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:

Twitc

h Te

nsio

n

Time

FT ST

-Neural stimulation level

-Muscle length

-Muscle shortening and lengthening velocity

-Time (i.e. it takes time for force to increase or decrease)

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

When resistance (force) is negligible, muscle contracts with maximal velocity. As the load increases, concentric contraction velocity slows to zero at isometric maximum Velocity

Forc

e (Low resistance, high contraction velocity)

isometric maximum

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.

Fc Fs

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.