Biomechanics

Kinetics & levers (chap 12/13)

  • Newton's third law of motion:  for every action there is an equal and opposite reaction

Friction

  • Force acting over the area of contact between two surfaces in the direction opposite of that motion

  • Quantified in units of force (N)

  • Magnitude of friction determines relative ease or difficulty of motion for two objects in contact

  • As weight increases, the normal reaction force increases

  • Not a lot we can do to decrease reaction force, maybe the weight

  • Push or pull easier? It depends on many different factors

  • Pushing = force exerted is typically diagonally downward

  • Pulling = force is diagonally upward

Friction in daily activities

  • Walking depends on friction between a person's shoes and supporting surface

  • If friction is too low (smooth soled shoes on ice), slipping will occur

Controversial in sports

Effect on performance vs. risk of injury

  • Artificial turf: high friction between turf and cleat does not allow rotation of a planted foot → knee injury

    • Further impacted by temperature of turf

    • Design of cleats specifically for turf

  •  Bowling: use of conditioning oil on the lanes, reduces friction

  • Cycling: rolling friction is inversely proportional to the wheel diameter; it decreases with tire width and increases with reduced tire pressure

Momentum

  • The quantity of motion that an object possesses

  • The product of an object's mass and its velocity

  • A static object has no momentum

  • Head-on collision: tendency for both object to continue moving in direction of motion of the object with greater momentum

  • In absence of external forces, momentum is conserved (inertia)

  • Friction and air resistance are forces that normally reduce momentum

Impulse

External forces change the momentum in a system

  • Depends on magnitude and length of time

  • Product of force and time = impulse

  • Can be small force acting over a large time interval or a large force active over a small time interval

Impulse generated by body from a vertical jump

  • The larger the impulse generated against the floor, the greater the change in the performers momentum and the higher the resulting jump

Impact

  • Collison characterized by the exchange of a large force during a small time interval (struck baseball and bat)

What happens after the collision depends on momentum:

  • Perfectly elastic impact; velocity of the system is conserved

  • Perfectly plastic impact; at least one of the bodies in contact deforms and does not regain it original shape

Work, power, & energy

  • Work: force applied against a resistance, multiplied by the displacement

  • Power: rate of work production, calculated as work divided by time.

    • Ability to produce mechanical power

    • Athlete's ability to exert mechanical power or the combination of force and velocity is critical 

    • peak

  • Energy: capacity to do work, mechanical energy is the capacity to do mechanical work

    • Kinetic energy: energy of motion, if motionless, Ek = 0, increase in velocity = increase its Ek

    • Potential energy: energy of position, includes strain energy to do work by virtue of a deformed body’s return to its original shape.

      • Lots of energy in arrow in archery while it hasn't been released yet

  • The work of a force is equal to the change in energy that it produces in the object acted on

  • The work-energy relationship is also evident during movement of the human body

Levers

  • A levers is a simple machine consisting of a relatively rigid, barlike body that may be made to rotate about an axis = fulcrum

  • Consist as a pivot point

  • First, second, third class levers 

  • Most levers within human body are third-class

    • Biceps of elbow

    • Patellar tendon at knee

    • Medial deltoid at shoulder

Using levers in sport

  • Levers are made up of joints and the bones that connect them to the objects being moved

  • Levers in body can be manipulated to improve speed and apple large forced at the same time

    • We use elbows to lift biceps for force rather than shoulder

  • Amount of leverage a person processes is dependant on length of their body (arms and legs)

  • Longer levers:  result in greater speed at the end of the lever arms, throwing and striking object

  • Short levers: can be moved with less force and at greater speeds. Good for moving body quickly and applying strength 

Oct. 22

Biomechanics and amputations

  • Lower extremity biomechanics post-amputation

  • kinetics / kinematics

  • Prosthetics

  • Bionics

Terry fox

  • Underwent an above knee amputation after being diagnosed with osteogenic sarcoma

  • Used a prosthetic designed for walking

  • Motivated researchers to develop protheses better suited for running

  • Development of materials, bionic technology

Running biomechanics: above knee ampute

  • The hip musculature had to perform the work that would otherwise have been done by the quadriceps and hamstring

    • Has no knee joints

  • The artificial knee also had to stay straight in full extension while the leg was bearing weight of the leg would buckle

Controlling the knee joint

  • Forces that affect the joint position and movement during stance

Ground reaction force:

  • Equal to magnitude and opposite in direction to sum of gravitational and contact forces

  • Evaluating its location with respect to joint axis

Preserving stability: amputees

  • Substituting another muscle in a closed chain

  • Moving joint axis to lower GRF moment about joint

  • Moving GRF to lower its moment about joint

Cons:

  • Conventional prosthesis was not designed to absorb the impact generated at heel strike for running

  • Slow “swing-through”

Pros:

  • Simple design = easy to repair on the go

  • Repaired prosthetic with car parts

  • Can be repaired by a welder

Terry fox inspiration

  • Inspired millions of canadians and researchers

  • Terry met with head of the prosthetics to review his prosthetic needs

  • Martel and his team received $17000 research grant to improve running prosthetics

    • Developed and test a lighter leg

Modern prosthetics

  • Create prosthesis that are lighter, stronger, more durable

  • Improved control and responsiveness of prosthetic knees

  • Develop prosthetic feet and knees for amputees who want to remain active

  • C-leg: first knee to include a microprocessor control hydraulic knee joint

  •  Microprocessor knees: used AI and magnetic fluid to adapt to change in users gait and changes in environment

  • Symbiotic leg: bionic knee and ankle system. Power knee, first device to provide powered lift

Running specific prostheses

  • Van phillps develop flex foot

    • Stored kinetic energy, releases at toe off

  • The flex-foot was first used in elite competition wa th 1988 paralympic games

  • Below knee amputees benefit particularly from this technology

Advantage controversy

  • Energy return and shorter swing-through times

  • prosthetic “blades” gave an unfair advantage 

  • They can only run as fast as their slowest limb

    • Not fair if the one leg cannot perform as much as the prosthetic can with all the developments

Oct 29.

Applying kinetics

  1. Why does a force directed through an axis of rotation not cause rotation at the axis

  • The moment arm is zero, there will be no torque due to the force applied

  1. To which lever classes do a golf club, a swinging door, and a broom belong?

  • Golf club: 3

  • Swinging door: 2

  • Broom: 3

  1. Position a pole across the back of a chair (serving as a fulcrum), and hang a 2 lb weight on one end of the pole. Position a 5 lb weight on the other side of the pole such that the weights are balanced

  • The larger mass can balance the system at a system at a smaller distance from the fulcrum

  • A weight at the end of the stick generates more torque.  

  1. 4. Perform curl-up exercises under the following conditions: a. Arms folded across the chest b. Hands behind the neck c. Holding a 5 lb weight above the head Write a paragraph explaining your findings, and draw a free body diagram showing the applied force, resistance, and axis of rotation

  • Farther from the center of mass with added weight are going to be more difficult to achieve. Hardest: c, b, a

  1. What factors affect the magnitude of friction?

  • Weight, and texture of surface and object. Coefficient of friction of surface 

  1. . Select one sport or daily activity, and identify the ways in which the amount of friction present between surfaces in contact affects performance outcome. 

  • Race car drivers: hand and the wheel 

  • Curling; ice and rock

  • Football players on turf: Higher friction, less effort required for acceleration, can lead to more injuries if not done on proper surface

  1. Select one of the following sport activities and speculate about the changes that take place between kinetic and potential forms of mechanical energy. 

  • A: single leg support during running: kinetic: if in motion we have kinetic energy all the time, must have potential energy to generate movement 

  • B: a tennis serve: potential; highest moment of serve, energy of the strings. Kinetic energy of springs and wrists to move the ball

  • C: pole vault: potential; highest energy from the ground, right before the movement happening from the top

  • D: spring board dive: potential energy; spring board going doing after jump and at the height of our dive. Kinetic; going down from dive

Oct. 31 

Biomechanics of hiking 

  • Navigate complex terrain in everyday lives

  • Energy demands increase when walking on natural complex surfaces

    • Height variations

    • Damping = dissipation of force to absorb vibration with locomotion

Factor contributing to greater henry expenditure

  1. Adjusting step parameter during locomotion 

  • Change COG, BOS  for stability and balance

  • Gait may change

  • Energy expenditure would be expected to increase with these changes

  1. Uneven terrain might also require more mechanical work from the legs, independent of the effect on step parameters

  • Toe-off is more present as it assist the propel forward; negative mechanical work

  • Muscles are compensating and actively do more positive work

  1. Increased energy expenditure due to coactivation of muscles 

  • Compensations of muscles help to stabilize the joints for uncertain conditions

  • More demand on muscles on uneven terrain, energy expenditure increases

Performing an analysis on uneven terrain

  • We can model it on a treadmill and overground walking

  • allows the collection of continuous kinematic and energetic data 

Predictions:

  • Steps width and length (wider/shorter)

Nov. 5 

 

Walking poles; Gait biomechanics 

  • Used for assistant propulsion for various sports

Parts on the pole

  • Grip: transfers force form arm to pole

Strap

  • Shaft

  • Basket

  • Tip: transfers force from pole to ground

Analysis:

Does walking with poles reduce loading to the lower extremity during level over ground walking? 

  • Analyzed different poling conditions:

    • Selected poles

    • Poles back

    • Poles front

    • No poles

Results:

  • Increased walking speed, stride length, stance time

  • Decrease in GRF braking impulse

  • Extensor impulse decreased (knee)

  • Less quad activation

Why the analysis?

  • Analyze the effects of walking with poles on the gait mechanics of healthy subjects

  • As a physio, we can analyze what benefits more for the patient

Hypothesis:

  • Poles would significantly reduce loading on the knee joint

  • Different polling techniques

Data collection:

  • Recorded using three-dimensional motion analysis system

  • Lower limb markers

  • Reflective markers on poles

  • Cameras to capture kinematic data