Download Abraham Musculoskeletal Levers Lab

KIN 320 LAB MANUAL: SPRING 2010
LAB SECTION:_________ NAME:________________________
LAB 11: Musculoskeletal Levers
Introduction:
Levers are rigid objects which rotate around a fixed axis. This axis, which is
sometimes referred to as a fulcrum, may be visible, like the axle shaft for a bicycle pedal
assembly, or merely a theoretical line around which the lever rotates, as happens for a
swinging baseball bat. In biomechanics we are interested in the ways in which levers in the
body transfer forces applied to them by one source to act on other objects. Functionally,
levers can do the following:
1) change the direction of an applied force;
2) increase or decrease the magnitude of the effective applied force; and
3) increase or decrease the speed at which the applied force acts.
The latter two effects are actually yoked in such a way that increasing the effective applied
force is accompanied by a decrease in the speed of action and decreasing the effective applied
force is accompanied by an increase in the speed of action.
In the musculoskeletal system we most commonly consider bones to function as levers,
particularly the long bones of the extremities. These levers rotate around axes at the joints,
under the influence of forces, including muscular forces, intersegmental contact forces, and
external forces. In this lab we are particularly interested in the ways in which muscle forces
acting on a lever (bone, or body segment) are modified by that lever as they are subsequently
exerted on an external object.
An example of how levers can act to change the direction of a force in important ways is
when pressing down on a bicycle pedal with the ball of the foot, you need muscular force to
plantarflex the foot at the ankle. There is no way that any muscle can pull the plantar surface
of the foot downward (no place for such a muscle to attach), but the pull of the calf muscles
upward on the calcaneus results in a rotation of the foot at the ankle so that downward force
can be applied to the pedal. What other examples can you describe?
With respect to calculating the changes in magnitude and speed of the effective applied force,
we use the concept of mechanical advantage. Mechanical advantage is the ratio of the
moment arm of the force applied to the lever to the moment arm of the effective force applied
by the lever. The moment arm is the perpendicular distance from the line of force to the axis
of rotation. When the mechanical advantage is less than 1.0, the lever applies less force than
the muscle exerts on it, but the force is applied at a greater speed than the muscle shortening
velocity. When the mechanical advantage is greater than 1.0, the lever applies more force
than the muscle exerts on it, but the force is applied at a slower speed than the muscle
shortening velocity. In the human body the majority of musculoskeletal lever systems have a
mechanical advantage less than 1.0, often on the order of 0.1 to 0.3, which means the effective
applied force may be a small fraction of the muscle force but the speed of the effective applied
force is much greater than the muscle shortening velocity.
In the diagrams below, a single representative muscle has been drawn to act on the
lever. Diagram A shows the net force of the muscle, the force applied at the end of the
lever, and the moment arms of these two forces. Notice that the length of moment arm
of the muscle is measured perpendicular to the line of force and is not calculated from
the point of attachment of the muscle on the lever. The moment arm of the applied
effective force is determined by the direction of the force applied (or resisted).
Diagram B shows the muscle force divided into two perpendicular components, which
originate at the point where the muscle force acts on the bone. The rotatory
component is always perpendicular to the lever (bone), while the nonrotatory
component is directed along the lever, either toward or away from the axis of rotation.
This shows that the muscle force not only acts to cause rotation of the lever (through
its rotatory component) but also can act to either pull the lever toward the axis of
rotation or away from the axis, which would be a destabilizing or dislocating effect.
The torque generated by the muscle is the product of the force applied and the length of the
moment arm. In Diagram A this would be the product of the total muscle force and its
moment arm. Diagram B shows that the same value can be calculated by multiplying the
rotatory component by its moment arm (the distance along the lever from the insertion to the
axis of rotation).
In cases of isometric contractions, the system is in static equilibrium, so the muscle torque is
balanced by an equal and opposite torque from the resistance force or load encountered. This
resistance torque is also a product of a force (the reaction force to the effective applied force)
and a moment arm (the perpendicular distance from the line of action of that force to the axis
of rotation). In this situation, notice that the mechanical advantage is clearly not only the ratio
of the moment arm of the force applied to the lever to the moment arm of the effective force
applied by the lever, it is also the ratio of the effective applied force to the total muscle force.
Purpose of Today’s Lab: Calculate the maximum force exerted by the elbow flexors (using the
biceps brachii as a model) during isometric flexion of the forearm with the elbow at
two different angles.
Equipment: cable tensiometer, cable, chain or strap, and handle, protractor or goniometer,
ruler, nomograph, calculator
Procedure: Work in groups to collect data using the equipment. First measure each person
in your group to determine the mechanical advantage in each position (90 degrees
and 135 degrees). Then see how much force can be exerted in each position.
Finally, use these data to calculate the total muscle force of the elbow flexors
(assuming all of this force is acting along the line of the biceps brachii.
Name
Questions:
1.
Joint Angle
MMA
RMA
FA
FM
Did you expect the calculated muscle force in each position to be the same for
each person? Why?
2.
Was it the same? Why?
3.
Describe a sport situation in which it is advantageous to have a large
mechanical advantage.
4.
Describe a sport situation in which it is advantageous to have a small
mechanical advantage.
What to Turn In:
Any data tables, graphs, or spreadsheets, typed answers to all questions.