Biomechanics - Kinematics & Kinetics

BIOMECHANICS

  • Kinematics & Kinetics

    • Kinematics

    • Definition: Description of motion.

    • Components considered in kinematics:

      • Time

      • Displacement

      • Velocity

      • Acceleration

      • Space factors related to a system's motion.

    • Kinetics

    • Definition: Study of forces associated with the motion of a body.

TYPES OF MACHINES FOUND IN THE BODY

  • The musculoskeletal system can be viewed as a series of simple machines.

  • Machines

    • Purpose: Used to increase mechanical advantage.

    • Analysis: Consider the mechanical aspect of each component in relation to the machine-like function of those components.

FUNCTIONS OF MACHINES IN THE BODY

  • Four Functions

    1. Balance multiple forces.

    2. Enhance force to reduce total force required to overcome resistance.

    3. Enhance range of motion and speed of movement, allowing resistance to be moved further or faster than the applied force.

    4. Alter the resulting direction of the applied force.

ARRANGEMENT OF MACHINES IN THE BODY

  • The musculoskeletal system's arrangement includes three types of machines that produce movement:

    • Levers (most common type)

    • Wheel-axles

    • Pulleys

  • Types of machines not present in the body include:

    • Inclined plane

    • Screw

    • Wedge

LEVERS IN THE BODY

  • Humans operate through a system of levers.

  • Characteristics of levers:

    • Levers themselves cannot be changed without surgery; however, their utilization can be more efficient.

    • Lever

    • Definition: A rigid bar that turns about an axis of rotation or fulcrum.

    • Axis

    • Definition: The point of rotation about which the lever moves.

MECHANICS OF LEVERS

  • Levers rotate about an axis due to force (effort, denoted as E) applied to cause movement against a resistance or weight.

  • In the human body:

    • Bones represent the bars.

    • Joints serve as the axes.

    • Muscles contract to apply force.

RESISTANCE VARIABILITY IN LEVERS

  • Resistance can range from maximal to minimal, often representing:

    • Only the weight of the bones or body segment.

  • All lever systems possess three components arranged in one of three possible configurations.

COMPONENTS OF LEVER SYSTEMS

  • Three Points Determine the type of lever and its suitability for different kinds of motion:

    • Axis (A): Fulcrum, the pivot point.

    • Point (F): Point of force application (typically muscle insertion or attachment site to bone via ligament).

    • Point (R): Point of resistance application (the lever's center of gravity or the location of external resistance).

TYPES OF LEVERS

  • 1st Class Lever

    • Configuration: Axis (A) is located between the force (F) and resistance (R).

  • 2nd Class Lever

    • Configuration: Resistance (R) is located between the axis (A) and force (F).

  • 3rd Class Lever

    • Configuration: Force (F) is located between the axis (A) and resistance (R).

MECHANICAL ADVANTAGE OF LEVERS

  • Mechanical advantage can be calculated using the following equations:

    • MA=Resistance ForceEffort ForceMA = \frac{Resistance\,Force}{Effort\,Force}

    • MA=Length of Force ArmLength of Resistance ArmMA = \frac{Length\,of\,Force\,Arm}{Length\,of\,Resistance\,Arm}

FIRST-CLASS LEVERS

  • Functions:

    • Produce balanced movements when the axis is midway between force and resistance (e.g., seesaw).

    • Produce speed and range of motion when the axis is closer to force (e.g., triceps during elbow extension).

    • Produce force motion when the axis is nearer to resistance (e.g., crowbar).

  • Examples of First-class levers:

    • Head balanced on the neck during flexion/extension.

    • Involved muscle contractions:

    • Agonist and antagonist muscle groups contract simultaneously on either side of a joint axis.

    • Agonist produces force while antagonist provides resistance.

  • Elbow Extension Example:

    • Triceps applies force to olecranon (F) while extending the non-supported forearm (R) at the elbow (A).

SECOND-CLASS LEVERS

  • Produce force movements, as larger resistance can be moved using less force.

  • Examples:

    • Wheelbarrow

    • Nutcracker

    • Loosening a lug nut

    • Raising the body on toes

  • Mechanics of Second-class levers:

    • Example: Plantar flexion of the foot where the ball of the foot (A) acts as the axis with ankle plantar flexors applying force to the calcaneus (F), lifting resistance of the body at the tibial articulation (R).

THIRD-CLASS LEVERS

  • Produce movements emphasizing speed and range of motion; most common in the human body.

  • They require significant force to move even small resistances.

  • Examples:

    • Paddling a boat

    • Shoveling, where lifting force is applied to a shovel with the lower hand while the upper hand acts as the axis of rotation.

  • Biceps Brachii Example in Elbow Flexion:

    • The biceps brachii uses the elbow joint (A) as the axis, applies force at its insertion on radial tuberosity (F) to rotate the forearm up, and the center of gravity (R) serves as resistance application.

TORQUE AND LENGTH OF LEVER ARMS

  • Torque

    • Definition: Moment of force; the turning effect produced by an eccentric force.

    • Eccentric force

    • Definition: A force applied in a direction that does not align with the center of rotation in an object with a fixed axis.

    • Importance in human anatomy: Muscle contractions create an eccentric force that causes bone rotation around a joint axis.

  • Amount of torque is determined by:

    • Torque=Force Magnitude×Force ArmTorque = Force\,Magnitude \times Force\,Arm

  • Force Arm Explained:

    • Definition: The perpendicular distance between the force application point and the axis.

    • Also referred to as: Moment arm or torque arm.

    • The length of the force arm influences the amount of torque produced; greater distance yields more torque.

RESISTANCE ARMS IN LEVERS

  • Resistance Arm

    • Definition: Distance from the axis of rotation to the point where resistance is applied.

  • To increase ease of movement against resistance, we can purposely increase the length of the force arm.

DEMONSTRATING MECHANICAL ADVANTAGE

  • First-class levers:

    • If the force arm and resistance arm are equal, a force equal to the resistance is needed to balance it.

    • Increasing force arm length decreases the required force to move a larger resistance.

    • Conversely, shortening the force arm increases required force, even for smaller resistances.

  • Second-class levers:

    • Placing resistance halfway between the axis and force application yields a MA of 2.

    • Moving the resistance closer to the axis increases MA but reduces distance resistance moves.

    • Closer positioning of resistance to force application decreases MA but increases movement distance.

  • Third-class levers:

    • Moving point of force application closer to the axis requires more force to overcome resistance, as resistance arm is always longer.

    • Positioning the force application point closer to the axis enhances range of motion and speed of movements.

    • Conversely, positioning closer to the resistance decreases force needed, enhancing efficiency.

EXAMPLE OF TORQUE IN LEVERS (BICEPS CURL FORMULA)

  • Equation: F×FA=R×RAF \times FA = R \times RA

  • Example breakdown:

    • F×0.1 meters=45 newtons×0.25 metersF \times 0.1\,meters = 45\,newtons \times 0.25\,meters

    • Calculating force F=112.5 newtonsF = 112.5\,newtons

    • Adjusting resistance arm to 0.05 meters, leads to:

    • F×0.1 meters=45 newtons×0.2 metersF \times 0.1\,meters = 45\,newtons \times 0.2\,meters

    • F=90 newtonsF = 90\,newtons.

APPLICATION AND IMPLICATIONS OF TORQUE

  • Human leverage is optimized for speed and range of motion, which comes at the cost of force.

  • Short force arms combined with longer resistance arms necessitate considerable muscular strength to achieve movement;

    • Example: Biceps and triceps attachment characteristics:

    • Biceps: Force arm of 1 to 2 inches.

    • Triceps: Force arm less than 1 inch.

LEVERAGE IN SPORTS

  • The human leverage system must utilize multiple levers for sports skills.

  • For instance, throwing a ball utilizes levers at shoulder, elbow, and wrist.

  • Longer levers enhance velocity; a tennis player can hit a ball harder using a straight-arm drive versus a bent elbow due to increased lever length and speed.

LONG LEVERS IN SPORTS PERFORMANCE

  • Long levers produce more linear force, beneficial for sports:

    • E.g., baseball, hockey, golf, field hockey, etc.

SHORT LEVER ARMS FOR QUICKNESS

  • Quickness benefits from shorter lever arms, demonstrated by:

    • A baseball catcher securing a quick throw by bringing their hand back to their ear.

    • A sprinter shortening knee lever through flexion to catch their spikes in the gluteal muscles.

WHEELS AND AXLES IN THE BODY

  • Functionality: Primarily to enhance range of motion and speed of movement.

  • Operate similarly to levers.

  • When the wheel or axle turns, both complete one turn simultaneously.

MECHANICS OF WHEELS AND AXLES

  • The center of the wheel and the axle acts as the fulcrum.

  • The radius of both the wheel and axle corresponds to the respective force arms.

MECHANICAL ADVANTAGE IN WHEELS

  • If the wheel radius is larger than the axle radius, the wheel benefits from a mechanical advantage due to a longer force arm.

    • Example: If the wheel's radius is 5 times that of the axle, it provides a mechanical advantage of 5 to 1.

CALCULATING MECHANICAL ADVANTAGE OF WHEEL & AXLE

  • Formula:

    • MA=Radius of WheelRadius of AxleMA = \frac{Radius\,of\,Wheel}{Radius\,of\,Axle}

REVERSE FORCE APPLICATION WITH WHEELS

  • Applying force to the axle generates movement in the wheel; this results in:

    • The wheel's outer edge turning at a speed 5 times that of the axle (if the wheel's radius is five times that of the axle).

RANGE OF MOTION WITH WHEELS IN THE BODY

  • Example: Upper extremity internal rotators attach to the humerus, which acts as the axle.

    • As the humerus rotates minimally, the hand and wrist (on the wheel's circumference) travel a significantly greater distance, increasing the speed of object thrown.

PULLEYS IN THE BODY

  • Single Pulleys: Function change the effective direction of force application with a mechanical advantage of 1.

  • Compound Pulleys: Combining multiple pulleys increases mechanical advantage; each added rope raises mechanical advantage by 1.

PULLEY EXAMPLE IN ANATOMY

  • Example: Lateral malleolus operates as a pulley for the tendon of peroneus longus.

    • When peroneus longus contracts, it pulls towards its belly (towards the knee), transmitting force to the plantar aspect of the foot, resulting in eversion and plantarflexion.

FORCE DEFINITION AND IMPORTANCE

  • Force: The weight (mass) of a body segment or the entire body multiplied by the acceleration.

  • Significance:

    • Critical in football and other activities involving parts of the body.

    • In throwing a ball, force applied equals the arm's weight times its acceleration.

    • Leverage factors are essential in maximizing force application.