PHTY103 Lecture Notes - Lecture 3: Ground Reaction Force, Resultant Force, Stop Motion
3. Biomechanics: Motion and Force
• Define basic biomechanical terminology
o Mechanics: Branch of physics that analyses the actions of forces on particles and
mechanical systems
o Biomechanics: tools of mechanics applied to living organisms
• Built on basic body of knowledge from
▪ Physics
▪ Chemistry
▪ Mathematics
▪ Physiology
▪ Anatomy
o Motion: change in place, position, or posture occurring over time and relative to
some point in the environment
• Differentiate kinematics and kinetics
o Kinematics
• Concerned with the characteristics of motion from a spatial and temporal
perspective
• Without reference to forces
• Description of position, velocity, acceleration
• Linear/angular
o Kinetics
• Examines the forces acting on a system, such as the human body
• Forces cannot be seen, but effects of forces can be observed
• Differentiate static and dynamic situations
• Define and provide examples of linear and angular motion
o Linear
• Movement along a straight or curved pathway in which all points on a body or
object move the same distance in the same amount of time
o Angular
• Motion around some point (axis) so that different regions of the same object
do not move through the same distance in a given amount of time
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• Distinguish between qualitative and quantitative approaches for analysing human
movement
o Qualitative
• Non numeric evaluation of motion based on direct observation
o Quantitative
• Numeric evaluation of motion based on data collected during the movement
• Describe different types of forces (both internal and external)
o Internal
• Produced from structures located within the body
• Active: generated by muscle contraction
• Passive: generated by tension in stretched tissue (e.g. ligaments, joint
capsules, intramuscular connective tissues)
o External
• Produced by forces acting from outside the body
• Gravity acting on mass of a body segment
• External load acting on mass of a body segment (e.g. hand weight)
• Describe forces and moments
o Force
• A push or a pull acting on a body
• Involves the interaction of 2 objects and produces a change in state of the
object
▪ Produces motion
▪ Stop motion
▪ Accelerate object
▪ Change direction of object
• Types
▪ Gravity or weight of body parts and attachments e.g. splints, casts,
weights
▪ Muscles which can produce forces on the bone segments by active
contraction or by being passively stretched
▪ Externally applied resistances/weight e.g. exercise pullies, manual
resistance
▪ Friction which can provide stability if optimum, retard motion if
excessive, or lead to instability if inadequate
• Characterised by
▪ Magnitude
▪ Direction
▪ Point of application
• Units are Newtons (N)
• Forces acting on a body or body segment may:
▪ Translate a body segment
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