Lecture 15, Biomechanics of the Body

Question 1

The Skeletal System

Answer 1

• bones and the tissues such as tendons, ligaments and cartilage that connect them
• teeth are also considered part of your skeletal system
• bones and joints are the basic components of the skeletal system
  ◦ joint is simply a connection between bones
• mechanically, this system is a series of rigid links connected to each other to allow for specific movement
• bones can protect, store minerals, can release material from the bone which makes it a dynamic tissue but for our purpose it is an attachment site for muscles where muscles are big elastic band there that could not do much if they did not have something to pull on - as bone and muscles work together to allow us to create movements (musculoskeletal system) - forces can affect bones positively and negatively

Question 2

Defining Force

Answer 2

• a force is a push or pull
  ◦ a force attempts to create a change (in motion)
• forces are exerted by objects on other objects
  ◦ forces can deform objects
  ◦ in rigid-body mechanics, we assume that objects do not change shape
  ◦ forces can accelerate objects
  ‣ create movement and stop movement
  ‣ speed up objects, slow down objects, and cause objects to change direction
• forces can create movement but…forces can also deform objects
• forces create, change and prevent movement
• bones can be deformed by forces - non-rigid body (cause permanent changes to a system)

Question 3

Area (mechanical stress)

Answer 3

• forces acting on a system impose loads that affect the structures of that body
• whether or not the system can handle these loads is dependent upon:
  ◦ size of the force
  ◦ area over which that force is applied
• mechanical stress = internal force (F) / cross-sectional area of the internal surface (A)
• stress = F / A (internal force applied over an area and if it is big enough it could dent the system)
• measured in Nm2 (or pascals)
• reflects the likelihood of injury or damage to the system
• the area over which you are exposed to force matters - is the force coming over us on wide area or a small area - the size of the force matters but if wear exposed to large force over large area we can better withstand it as opposed to if its the large force is concentrated in a small area in a specific spot
• mechanical stresses deals with forces and movement - the size of the force and the area over which it is applied - key: happens inside a system (is your arm strong enough to withstand the force, how wide is the attachment site, how much force is generated in the body and how big is the area)
• force is too big (excessive) or the area over which you apply is really small which creates big mechanical stress which can potentially be damaging

Question 4

Mechanical Pressure

Answer 4

mechanical pressure = external force (F) / cross-sectional area of the external surface (A)
- pressure = F / A
- measured in Nm2 (or pascals)
- external force (comes from outside the body) and the area over which it is applied
- someone hopes on somebodies back - carrying others, an external force being applied on someone and the area over which it is applied (if the force is too big or area is too small - the bones can potentially break)
- mechanical pressure is same thing as mechanical stress - just external and internal (true or false question)
- factors the influence bone strength: size of force, the area over which the force is applied, which way you are getting hit (the direction is which the force comes from is also just as important)

Question 5

Force vs. Direction (what are the 3 directions?)

Answer 5

• a force applied to a system will place a mechanical load on that object
• the number, direction, and location of the force will define the type of load imposed
  tension, compression and shear
• which way are you getting hit matters
• load refers to size of the force and the direction in which it is applied

Question 6

Tension - Tensile Stress

Answer 6

the resistance of an object to a force tending to tear it apart
- tension is a type of axial load
- a force administered along the lines of an axis
- tension force is trying to pull two things away from another (every time you throw you are throwing your arm out of the socket) - kicking your leg away from the body is tensile stress
- joints, muscle and ligaments hold everything together and counteract tensile loads

Question 7

Compression - Compressive Stress

Answer 7

squeezing force directed through the center of a body
- tension is a type of axial load
- a force administered along the lines of an axis
- joints, muscle and ligaments hold everything together and counteract tensile loads
- with compression you are trying to push things together (the ends of the object) along the long axis which is opposite of tensile
- bending is a combination of tension on top and compression on the bottom

Question 8

Shear - Shear Stress

Answer 8

pushing forces acting in the opposite direction
- shear is a transverse stress
- acts parallel to the surface
- in shear stress you are acting perpendicular to the long axis - can be described as pushing
- in shear you have forces going in opposite directions - pushing force which chops the object down
- our bones are pretty good at dealing with compression and tension but no so much with shear

Question 9

Bone Strength vs. Mechanical Loading

Answer 9

• the three principal stresses are tension, compression, and shear
• internal forces generated within the body
• bones tend to be strongest during compression and weakest during shear
• your bones completely remodel the moment you stand up - bones lay down more bone tissue - reinforce themselves in a way so that you can handle you standing on top of yourselve which is compression (the more your move you beginning throwing you legs where you bones are able to also withstand that tension)
• not exposed to shear force very often so your bones cannot withstand it

Question 10

Bone Strength vs. Mechanical Loading (where is bone weakest and strongest?)

Answer 10

bone is the strongest is resisting compression
- compressive stresses experienced when walking (with extra mass on your shoulders)
bone is weakest in resisting shear
- shear stresses experienced during a soccer tackle (which is too much for the body)
- from more than one direction which we get caught and see breaks
- the force that you are exposed to and the makeup on the bone (a long bone - have larger ends provide attachment sites and connection with other bones - when you move, it moves it sorts of direction so you are exposed to forces from all direction)
- bone is an important warehouse which is the site of blood cell production and it turns into yellow marrow after which turns into fat - we can reinforce but there is a limitation to how much we can reinforce
- people who are not very active, there are not very strong so it takes less shear stress but the tolerance can be higher if you are move active

Question 11

The Effects of Loading (strain and stress)

Answer 11

• stress looks at the amount of force you are exposed to and strain or deformation (force vs deformation and stress vs strain)
• how much force you can opposed to and how much change in shape do you see a system undergo

Question 12

The Effects of Loading - Elastic Behaviour

Answer 12

• if a system is in an elastic phase of loading it will undergo a change big or small but when you release the force, it bounces back to its original shape (no dents are created) - never know that it was originally stretched
• stiff objects: huge amounts of force but bend is small - not noticeable of there is any change in shape
• whereas with a piece of paper there is a small force that creates much bigger change in shape
• piant bends a lot more with little force - system bounces back if it is in the elastic phase when the force is remove

Question 13

The Effects of Loading

Answer 13

• how much force versus how much shape (all system undergo same general process)
• all system have yield point where you made some sort of mark in the system, when you take it a little too far and now there is a permanent change in shape and it always remains can no longer go back to original shape - it is now weaker (for example: a paper get creased or water bottle getting dented)
• plastic phase because the system is now weaker, any little bit of force applied is going to create much bigger change in system as the system is now weaker (plastic behaviour) (eventually the paper rips)
• elastic phase: force is exposed but it returns to its original shape whereas plastic behaviour system is exposed to force but there is a permanent deformation (every bit of force added above bend, crease, fold gets bigger)
• plastic behaviour - if the load exceeds a certain size, some permanent deformation may occur
• every system has ultimate strength (most amount of force is can be exposed to before it breaks)
• failure strength system breaks - created a break, rip, hole and system becomes permanently damaged - going beyond ultimate strength (microtears in bones for example)
• factors that affect whether a bone can withstand force: can look at size, direction, area over which force is applied and whether the bone is in elastic or plastic phase, has the system been damaged previously or not, how often have you been exposed to the force

Question 14

Frequency of Loading (acute and chronic injury)

Answer 14

• look at size of force and the frequency over which we are exposed to that force (how often are we exposed to the force)
• if the value of the force you are exposed to is below the injury threshold you are fine/safe and anywhere above the line you are in trouble
• if you are exposed to the force very often it is enough to cause a break when you go above the injury threshold line
• if you are exposed to one force once but it is big enough to cause injury in that one moment it is called acute injury (jumping off something, taking one wrong step)
• single force of high magnitude that causes injury or damage: acute injury (single force, exposed to it once but it was enough to cause acute injury)
• damage from repeated applications of a sub-acute force: chronic injury (exposed to force over long period force: weeks, months, years - repeated exposure to one low force) high exposure, low force

Question 15

Frequency of Loading (level of conditioning)

Answer 15

level of conditioning or training is a factor in risk of injury
movement, force, physical activity, sitting or standing are all potential factors
- force is a good thing because you need some degree of mechanical stress to strengthen the bone in the first place - bones responds to size and direction of the force and reinforces itself in way so it can withstand the forces in later interactions
- whether activity could shape bone development - done on young athletes who are just coming through puberty - looking and muscle strength and bone - runners, swimmers and cyclists (cyclists had strongest muscles then swimmers and last runners but runners had the strongest bones) - muscle pulling on bone does something but key is physical loading (constant push on the bone development) - want to have some activity as doing nothing is not the answer as bone health declines but if you do way too much that is also negative and cause body to breakdown
- level of activity, frequency of stimulation, if there has been any damage before or afterwards and the direction, area and size of force all contributes to one having healthy bones

Question 16

The Muscular System (musculoskeletal)

Answer 16

• organ system consisting of skeletal, smooth and cardiac muscles
• together with the skeletal system it forms the musculoskeletal system, which is responsible for movement of the human body
• bones and joints of the skeleton provide the framework of the body
  ◦ active force generation of muscles provide stiffness to the joints
• muscles are able to create the force (our focus in on skeletal)

Question 17

The Muscle Model

Answer 17

• contractile component that is muscle cell and elastic component which describes two different types (elastic component found over the muscle cells which contributes to force development called the parallel elastic component because the muscle cells run in parallel with the connective tissue)
• the tendon is the series elastic component which is a fancy way of saying “next to” (next to muscle)
• hills model shows how muscle and connective tissue play rule in force development
• parallel elastic components when the muscle run parallel with the connective tissue

Question 18

Hill’s Muscle Model

Answer 18

three sites contribute to the development of force within a muscle

1) the contractile element
- the site in the muscle where force generation actually occurs
- this is the sarcomere, the basic contractile unit of muscle
- also known as the active component (shrinks and stretches)
- is the muscle cell - actively generates force for something to happen inside the muscle for force generation (actin and myosin forming cross bridges; cross bridges pull on one another and sarcomeres shrink)

2) the series elastic component
- as muscle lengthen, the tendons stretch
- the connective tissue when stretched can contribute to force development
- also known as the passive component (does not do anything as it only acts as a result of muscle moving)
- the layer of connective tissue is going to stretch or shrink - the tendon (moves in response to muscle moving not on its own)
- when the actin and myosin shrink or stretch, there is a layer of connective tissue that surrounds it is also going to shrink or stretch

3) the parallel elastic component
- as muscle lengthen, the connective tissue inside the muscle stretch (what surrounds the muscle)
- the connective tissue when stretched can contribute to force development
- also known as the passive component
- the majority of stretch comes from this component as the tendon does not stretch that much

Question 19

Hill’s Muscle Model (final exam question)

Answer 19

• contributes to force generating capacity: its length of the muscle and component the of the muscle that is a part of it and contributes to the force (active - responsible for force and passive - stretch that play role in force production)
• on x axis you have muscle length (shorter or longer)
• contribution of active cell (muscle cell) and contribution of what surrounds the muscle cell (looks at them independently) but the force generating capacity will be the combination of all of it together
• every muscle has a position where it generates the most amount of force and at a given length it generates the most amount of force (prime operating length)
• when you you got to shorten a muscle beyond optimal length or stretch beyond that point, the force generating capacity decreases
• rubbers band do not do a lot when they are shortened but can do a lot when stretched - so you want to also look at passive component
• passive component: when muscle is shortened, the force generating capacity are not a lot but as it stretches the capability increases
• total: as you increase the length the muscle get stronger and stronger up to a point where they decrease a little and then get stronger and stronger
• when you go to shorten a muscle you are not that strong as you predominantly relying on the active component as the passive component does not contribute but much stronger when you lengthen a muscle, there is lots of “rubberbands” stretching together and generating tension
• stronger during eccentric action as compared to concentric actions

Question 20

Contraction Mechanics

Answer 20

a twitch, is a small, local, muscle contraction and relaxation
a single muscle action in response to a brief threshold stimulation
- look at force with respect to time - when you stimulate a muscle nothing happens in terms of muscle generating force (latent - delay) - then you get phase of contraction where it was produce peak and eventually relax
- during latent period, calcium gets released from the SR and opens up the binding sites, then comes contraction phase and then calcium goes back into SR and then cross bridges detach

Question 21

Characteristics of Skeletal Muscle Fibers

Answer 21

fast twitch (FT) fibers reach peak tension and relax more quickly than slow twitch (ST) fibers
- how quickly does this happen and how big is the force
- fast twitch generates high amount of force in comparison to slow twitch which takes more time
- they do not all get to the same peak force
- shortening or lengthening, what fiber type is being engaged, passive or active
- peak tension is typically greater for FT than for ST fiber

Question 22

The Force-Velocity Relationship (concentric)

Answer 22

• velocity is zero at origin and then gets faster and faster - strongest when you are not moving if you have a number right off the start
• this relationship exists for concentric actions - when a muscle shortens
• stronger during isometric actions (not moving) as opposed to concentric muscle action (then shortening)
• if you want to generate fast, force needs to be small and vice versa
• muscle contraction with maximal velocity when a lot of force is required, concentric actions most occur slowly the maximum amount of force can be generated during isometric actions

Question 23

The Force-Velocity Relationship (eccentric)

Answer 23

for eccentric action
- opposite - velocity gets bigger as we move to left
- the graph does not start from origin
- if you are not moving you are generating a certain amount of force but as you start to move (more and faster) the stronger you become
- the more you move and faster you move the stronger you become
- the faster a muscle lengthens, the more force it can generate
- this graph starts at isometric muscle action not at origin (some level of force being generated so not at zero but when you stretch beyond that force increases)
- the origin is an isometric muscle action - going away from this velocity is going to increase
- when you shorten the active is mainly active (less time for crossbridges to form) and thus less force being generating - related to active muscle component as less crossbridges are being made making you weaker
- when you stretch you rely primarily on passive (the faster you stretch the elastic band the more it is ready to generate force for you)
- are you relying on active or passive component

Question 24

The Length-Tension Relationship

Answer 24

graph represents the active (contractile) component in Hill’s Muscle Model
- force generated by a muscle is influenced by the degree of protein-filament overlap in the sarcomere
- if you stretch beyond a length or shorten beyond that length your ability to generate force decreases as it is dependent on number of active crossbridges you can create between actin and myosin and the more crossbridges that are formed the more opportunity to pull therefore more force you can generate
- when the lengthen above optimal length the proteins cannot come in reach of one another
- when you shorten and go below optimal length now you have protein banging into one another not being able to form the crossbridge connections or pull at an optimal position when force decreases
- the action between actin and myosin

Question 25

Muscle Architecture (3)

Answer 25

parallel, convergent, pennate

Question 26

Parallel Arrangement

Answer 26

• long fibers act over a great distance (run in parallel to one another)
• are not that strong but high in endurance
• slow twitch muscle fibres in parallel - long activity low force (to be able to hold a given position) - running over long distance (ex. postural muscles - in your back to hold a position)

Question 27

Convergent Arrangement

Answer 27

• the fibers converge at the insertion point to maximize the muscle action
• provides increased strength
• run over shorter distance and one wide attachment site where fibres converge over at a point - generate more force
• chest, shoulder muscles where we put more fibres in a area running over a smaller area (look like a fan) - pectoralis major and deltoid (stretch from front to back and converge at a point)

Question 28

Pennate Arrangement

Answer 28

• fibers span short distances in feather-like arrangement
• strong fascicle arrangement (more force than parallel)
• shortest muscle cell length - more muscle fibres shoved into an area (short distance - looks like a feather)
• we have the strongest muscle arrangement
• so what factors determine force generating capabilities of a muscle: whether you are relying on the active or passive component, whether you are stretching or lengthening a muscle, whether you are relying on fast twitch or slow twitch muscle fibre types, length of a muscle cell and the arrangement of the fibre and whether you have done a previous attempt or not

Question 29

Body Temperature

Answer 29

higher maximum isometric tension and high maximum velocity of shortening is possible at a given load at a warmer temperature
- the faster we move the weaker we become
- when we are warmer (done a warmup), the force velocity relationship shifts to the right - the relationship still holds true in that the faster we shorten a muscle the weaker we become however for the same given velocity you can generate more force
- when warmer you can produce more force for same amount of velocity (stronger once you have warmed up and had a few opportunities to do something)
- generate more force for a given velocity
- do a warm up because you can be better
- there is a limit because if you do much you will eventually run out of energy and fatigue