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 -Muscle length FT ST -Muscle shortening and lengthening velocity -Time (i.e. it takes time for force to increase or decrease) Twitch Tension 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 When resistance (force) is negligible, muscle contracts with maximal velocity. As the load increases, concentric contraction velocity slows to zero at isometric maximum Force isometric maximum Velocity (Low resistance, high contraction 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
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 Ligament Force
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. F s F c F w
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. Loads on the Spine