Download Internal forces on the body

Physics 160 Biomechanics
Internal Forces on Humans
Forces from Inside the Body
• Joint Contact Force: compressive force resulting
from bone-on-bone contact
• Musculotendon Force: active and passive forces
generated by a muscle-tendon unit
• Ligament Force: passive force produced by
stretching of a ligament
• Intervertebral Force: force acting on the disk
between spinal vertebrae
• Resultant Joint Force: net force acting across a joint
(due to all sources)
Joint Contact Force
• Results from contact of
two articular surfaces (i.e.
bone-on-bone)
• Joint contact forces
always compressive
(directed into the bone)
• Cartilage causes friction
to be very small thus joint
contact forces are normal
to the articular surface
Muscle Attachments
Musculotendon Force
• Acts through the muscle’s
tendons onto the bone at
the origin and insertion
• Produces tensile forces on
bone in the direction given
by the tendon’s angle of
insertion into the bone
• Forces produced at the
origin and insertion are
equal
origin
insertion
Muscle Force Properties
• Generates passive force when stretched
• Generates active force which depends on:
-Neural stimulation level
FT
-Muscle length
ST
-Time (i.e. it takes time
for force to increase or
decrease)
Twitch Tension
-Muscle shortening and
lengthening velocity
Time
Tension vs. Length for Muscle
Tension present in a
stretched muscle is the
sum of the active tension
provided by the muscle
fibers and the passive
(SEC and PEC) tension
provided by the tendons
and membranes.
Force vs.Velocity for Muscle
isometric maximum
Force
When resistance
(force) is negligible,
muscle contracts
with maximal
velocity. As the
load increases,
concentric
contraction velocity
slows to zero at
isometric maximum
(Low resistance,
high contraction
velocity)
Velocity
Skeletal Muscle Function
active insufficiency: failure to produce force when
muscles are slack (decreased ability to form a
fist with the wrist in flexion)
Skeletal Muscle Function
passive insufficiency: restriction of joint range of
motion when muscles are fully stretched
(decreased ROM for wrist extension with the
fingers extended)
Tendon Force Properties
• When stretched, tendon
produces a tensile force
between the muscle and the
bone
• Tendon force on bone =
Muscle force on tendon
• Can function to store energy
during eccentric contractions
• Capable of handling high loads
Ligament Force
• When stretched,
ligaments produce a
tensile force that acts
onto the bone at the
origin and insertion
• Direction of force given
by the ligament’s angle
of insertion into the
bone
• Forces at the origin and
insertion are equal
• Ligaments get stiffer as
they’re stretched
Resultant Joint Force
• In most cases, contact and
muscle forces acting at a joint
cannot be determined
individually
• Resultant joint force = net force
produced by joint contact and by
all the structures that act across
a joint (muscle, ligament, tendon,
etc.)
• Acts at the joint center and is the
vector sum of all forces acting
across the joint.
Intervertebral Force
• Disk has complex
structure and behavior
• Nucleus expands
against annulus when
compressed
• Annulus fibers provide
restraining force as
stretched
• Nucleus experiences
large compressive stress
• Fibers of annulus
experience very large
tensile stress
Loads on the Spine
In normal standing position, body
weight acts anterior to the spine,
creating a forward bending load
(moment) on the spine.
Loads on the Spine
Because the spine is
curved, body weight,
acting vertically, has
components of both
compression (Fc) and
shear (Fs) at most
motion segments.
Fs
Fc
Fw
Loads on the Spine
When the trunk
flexes, extends, or
laterally flexes,
compressive force
develops to the
side of the bend
and tension force
develops on the
opposite side.
Loads on the Spine
When the trunk
rotates, half of the
fibers of the annulus
fibrosus become taut,
and the rest relax.
This creates tension
force in the fibers
running in the
direction of the
rotation and shear
force across the plane
of rotation.