Levers in the Body:
They Are Not What You Might Think!
Activities 3A, B, C, D, E
Using a model of the human arm, students will be able to:
u Investigate lever systems in the human body
u Observe how muscles and bones work together to move joints
u Compare arm anatomy to model
u Explain that muscles must pull against the weight of various
body segments (resistance)
u Distinguish between joint flexion and extension
u Observe that muscles can work only by pulling (contracting)
u Graph and analyze results
Activity Description:
Activity Overview
Activity Objectives:
Activity Background:
As students discovered in the previous activity, “Just a Little Bit of Effort:
Exploring Levers”, there are three classes of levers; first, second and third
class. The classification of levers is based upon the relative position of
the Fulcrum (F), Effort (E) and Resistance (R). *(Resistance is often called
load (L)). Lever systems are important to human movement and are
found throughout the human body. The field of study that applies the
principles of mechanics and anatomy to human movement is called
kinesiology (ki`neesee'âlujee). Before moving on, a cautionary statement is necessary. There is a significant amount of disagreement in the
professional literature about the classification of lever systems in the
Positively Aging®/M.O.R.E.
2007©The University of Texas Health Science Center at San Antonio
Discrepant Design
Students will be surprised to learn that the levers in
their arm don’t behave as expected! They will use a model
of the human arm, including an artistically modified
scapula, humerus, radius and ulna. After measuring the
weight of the forearm on their model with a spring scale,
they will assemble the bones so that the elbow joint is
moveable. Once students have assembled the model, they
will attach string “biceps and triceps muscles” to the model
at origin points labeled on the model. An approximation of
the actual insertion point will be labeled on the model and
the other end of the string “muscle” attached to that location.
The student will then investigate how the biceps muscle
operates the joint as a lever system, exploring angles and the resistance
and effort forces. Next, students will move the insertion point of the
string muscle to examine how the lever system changes. Both the triceps
and biceps will be studied in this part of the activity. Finally, using body
segment weight percentages, students will relate their findings to their
own bodies.
LESSON 3
ACTIVITY 3A, B,
C, D, E
1
An example of a first class lever in the body includes the splenius muscle
acting to balance head on atlanto-occipital joint. This lever allows us
to tilt our head back. Figure 1, First Class Lever in the Body. It is
important to note that there are few first class levers in the human body.
RESISTANCE
(Load)Front of skull
FULCRUM
cranium
Atlanto-occipital
joint
face
EFFORT
Muscles at back
of head
EFFORT
RESISTANCE
t
(Load)
FULCRUM
Figure 1 First Class Lever in Body
Very few, if any, second class levers are found in the body - rising on the
toe is identified and often disputed as a second class lever. The human
body is not designed to apply a large force in a lever system, as occurs
in a second class lever. Please note that many materials designed for
students make definitive statements that rising on the toe is a second class
lever – all experts do not agree on this point!!
Positively Aging®/M.O.R.E.
2007©The University of Texas Health Science Center at San Antonio
Discrepant Design
Just as the levers already studied, levers in the human body consist of a
rigid bar (bone), a fulcrum (joint), an effort force (pull of muscle on
bone) and a resistance (load) force (weight of the body part being
moved – this may include weight being held). An example is the elbow
joint, operated by the biceps and triceps muscles. The elbow is a complex joint held together by strong connective tissue which forms ligaments. Supporting muscles, ligaments and the shape of the bones themselves help to keep the bones of the joint from sliding sideways during
movement. The position of the connection between muscle and bone
can alter the amount of rotation and speed with which joint moves.
Speed is produced at a cost; reduction of power in the joint.
Activity Overview Continued
body. Human joints are complex, often incorporating many muscle
connections and ligaments that allow a joint to function in several ways
and creating complex sets of forces in the joint lever system. This complexity makes it difficult definitively classify joints as one type of lever or
another. All forces involved in moving a joint must be carefully defined
and even experts in the field cannot always agree. This disclaimer
should be carefully explained to students.
LESSON 3
ACTIVITY 3A, B,
C, D, E
2
RESISTANCE
FULCRUM
Elbow
Weight of
forearm
Figure 2 Third Class Lever in Body
Activity Materials: (per group)
• Pattern for arm model
• Stiff cardboard for arm model (note: more permanent models can
be made with wood*)
• 1 1 N (100g) hanging weight
• 1 1/4” X 3/4” bolt with wing nut (if using wooden models)
• 1” diameter plastic pulley
• Heavy brads (if using cardboard models)
• Rubber Cement or Craft Spray Adhesive (Liquid Nails® to attach
laminated pattern to wood))
• Ring Stand
• 10 N Spring scale
• Single hole punch
• String (2 colors)
• 6 #216 screw eyes used to attach string to
wooden models (reduces friction)
• Protractor
• Ruler
• Student Activity Page packet for each group
• Student Data Page packet for each student
* 1 2’ X 4’ piece of 1/4” thick wood makes about 18 models
Positively Aging®/M.O.R.E.
2007©The University of Texas Health Science Center at San Antonio
Discrepant Design
EFFORT
Contracting
Biceps
Activity Overview Continued
Third Class levers are the most common type of lever in the body –
almost all movable joints function as third class levers. Examples include
the Biceps muscle moving the forearm. Students will be surprised to
discover that third class levers actually operate at a mechanical disadvantage, in that more effort must be applied to overcome the resistance. Be sure to explain this carefully when you “process out” the activity. The advantage of this design comes from the speed at which joints
can rotate. The fact that most levers in the human body function in this
manner indicates that our bodies are designed for speed over strength.
LESSON 3
ACTIVITY 3A, B,
C, D, E
3
Activity Management:
Prior to doing the activity, compare parts of the model to parts of the
arm (Figure 1).
Some students may be sensitive about using their own weight for some
of the calculations in this activity. These students can do the calculations
using a body weight of 80 lbs.
Extension:
1. Investigate the knee joint by identifying the forces involved in knee
flexion. Determine what type of lever system is represented by knee
flexion. Examine knee extension to identify the forces and type of
lever system.
2. Observe how the triceps muscle serves as an opposable muscle to
operate the elbow joint.
3. Students can investigate the concept of torque as it applies to levers
in the body.
References Used:
Broer, MR. (1973) Efficiency of human movement (Third Edition).
Philadelphia: W.B. Saunders Company.
Delezene, Lucas, Fellows Lesson Website:
http://gk12.asu.edu/curriculum/life_science/levers_body/index.html
Gowitzke, BA & Milner, M. (1988). Scientific bases of human movement
(Third Edition). Baltimore: Williams & Wilkins.
Gray, H. (1918). Anatomy of the human body (20th Edition). Philadelphia:
Lea & Febiger.
Hamill, J. & Knutzen, KM. (2003). Biomechanical basis of human movement
(Second Edition). New York: Lippincott Williams & Wilkins.
Inspiration SoftwareΤΜ, Inc.
http://www.inspiration.com
Medline Plus Medical Dictionary
http://www.nlm.nih.gov/medlineplus/mplusdictionary.html
Plagenhoef S, Gaynor Evans F, and Abdelnour T. Anatomical data for
analyzing human motion. Res Quart Exerc Sport 54: 169-178, 1983
Zatsiorsky. V. (2002). Kinetics of human motion. Champaign, IL:
Human Kinetics.
Positively Aging®/M.O.R.E.
2007©The University of Texas Health Science Center at San Antonio
Discrepant Design
Ask the Industrial Technology class to cut the pattern pieces out of 1/4”
wood for more durable models. You can glue laminated pattern pieces
onto the wood for models you will be able to use year after year. A
2’ X 4’ sheet of wood makes about 18 models.
Activity Overview Continued
Be sure to explain to students that there is a significant amount of
disagreement among experts about the classification of lever systems in
the human body.
LESSON 3
ACTIVITY 3A, B,
C, D, E
4