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How can skeletal muscles in the human body be modeled as levers?

How can skeletal muscles in the human body be modeled as levers?


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From what I understand, human arms can be thought of as third class levers, so that the distance from your elbow to the place where the muscle attaches is effectively the distance to the fulcrum, so that arms produce proportionally less strength on the end, and proportionally greater distance when lifting a load. So this is somewhat of an anatomy question (although I suppose if it was to be used for an exoskeleton, it could be robotics) but can leg muscles and the muscles that attach to the ends of arms and legs be roughly modeled in such a way? What muscles/muscle groups don't classify that way?


Lever Systems

The operation of most skeletal muscles involves leverage – using a lever to move an object. A lever is a rigid bar that moves on a fixed point called the fulcrum, when a force is applied to it.

The applied force, or effort, is used to move a resistance, or load. In the human body, the joints are fulcrums, and the bones act as levers. Muscle contraction provides the effort that is applied at the muscle’s insertion point on the bone. The load is the bone itself, along with overlying tissues and anything else you are trying to move with that lever.


How can skeletal muscles in the human body be modeled as levers? - Biology

p-ISSN: 2163-2669 e-ISSN: 2163-2677

The Anatomy of a Human Body, a Model to Design Smart High Building

Katayoun Taghizadeh , Matin Bastanfard

Department of Architecture, University of Tehran, Tehran, 1417466191, Iran

Correspondence to: Katayoun Taghizadeh , Department of Architecture, University of Tehran, Tehran, 1417466191, Iran.

Email:

Copyright © 2012 Scientific & Academic Publishing. All Rights Reserved.

Today, population growth and the rise in real estate price have made the growth and development of cities in a vertical manner inevitable. The benefits of developing a city vertically include land preservation, the reduction of environmental damage, traffic and transportation as well as reducing environmental pollution and energy consumption caused by horizontal expansion. Designing a high building is a complex process, particularly in regards to its structure. Since the beginning of civilization, the nature with its secrets has been a valuable tool for solving such problems. Bionic is a science that can eradicate this gap between nature and technology. Bionic is the science that is formed from the combination of various natural and engineering science concepts. It is the study of complex vital systems of live organisms and natural forms and maybe a good solution for solving technical and structural issues of engineers. The structure of human body has been considered as a good example of a tall structure with a reasonable ratio of the height to the base. This model may serve for designing high buildings with particular attention to three aspects. First human body structure: in this section the body's skeleton, the spinal cords curves, its connections, and the routes of energy transmission are analyzed. Second the human's brain, intelligent controller of the body. The human brain has two major functions reaction to stimuli and commanding other organs. The distribution and connection of nerves in the brain and throughout the body are discussed in this section. Third human skin, a model for the rind of high buildings: skin with its complex ability has a significant role in converting human body to a micro climate. Self-repairing, energy absorption, and thermal insulation are among other features are discussed in this section. At the first step, the method of research in this study is based on the study of human's anatomy and similar subjects, and the discussion with medical and experimental sciences experts. We will discuss and analyze each topic separately in details. The goal of this research is to start a path toward providing a model for designing intelligent high buildings with the use of the human body as a model.This document gives formatting instructions for authors preparing papers for publication in SAP. The authors must follow the instructions given in the document for the papers to be published. You can use this document as both an instruction set and as a template into which you can type your own text. The body of abstract immediately follows abstract heading in the same paragraph. For example, this paragraph begins with abstract heading

Keywords: Tall Building, Bionics, Human Body, Structure, Skin, Intelligent Control


Static Equilibrium in Levers

For all levers the effort and resistance ( load ) are actually just forces that are creating torques because they are trying to rotate the lever. In order to move or hold a load the torque created by the effort must be large enough to balance the torque caused by the load. Remembering that torque depends on the distance that the force is applied from the pivot , the effort needed to balance the resistance must depend on the distances of the effort and resistance from the pivot. These distances are known as the effort arm and resistance arm (load arm). Increasing the effort arm reduces the size of the effort needed to balance the load torque. In fact, the ratio of the effort to the load is equal to the ratio of the effort arm to the load arm:

(1)

Every Day Examples: Biceps Tension

Let’s calculate the biceps tension need in our initial body lever example of a holding a 50 lb ball in the hand. We are now ready to determine the bicep tension in our forearm problem. The effort arm was 1.5 in and the load arm was 13.0 in, so the load arm is 8.667 times longer than the effort arm.

That means that the effort needs to be 8.667 times larger than the load, so for the 50 lb load the bicep tension would need to be 433 lbs! That may seem large, but we will find out that such forces are common in the tissues of the body!

*Adjusting Significant Figures

Finally, we should make sure our answer has the correct significant figures . The weight of the ball in the example is not written in scientific notation , so it’s not really clear if the zeros are placeholders or if they are significant. Let’s assume the values were not measured, but were chosen hypothetically, in which case they are exact numbers like in a definition and don’t affect the significant figures. The forearm length measurement includes zeros behind the decimal that would be unnecessary for a definition, so they suggest a level of precision in a measurement. We used those values in multiplication and division so we should round the answer to only two significant figures, because 1.5 in only has two (13.0 in has three). In that case we round our bicep tension to 430 lbs, which we can also write in scientific notation: .

*Neglecting the Forearm Weight

Note: We ignored the weight of the forearm in our analysis. If we wanted to include the effect of the weight of the forearm in our example problem we could look up a typical forearm weight and also look up where the center of gravity of the forearm is located and include that load and resistance arm . Instead let’s take this opportunity to practice making justified assumptions . We know that forearms typically weigh only a few pounds, but the ball weight is 50 lbs, so the forearm weight is about an order of magnitude (10x) smaller than the ball weight [7] . Also, the center of gravity of the forearm is located closer to the pivot than the weight, so it would cause significantly less torque . Therefore, it was reasonable to assume the forearm weight was negligible for our purposes.


Skeletal, Muscular, and Integumentary System

2. Explain how skeletal muscle contracts, including fiber structure, the sliding filament model, and how contractions are controlled: Muscle contractions go from end to end, cells generate such force from two kinda of muscle protein filaments interactions the thick filaments (myosin) and the thing filaments (actin) which are packed into filament bundles called myofibrils. Together actin and myosin form a Z line, together with the rest of the filaments they make up a sarcomere. In the process of muscle contraction myosin filaments form cross-bridges with actin filaments which eventually change shape, pulling the actin filaments in the direction of the center of the sarcomere. Skeletal muscles are useful only if they contract in a controlled fashion, the motor neurons are the main source of control to the contraction of muscle fibers. (Miller – 32.2, 931)

3. Explain how muscle contractions produce movement: When a muscle intends to move, signals are transmitted through motor nerves from the brain, down through the spinal cord to the muscles. Muscles also tend to aid each other in movement, for example the bicep and the tricep work together when one moves that is why in certain exercises it is possible to train both muscles at the same time. (Miller – 32.2, 932)

4. Explain 2 major disorders that occur within this system, include:

Pelvic floor muscle disorder:

PFD is usually related to the presence of too much tension, However, sometimes patients with PFD can have a combination of muscles that are too tense and too relaxed.

Signs and symptoms: Frequent urination, pain in the pelvic region, constipation

Prevalence: Overall, 23.7% of women and men reported symptoms of at least one pelvic floor disorder.

Treatment options: Self-Care

Medicines :Physical therapy

“Pelvic Floor Dysfunction.”, Pelvic Muscle Dysfunction, IC and Painful Sex. Interstitial Cystitis Association, n.d. Web. 06 Apr. 2014. <http://www.ichelp.org/page.aspx?pid=361#floor>.

Laminopathy: It is a genetic disorder in which mutations tend to occur in certain genes.

Sign and symptoms: Skeletal and/or cardiac muscular dystrophy, lipodystrophy anddiabetes, dysplasia, dermo- or neuropathy, leukodystrophy, and progeria

Prevalence: More common in females rather than males, usually diagnosed from the ages of 6 months to 54 years old.

Treatment options: No cure or treatments

1. Basic Function: The Integumentary system acts as a barrier protecting the body from various types of damage coming in. (Miller – 32.3, 935)

2. Epidermis: The epidermis is the outer layer of the skin and has two layers, the outer layers the dead cells and the inner layer living and stem cells. (Miller – 32.3, 936)

Dermis: The dermis is located under the epidermis. It contains the protein collagen, blood vessels, nerve endings, glands, sensory receptors, smooth muscles, and hair follicles. The dermis aids maintaining homeostasis by regulating body temperature.(Miller – 32.3, 937)

Hair: Hair on the head protects the scalp from ultraviolet light from the sun and provides insulation from the cold. The hairs in the nostrils, external ear canals, and around the eyes block dirt and other particles from entering the body. Hair is produced by follicles. (Miller – 32.3, 937)

Nails: Nails grow from the nail root, which are located near the tips of the fingers and toes. They grow at an average rate of 3 millimeters per month, fingernails grow three times faster than toenails. (Miller – 32.3, 937)

4. Melanoma is a cancer that usually starts in the skin, either in a mole or in normal-looking skin. About half of all melanomas start in normal-looking skin. The number of people developing melanoma is continuing to rise. More than 10,600 people in the UK are diagnosed with melanoma each year. Melanoma is more common in women, particularly young women. In the UK it’s the most common cancer in people aged 15–34. UV radiation damages the DNA in our skin cells and can cause skin cancers, such as melanoma. Many severe sunburns that cause the skin to blister, especially during childhood, can increase the risk of melanoma in the future. So it’s important that adults and particularly children avoid getting sunburnt. It can be difficult to differentiate the difference of a normal mole and melanoma. These are a few symptoms of what melanoma could look like: Asymmetry – Melanomas are likely to be irregular or asymmetrical. Ordinary moles are usually symmetrical (both halves look the same). Border – Melanomas are more likely to have an irregular border with jagged edges. Ordinary moles usually have a well-defined, regular border. Colour – Melanomas tend to be more than one colour. They may have different shades, such as brown mixed with a black, red, pink, white or bluish tint. Normal moles tend to be one shade of brown. Diameter (width) – Melanomas are usually more than 7mm in diameter. Moles are normally no bigger than the blunt end of a pencil (about 6mm across)(Macmillan). The main treatment for melanoma is surgery. Many melanomas are cured with surgery however they cannot always be treated. There has been more than 76,600 cases of melanoma cases of skin cancer in 2013(Macmillan).

Acne develops when sebum and dead skin cells from plugs in hair follicles. (Miller – 32.3, 938)

Signs and symptoms: Blackheads, whiteheads, papules, pustules (what many people call pimples), cysts, nodules, scars, depression, low self-esteem, dark spots on the skin

“Var R_text = New Array () R_text[0] = “Excellence in Dermatology™” R_text[1] = “Excellence in Dermatologic Surgery™” R_text[2] = “Excellence in Medical Dermatology™” R_text[3] = “Excellence in Dermatopathology™” Var I = Math.floor(4*Math.random()) Document.write(r_text[i]).”Acne: Signs and Symptoms. American Academy of Dermatology, n.d. Web. 04 Apr. 2014. <http://www.aad.org/dermatology-a-to-z/diseases-and-treatments/a—d/acne/signs-symptoms>.

Prevalence: Up to 85% of people experience acne during adolescence and young adulthood. (Miller – 32.3, 938)

Treatment options: Medications that can be purchased over the counter or if case is severe then a dermatologist should be consulted. (Miller – 32.3, 938)


Make a Model of the Human Skeleton

The skeleton forms the frame for the body and makes up about one fifth of the body&rsquos weight. It is made up of 206 bones. It also includes cartilage, joints, and ligaments. Besides for forming our body frame, the skeleton has several other jobs. It is the anchor and support for all our muscles and even our organs. It protects our vital organs like the brain, spinal cord, heart and lungs. It allows us to move with muscles attached by tendons, using the bones as levers. It is a place for our body to store fat and minerals, like calcium. It is where the body makes most of its new blood cells. Bones come in many shapes and sizes. The long bones have a long shaft and two bigger ends. These include the bones of the arms and legs. The largest bone in the body, the femur, is a long bone. It is 2 feet long and hollow, to make it lighter. It is very strong to support the body&rsquos weight. The short bones are cube-shaped and include the wrist &ndash the carpals, and the bones of the ankle &ndash the, tarsals. The flat bones are thin, curved and flattened like the sternum and skull. Lastly, there are irregular bones such as the vertebra and pelvis. Each section of the skeleton has a job. Below see all the parts of the skeleton and how they work together to make the body a strong, moving machine.

The Skull: The skull surrounds and protects the brain and the organs of hearing and balance. The facial bones form the structure of the face, hold the eyes, and the organs for taste and smell and anchor the teeth. They have the openings for air and food. The whole skull anchors muscles that hold the head up, allow us to chew, and form facial expressions.

Arms and Hands: Each upper limb includes the humerus (arm), the radius and ulna (forearm) and the bones of the hand: 8 carpals (wrist), 5 metacarpals (palm) and 14 phalanges (fingers and thumb). The arm and forearm bend at the elbow in a hinge joint, which is not as flexible as the shoulder joint, but is much more stable. The hand with its many joints is made to be flexible and agile. It can grasp and lift a heavy suitcase or careful pick up a pin. The upper limb is made to do both and everything in between.

The Pelvis: is formed by 2 hip bones attach to the sacrum of the backbone. On the outside of each side is a deep socket, called the acetabulum, where the head the leg bones (the femur) sits. The pelvis supports and protects internal organs, attaches the lower limb to the body and with the lower limb supports the weight of the whole upper body. The hip joint is very important for leg movement and is supported by strong muscles and ligaments. Though it is a ball and socket joint like the shoulder, it is more stable and less moveable than that joint.

Legs and Feet: The lower limb includes the femur (thigh), tibia (shinbone) and fibula (leg), and bones of the foot: 7 tarsals (ankle), 5 metatarsals (foot) and 14 phalanges (toes). The kneecap (patella) sits in front of the knee joint, inside a muscle tendon. The femur is the largest bone in the body and makes up about one fourth of a person&rsquos height. It forms a ball and socket joint at the hip, with the pelvis, and a hinge joint at the knee, with the tibia. The knee, a hinge joint, has less flexibility than the hip but is more stable, because the knee carries the bulk of the body&rsquos weight. It is often injured. The entire weight of the body sits on the foot. The foot acts as a lever to move the body forward when we walk or run.

Have students r ead about the skeletal system (see pdf at bottom) and take the two short answer quizzes .

They can study the labeled skeleton and then try to label a whole skeleton themselves. Now they are ready to build their own labeled skeletons.

1. Print out the pdf bones of the body hands, feet, arms, forearms legs, shins, skull,
and ribcage.
2. Copy enough for each student to have a set.
3. Have them cut out each bone section, e.g a whole hand (not each individual
bone).
4. Get a large roll of paper (3 foot wide).
5. Cut off lengths about the heights of your students.
6. Have each student lie down on a length paper and trace their outline
with a permanent marker. Have them write their name on their body
sheet.
7. Have them place their bones in the right places on their body
outlines. Use glue sticks to secure organs in place.
8. Label each of the bones in your skeleton.


In a first-class lever, the fulcrum is the middle component and lies between the effort and load. Examples of a first-class lever in the body are rare as few exercises utilise a first-class lever system although extension (straightening) at the elbow is one example. Extension at the elbow can be seen during a throwing action or tennis stroke.

In the image below, the triceps is the effort, the fulcrum is the elbow joint and the load is the weight of the arm and Javelin. First-class levers increase both the effects of effort and the speed of the body.

There is sometimes more than one lever system operating at the joint. The elbow joint is one example. During extension of the elbow, the effort is created by the triceps via its point of insertion on the ulna, so is a first-class lever. However, during flexion at the elbow, as in a bicep curl, the effort comes from the point of insertion of the biceps on the radius, this is an example of a third-class system.


Skeletal muscle created from stem cells

UCLA scientists have developed a new strategy to efficiently isolate, mature and transplant skeletal muscle cells created from human pluripotent stem cells, which can produce all cell types of the body. The findings are a major step towards developing a stem cell replacement therapy for muscle diseases including Duchenne Muscular Dystrophy, which affects approximately 1 in 5,000 boys in the U.S. and is the most common fatal childhood genetic disease.

The study was published in the journal Nature Cell Biology by senior author April Pyle, associate professor of microbiology, immunology and molecular genetics and member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA. Using the natural human development process as a guide, the researchers developed ways to mature muscle cells in the laboratory to create muscle fibers that restore dystrophin, the protein that is missing in the muscles of boys with Duchenne.

Without dystrophin, muscles degenerate and become progressively weaker. Duchenne symptoms usually begin in early childhood patients gradually lose mobility and typically die from heart or respiratory failure around age 20. There is currently no way to reverse or cure the disease.

For years, scientists have been trying different methods that direct human pluripotent stem cells to generate skeletal muscle stem cells that can function appropriately in living muscle and regenerate dystrophin-producing muscle fibers. However, the study led by Pyle found that the current methods are inefficient they produce immature cells that are not appropriate for modeling Duchenne in the laboratory or creating a cell replacement therapy for the disease.

"We have found that just because a skeletal muscle cell produced in the lab expresses muscle markers, doesn't mean it is fully functional," said Pyle. "For a stem cell therapy for Duchenne to move forward, we must have a better understanding of the cells we are generating from human pluripotent stem cells compared to the muscle stem cells found naturally in the human body and during the development process."

By analyzing human development, the researchers found a fetal skeletal muscle cell that is extraordinarily regenerative. Upon further analysis of these fetal muscle cells two new cell surface markers called ERBB3 and NGFR were discovered this enabled the reserchers to precisely isolate muscle cells from human tissue and separate them from various cell types created using human pluripotent stem cells.

Once they were able to isolate skeletal muscle cells using the newly identified surface markers, the research team matured those cells in the lab to create dystrophin-producing muscle fibers. The muscle fibers they created were uniformily muscle cells, but the fibers were still smaller than those found in real human muscle.

"We were missing another key component," said Michael Hicks, lead author of the study. The skeletal muscle cells were not maturing properly, he explained. "We needed bigger, stronger muscle that also had the ability to contract."

Once again, the team looked to the natural stages of human development for answers. Hicks discovered that a specific cell signaling pathway called TGF Beta needs to be turned off to enable generation of skeletal muscle fibers that contain the proteins that help muscles contract. Finally, the team tested their new method in a mouse model of Duchenne.

"Our long term goal is to develop a personalized cell replacement therapy using a patient's own cells to treat boys with Duchenne," said Hicks. "So, for this study we followed the same steps, from start to finish, that we'd follow when creating these cells for a human patient."

First, the Duchenne patient cells were reprogrammed to become pluripotent stem cells. The researchers then removed the genetic mutation that causes Duchenne using the gene editing technology CRISPR-Cas9. Using the ERBB3 and NGFR surface markers, the skeletal muscle cells were isolated and then injected into mice at the same time a TGF Beta inhibitor was administered.

"The results were exactly what we'd hoped for," said Pyle. "This is the first study to demonstrate that functional muscle cells can be created in a laboratory and restore dystrophin in animal models of Duchenne using the human development process as a guide."

Further research will focus on generating skeletal muscle stem cells that can respond to continuous injury and regenerate new muscle long-term using the team's new isolation and maturation strategy.

The newly identified strategy to generate skeletal muscle from human pluripotent stem cells is covered by a patent application filed by the UCLA Technology Development Group on behalf of The Regents of the University of California, with Michael Hicks and April Pyle as co-inventors.

The research was supported by grants from the National Institutes of Health (K01AR061415), the National Institute of Arthritis and Musculoskeletal and Skin Diseases (R01AR064327), the National Center for Advancing Translational Sciences, the Department of Defense (W81XWH-13-1-0465), the California Institute for Regenerative Medicine (RB5-07230, DISC1-08823, DISC2-08824), the Center for Duchenne Muscular Dystrophy at UCLA, a Cure Duchenne Fellowship, a National Institutes of Health Paul Wellstone Center Training Fellowship and the UCLA Broad Stem Cell Research Center's Rose Hills Foundation Research Award and Shaffer Fellowship.


UCLA researchers create skeletal muscle from stem cells

UCLA scientists have developed a new strategy to efficiently isolate, mature and transplant skeletal muscle cells created from human pluripotent stem cells, which can produce all cell types of the body. The findings are a major step towards developing a stem cell replacement therapy for muscle diseases including Duchenne Muscular Dystrophy, which affects approximately 1 in 5,000 boys in the U.S. and is the most common fatal childhood genetic disease.

The study was published in the journal Nature Cell Biology by senior author April Pyle, associate professor of microbiology, immunology and molecular genetics and member of the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA. Using the natural human development process as a guide, the researchers developed ways to mature muscle cells in the laboratory to create muscle fibers that restore dystrophin, the protein that is missing in the muscles of boys with Duchenne.

Without dystrophin, muscles degenerate and become progressively weaker. Duchenne symptoms usually begin in early childhood patients gradually lose mobility and typically die from heart or respiratory failure around age 20. There is currently no way to reverse or cure the disease.

For years, scientists have been trying different methods that direct human pluripotent stem cells to generate skeletal muscle stem cells that can function appropriately in living muscle and regenerate dystrophin-producing muscle fibers. However, the study led by Pyle found that the current methods are inefficient they produce immature cells that are not appropriate for modeling Duchenne in the laboratory or creating a cell replacement therapy for the disease.

“We have found that just because a skeletal muscle cell produced in the lab expresses muscle markers, doesn’t mean it is fully functional,” said Pyle. “For a stem cell therapy for Duchenne to move forward, we must have a better understanding of the cells we are generating from human pluripotent stem cells compared to the muscle stem cells found naturally in the human body and during the development process.”

By analyzing human development, the researchers found a fetal skeletal muscle cell that is extraordinarily regenerative. Upon further analysis of these fetal muscle cells two new cell surface markers called ERBB3 and NGFR were discovered this enabled the reserchers to precisely isolate muscle cells from human tissue and separate them from various cell types created using human pluripotent stem cells.

Once they were able to isolate skeletal muscle cells using the newly identified surface markers, the research team matured those cells in the lab to create dystrophin-producing muscle fibers. The muscle fibers they created were uniformily muscle cells, but the fibers were still smaller than those found in real human muscle.

“We were missing another key component,” said Michael Hicks, lead author of the study. The skeletal muscle cells were not maturing properly, he explained. “We needed bigger, stronger muscle that also had the ability to contract.”

Once again, the team looked to the natural stages of human development for answers. Hicks discovered that a specific cell signaling pathway called TGF Beta needs to be turned off to enable generation of skeletal muscle fibers that contain the proteins that help muscles contract. Finally, the team tested their new method in a mouse model of Duchenne.

“Our long term goal is to develop a personalized cell replacement therapy using a patient’s own cells to treat boys with Duchenne,” said Hicks. “So, for this study we followed the same steps, from start to finish, that we’d follow when creating these cells for a human patient.”

First, the Duchenne patient cells were reprogrammed to become pluripotent stem cells. The researchers then removed the genetic mutation that causes Duchenne using the gene editing technology CRISPR-Cas9. Using the ERBB3 and NGFR surface markers, the skeletal muscle cells were isolated and then injected into mice at the same time a TGF Beta inhibitor was administered.

“The results were exactly what we’d hoped for,” said Pyle. “This is the first study to demonstrate that functional muscle cells can be created in a laboratory and restore dystrophin in animal models of Duchenne using the human development process as a guide.”

Further research will focus on generating skeletal muscle stem cells that can respond to continuous injury and regenerate new muscle long-term using the team’s new isolation and maturation strategy.

The newly identified strategy to generate skeletal muscle from human pluripotent stem cells is covered by a patent application filed by the UCLA Technology Development Group on behalf of The Regents of the University of California, with Michael Hicks and April Pyle as co-inventors.

The research was supported by grants from the National Institutes of Health (K01AR061415), the National Institute of Arthritis and Musculoskeletal and Skin Diseases (R01AR064327), the National Center for Advancing Translational Sciences, the Department of Defense (W81XWH-13-1-0465), the California Institute for Regenerative Medicine (RB5-07230, DISC1-08823, DISC2-08824), the Center for Duchenne Muscular Dystrophy at UCLA, a Cure Duchenne Fellowship, a National Institutes of Health Paul Wellstone Center Training Fellowship and the UCLA Broad Stem Cell Research Center’s Rose Hills Foundation Research Award and Shaffer Fellowship.


Third-Class Levers in the Human Body

A lever is a type of simple machine where a rigid arm is arranged around a fixed point or fulcrum. Input, the force you put in, directed into an output force. The classic example of a lever is a seesaw. The fulcrum is in the middle, and when you push down on your side of the seesaw (input), it makes the person on the other side of the seesaw go up (output).

There are three main classes of levers. If the fulcrum is in the between the output force and input force as in the seesaw, it is a first-class lever. In a second-class lever, the output force is in between the fulcrum and the input force. An example of a second class lever is a wheelbarrow. The fulcrum is the wheel, the load of stuff in the wheel barrow requires the output force to be lifted, and the person at the handle supplies the input force. In a third-class lever, the input force is in between the output force and the fulcrum. An example of this class of lever is a baseball bat. The handle of the bat is the fulcrum, you supply the input force near the middle, and the other end of the bat that pushes the ball with the output forces. In a third-class lever, the input force is greater than the output force but the output load is able to move farther.

You have several third-class levers in your body. One that is easy to investigate is your forearm.

Problem: How is your arm a third-class lever?

Materials

  • Table
  • Bucket with strong handle
  • Sand or other material to put in the bucket.
  • Helper
  • 2-3 ft piece of one-inch wide PVC pipe, or a strong yardstick
  • Cardboard
  • Scissors
  • 3 paperclips
  • String
  • Small weights

Procedure

  1. Fill the bucket halfway with sand.
  2. Place your arm flat on the table. Your hand and about four inches below your wrist should extend over the table&rsquos edge. The inside of your elbow should be facing upward.
  3. Have your friend hang the bucket across the palm of your hand.
  4. Keeping your elbow on the table, lift the bucket up.
  5. Repeat, this time with the bucket full of sand. Does this require more or less effort?

  1. Remove sand until the bucket is again halfway filled with sand.
  2. Put the handle of the bucket around the PVC pipe or yardstick.
  3. Set your arm on the table as before, but hold the PVC pipe or yardstick with the bucket hanging off of it. The bucket should be 2-3 feet farther from the edge of the table.
  4. Lift the bucket again. How does the effort to lift the bucket on the long stick compare to the effort of moving it when it was in your hand? Does it feel like the bucket contains more or less sand than the first trial?
  5. Slide the bucket to different places on the long stick and note the different amounts of effort.
  6. Now, you'll make a model of your forearm as third-class lever. Measure the length of your upper arm and forearm and outline your model on the cardboard. Keep the upper arm and forearm pieces separate, and at the end of the forearm, trace your hand.
  7. Cut out your model arm from the cardboard.
  8. Use a brad to join the cardboard upper arm and forearm. This is your elbow joint and the fulcrum of your lever.
  9. Tape three paper clips on to the arm model, and thread the string through. The two paperclips on your upper arm represent the bicep muscle on your arm, the paper clip on the forearm represents where the muscle attaches. The distance between the three paperclips represents the length of the input effort arm of your lever.
  10. Tie one end of your string to the thumb of your cardboard hand. Tie or tape the other end to the top of the forearm (where the shoulder would be). The total string length represents the output arm of your lever.
  11. Now, lightly pull the string between the two paperclips on the upper arm. This represents the contraction of the bicep. What happens? How does the distance the hand moves compare with the distance you moved the string?

  1. If your model seems sturdy, try adding a bit of weight tied with a string around the paper hand and tug the muscle again.

Results

When the bucket contains more sand, it takes more effort to lift. What might be more surprising is when the bucket is hanging off the PVC pipe or yardstick it takes more force to lift it. It feels like there is more sand in the bucket. When you tug at the string between the two paper clips on the model&rsquos forearm, the hand lifts up, covering more distance than the distance you pulled the string.

When you added more sand to the bucket, you increased its weight, so you needed more input force to lift it. When you hung the bucket off the piece of PVC pipe or yardstick, you increased the length of the output arm, which also increased the amount of input force you needed to add. In all third-class levers, the length of the output arm is longer than that of input arm. When you added length to your arm by hanging the bucket on the stick, you exaggerated this characteristic. When you tugged at bicep area of your model, you made the hand move farther than the amount if string you pulled. In all third-class levers, the distance moved by the output load is greater than the distance moved in the input force.

Want to investigate your &ldquoarm machine&rdquo more? Put two more paper clips on the other side of your model upper arm, and another paper clip on the other side of the one on your forearms. Thread another piece of string through the paperclips. You have created a model of your tricep muscle, which straightens your arm. Tug the bicep muscle so it bends the arm, then tug the string between the two paperclips on the upper arm. The arm should straighten.

Disclaimer and Safety Precautions

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