biomechanics brief study guide

in class

biomechanics definition

the study of the effects and control of forces acting on and produced by living bodies

what movements happen in a saggital plane

• Definition: The sagittal plane (or anteroposterior plane) divides the body into left and right halves. - Movements: extension, flexion, dorsiflexion, plantar flexion

what movements happen in a transverse plane

• Definition: The transverse plane (or horizontal plane) divides the body into upper and lower halves.

• Movements: medial and lateral rotation, pronation, supination, horizontal adduction - cross flexion, horizontal abduction - cross extension

what movements happen in a frontal plane

• Definition: The frontal plane (or coronal plane) divides the body into anterior (front) and posterior (back) halves.

• Movements: adduction, abduction, lateral bending, scapular elevation and depression, ulnar deviation, radial deviation, inversion, eversion

axis of rotation

• Definition: An axis of rotation is an imaginary line perpendicular to a plane, around which rotation occurs at a joint. Each plane of movement has a corresponding axis.

• Types of Axes:

• Mediolateral - sagittal plane rotations

• anteroposterior - frontal plane rotations

• longitudinal - transverse plane rotations

types of fibres

slow twitch and fast twitch

examples of a slow twitch fibre and exercise

include marathon running and cycling. Slow twitch fibers are more resistant to fatigue and are adapted for endurance activities.

examples of fast twitch fibre and excersize

muscle fibers that contract quickly and powerfully but fatigue rapidly. They are primarily used in explosive movements, such as sprinting or weightlifting.

injury tissue load

compression, shear, torsion, and tensile forces that can cause damage to muscles, tendons, or ligaments.

compression force and what it looks like

• Definition: Forces that press or push tissue together, leading to a shortening and widening of the structure.

• can look like
  Squishing”
  • Vertebrae during
  upright posture
  • Act upon
  longitudinal axis

  
  Looks Like: Bones are typically strong under compression. However, excessive or repetitive compression can lead to: - Bone fractures: Vertebral compression fractures (e.g., from a fall or osteoporosis). - Cartilage damage: Meniscus tears in the knee from squatting with heavy loads. - Disc herniation: Intervertebral discs being compressed and bulging.

shear force and what it looks like

• Definition: Combination of compressive and tensile forces
  • “Sliding”
  • Force acts parallel to surface

• Looks Like: - Ligament/tendon tears: Anterior cruciate ligament (ACL) tears often involve shear forces on the knee joint when the tibia slides forward relative to the femur. - Blisters: Skin shear from rubbing against a surface. - Dislocations: Joints can be displaced due to excessive shear forces. - Vertebral shear: Spondylolisthesis, where one vertebra slides forward over another.

torsion force and what it looks like

• Structure “twists”
  along its
  longitudinal axis
  • Fractures to tibia in
  football and skiing
  (foot is fixed)
  • Spiral Fracture

  
  Meniscus tears: Rotational forces on a weight-bearing knee can tear the menisci. - Ligament sprains: Excessive twisting of a joint beyond its normal range (e.g., ankle sprain). - Tendonitis/ruptures: Repetitive torsional stress can inflame or tear tendons.

tensile force and what it looks like

Tensile force is a pulling force that stretches or elongates an object.

It looks like arrows pointing away from each other, pulling on opposite ends of a material.

• Muscle strain – when a muscle is overstretched or torn (common in hamstrings or calves)

• Tendon tear – like an Achilles tendon rupture from sudden stretching

• Ligament sprain – e.g., an ankle sprain when ligaments are pulled beyond their limit

• Avulsion fracture – when a tendon or ligament pulls off a small piece of bone due to extreme tension

roles of skeletal muscles

responsible for moving the skeletal system, maintaining posture, and producing heat through contraction. They enable voluntary movements by working in pairs, where one muscle contracts while the opposite relaxes.

Muscle Activated
Tension length of muscle, tendons to
attachment of bone
Muscle pulls on bone creating “Moment of
Force” (Torque)
(MoF = moment arm x Force)
Net Moment determines:
a) If movement occurs, and
b) direction of movement

waht do muscles do…why do we have them?

• Primary Function: Generate force and produce movement by contracting and pulling on bones.

• ONLY tissue capable of actively Developing
  tension
  • Relies on Nervous system for stimulation
  • Has key features:
  i. Extensibility
  ii. Elasticity
  iii. Responsiveness
  iv. Contractility

• Movement: They pull on bones to make us walk, run, smile, or even blink.

• Posture: They hold us upright and stable, even when we’re standing still.

• Circulation & breathing: The heart and diaphragm are muscles that pump blood and help us breathe.

• Protection: They cushion and support internal organs.

• Heat production: Muscle activity generates body heat to maintain temperature.

different types of bones and their difference and examples

• Long bones: support weight & movement → example: femur

• Short bones: provide stability & limited motion → example: carpals

• Flat bones: protect organs & attach muscles → example: sternum

• Irregular bones: complex shapes for support/protection → example: vertebrae

• Sesamoid bones: reduce friction in tendons → example: patella

ground reaction force

is the force exerted by the ground on a body in contact with it, equal and opposite to the force applied by the body.

• newtons 3rd law: for every action, there is an equal and opposite force

what is a force

any influence that causes an object to change its velocity, direction, or shape. any type of resistance, load, etc

air resistance, body weight, friction, muslce, and reaction forces

body weight

• the force exerted on a body by gravity, typically measured in Newtons or pounds, representing the downward pull of Earth's gravitational field.

• example is a 70kg runner experiences 686 N of downward force due to gravity

• we have our force, it will = the mass of the person multiplied by 9.8 = 686

that is a negative effect on the body

air resistance

• opposes forward motion through the air

• increases with bdoy s[peed and surface area

• important for running, cycling, and sprinting

• a sritner may lean forward to reduce air drag

the air resistance is a negative force acting against motion, slowing descent

friction

• acts at the contact point with the ground, allowing forward movement

depends on which direction its going into

ice: friction goes in your direction where ur trying to go so : positive movement

running in mud: friction goes in opposite direction where ur trying to go so :negative movement

muscles forces

• generated by muscles to move the body

• act internally to move limbs and externally thorgh the feet against the ground

• create movement and maintain posture

muscle forces that act on something, helping me push off when im running: positive

peak height questions

• Definition: In biomechanics, peak height typically refers to the maximum vertical displacement achieved by an object or the center of mass of a body during a ballistic movement, such as a jump or projectile motion. - Calculation: Often analyzed using kinematic equations, where initial velocity (u), acceleration due to gravity (g = 9.81 m/s^2), and final velocity at peak height (v = 0) are used. The formula for vertical displacement (h) can be derived from (v^2 = u^2 + 2gh), leading to (h = u^2 / (2g)). - Significance: A key metric in performance assessment for sports involving jumping (e.g., basketball, volleyball) and in analyzing human movement efficiency. Factors like ground reaction force, muscle power, and technique influence peak height.

concentric contraction

for example: bicep curl : coming close together

isometric contraction

A type of muscle contraction where the muscle generates force without changing length, often used to stabilize joints during various activities. such as planks or wall sits.

eccentric contraction

a type of muscle contraction where the muscle lengthens while generating force, such as during the lowering phase of a bicep curl.

cortical bone

The dense outer surface of bone that provides strength and structure, crucial for supporting weight and protecting underlying tissues.

cancellous bone

a type of bone that is spongy and porous, found at the ends of long bones and in the interior of others, providing structural support and flexibility.

newtons law of motion

Three fundamental principles describing the relationship between an object and the forces acting upon it, which govern its motion.

when im at rest, the net force is 0

The first law states that an object at rest stays at rest and an object in motion stays in motion unless acted upon by an external force. The second law quantifies force as the product of mass and acceleration, while the third law states that for every action, there is an equal and opposite reaction.

tendon

A flexible band of fibrous connective tissue that attaches muscles to bones, transmitting the force generated by muscles to facilitate movement.

ligament

connective tissue that connects bones to other bones at a joint, providing stability and support.

factors that effect joint stability

• Definition: Joint stability refers to the resistance of a joint to displacement, or its ability to maintain its structural integrity while allowing movement. -

• The 'fit' of the articulating bone surfaces. Deep sockets (e.g., hip) offer more stability than shallow ones (e.g., shoulder).

• Ligaments: Strong, fibrous connective tissues that connect bones to bones, restricting undesirable movements and preventing excessive motion.

• Joint Capsule: A fibrous sac enclosing the joint, providing structural support and containing synovial fluid

• - Muscle Tone/Strength: Muscles and their tendons crossing a joint provide dynamic stability. Muscle contraction can pull bones closer together or prevent excessive movement.

• Negative Intra-articular Pressure: In some joints (e.g., hip, shoulder), the slight vacuum created within the joint capsule helps hold the bones together.

• Menisci/Labrum: Fibrocartilaginous structures (e.g., in the knee or shoulder) that deepen the joint socket and improve congruence.

net force of an object

at rest the net force is 0

• Definition: The net force (F{net}) acting on an object is the vector sum of all individual forces acting on it. It determines the object's acceleration according to Newton's Second Law: (F{net} = ma). - Equilibrium: When an object is at rest or moving with a constant velocity, its acceleration is zero, meaning the net force acting on it is zero (F{net} = 0). This is known as static or dynamic equilibrium. - Non-equilibrium: If (F{net}
  eq 0), the object will accelerate in the direction of the net force, changing its velocity (speed or direction, or both).

bone strength and integrity

refers to the ability of bone to withstand forces without breaking, influenced by factors such as density, mineral content, and overall health.

• trying to strengthen up a bone by using a logitudinal force applied along its length.

what componets are within a diarthitic joint vs synarthitic joint

Diarthritic joints include a synovial cavity, articular cartilage, ligaments, and a joint capsule, allowing for free movement. In contrast, synarthritic joints lack a synovial cavity, offer no movement, and are connected by fibrous or cartilaginous tissue.

body diagram

A body diagram visually represents the major systems and structures of the human body, often used to illustrate biomechanics concepts such as forces, movements, and joint interactions.

stress and stain curves

visual representations of the relationship between stress (force per unit area) and strain (deformation) in materials, indicating elasticity, yield point, and ultimate strength.

differences between the applied load

• Any external force acting on the body or a structure. Its characteristics affect how the body responds.

• Magnitude: Size of force → bigger force = more stress (e.g., 50 N vs 500 N)

• Direction: Type of force → compressive, tensile, shear, or torsional

• Point/Area: Where it’s applied → single point or spread out

• Duration: How long → short (impact) vs long (posture)

• Rate: How fast → quick = more damage (impact vs slow stretch)

• Frequency: How often → repeated = overuse, single = acute injury

• Type: Static (constant), Dynamic (changing), Cyclic (repeated), Impact (sudden)

velocity and distance

are key concepts in biomechanics that measure the rate of change in position over time and the total path traveled by an object, respectively. These metrics are essential for analyzing movement dynamics.

shoulder joint and stability

Shoulder (Glenohumeral) Joint – Summary:

• Type: Ball-and-socket joint (humerus head + glenoid fossa)

• Function: Most mobile joint in body (~180°+), low stability

• Stability factors (weak):

  • Glenoid labrum → deepens socket (~50%)

  • Joint capsule → loose for movement

  • Ligaments → minor passive support

  • Rotator cuff (SITS) → main dynamic stabilizers (active muscle support)

shoulder rotation

• Shoulder Rotation – Summary:

  • Definition: Turning of the humerus around its long axis at the shoulder joint.

  • Types:

    • Internal (medial) rotation: Humerus turns toward midline; with adduction & extension.

    • External (lateral) rotation: Humerus turns away from midline; with abduction & flexion.

  • Muscles involved: Rotator cuff (SITS) + pectoralis major + latissimus dorsi..

shoulder axis

• Glenohumeral Joint – Axes of Movement:

  • Type: Multi-axial (ball-and-socket) joint → movement in 3 planes

  Primary Axes:

  • Anteroposterior (AP) axis: Front ↔ back → abduction & adduction (frontal plane)

  • Mediolateral (ML) axis: Side ↔ side → flexion & extension (sagittal plane)

  • Longitudinal (vertical) axis: Top ↔ bottom → internal & external rotation (transverse plane)

shoulder built more for stabilty or mobility and why?

• Shoulder Joint – Mobility vs. Stability:

  • Built for: Mobility > Stability

  • Reasons:

    • Shallow glenoid fossa: Small, shallow socket → minimal bony support

    • Loose joint capsule: Allows wide motion, less restriction

    • Muscular stability: Rotator cuff & larger muscles stabilize but mainly move the joint → less structural stability

  • Result:

    • Allows 360° movement (circumduction + motion in all planes)

    • Makes joint more flexible but prone to dislocation

weakest position for the shoulder

• Shoulder – Weakest Position (Most Prone to Dislocation):

  • Position: Abduction + External Rotation (AER)

    • Arm at 90°+ abduction, externally rotated (e.g., “cocked” throwing position)

  Why it’s weak:

  • IGHL (Inferior Glenohumeral Ligament) is stretched → less support

  • Humeral head loses bony protection from glenoid

  • Forceful rotation can push humeral head out of socket (anterior/inferior) → may cause labrum tear (Bankart lesion) or rotator cuff damage

what type of joint is the shoulder

The shoulder is a ball-and-socket joint that allows for a wide range of motion in various directions.