Biomechanics Week 2

Static Posture

The positioning of the musculoskeletal system while the body is motionless.

Static posture examples

standing, lying, kneeling

Dynamic posture

Positioning of the body during any movement

Dynamic posture examples

walking, running, jumping, throwing, lifting

COM aka

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

just below the axilla

CoM stays the same in a

rigid segment

1 multiple choice option

CoM changes in a

non-rigid body

2 multiple choice options

Line of Gravity

imaginary vertical line that passes through the center of gravity

The line of gravity is a

gravitational vector

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

New CoM will shift toward

addt'l weight

If a mass is subtracted the CoM will

Deviate towards the heavier side

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

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

visual, vestibular, somatosensory systems

In children less than 2, which 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%

CNS selects responses based on

past experiences and the goal of the response

Reactive responses

Reactions to external forces that displace the bodies CoM

compensatory response

a response that offsets the effects of the upcoming unconditioned stimulus

Reactive aka

compensatory

Proactive aka

anticipatory

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 flexrs

hip-adductors and abductors

-DF and PF

pertubation

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

sensory perturbation

caused by altering visual input

mechanical perturbation

changes in BOS by an outside force

Perturbations of breathing while standing are counterbalanced by

muscles in the trunk and LE

Postural responses to perturbations caused by external forces are

reactive

synergies

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 during perturbation and recovery of equilibrium

Fixed support strategies by position

ankle and hip

Changes in support strategies

Shifts in BoS or entire body

Stepping, Grasping

Head stabilizing strategies

Head-stabilizing strategies

Stabilize head with trunk

Stabilize head in space

Stabilize with trunk

vertical position of the head during movement

Stabilize head with trunk

Ankle strategies

small disruptions to balance, ankle synergy displaces center of gravity with movement at the ankle joint

Ankle strategies occur in what fashion

distal to proximal

Anterior displacement of CoM and Log

POSTERIOR MUSCLES MUST COUNTERBALANCE

Hip strategy

Used when perturbation is fast or large amplitude

Surface is unstable or shorter than feet

Muscles recruited proximal-to-distal

Head movement out-of-phase with hips

hip strategy guideline

proximal to distal

Stepping strategy

largest, fastest perturbation; more frequent with aging

Young people may take 1

large step

older people may

take many smaller steps

Older Center of pressure is

closer to the edge

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?

laxity as well as the joint deals with lots of mass

size of perturbation for ankle strategy

small

size of perturbation for hip strategy

larger

size of perturbation for stepping strategy

largest

order of muscle activation for ankle strategy

distal then proximal

order of muscle activation for hip strategy

proximal then distal

sequence of strategies

Ankle-->hip--;stepping

External forces

intertia, gravity, GRF

Active internal forces

generated by muscle

pasive internal forces

Ligaments, capsules, soft tissue

to be in equilibrium

internal and external forces must equal 0

Optimal standing posture has no

acceleration

postural sway

Swaying back and forth when standing upright

sway envelope

path of the body's movement during standing

Normal sway

•12° sagittal plane

•16° frontal plane

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

Ground reaction force

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

GRF represents

magnitude and direction of loading applied to the feet

Center of pressure

center of distribution of total force applied to supporting surface

Point of application of GRF

center of pressure (COP)

Located between feet in bilateral stance

CoP

Path of CoP is determined by

pressure applied to a foce plate

coincident action lines

LoG and GRF

LoG is equal in magnitude and in same direction as

force of the person on the ground

LoG =

GRF in equilibrium