Chapter 12. Biomechanics

Muscular Antagonism

Although a number of muscles may be involved in an action, the principal muscle involved is called the prime mover, or agonist. To lift a cup, a muscle called the biceps brachii is the prime mover; however, because this muscle can be assisted by the brachialis, the brachialis is called a synergist in this action (Figure 12.1). A synergist can also be a fixator, which stabilizes the bone that is the attachment for the prime mover’s origin.

A muscle with the opposite action of the prime mover is called an antagonist. Antagonists play two important roles in muscle function: (1) they maintain body or limb position, such as holding the arm out or standing erect, and (2) they control rapid movement, as in shadowboxing without landing a punch, and thereby check the motion of a limb.

For example, to extend the knee, a group of four muscles called the quadriceps femoris in the anterior compartment of the thigh is activated. These muscles would be called the agonists of knee extension. However, to flex the knee, an opposite or antagonistic set of muscles called the hamstrings is activated. Flexing the knee involves the hamstrings muscles as the agonists and the quadriceps femoris muscles as the antagonists. As you can see, the terms agonist and antagonist are associated with a particular movement, and these terms can be reversed for opposing movements. See Table 12.1 for a list of some common agonists and antagonists.

Muscles Without Attachments to the Skeleton

Some skeletal muscles do not pull on the skeleton to cause movements. For example, the muscles that produce facial expressions have their insertions and origins in the skin, and particular muscles contract to form a smile or frown, form sounds or words, or raise the eyebrows. There are also such skeletal muscles in the tongue, as well as in the external urinary and anal sphincters that allow for voluntary regulation of urination and defecation, respectively. Another example is the diaphragm, which contracts and relaxes to change the volume of the pleural cavities without moving the skeleton.

Part 2: Lever Systems

Skeletal muscles do not work by themselves. Muscles are arranged in pairs based on their functions. For muscles attached to the bones of the skeleton, the connection determines the force, speed, and range of movement. These characteristics depend on one another and can explain the general organization of the muscular and skeletal systems.

General Characteristics

The skeleton and muscles act together, as a lever, to move the body.  Have you ever used the back of a hammer to remove a nail from wood? The handle acts as a lever and the head of the hammer acts as a fulcrum, the fixed point that the force is applied to when you pull back or push down on the handle. The effort applied to this system is the pulling or pushing on the handle to remove the nail, which is the resistance to the movement of the handle in the system. The resistance is also sometimes called the load. Our musculoskeletal system works in a similar manner, with bones being stiff levers and the articular endings of the bones—encased in synovial joints—acting as fulcrums. The resistance would be an object or body part being moved, or any resistance to a movement (e.g. your head when you are lifting it). The effort, or applied force, comes from contracting particular skeletal muscles.

The characteristics and operation of a particular lever system are mainly determined by distances and forces. The two opposing forces are the effort and resistance. There are two main distances to consider: (1) the effort arm, which is the distance from the fulcrum to the insertion point of the skeletal muscle delivering most of the effort, and (2) the resistance arm, which is the distance between the fulcrum and the bulk of the resistance.

In order for movement to occur (e.g. when lifting a weight in your hand), the effort produced by one or several muscles needs to overcome the resistance. When the lever system is balanced, the two opposing forces applied at their respective distances from the fulcrum work to maintain a body posture or carry a particular weight without moving it. Thus, in a balanced lever system, the effort times the effort arm is equal to the resistance times the resistance arm. This is the basic formula used to calculate relationships between forces and distances in a lever system:

Effort x Effort arm = Resistance x Resistance arm

Lever systems that can move heavy loads over short distances using little effort are referred to as “power levers”. Conversely, lever systems that can quickly move light loads over large distances using a large amount of effort are referred to as “speed levers”. The location of the insertion point of a muscle (or muscles) delivering the effort relative to the resistance determines whether a lever system operates as a power lever or speed lever. For example, a lever system with a muscle insertion point close to the joint (fulcrum) and a resistance far away from the joint will move a particular object relatively fast over a large distance with a great range of motion. However, the muscle(s) involved need(s) to deliver a large effort for the movement to occur. Another lever system with a resistance close to the fulcrum which is supported by a muscle (or muscles) inserting far away from the fulcrum will require a small effort to support or move the resistance. However, the resistance will only be moved over a short distance and slowly.

There are several types of lever systems in the body, identified as either first-class, second-class or third-class levers.

First-class levers

First-class levers are the simplest types of lever, where the two forces, the effort and the resistance, are applied on opposite sides of the fulcrum (Figure 12.2). In the body, the best example of a first-class lever is the way your head is raised off your chest (Figure 12.3). The posterior neck muscles produce the effort, the facial skeleton is the resistance, and the atlanto-occipital joint behaves as the fulcrum.

 

 

Second-Class Levers

Second-class levers have the resistance between the effort and the fulcrum (Figure 12.4). The effort is closer to the resistance than the fulcrum, which allows a large resistance to be moved by a small amount of effort. However, this means that the resistance will be moved at a relatively slow pace and can only be moved a short distance. Any time you stand up on your toes, as shown in Figure 12.5, you are using a second-class lever. The weight of your body acts as the resistance, your calf muscles produce the effort, and the joints in the balls of your feet act as fulcrums.

Third-Class Levers

Third-class levers are the most common type of levers in your body (Figure 12.6). The effort is applied between the fulcrum and the resistance, which allows the resistance to be moved relatively quickly over large distances. When you lift your hand by flexing your biceps brachii, you are using a third-class lever. The elbow joint acts as the fulcrum, the biceps brachii produces the effort, and the weight of your hand is the resistance being lifted (Figure 12.7).