Kinesiological Principles—Forces, Levers and Muscle Actions

Part 1 – Forces
Definition & Fundamental Characteristics
  • Force = Push or Pull
  • Always occurs in action–reaction pairs (Newton’s 3rd law)
  • Capable of causing, stopping or altering motion
  • SI Unit = Newton (N)
  • Mathematical symbol F
Measuring & Representing Force
  • Vector quantity → has magnitude, direction and point of application
  • Common graphic representation = arrow (vector)
  • Basic magnitude equation when in straight-line motion: F = m \times a (Newton’s 2nd law)
Broad Classification of Forces
  • External Forces (act on the system from outside)
    • Contact Forces
    1. Normal contact (ground-reaction) force – perpendicular to the contact surface
    2. Frictional force – parallel to the contact surface; resists sliding
    3. Fluid (drag or lift) forces – occur in water or air
    • Non-Contact Force
    • Gravity / Weight force F_{WT}=m \times g acting through the Centre of Gravity (COG)
  • Internal Forces (originate within the body / system)
    • Tensile force – pulling along the length of tissues (e.g. tendons, ligaments)
    • Compressive force – pushing along the length (e.g. bones, articular cartilage)
    • Shear force – acts parallel to the analysis plane; tends to cause sliding between parts
    • Excessive tensile, compressive or shear loading ⇒ deformation or structural failure (fracture, tear). Structures typically deform and then break; the extent of deformation depends on load and structure.
Mechanical Loads on the Human Body
  • Muscle contractions, gravity and external impacts combine, generating unique internal loading profiles
  • Understanding load type allows prediction of injury risk & design of conditioning plans
Part 2 – Torque, Moments & Lever Systems
Torque (Moment of Force)
  • Torque (τ) = turning effect of a force about an axis
  • Equation: \tau = F \times r where r = moment arm (perpendicular distance from axis to line of action)
  • Mechanical advantage decreases or increases by manipulating r
  • Vectors obey right-hand rule (direction of rotation)
Muscular Torque
  • Muscles create torque that rotates body segments about joints
  • Example: Biceps brachii produces elbow-flexion torque by pulling on the ulna, with the elbow as the fulcrum and the biceps insertion as the point of effort.
Components of Every Lever
  1. Fulcrum (pivot/axis)
  2. Effort (applied force)
  3. Load (resistive force)
Lever Classes & Functional Emphasis

  • 1st-Class Lever

    • Arrangement: Load – Fulcrum – Effort
    • Trade-off between distance & force; can favor speed or force depending on position of fulcrum (e.g. head nodding on atlanto-occipital joint, seesaw, scissors)

  • 2nd-Class Lever

    • Arrangement: Fulcrum – Load – Effort
    • Force multipliers → smaller effort lifts larger load but through shorter distance
    • Body example: rising on tiptoes (ball of foot = fulcrum, body weight = load, gastrocnemius force = effort); also wheelbarrow

  • 3rd-Class Lever

    • Arrangement: Fulcrum – Effort – Load
    • Distance/speed multipliers → greater effort needed, but large, fast load displacement
    • Classic body example: elbow flexion (load = hand/forearm, effort = biceps on radial tuberosity, fulcrum = elbow) – note effort (muscle insertion) lies between load and fulcrum; also tweezers; most muscle attachments in the body are third-class levers.

  • Comparative Summary

    • 1st: Balance of **distance **↔

    ** force** (trade-off)

    • 2nd: Maximises force
    • 3rd: Maximises distance / velocity
Part 3 – Joint (Kinesiological) Actions & Muscle Contractions
Types of Muscle Contraction
  • Concentric – muscle shortens while producing force (accelerates segment)
  • Eccentric – muscle lengthens while producing force (controls or decelerates movement, often resisting gravity during lowering or deceleration)
  • Isometric – muscle length unchanged (stabilises)
Guided Application Questions (from slides)
  • Shoulder – Up phase of pull-up
    • Action: ext{Extension / Adduction} (depending on grip)
    • Latissimus dorsi performs concentric contraction
  • Shoulder – Down phase of pull-up
    • Action: ext{Flexion / Abduction}
    • Latissimus dorsi performs eccentric contraction (controlling descent)
  • Knee – Down phase of squat
    • Action: ext{Flexion}
    • Quadriceps = eccentric (control descent)
  • Hip – Down phase of squat
    • Action: ext{Flexion}
    • Gluteus maximus = eccentric (control hip flexion)
  • Horizontal adductors – Return phase of push-up
    • Action: Horizontal adduction of shoulder
    • Contraction type = concentric
  • Chest Press – Return / Lowering Phase
    • Action: Horizontal Abduction of shoulder
    • Muscles involved: Horizontal Adductors (e.g., Pectoralis Major)
    • Contraction type = eccentric (controlling the descent of the weight)
Additional Notes & Cross-References
  • Ground Reaction Force (GRF) is a practical measurement of normal contact force in gait & jumping analyses.
  • Fluid drag increases with square of velocity F_{d} \propto v^{2} → critical in swimming/cycling technique.
  • Centre of Gravity location shifts with body position; essential for balance training & sports performance.
  • Moment arm optimisation through posture affects torque production (e.g. bent-knee vs straight-knee sit-ups).
  • Wolff's Law: Bones adapt to the loads placed on them; increased load (e.g., compressive forces on vertebrae) leads to