Biomechanics of the Biceps and Triceps Provocacion: Using a 6 foot long 2x8 and a 2x2 fulcrum, can you lift another person off of the ground? A lever is a rigid rod which can rotate about a pivot or fulcrum. Levers don't make you stronger, but they allow you to trade force for distance, or distance for force. All levers have three parts: the fulcrum (or pivot) which is the place where a lever rocks back and forth the load or resistance the effort In a first class lever, the fulcrum in located in between the effort and the load. (See diagrams on next page.) If the distance from the fulcrum to the load is equal to the distance from the fulcrum to the effort, the amount of effort you push down equals exactly the amount of load you lift on the other end (like a seesaw). Moving the fulcrum closer toward the load (what you are trying to lift) will make the lifting require less effort, but the load won't be lifted as great a distance. We "trade" effort for distance - that is, lifting the load will require less effort, and the effort needs to be applied over a greater distance, but we won't lift the load as high. On the next page are diagrams of the three types of levers. Our bodies use many third class levers.

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effort load

fulcrum First Class Lev er

effort load

effort load fulcrum fulcrum Second Class Lev er

Third Class Lev er

Arm Model: Biomechanics of Biceps and Triceps Materials: per group or per student one 25 cm piece of black mat board (upper arm) one 35 cm piece of black mat board (lower arm and hand) one 20 cm piece of board to strengthen the upper arm piece (optional) white crayon or chalk clear packaging tape 2 brads (paper fasterners) 2 pieces of string, about 40 cm each masking tape push pin ball point pen 3 hole punch Lori Lambertson Exploratorium Teacher Institute Page 2 © 2005 Exploratorium, all rights reserved

Drawing the bones: 1. Draw the anterior view of the right humerus on the 25 cm board, then draw the anterior view of the bones of the right forearm - radius, ulna and hand on the 35 cm long foam board. Be sure to draw the olecranon process at the top of the ulna so that its top comes to the end of the board. The olecranon process makes up the prominence of the elbow. Notice that the top of the radius is below the olecranon process. Some anatomy: From Anatomy and Physiology, 15th Edition, by Kimber, Gray and Stackpoole, published in 1967, by the Macmillan Company, New York. Skeletal muscles usually pass over joints, some of which are moveable and some of which are not. The origin of a muscle is the end that attaches to the relatively non moving bone, and the insertion is the end that attaches to the moving bone. Biceps brachii (biceps) has two originations – one for each head. The short head arises from the coracoid process, on the scapula, and the long head originates from a tendon that arches over the head of the humerus and attaches to the glenoid process (also on the scapula, it is the curved section into which the head of the humerus fits). Our model does not include the scapula, and we will be approximating the location of the origination of the biceps. The two heads of the biceps terminate into a flat tendon that attaches to the radius. The action of the biceps is to flex the elbow joint. Triceps has three originations: the long head from the infraglenoid tuberosity of the scapula, and the medial and lateral heads from the humerus. The triceps terminate into a tendon that is attached to the olecranon of the ulna. The action of the triceps is to extend the elbow joint. This pair of muscles is antagonistic, each one performing the opposite action at the elbow. Continue the assembly: 2. Tape the 20 cm piece of board to the back of the humerus, so that the thickness of the humerus board is doubled in the center of the board (leave 2 cm or so at each end that is not doubled in thickness). 3. Tape the arm together: Lay the humerus over the top of the ulna and radius so that the board overlap by about 2 cm and the arm appears to be extended. Add a piece of tape across the width of the boards, making a hinge. 4. Fold the humerus over the lower arm, and make another tape hinge on the back side of the model. The elbow should be able to flex and extend.

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5. Make a small hole to accommodate a brad in the board at the insertion of the biceps (about 5 cm from the end of the board with the olecranon process). Push the brad through the hole (which should be at the top of the radius. Tie a string just under the head of the brad and then flatten the ends on the other side. This string will model the biceps. 6. Make another small hole to accommodate a brad in the board at the insertion of the triceps (about 1 cm from the end of the board with the olecranon process). Push the brad through the hole (which should be at the top of the radius. Tie a string just under the head of the brad and then flatten the ends on the other side. This string will model the triceps. 7. Use the 3 hole punch to make a hole through the board at the top of the humerus. 8. Thread the biceps string through the hole at the top of the humerus (from front to back). Add a piece of masking tape to this string and label it “Biceps”. 9. Thread the triceps string through the hole at the top of the humerus from back to front. Label this string with masking tape, “Triceps”. To Do and Notice: Notice when the biceps contracts (when you pull on the string), the lower arm moves toward the upper arm. If you let go of the string, the arm extends without a muscular contraction – gravity can work on this lever to extend the elbow. In order to get the model (or your own triceps) to contract against gravity, hold the upper arm parallel to the ground, with the lower arm hanging straight down. Notice that contraction of the triceps causing elbow extension. What type of lever is demonstrated by the action of the biceps at the elbow? What type of lever is demonstrated by the action of the triceps at the elbow? Math Extension: Measure the length of the biceps at rest (when the elbow is extended), and compare it to the length of the biceps when the elbow is flexed. Our muscles can contract up to 60% of their resting length. How does our model compare? Is the flexed length 60% or greater of the resting length? Try this with the triceps, too. What’s Going On? Skeletal muscles are voluntary muscles (unlike cardiac muscle and smooth muscle, which are not under voluntary control). They are attached to bone by tendons. When skeletal muscles contract, they shorten in length, bringing the bones to which they are attached together. Many muscles in our bodies work in antagonistic pairs - the biceps flexes the arm at the elbow, and the triceps extends the arm. Both actions are produced by the contraction and the shortening of the muscle. The only way a muscle can lengthen is by relaxing (which is not an action, but the lack of the action of contracting). Lori Lambertson Exploratorium Teacher Institute Page 4 © 2005 Exploratorium, all rights reserved

The action of the biceps at the elbow is an example of a third class lever. In this example, the effort (the attachment of the biceps to the radius) is located in between the load (your forearm plus anything you have in your hand) and the fulcrum (the elbow joint). Because the effort is very close to the fulcrum, it moves through a small distance as the hand moves through a large distance. The action of the triceps at the elbow is an example of a first class lever. In this example, the fulcrum (the elbow joint) is located in between the load (your forearm plus anything you have in your hand) and the effort (the attachment of the triceps to the ulna). Again, the effort is located very close to the fulcrum, and it moves through a small distance as the hand moves through a large distance.

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