Principles Of Biomechanics

Question 1

What does the term biomechanics mean?

Answer 1

Biomechanics is the science that examines forces acting upon and within a biological structure and effects produced by such forces - Hay, 1973

Question 2

What does the term ‘Podiatric Biomechanics’ mean?

Answer 2

Podiatric biomechanics is the study of forces acting on the human body its structure and function with particular reference to the lower limb, the foot and related pathology….

…and the utilisation of this information in various treatment regimes to change these forces for more efficient function.

Question 3

What is Newton’s 1st law of motion?

Answer 3

A body will continue in a state of rest or uniform motion in a single direction unless compelled to change its state by the action of an external force

Example - a ball rolling down a hill will continue to roll unless friction (i.e from the ground surface/ grass) or another force stops it

Question 4

What is Newton’s 2nd law of motion?

Answer 4

The rate of change of the momentum of an object is directly proportional to the force acting on it, and takes place in the direction in which the force acts.

Example - When a ball is kicked, a certain amount of force is exerted upon it in a specific direction so it will travel in said direction. The stronger the ball is kicked, the farther it will travel in that direction as the acceleration with which the ball moves is directly proportional to the force applied to it

Question 5

What is Newton’s 3rd law of motion?

Answer 5

To every action (force) in nature there is an equal and opposite reaction

Example - As a result of gravity, the weight of the body acts vertically downward on the ground, an equal and opposite force must act upward from the ground on the foot - known as ground reaction force (GRF)

Question 6

What is ‘Root Theory’ ?

Answer 6

A model/ theory for optimising the foot’s bio-mechanical function developed by Dr Merton Root in the late 1950s (AKA STJ neutral theory)

The author believed that in order for the foot to function ‘normally’, the STJ should be in a neutral position (i.e not inverted/supinated or everted/pronated) with the midtarsal joint fully locked during mid-stance to heel off.

This theory is based on a series of static measurements that was believed by Root to predict kinematic function - any deviation from the stated STJ alignment is considered abnormal and should therefore exhibit mechanical dysfunction

Root believed that the degree of abnormal alignment or movement would correlate with the degree of compensation

Question 7

What are the pitfalls/ limitations of Root Theory?

Answer 7

We now understand that variance in joint positioning and the location of the STJ axis is normal and does not necessarily lead to pathology. Root’s theory was based on the idea that these variations would directly influence gait kinematics.

We also now understand that the STJ is only held in a neutral position for a fraction of the gait cycle during heel contact (terminal stance/propulsion), so it is proposed that creating a orthoses to encourage STJ neutral is not ideal as the STJ needs to be able to move

There is also some debate over the reliability, repeatability and accuracy of the tests Root proposed to find STJ neutral

And given that the heel has a significant amount of fat and other soft tissues beneath it, how much control can an extrinsic wedge/prescribed orthotic actually provide

Question 8

What is ‘Tissue Stress Theory’

Answer 8

A biomechanical theory first proposed by Kirby in 1992, who stated that foot deformity measurement does not give us nearly enough information to predict the mechanical behaviour of the foot during weight-bearing activities and therefore is insufficient to prescribe the ideal orthoses for patients.

Tissue stress theory relies on the fact that mechanically-based pathologies of the foot and lower extremity all result from pathological magnitudes of stress acting within the structural components of the foot and lower extremity.

This theory focuses on identifying the structures that are painful, determining the structural or functional variables that may be the source of pathological forces acting on the injured structure(s), and formulate a treatment plan including specific orthoses modifications, footwear modifications, bracing, stretching, strengthening, injection therapy, surgery etc

The three goals of prescription foot orthosis therapy using tissue stress theory are: reducing the pathological loading forces on the injured structural components of the foot and lower extremity; optimizing overall gait function; and preventing other pathologies from occurring

Question 9

What is Saggital Plane Facilitation Theory?

Answer 9

The amount of saggital plane motion required to shift the body’s centre of mass anteroposteriorly during each step, is 500% that of transverse or frontal plane motion. In order for the motion to be transmitted smoothly and efficiently through the saggital plane, it relies on three key processes in stance phase of gait:
The Heel Rocker - When the calcaneum strikes the ground, the foot slowly plantarflexes to lower the forefoot
The Ankle Rocker - Where the tibia moves over the foot (now flat on the ground) prior to heel off
The Forefoot Rocker - Where the MTPJs dorsiflex as the foot propulses, activating the windlass mechanism providing stability to the midfoot

Based on this, saggital plane movement can be disrupted by restrictions in movement at these pivotal sites (Ankle equinus and 1st MTPJ ROM restriction)

Saggital plane facilitation theory was first published by Dananberg in 1986, who emphasised the importance of the foot as a pivot that rocks forward from the heel to toe allowing adequate hip extension, which he proposed was necessary for a normal stride and therefore an efficient gait. While this theory can help determine the etiology of certain foot pains and deformities, Dananberg highlighted the effect that restrictions at the various foot rockers had on more proximal posture related issues such as lower back pain. This theory focuses on the use of orthoses to facilitate saggital plane motion at these sites, which is assessed via video gait analysis and in-shoe pressure system assessment.

Question 10

What is the SALRE Theory?

Answer 10

Subtalar Axis Location & Rotational Equilibrium (SALRE) theory is another biomechanical theory developed by Kevin Kirby (2001) that suggested that the location of the STJ axis influenced the biomechanics of the foot and that this should be measured to help identify any medial or lateral deviation of the STJ axis.

An object is in rotational equilibrium if the sum of all the external torques acting on it is equal to zero. The STJ will continue to pronate/supinate until the pronation and supination moments acting on the joint are equal to each other.

Question 11

What is tissue stress?

Answer 11

Tissue stress is a measure of the internal resistance developed by a tissue within the body when subjected to an external loading force and one measures this by dividing the cross-sectional area of that object by the loading force being applied

Stress = The Magnitude of Force/ The Cross Sectional Area of The Tissue

The standard unit of measurement of stress is the pascal (Pa), defined as 1 Newton (N) distributed over 1.0 m2 (1 Pa = 1 N/m2). In scientific studies of tissue mechanics, the megapascal (1 MPa = 1 N/mm2) is the most common unit to measure stress

Question 12

What is tissue load?

Answer 12

Tissue load is essentially the amount of force exerted on a tissue.

Question 13

How is can tissue load be classified?

Answer 13

External forces/loads can be classified based on the way they deform the structures on which they act:

1. Axial Loading - Forces that are applied along the longitudinal axis of an object or tissue E.G
   Compression loading - Where the tissue is shortened
   Tension Loading - Where the tissue is elongated
2. Tangential/ Shear Loading - A tangential force is a force that acts parallel to the cross sectional area of an object, when this force is applied the object deforms along the plane of the force. Shearing forces are a pair of forces acting on an object with the same magnitude but in the opposite direction, and are a form of tangential force

Question 14

How is a lever defined in the context of biomechanics?

Answer 14

A lever is a rigid bar that moves on a fixed point known as a fulcrum/pivot when force is applied to it. Their main function is to generate a larger force from the small force applied to it, creating what is called a ‘mechanical advantage’ and facilitating movement.

There are four aspects to a lever…
The Lever Arm - A rigid structure
The Fulcrum - A fixed point
The Resistance/Load - A weight/ load on the rigid structure
The Force/Effort - A force acting on the rigid structure to produce a turning movement

The product of the force, and the horizontal distance between the point of application of the force and the fulcrum is known as the ‘moment’ of force

Question 15

What acts as the 4 components of a lever within the human body?

Answer 15

The Lever Arm - A bone

The Fulcrum - A joint

The Resistance/Load - Body weight, shoe weight, gravity

The Force/Effort - The muscle(s) acting on a joint

Question 16

Define a first class lever and give an example pertaining to the human body

Answer 16

First Class Lever = The fulcrum is located at the centre, with the effort and load on either end.

Example = The Occipital-Atlanto joint that connects the skull to the neck

The neck muscles provide the effort, the neck is the fulcrum and the weight of the head is the load

Question 17

Define a second class lever and give an example pertaining to the human body

Answer 17

Second Class Lever = The load is in the middle of the effort and the fulcrum

Example = The Ankle Joint - The MTPJs act as the fulcrum, the weight of the body acts as the load, and the contraction of gastrocnemius acts as the effort to plantarflex the joint

This type of lever always has a high mechanical advantage as the effort arm (the distance between the fulcrum and the effort) is always longer than the load arm (the distance between the fulcrum and the load), the farther away the effort from the fulcrum, the less effort is required to move it.

Question 18

Define a third class lever and give an example pertaining to the human body

Answer 18

Third Class Lever = The effort is in the middle of the fulcrum and the load

Example = The Elbow Joint (During a weighted bicep curl) - the joint itself acts as the fulcrum, the weight of the forearm and the weight of a dumbbell held in the hand, and the contraction of the biceps acts as the effort, flexing the elbow and pulling the weight upwards and towards the body

Question 19

Which lever(s) typically have a mechanical advantage and which typically lever(s) do not?

Answer 19

First Class Levers = Levers of balance, so they may work at an advantage or disadvantage depending on how close the effort is to the fulcrum in comparison to the load

Second Class Levers = Levers of strength, always work with a mechanical advantage

Third Class Levers = Levers of speed and range of movement, always work at a mechanical disadvantage as the effort is closer to the fulcrum than the load arm

Question 20

What is a moment of force?

Answer 20

A moment (also known as torque) is the action of a force causing a body to rotate/turn about a point (the fulcrum or turning centre)

The moment of a force depends on the magnitude of the force and the perpendicular distance of the force application from the axis of rotation

Moment (Nm) = Force (N) x Distance (m)

The same magnitude of force can be applied to two identical levers of the same length, but will produce different moments of force depending on where the force is applied on the lever. The further away the force is applied, the greater the moment of force (thus the greater the turning effect)

Question 21

What extrinsic factors can impact tissue load?

Answer 21

Activity - Increased activity levels increases tissue loading

Velocity & Speed - Increased speed exerts increased load on the tissues

Distance/Intensity - Running/walking longer distances and/or increasing training intensity increases the amount of load

Weight - Increased body weight or training weights (such as barbells, dumbbells, wearable weights etc) increases tissue loading

Surfaces - Training or running on hard, solid surfaces such as concrete or road increases tissue load.

Repetitions - Increasing training repetitions increases the level of tissue loading

Footwear - Changes in footwear (i.e switching to a shoe with less shock absorption or wearing a pair of new shoes for intense training that are new or different to your usual footwear) can increase tissue load

Question 22

What is tissue capacity?

Answer 22

A tissue’s ability to take load (AKA tissue tolerance)

Tissue capacity varies between individuals, the more a tissue is trained to tolerate load, the greater it’s capacity

A tissue at full capacity is able to perform functional movements at the volume and frequency required without exacerbating symptoms or causing tissue injury

As functional movements require full capacity in a number of musculoskeletal tissues, injury occurs when the capacity of the weakest link in the lower limb is exceeded

Question 23

What factors can influence tissue capacity?

Answer 23

Previous or Current Tissue Injury - Previously/ currently injured tissues will have decreased capacity in comparison to normal tissue

Age - Collagen production and remodelling decreases with age, thus decreasing tissue capacity and it’s ability to heal from injury

Illness - Illness often causing an interruption in training for rest which reduces tissue capacity

Lack of Sleep - Linked to increased injury risk (especially in adolescents)

Increased Stress - Evidence suggests that increased stress may slow the healing process by as much as 40-60%

Poor Nutrition

Reduced Water Intake

Hormonal Changes - Decreased oestrogen production in menopause causes decreased collagen production and increased levels of inflammation

Question 24

How can tissue load & capacity be measured?

Answer 24

Wearable Devices - Such as step counters, to track distance, heart rate, step count, speed, number of reps e.t.c

Laboratory Testing (following exercise) - Such as blood lactate levels, creatinine kinase etc

Heart Rate Variability - A measure of the variation in time between each heartbeat, suggested as a practical, non invasive method for assessing an individuals response to internal load. This can be done using electrocardiography devices, wearable photoplethysmography devices etc (although there is debate over the accuracy of these devices)

Rate of Perceived Exertion - A NRS from 1-10 of an individual’s perceived level of exertion

Acute:Chronic Workload Ratio/ ACWR

Question 25

What is ACWR and how is it calculated?

Answer 25

The ratio between an individual’s acute (total workload over the past 7 days) and chronic workload (average weekly workload over the previous 28 days)

It is used to help measure an individual’s risk of a sports related injury and whether they are progressing their tissue load and capacity at an adequate pace

It is calculated by dividing the acute workload by the chronic workload, you can use various metrics for workload such as distance, number of repetitions, speed, weight, rate of perceived exertion e.t.c

Example:
An athlete ran 30km in the past 7 days, with an average of 28.5km per week in the past 28 days

Athletes ACWR = 30/28.5
= 1.05
Meaning their workload in the past week was equivalent to 105% of their average weekly workload over the previous 28 days, so they had a total workload increase of 5%

Question 26

How are ACWR values interpreted?

Answer 26

<0.80 = Undertraining (not training enough to maintain load capacity) and higher relative injury risk

0.80 - 1.30 = Optimal workload and lowest relative injury risk

> 1.50 = The ‘Danger Zone’, indicating that tissue load is high and is associated the highest relative injury risk (risk of injury is not the same as injury rate, so this does not indicate that you are going to get more injuries than the average person but are at a higher than average risk of one)

Question 27

How are ACWR values interpreted?

Answer 27

<0.80 = Undertraining (not training enough to maintain load capacity) and higher relative injury risk

0.80 - 1.30 = Optimal workload and lowest relative injury risk

> 1.50 = The ‘Danger Zone’, indicating that tissue load is high and is associated the highest relative injury risk (risk of injury is not the same as injury rate, so this does not indicate that you are going to get more injuries than the average person but are at a higher than average risk of one)