Levers in the Human Body

21/09/2018

First Class Levers Axis is between the other two components F A R Teeter Totter Either speed or strength Only lever that can be balanced Only lever that can have a mechanical advantage equal to one Can have a mechanical advantage of less than or greater than one In a first class lever, the axis is located between the other two components of force and resistance (not necessarily in the “middle”). A crow bar and see-saw (or teeter totter) are classic examples. A first class lever is basically designed to produce balanced movements when the axis is located between the force and resistance. If the axis is closer to the force compared to the resistance, then the lever produces speed and range of motion. However, when the axis is closer to the resistance the lever produces greater strength and can be used for force.

Classes of Levers 1st class The fulcrum (axis) lies between the effort and the load 21/09/2018

Classes of Levers 1st class As the fulcrum moves toward the load the mechanical advantage increases and the load becomes easier to move but its range of motion and distal end speed decreases. As the fulcrum moves towards the force the range of motion and the distal end speed increases but the mechanical advantage decreases. 21/09/2018

Classes of Levers 1st class A first-class lever is a lever in which the fulcrum (axis) is located between the input effort and the output load. In operation, a force is applied (by pulling or pushing) to a section of the bar, which causes the lever to swing about the fulcrum, overcoming the resistance force on the opposite side. The fulcrum may be at the centre point of the lever as in a seesaw or at any point between the input and output. This supports the effort arm and the load. 21/09/2018

Class 1 Levers

Anatomical First Class Lever Tilting the Head Backwards R A F Tilting the head backwards is an example of a first class lever in the human body. The top of the vertebral column serves as the axis which is between the other two components (middle green dot). The resistance is near the middle of the head (the first green dot from the left). And the muscle insertion is at the back of the head (green dot near the base of the skull). During elbow extension, the triceps inserts on the olecrenon process of the ulna which is farther back from the center of the elbow joint (placing the elbow – the axis- in between the other two components). The weight of the hand and lower arm is the resistance. Thus extending the elbow is an example of a first class lever. These are the two primary examples of first class levers in the human body. Performance is critical to analyzing any movement. The analysis of class of levers is a good example. Elbow extension with the hand moving freely in space is a first class lever. However when the hand is fixed, like in a push-up, the system becomes a second class lever. The hand against the floor is the axis, the resistance is the body weight at the elbow and the force is the insertion on the end of the ulna at the olecrenon process. Thus the order is A, R, F making it a second class lever. It is surprising for many to find out that the body is not designed for balance. Rather, it is designed to move.

Second Class Levers Resistance is between the other two components A R F Wheelbarrow Built for strength Mechanical advantage is greater than one In a second class lever the resistance is the in between component. A wheel barrow is the classic example because a person can move very heavy loads rather easily by using this lever system. The arrangement of the components provides for a mechanical advantage of greater than one in all cases. This high mechanical advantage is part of what defines the second class lever. In other words a little force is required to move or overcome a much larger force. A nut cracker, and crow bar are other examples of a second class lever. Anatomical second class levers are discussed on a later slide.

Classes of Levers 2nd class The fulcrum (axis) is at one end, the effort at the other end and the load lies between the effort and the fulcrum 21/09/2018

Classes of Levers 2nd class As the load moves toward the fulcrum the mechanical advantages increases and it becomes easier to move but its range of motion and speed decrease. As the load moves towards the force the range of motion and speed increases but the mechanical advantage decreases. 21/09/2018

Classes of Levers 2nd class In a second class lever the input effort is located at the end of the bar and the fulcrum (axis) is located at the other end of the bar, opposite to the input, with the output load at a point between these two forces. 21/09/2018

Class 2 Levers

Anatomical Second Class Lever Raising up on the toes F R A (blue picture) There are only a few second class human levers. Raising up on the toes is one of them. The floor serves as the axis of rotation at the toes. The body weight falls through the tarsals in between the other two components and the force application is the insertion of the Achilles’ tendon on the calcaneus. Thus the order is A, R, F to make a second class lever. The push-up example used in the first class lever slide can also become a second class lever as explained there. It is surprising to find out that the body is not designed for strength, the primary quality of the second class lever.

Third Class Levers Force is between the other two components A F R Shoveling Built for speed Mechanical advantage is less than one Third class levers are abundant in the body and further examples will be provided later in this presentation. In a third class lever, the force is between the other two components (not necessarily in the “middle” as pictured in most books). Shoveling is a good example with the upper hand serving as the axis, the lower hand on the handle providing the force application, and the dirt or snow in the shovel pan serves as the resistance. Other examples of third class levers include a screen door operated by a short spring. Paddling in a boat with the upper hand on the paddle being the axis of rotation, the lower hand is applying the force and the water against the paddle is the resistance.

Classes of Levers 3rd class The fulcrum (axis) is at one end, the load at the other end and the effort lies between the load and the fulcrum 21/09/2018

Classes of Levers 3rd class As the force moves toward the fulcrum the mechanical advantage decreases and the load becomes more difficult to move but its range of motion and distal end speed increases. As the force moves towards the load the range of motion and the distal end speed decreases but the mechanical advantage increases. 21/09/2018

Classes of Levers 3rd class For this class of levers, the input effort is higher than the output load, which is different from second-class levers and some first-class levers. However, the distance moved by the resistance (load) is greater than the distance moved by the effort. Since this motion occurs in the same length of time, the resistance necessarily moves faster than the effort. Thus, a third-class lever still has its uses in making certain tasks easier to do. In third class levers, effort is applied between the output load on one end and the fulcrum (axis) on the opposite end. 21/09/2018

Class 3 Levers

Anatomical Third Class Lever F A The body is designed with a predominance of third class levers. They are the most abundant in the body. The third class lever provides for speed and range of motion. Thus the body is primarily designed for speed and range of motion. In the example in the picture, the knee joint serves as the axis of rotation, the point of force application is where the quadriceps tendon inserts on the tibial tuberosity and the resistance being overcome is the weight of the leg and the attached weight on the foot. Other examples of third class levers in the body include the biceps brachii in elbow flexion, the brachialis in elbow flexion, the hamstrings in knee flexion and the iliopsoas in hip flexion.

Levers in the Human Body The human body is built around a sturdy frame called the skeleton. Muscles are attached to the skeleton and generate movement. Many levers exist within the skeletal structure. Nature has designed the human body for movement rather than maximising force. 21/09/2018

Levers in the Human Body The forearm is a classic example of nature's way of maximising motion rather than force. The biceps is a muscle that flexes the arm. Tendons attach this muscle close to the elbow. 21/09/2018

Levers in the Human Body The forearm is an example of a third order lever. Identify the fulcrum, load and effort. The effort is where the muscle is attached. You can flex your biceps and find where they attach on your forearm. What type of lever is the forearm? Is it designed to maximise distance or force?. In nature why is it more important to multiply the distance rather than the force?. 21/09/2018

Levers in the Human Body The thigh muscles (quadraceps) are attached to the shin bone (tibia) just below the knee joint. Look at the athlete on the right as he performs the leg extension exercise. Identify the fulcrum, load and effort of this particular lever system. What type of lever is this? The muscle generates movement by shortening and pulling on a bone. Compare the amount the muscle shortens with the amount of movement generated in the lower leg. Is this lever system designed for force or speed multiplication? 21/09/2018

Levers in the Human Body The action associated with kicking a soccer ball involves a number of levers working together. Consider the kicking motion shown on the right. 21/09/2018

Levers in the Human Body The kicking action is animated on the right. What muscles are involved to start the femur moving? Where are these muscles attached? Identify all the levers involved. What type of lever is each? 21/09/2018

Levers in the Human Body The forearm is controlled by two opposing set of muscles. The movement of the forearm is shown on the right. What type of lever is formed when the biceps contract? What type of lever is formed when the triceps contract? Why are both sets of muscles attached so close to the elbow (fulcrum)? 21/09/2018

Levers in the Human Body The picture on the right shows the bone structure of the leg. Sitting down on a chair raise your shins to a horizontal position. Your thigh muscle tenses in order to lift the lower part of the leg. Using the shin as an example identify the fulcrum, load and effort. What type of lever is this? Is this the best type of lever to use for the shin? Explain. 21/09/2018

Levers in the Human Body What type of lever is this? Third order lever. Is this the best type of lever to use for the shin? Yes. Explain. To maximize movement this is the ideal lever. With the effort close to the fulcrum we can multiply the distance the load travels. Notice how the thigh muscle is attached just below the knee joint. Small changes in muscle length result in large changes in the distance travelled by the load. The thigh muscles are a very strong group of muscles. A third order lever gives no mechanical advantage and therefore strong muscles are needed to generate significant forces resulting in leg movement. 21/09/2018

Human Levers exercises The picture on the right shows the bone structure of the hand. Identify the types of levers present and label the fulcrum, load and effort. What do these levers multiply? 21/09/2018

Human Levers exercises The picture on the left shows the bone structure of the foot. What muscle controls the movement of the foot as we jump off the ground? Where is the Achilles' tendon attached? Identify the type of lever present and label the fulcrum, load and effort. What does this lever multiply? 21/09/2018

Human Levers exercises Look at the ball and socket joint of the femur and hip on the right. Look at the attachment of the muscle to the femur. What type of lever is this? What advantage does this lever offer? 21/09/2018

The foot Second order lever in action. By now it should be obvious that most of the levers found in the body are third order levers. Nature sacrifices mechanical advantage for an increase in distance travelled. Since the effort is placed so close to the fulcrum a great deal of force is needed to lift any significant load. The foot however is an example of a second order. The foot is used to spring the body off the ground. Upon landing the foot is gently placed on the ground by the contraction of muscles at the front of the lower leg. 21/09/2018

Human Levers exercises Look at the animation above. During stage "A" the foot springs the body off the ground. What type of lever does the foot represent during stage "A"? Does this lever give a mechanical advantage or is it a distance multiplier? Explain. Label the load, fulcrum and effort. During stage "B" the foot lands on the ground. What type of lever does the foot represent during stage "B"? Does this lever give a mechanical advantage or is it a distance multiplier? Explain. The gastrocnemius is a very powerful muscle, one of the strongest muscles in the body. Why does this muscle need to be so powerful? 21/09/2018

First class levers in the Human Body A lever that has its fulcrum (point of support or axis of rotation) between the point of resistance (load) and the point of effort (applied force). In the human body, a first class lever is used when the head is raised off the chest. 21/09/2018

Summary Classes of levers. (a) In a first-class lever, the fulcrum (F) is set up between the resistance (R) and the effort (M). (b) In a second-class lever, the resistance is between the fulcrum and the effort. (c) In a third-class lever, the effort is between the fulcrum and the resistance. 21/09/2018

Summary 21/09/2018

Forces in Your Body When you walk, there is 2 to 3 times your body weight transmitted through each foot. When you run, this is 7 to 8 times The foot has 26 bones – everything must be functioning correctly to avoid injury E.g. flat feet

Forces in Your Body

Reaching for your toes is a 300% increase Forces in Your Body When you sit with bad posture, there is a 200% increase pressure in your spine. Reaching for your toes is a 300% increase Human movement is very complex. There may be dozens of joints and muscles working to achieve a desired movement.