that allows the body to walk forward
• Static Friction: Friction of an object that does not move.
• Sliding Friction: Friction between two surfaces where one or both are moving against one another.
• Rolling Friction: The force that resists a surface rolling across another

Principles of Biomechanics

• Explained by laws of motion and kinetics.
• Kinetics: The study of forces acting on a mechanism.

Categories of Biomechanics:

• Stability
• Maximum effort (maximum amount of force or velocity)
• Linear motion
• Angular motion

Principles of Biomechanics Overview

• Stability: The ability to maintain control (i.e., resist change) of a joint or position.
• Production of Maximum Force: The maximum amount of force produced by a muscle or group of muscles.
• Production of Maximum Velocity: The maximum movement velocity, or muscular contraction speed, for a muscle or group of muscles.
• Force-Velocity Relationship: The greater the applied force on the same object, the greater the velocity.
• Direction of Movement: Movement occurs in the direction opposite the applied force.
• Ground Reaction Forces (GRFs): The force exerted by the ground to a body in contact with it.
• Angular Motion: The motion of an object around a fixed point or axis. All lever actions are angular, and therefore most joint movements are angular.
• Conservation of Angular Momentum: Angular momentum is constant until an external force acts on it.

Angle of Muscle Pull

• Strength varies throughout ROM based on the angle the muscle is pulling.
• Mechanical Advantage: The ratio of force that creates meaningful movement compared to the force applied to generate the movement.

Work

• Moving the body and exercising are examples of measurable work
• $W = F \times D$, where:
  • $W$ = work
  • $F$ = force
  • $D$ = distance or displacement.
• Work: Force times distance measured in foot-pounds.

Power

• The amount of work done in a unit of time.
• $W = 300 \text{ pounds } \times 3 \text{ feet} = 900 \text{ feet/pound}$
• $900 \text{ feet/pound} \div 3 \text{ seconds} = 300 \text{ feet/pound of work per second}$
• $900 \text{ feet/pound} \div 2 \text{ seconds} = 450 \text{ feet/pound of work per second}$
• Power: The combination of strength and speed-the ability for a muscle to generate maximal tension as quickly as possible.

Levers

• The most common mechanical machines within the human body.
• A lever consists of a rigid bar and an axis or point of rotation the lever moves around.
• Levers: A rigid or semirigid bar rotating around a fixed point when force is applied to one end.
• Fulcrum: The point on which a lever rests or is supported and on which it pivots.

Lever Components

• Bones: levers
• Joints: axis (fulcrum)
• Muscles: contract to apply force
• Effort Arm: The portion of the lever arm between the applied effort and the axis.
• Lever Arm: The rigid bar portion of a lever that rotates around the fulcrum.
• Resistance Arm: The portion of the lever arm between the load and the axis.
• Moment Arm: The perpendicular distance between the fulcrum and the line of the force being applied.

Three Classes of Levers

• Differentiated by the location of the fulcrum, resistance, and effort.
• First-Class Lever: Fulcrum is between the effort and load (e.g., neck extension).
• Second-Class Lever: Load is between the fulcrum and effort (e.g., plantar flexion).
• Third-Class Lever: Effort is between the fulcrum and load (e.g., elbow flexion).

Torque

• Force applied rotationally.
• Torque is force applied that results in rotation about an axis.
• Torque: Force applied that results in rotation about an axis.
• Rotary Motion: The movement around a fixed axis moving in a curved path.
• Force Arm: The distance between the fulcrum and the force or load application in a lever.
• Determined by multiplying force by the length of the force arm.

Muscles as Movers

• Create movement by generating force and transferring it to bones via tendons.
• Origin: The proximal muscular attachment point to a bone.
• Insertion: The distal muscular attachment point to a bone.
• Agonist: The primary muscle used for a mechanical movement.
• Synergists: Muscle(s) supporting the mechanical movement of a prime mover.
• Antagonist: Muscle(s) opposing the mechanical movement of a prime mover.
• Sherrington's Law of Reciprocal Inhibition: A law that states that for every muscle activation, there is a corresponding inhibition of the opposing muscle.
• Stabilizer Muscles: The muscles playing the role of stabilizing or minimizing joint movement.

Muscle Relationships

• An example of a group of muscles working together in this fashion is a dumbbell curl. The biceps brachii is the agonist, the brachioradialis is a synergist and the triceps are the antagonist.
• A single muscle may act as agonist, synergist, or antagonist depending on movement.
• Length-Tension Relationship: The amount of tension a muscle can produce with respect to its length.
• Force-Couple Relationship: Two or more muscles acting in different directions that influence the rotation of a joint in a specific direction.
• Muscle Synergies: The activation of a group of muscles to generate movement around a particular joint.
• Innervation: The distribution or supply of nerves.