How Muscle Levers Affect Muscle Efficiency

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  • 0:06 Skeletal Muscle Contraction
  • 1:42 First-Class Levers
  • 2:16 Second-Class Levers
  • 3:36 Third-Class Levers
  • 5:26 Lesson Summary
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Lesson Transcript
Instructor: John Simmons

John has taught college science courses face-to-face and online since 1994 and has a doctorate in physiology.

Muscular contraction creates force to move the weight of our body and body parts. This lesson describes the three lever systems utilized by our body to create movement. The efficiency of each lever type in terms of strength, speed and distance are identified along with examples.

Skeletal Muscle Contraction

Skeletal muscles are attached to bone on each end by tendons. This attachment causes bones, and thus body parts, to move when our muscles contract. Furthermore, the nature of the attachment affects the efficiency of movement in terms of strength, speed and distance moved. Muscles work by way of lever systems which is a rigid lever, such as a bone, that moves on a fixed point called a fulcrum.

A teeter-totter is a good example of a lever system. The weight of each person on either side represent the force and load, while the pivot in the middle represents the fulcrum. If the fulcrum is in the middle of the board, two people with the same weight will balance the board.

Moving the board relative to the fulcrum will result in unbalance, making it easier for one person to push the other up. Just as moving the fulcrum of a teeter totter can increase the effectiveness of a force, the nature of muscle attachment to bone relative to the joint increases the efficiency of muscle contraction and thus, body movement. A lever moves when a force (F), or effort, is great enough to overcome a load (L), or resistance, that would otherwise resist movement. This lesson describes the three basic lever systems utilized by our muscles and bones for body support and movement.

First-Class Lever

Let's take a look at first-class levers. While the body has relatively few first-class levers, they are easy to understand as they resemble a teeter-totter. In such a lever, the load is located on the opposite side of the fulcrum as the applied force.

For example, head extension is a good example of a first-class lever system. Here, neck and back muscles apply a force to move the head. The weight of the head is the load, and the fulcrum is the joint between the skull and the vertebrae.

Plantar flexion is an example of a second-degree lever
Plantar Flexion Lever

Second-Class Lever

Let's move on to second-class levers. In a second-class lever, the load is located between the force and the fulcrum. A wheel barrow is a great example of a second-class lever system. Here, the wheel is the fulcrum, or the pivot point, the weight is the load and lifting the handles applies the force to move the load.

What about in the body? Plantar flexion, or standing on balls of our feet, is an example of a second-class lever in the body. Here, the ball of the foot is the fulcrum, the weight of the body is the load, and the contraction of the calf muscles provides a lifting force, or effort. This type of system allows a small force to move a relatively large load. In fact, moving the force further away from the load increases the strength of the lever system. However, speed and distance of movement are compromised at the expense of this increased strength. In other words, a relatively small force moves a large load a relatively short distance and moves it slowly.

Third-Class Lever

Now let's talk about third-class levers. Third-class levers are the most common in the human body. Here, the force is applied between the load and the fulcrum. A broom is a good example of a third-class lever where the hand on top of the broom represents the fulcrum, or the pivot point. The force is applied in the middle of the broom by your other hand, which will then move the broom (the broom being the load).

An example of a third-class lever in the body is forearm flexion, where the elbow is the fulcrum, the biceps apply upward-directed force to the forearm. The load is the forearm itself and anything that may be held in the hand.

Forearm flexion allows a load to move faster, but requires more strength
Forearm Flexion Lever

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