Biomechanics 3: Bone, Joint, and Muscle Biomechanics

bone function

stability, mobility, protection

bone stability

provide a frame to support the body

bone mobility

provide attachment sites for muscles, allowing movement of limbs

move the bones

goal of movement

cortical (hard) bone

dense, forms hard outer layer of a bone segment

• higher density

trabecular (spongy) bone

low density, fills the interior of a bone segment

apply excessive amount of forces to the bone

how can you fracture a bone?

fracture

different force patterns can lead to different types of

compression force/fracture

two forces move towards each other in the same line of action

tension force

two forces move away from each other in same line of action

• ex: transverse patella fracture

bending force

compression force applies to one side of the object while tension force applies to the other side of the object

Colles’ fracture

fall onto an outstretched hand (FOOSH)

• dinner fork deformity if untreated

shear force

two forces move towards/away from each other in different lines of action

shear force

tibia fracture

• direct blow to the tibia from the side while the foot is fixed on the ground

torsion

two torques rotate the object in different directions

• ex: tibia fracture; turning the tibia forcefully while the foot is fixed on the ground

weight bearing exercise

how do we strengthen bones?

Wolff’s law

if loading on a particular bone increases, the bone will remodel itself over time to become stronger to resist that sort of loading

stress, strain

2 parameters to determine stiffness

stress

the force that acts on the object

strain

change of shape of the object

stiff

the object is ______ if large amount of force can only cause small amount of shape change

stiffness

slope of the stress/strain curve

effect of immobilization

synarthrosis

joints that can barely move

ex: skull

amphiarthroses

joints that can move slightly

• usually connected by fibrocartilage (intervertebral joint)

diarthroses/synovial

joints that can move freely with certain amounts of “degrees of freedom”

degrees of freedom

number of axis that the joint can rotate around

synovial

_________ joints have at least 6 different configurations of degrees of freedom

plane joint

articular surfaces are flat

• ex: intercarpal joints

allow only gliding movements

hinge joint

a joint that is modeled like a door hinge

ex: humeroulnar joint

allow 1 degree of freedom

pivot joint

a ring-shaped bone rotates on the longitudinal axis of the other bone

• ex: atlantoaxial joint, radioulnar joint

allow 1 degree of freedom

saddle joint

joint surface has both concave and convex configurations

• ex: CMC joint

allows 2 degrees of freedom

• the joint configuration allows flexion/extension, abduction/adduction, but limits transverse rotation

condyloid joint

an oval head sits on an elliptical cavity

• ex: radiocarpal joint

allow 2 degrees of freedom

• the joint configuration allows flexion/extension, abduction/adduction, limits transverse rotation

ball and socket joint

ball head sits on a round socket

• ex: hip and shoulder

allow 3 degrees of freedom

socket could be “shallow” or “deep”

joint configuration, soft tissues, joint position

factors contributing to joint stability

joint configuration

allows less degrees of freedom, tend to be more stable

soft tissues

ligaments, capsules and muscles provide joint stability

close-packed position

the joint space is minimized, most ligaments and capsules surrounding joint are taut

ex: position for knee joint: full extension and external rotation

loose-packed position

the joint space is maximized, most ligaments and capsules surrounding the joint are loose

ex: position for knee joint: 25 degrees of flexion

muscle

muscles play a major role in moving the joints

• muscle belly ← muscle fibers ← sarcomeres

• the basic unit of a muscle is sarcomere

type I muscle fiber

slow twitch oxidative fibers (uses oxygen to generate energy)

high endurance, low power

type IIa muscle fiber

fast twitch oxidative fibers

intermediate power and endurance

type IIb muscle fibers

fast twitch glycolytic fibers

high power, low endurance

type I

posture muscles, such as the erector spinae are more

type II

power muscles, such as the quads, hamstrings, and biceps are more

adaptable

muscle fibers are _________ and can change their functional and metabolic properties in response to training

type I

endurance athletes tend to have a greater proportion of ______ __ fibers

type II

power athletes tend to have a greater proportion of _____ __ fibers

effects of aging on muscle fibers

changes in muscle fiber proportion

all muscle fibers become smaller

atrophy is greater in type II muscle fibers than type I muscle fibers

motor unit

muscle fibers are part of a

alpha motor neuron → axon → muscle fibers innervated

parts of the motor unit

CNS

controls muscle activities through controlling motor units

size of motor unit

defined by the number of muscle fibers it controls

type I

small size of motor unit controls

type IIa

medium size of motor unit controls

type IIb

large size of motor unit controls

smallest, largest

motor units are recruited from ______ to _______

• helps us preserve energy

low, high

___ intensity, ____ repetition exercises could better improve endurance

high, low

____ intensity, ___ repetition exercises could better improve power

parallel muscle arrangement

parallel to the longitudinal axis of the muscle

ex: sartorius

this type of muscle is usually long and can travel a long distance, which results in great ROM

pennate muscle arrangement

the muscle fibers are at an angle to the longitudinal axis of the muscle

• angle called pennation angle

ex: deltoid

pennation

there can be more fibers in a muscle of a given volume, which creates more force

• force advantage

stress-strain curve

shows the typical relationship between stress and strain observed in soft tissues such as ligaments and muscles

• toe region: fibers are re-aligned but not fully straightened

• linear region: fibers are straightened

• in these regions, fibers can return to the original shape when the force is removed (elastic)

• final point of linear region: yield point

• fibers begin to have permanent, irreversible change in shape after being stretched over the yield point (plastic region)

• failure point: fibers are torn

straighten

in stretching, we first ________ the muscle fibers to enter the linear region

yield point

the goal is to bring the muscle beyond the ______ ______ so that the muscle length won’t go back to the origin

• stay away from failure point

active tension

when the muscle is doing active contraction

passive tension

when the muscle is being stretched, there is a positive relationship between the muscle length and tension

at resting length

in which position can we produce the largest amount of active tension?

passive, but only when muscle stretched a lot

can we generate greater active tension or greater passive tension?

two-joint muscle

muscle that crosses over two different joints

• ex: hamstrings

  • crosses both hip and knee

  • action: knee flexion and hip extension

active insufficiency

muscle becomes weak so it has reduced ability to drive the joint through its full range of motion

passive insufficiency

the muscle is too tight so its restricts joint range of motion in the opposite side

force-velocity curve

Y: force generated by the muscle

X: velocity of muscle shortening or lengthening