biomechanics definitions

Kinematics

Describes the motion of an object

Kinetics

Describes the forces associated with motion

Scalar

A variable which only has madnitude, but not direction

Force

Any action that alters the state of an object

Pressure

Force normalised to area (F/A)

Mass

Quantity of matter making up a body

Weight

Amount of force due to gravitational acceleration exerted on a body

Centre of Mass

The point at which the mass of the body is evenly distributed in all directions

Speed

Distance travelled over a given time m

velocity

Displacement over a given time

Acceleration

Vector - Change in velocity with respect to time

Strain

Proportionate change in length

Wolff’s law

Bone remodels in response to mechanical stress

Potential energy

A capacity to work due to gravity (in joules)

Kinetic energy

A capacity to work due to motion (in joules)

Work

Transfer of energy from one body to another through the application of force resulting in displacement (in joules)

Power

Rate of doing work (in watts, J/s)

GRF

Force exerted by the ground on a body in contact with it

Stride time

Time taken to complete one stride

Stride length

Distance travelled during one complete stride

Stride frequency

Number of strides per second

Stance phase

Portion of a stride in which a foot is in contact with the ground

Swing phase

Portion of a stride in which a foot is not in contact with the ground

Duty factor

Stance phase as a proportion of stride duration/time

Newton’s first law

Inertia - a body will keep doing what it’s doing unless another force acts upon it

Newtons 2nd law

Acceleration - if a force is applied to a body it will keep moving in the direction of the applied force

Newtons 3rd law

Action-reaction - for every action there is an equal and opposite reaction

Ground reaction force

Force exerted by the ground on a body in contact with it

What are the 3 methods of kinematic analysis

Optical Motion capture

Videography

IMUs

Optical Motion Capture

Pros:

Highly accurate and precise

3D analysis

Multiple strides can be recorded

Cons:

Expensive

Not easy to use in field conditions

Videography

Pros:

easy to set up

Inexpensive

Cons:

2D analysis only

Distortions & perspective errors

IMUs

Pros:

Inexpensive & portable

Easily used in field conditions

More accurate for measuring acceleration

Cons:

Integration error - less accurate displacement

Sensors need to be attached to subject

Battery/recording capacity

Efficient locomotion is reliant on?

1. Moving the COM with minimal energy expenditure

2. Minimising the amount the COM is displaced from the line of progression

   overall it is the minimising loss to other energy forms

Spring-mass model

Used for running gaits

Consists of compressing (braking phase) and recoiling (propulsive phase)

Metabolic energy is saved by replacing concentric contractions with passive biological springs (tendons) controlled by isometric contractions which are less ‘expensive’

How does vertical displacement vary between trot and canter/gallop

In trot the upper body is raised and lowered twice, in canter/gallop it is raised once

The faster the speed, the lower the vertical movement

Resting BMR

Energy expenditure at rest

Net cost of locomotion (NCOT)

Energy input required to move the animal by 1m

Total cost of locomotion

Total amount of energy to move 1m

How does speed increase energy cost

More speed → more muscle force → more muscle contraction → higher rate of metabolic energy supply

Energy cost vs body size

Larger animals = more energy expenditure (increased net cost of locomotion)

But larger animals use relatively less energy per kg of body mass to travel a given distance (decreased specific cosy of locomotion)

Higher NCOT, lower specific cosy of locomotion

Advantages of longer straighter limbs

Lower stride frequencies + longer limbs = greater distance covered with each stride

Fewer steps = fewer muscle contractions

Factors influencing cost of locomotion

Inclines

Uneven/slippery surface

External temperature

How is stability increased

Lower COM

Increased area of contact

Larger area enclosed by contact points

Bones

High stiffness and strength

Can withstand high compressive loads

Weak when subjected to shear or tensile stress

Strategies to reduce stress on bones

Thicker bone (P=F/A) (areas = thickness)

Greater thickness = lower pressure

Animals move slower (lower forces)

Straighter limbs - higher EMA, lower muscle force w required, decreased musculoskeletal stress

Effective mechanical advantage (EMA)

Measure of a given joints leverage against the GRF (relative suitability of the joint to resist gravity)

EMA = r/R

Advantages of an upright posture

GRF vector is more closely aligned with the long axes of the bones - reducing the bending moments in the bones

Advantages of crouched posture

More propulsion

How does animal size affect EMA

Small animals have crouched posture - lower EMA

Large animals have upright posture - higher EMA

How do tendons benefit locomotion

Crimped - allow lengthening without tensile forces

Stretch to store and release strain energy reducing energy expenditure

Beyond 8-10% strain - macroscopic failure

SDFT - most commonly injured structure

Disadvantages of efficient running locomotion (horses)

7% of energy stored in tendons released as heat causing core lesions in SDFT

Reduced bone mass increase risk of fracture in distal limb

How do ligaments support locomotion

Shorter and wider

More varied load thus collagen arrangement more variable

Scaling - square-cube law

Geometry - two systems are geometrically similar if any measurement from one is equal to an equivalent from the other multiplied by a constant.

An organism which doubles in length would result in 4x increase in SA

Volume - if we double the length, the volume and mass will increase by 8x

Allometry

How the size of a particular feature of an organism changes in relation to body size

Negative allometry - trait scales less than allometry

Positive allometry - trait scales more than allometry

Froude number: at what values does gait change?

At the same freude number, geometrically similar animals move in a dynamically similar fashion.

They use the same gait at the same froude number and change gaits at roughly the same froude number.

Walk to trot - 0.3-0.5

Trot to gallop - 2-3

Jumping

On landing, limbs experience forces equivalent to up to 4.5BW