Biomechanics Week 2

Static Posture

The body and its segments are aligned and maintained in certain positions

Static posture examples

standing, lying, kneeling

Dynamic posture

The body or its segments are moving

Dynamic posture examples

walking, running, jumping, throwing, lifting

CoM is also known as

center of gravity

CoM

Hypothetical point at which all mass of an object is concentrated

The center of mass is essentially

a balancing point

Adult CoM

Anterior to S2

The Center of Mass lies closer to

heavier region

The CoM may not

be located in the body

With multiple segments, COM of combined unit will change if

segments are rearranged

Seated CoM for the entire trunk

Located just below the axilla

CoM stays the same in a

rigid segment

CoM changes in a

non-rigid body/segment

Line of Gravity

imaginary vertical line that passes through the center of gravity

Gravitational vector

Line of Gravity

The line of gravity is always

vertically pointing downwards

Plumb line

A string with a weight used to provide a vertical reference line

CoM doesn't change location in a rigid body however, what does?

LoG

BoS

Base of support

Standing on 2 feet

bipedal

Bipedal stance grants

use of UE

Quadruped vs. Biped

quadruped position is more stable d/t wider BoS

Line of gravity must lie within

BoS

If the LoG leaves the BoS

The individual may fall

Large BoS

LoG stays inside and the person is more stable

Small BoS

LoG moves outside = less stable

Higher CoM

LoG moves outside of the BoS and is less stable

Lower CoM

LoG stays inside BoS and is more stable

Internally adding mass

weight gain or pregnancy

Externally adding mass

added force by a bag or external object on the body

With additional weight, New CoM will shift toward

addt'l weight

If a mass is subtracted the CoM will

Deviate towards the heavier side

The shift of new CoM will be _____ to the additional weight

Proportional

If a cuff weight is added to the wrist, what happens to the extremity's Center of Mass

the CoM deviates and moves towards the wrist

How does the shifted Center of mass affect the muscles that lift the arm?

they have to work harder to lifted the added weight

What would happen to the body's overall Center of Mass if a person had a below knee amputation on the right leg?

The center of mas would shift to the left side and move up because it's heavier to the left and proximally

Postural control

A person's ability to maintain stability of the body and it's segments in response to the forces that disturb its equilibrium

Three balance systems that contribute to overall postural control

1. Vestibular

2. Visual

3. Somatosensory: Proprioceptors in joints, ligaments, tendons, and muscles

In children less than 2, which postural control systems are of higher importance

Somatosensory proprioceptors and vestibular system

30% of stability is lost with

eye closure

Those over 60 yo, ___ of stability is lost with eye closure

50%

What is a major factor in balance in the elderly population

Vision

CNS selects responses based on

past experiences and the goal of the response

Reactive responses

Reactions to external forces that displace the body’s CoM

Compensatory response

a response that offsets the effects of the upcoming unconditioned stimulus

Reactive response aka

compensatory response

Proactive response

A response that occurs in anticipation of internally generated forces

Proactive Response aka

anticipatory response

Anticipatory response

Occur in anticipation of internal forces

Goals of control

- Keep body's spatial orientation

- Keep the CoM over the BoS

- Stabilize head vertically for gaze

Basic elements of control

Absent or altered inputs

Absent or altered inputs

can't relay sensory info to brain

Absent or altered outputs

Muscles can't respond appropriately, atrophy, disease

Anti-gravity muscles definition

muscles that resist gravity, primarily posterior muscles, used in extension and hyperextension and plantar flexion (hip and knee extensors and trunk and neck extensors)

Anti-gravity muscles

• Hip extensors

• Knee extensors

• Trunk extensors

• Neck and back extensors and flexors

• Hip-adductors and abductors

• DFs and PFs

Perturbations

Any sudden change in conditions that displaces the body posture away from equilibrium

Sensory perturbation

caused by altering visual input

Mechanical perturbation

changes in the relationship of the body’s CoM to the BoS by an outside force

Perturbations of breathing while standing are counterbalanced by

movements in the trunk and LE

Postural responses to perturbations caused by external forces are

reactive/compensatory responses

Synergies/Strategies

Patterns of muscle activity organized by the CNS in response to perturbations in the standing position

Synergies are

task specific and vary according to many factors

Fixed support strategies

Occur when BoS remains fixed during perturbation and recovery of the equilibrium

Fixed-support-strategies by position

ankle and hip

Changes-in-support strategies

Shifts in BoS or entire body

• Stepping, Grasping

• Head stabilizing strategies

Stepping and grasping differ from fixed support strategies b/c

they either move or enlarge the BoS so that it remains under the body’s CoM

Head-stabilizing strategies

Stabilize head with trunk

Stabilize head in space

Stabilizing Head with trunk strategy

the head moves as a unit with the trunk

Stabilizing Head in space strategy

is an anticipatory response to CoM displacement in the body. Keeps the head vertical even if the trunk is doing another task

Ankle strategies occurs with

small perturbations, ankle synergy displaces center of gravity with movement at the ankle joint

Ankle strategies occur in what fashion

distal-to-proximal

Ankle Strategy: Anterior displacement of CoM and LoG

Posterior muscles must counterbalance:

• Gastrocnemius

• Hamstrings

• Paraspinal

Ankle Strategy: Posterior displacement of CoM and LoG

Anterior muscles must counterbalance:

• Tibial Anterior

• Quads

• Abdominals

Hip strategy occurs with

larger perturbations, but not large enough for stepping strategy

Surface is unstable or shorter than feet

Head movement out-of-phase with hips

Hip strategy occurs in what fashion

proximal-to-distal

Hip Strategy: Posterior displacement of CoM & LoG

Anterior muscles must counterbalance:

• Abdominals

• Quads

• Tibialis Anterior

Hip Strategy: Anterior displacement of CoM & LoG

Posterior muscles must counterbalance:

• Paraspinals

• Hamstrings

• Gastrocnemius

Stepping strategy occurs with

largest, fastest perturbation; more frequent with aging

Stepping strategies: Young people may take

1 large step

Stepping strategies: Older people may

take many smaller steps

Older individual’s Center of pressure is

closer to the edge of BoS

Reactive vs. Proactive

Reactive - Acting in response

Proactive - Acting in anticipation

Why is a person likely to re-sprain an ankle after the initial sprain injury?

after the sprain, a person is less able to detect passive ROM and is more unstable on the injured ankle. It will become a chronic instability if not trained properly, thus becoming more likely to be sprained again

Size of perturbation for ankle strategy

small

Size of perturbation for hip strategy

larger than ankle but smaller than stepping

Size of perturbation for stepping strategy

largest

Sequence of strategies

Ankle → Hip → Stepping

External forces acting on the body

inertia, gravity, GRF

Active internal forces acting on the body

generated by muscle

Passive internal forces acting on the body

Ligaments, capsules, soft tissue

For the body to be in equilibrium

internal and external forces must equal 0

Optimal standing posture has little to no

acceleration except for the postural sway the body undergoes

Postural sway

Swaying back and forth when standing upright

Sway envelope

Postural sway; path of the body's movement during standing

Normal sway in stance with the feet apart 4 inches

•12° sagittal plane

•16° frontal plane

Is postural sway accounted for when examining forces acting upon the body?

no because it is a natural occurence

Limit of stability

The distance outside of the base of support that he or she can move into without losing control of his or her center of gravity

Edge of the sway envelope

Limit of Stability

Ground reaction force

An equal and opposite external force that is exerted back onto the body by the ground.