The Elbow is Considered a Third Class Lever Because __________

Have yous ever noticed how much easier information technology is to do a dogie enhance with 50 lbs than it is to perform a l-lb bicep curl? Is it considering your bicep is smaller or weaker than your gastrocnemius? Peradventure! Only even if your muscles were equally every bit strong, it would still be easier to perform a calf raise than a bicep scroll. Why is this?

Our bodies are composed of a variety of synovial joints that function as lever systems. The reason it’due south easier to perform a weighted calf raise than a bicep curl is considering the lever organization involved in a bicep ringlet is mechanically less efficient than the lever organisation involved in a calf heighten.

The gastrocnemius muscle. Paradigm from Muscles & Kinesiology.

Parts of a Lever

Synovial joints are moveable joints; a few examples of these are the shoulder, spine, knee, elbow, and ankle. To understand why some synovial joints take more efficient lever systems, we must starting time understand the relationships betwixt the 3 lever parts: an effort or force practical to the lever, a fulcrum, and a load. A synovial joint likewise has these: try (a muscle’s applied force at the insertion site of a bone), a fulcrum (the joint axis), and a load (the trunk office/weight to be moved).

The Three Types of Levers

At that place are 3 different kinds of levers: first class, second grade, and third form. Each of these lever classes take unique arrangements of the muscle’s insertion (endeavour) and bones (lever/arm) effectually the joint (fulcrum). See the nautical chart below to visualize the deviation between the levers:

Effort Arm vs. Load Arm

The efficiency of a lever relies on the ratio of the
endeavor arm
to the
load arm. The
effort arm (EA)
is the distance between the fulcrum and the endeavor; in the body, this is the distance between the joint and the muscle’southward insertion site. The
load arm (LA)
is the altitude between the fulcrum and the load; in the trunk, this is the distance between the articulation and the loaded body part.

The greater the ratio of the endeavor arm to the load arm, the more efficient the lever system is (i.due east. the easier it is to move the load). Therefore, if the distance between a muscle’due south insertion site and the articulation is greater than the altitude between the load and the joint, your muscle is at an advantage. This is why your gastrocnemius tin lift more than weight than your bicep, even if they are every bit equally stiff!

The levers involved in plantarflexion and elbow flexion. Illustrations from Beefcake & Physiology.

To empathize this concept, let’s expect at a basic example of a outset class lever.

Pretend you need to motion a really heavy stone. To lift it, you could create a first class lever organisation with a shovel and cinder cake. You strategically position the cinder cake so that when you use it as leverage, the middle of the shovel handle rests on it. Now, when you put your easily on the end of the shovel to apply a downwards force (effort) that leverages the stone (load), your hands and the rock are equally distant from the cinder block (fulcrum). Thus, your try arm and load arm are equal.

To make information technology even easier, you lot could motility the cinder cake closer to the stone, moving the fulcrum closer to the load. This decreases the load arm and increases the effort arm, making the lever more efficient and allowing you to lift the rock while applying less force.

Atlanto-Occipital Joint as a Kickoff Form Lever

An example of a starting time class lever in the human being torso is the caput and neck during neck extension. The fulcrum (atlanto-occipital joint) is in between the load (front of the skull) and the effort (cervix extensor muscles). The muscles are attached to the posterior role of the skull to allow for the greatest effort arm. The atlanto-occipital joint in the middle provides leverage, and when the muscles contract, pulling the occipital os down, the forepart of the skull is lifted. Can y’all recall of any other examples?

The atlanto-occipital joint every bit a starting time class lever. Screenshot from Human Beefcake Atlas.

Neck flexion in activity. Video footage from Muscles & Kinesiology.

Elbow Joint as a Third Grade Lever

Examples of the 3rd course lever system are arable in the human body. In our case of a weighted bicep curl and a calf raise, the lever system involved in a bicep scroll is mechanically less efficient than the lever organisation involved in a dogie raise. The elbow articulation is an instance of a third class lever, operating with the endeavor between the load and fulcrum.

The distance between the elbow joint and the insertion site of the bicep tendon is very small, particularly when it’due south compared to the distance between the elbow joint and the weight in your hand. The effort arm is significantly shorter than the load arm, making the ratio very minor. When you try to scroll the weight, your bicep has to work harder considering it is at a mechanical disadvantage.

The elbow joint as a third class lever. Screenshot from Human being Anatomy Atlas.

Elbow flexion. Video footage from Muscles & Kinesiology.

The Calf equally a 2nd Class Lever

The gastrocnemius in your calf, yet, is at a mechanical advantage. In plantar flexion, the lower leg acts as a 2d class lever. A second form lever is the merely lever that tin promise that the endeavor arm volition always be greater than the load arm. This system results in a bigger attempt arm to load arm ratio, making the second class lever the most mechanically advantageous.

In a calf heighten, the effort comes from the gastrocnemius musculus, which is attached to your calcaneus bone. The load comes from your body weight and the extra weight yous are holding; this force acts on the lever arrangement through the tibia. The fulcrum is made up of the metacarpophalengeal joint.

In this arrangement, the load is in the eye, and the effort is farthest from the fulcrum. Therefore, the human action of plantarflexion tin move much more weight than elbow flexion, even if your bicep is just equally potent as your calf.

The calf as a 2d form lever. Screenshot from Human Anatomy Atlas.

Plantarflexion in activeness. Video footage from Muscles & Kinesiology.

All of these lever systems are used throughout the entire human body.  However, it can exist tricky to figure out which system a joint uses! If you lot become stuck, remember that to allocate a lever, yous have to analyze where the forces are beingness applied. For case, some muscles exist on 1 side of a articulation, but their tendon crosses over the articulation and applies a force to a os on the opposite side. This is what happens in the knee. Can you figure out which lever organisation the knee uses?

Knee extension. Video footage from Muscles & Kinesiology.

Like in the images in a higher place, you can use the draw tool in Human Anatomy Atlas and Muscles & Kinesiology to draw diagrams to help yous determine which kind of lever system a sure joint uses. Take fun learning!

Speaking of musculus deportment and the knee joint…bank check out our Knee Flexion eBook to acquire more about how this important movement works!

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