1.8: Biomechanics

Learning Objectives

1. Describe each level type.
2. Understand muscle antagonist movement.
3. Describe how human biomechanics may be examined.

Biomechanics is the study of how the body physically moves through the lens of physics and mechanical principles. As such it combines multiple areas of study including biology, physics, engineering, and kinesiology to explain and study how the body moves and responds to internal and external forces. Biomechanics also helps us understand the “how” and “why” in many other fields of kinesiology. Many physical skills can be broken down into physical directions of the body. Imagine writing down the motion of the body of someone throwing a ball.

Some of the key concepts in advanced biomechanics of the human body include

1. Kinematics: The study of motion, including linear and angular motion, displacement, velocity, and acceleration.
2. Kinetics: The study of forces and their effects on motion, including Newton's laws of motion and the concept of momentum.
3. Joint mechanics: The study of the structure and function of joints, including the types of joints, their movements, and the forces acting on them.
4. Muscle mechanics: The study of the properties of muscle tissue and their role in generating force and movement.
5. Biomechanical modeling: The use of mathematical models and simulations to study and predict movement patterns and forces in the body.
6. Performance analysis: The use of biomechanical analysis to evaluate and optimize human performance in sports and other activities.
7. Injury prevention and rehabilitation: The use of biomechanical analysis to identify and address factors that contribute to injury, and to design interventions to prevent or treat injuries.

Critical Thinking

Before moving on, what are some ways your understanding of biomechanics would be important for other fields of kinesiology? For example, how would a PE teacher use biomechanics to explain how to kick a soccer ball to a grade school kid?

One of the main goals of biomechanics is to understand the factors that influence movement efficiency and athletic performance. This includes examining the effects of muscle strength, joint range of motion, and other factors on movement like injuries. Biomechanics also study how the body responds to external forces impacting movements like tripping, carrying a weighted backpack, or slippery walking surfaces.

The basics of physics and biomechanics combine understanding movement (review of Chapter 1) and how the physical body works as a system of simple machines (our muscles, bones, and joints from Chapter 5).

Sir Isaac Newton developed three laws of motion that are the foundation of understanding physical movement. Simply stated they are as follows:

1. The Law of Inertia. If a body is at rest or moving, it will remain at rest or moving until acted upon by an outside force.
2. Force is equal to mass x acceleration. This law describes the relationship between something’s mass, acceleration, and force.
3. The Law of Action and Reaction. For every action, there is an equal and opposite reaction.

Skeletal muscles do not work by themselves. Muscles are arranged in pairs (agonist muscle groups) 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.

The skeleton and muscles act together 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 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 the 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 of the effort arm is equal to the resistance times of the resistance arm. This is the basic formula used to calculate relationships between forces and distances in a lever system:

\[\text { Effort x Effort arm = Resistance } \times \text { Resistance arm }\nonumber\]

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) inserted 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.

Critical Thinking

While moving through the rest of this chapter take periodic breaks and experiment with moving your own body and talking about the concepts with other classmates.

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

First-class levers are the simplest types of levers, where the two forces, the effort, and the resistance, are applied on opposite sides of the fulcrum (Figure 2). In the body, the best example of a first-class lever is the way your head is raised off your chest (Figure 3). The posterior neck muscles produce the effort, the facial skeleton is the resistance, and the atlantooccipital joint behaves as the fulcrum.

Figure 1: Resistance being balanced by equal effort across a fulcrum. "First Class Lever" by Douglas College Biology Department is licensed under CC BY 4.0

Figure 3. Graphical overlay showing a first class lever system in the human neck. First-class lever as seen in the body. (credit: sarahmckinnon/flickr.com, original image: celtibere/pixabay.com)

Second-class levers have resistance between the effort and the fulcrum (Figure 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 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.

Figure 4. Resistance being held up by effort. "Second Class Lever" by Douglas College Biology Department is licensed under CC BY 4.0

Figure 5. Graphical overlay showing a second class lever system in the human ankle. (credit: sarahmckinnon/flickr.com, original image: thenarratographer/flickr.com)

Third-class levers are the most common type of lever in your body (Figure 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 7).

Figure 6. Effort pulling resistance up. "Third Class Lever" by Douglas College Biology Department is licensed under CC BY 4.0

Figure 7. Graphical overlay showing a third class lever system in the human elbow. (credit: sarahmckinnon/flickr.com)

To move the skeleton, the tension created by the contraction of the fibers in most skeletal muscles is transferred to the tendons. The tendons are strong bands of dense, regular connective tissue that connect muscles to bones. The bone connection is why this muscle tissue is called skeletal muscle.

To pull on a bone, that is, to change the angle at its synovial joint, which essentially moves the skeleton, a skeletal muscle must be attached to a fixed part of the skeleton. The moveable end of the muscle which attaches to the bone being pulled is called the muscle’s insertion, whereas the end of the muscle attached to a fixed (stabilized) bone is called the origin.

Watch this video on how proper use of levers and fulcrum arms can improve human movement.

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 1). A synergist can also be a fixator that 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 shadow boxing without landing a punch, and thereby check the motion of a limb.

Figure 1. Drawing of how the arm lifts a glass of water. "Prime Movers and Synergists" by Douglas College Biology Department is licensed under CC BY 4.0

The biceps brachii flexes the lower arm. The brachioradialis, in the forearm, and brachialis, located deep to the biceps brachii in the upper arm, are both synergists that aid in this motion.

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 1 for a list of some common agonists and antagonists.

Agonist	Antagonist	Movement
Biceps brachii (in the anterior compartment of the arm)	Triceps brachii (in the posterior compartment of the arm)	The biceps brachii flexes the forearm, whereas the triceps brachii extends it
Hamstrings (group of three muscles in the posterior compartment of thigh)	Quadriceps femoris (group of four muscles in the anterior compartment of thigh)	The hamstrings flex the leg, whereas the quadriceps femoris extend it
Flexor digitorum superficialis and flexor digitorum profundus (in the anterior compartment of the forearm)	Extensor digitorum (in the posterior compartment of the forearm)	The flexor digitorum superficialis & profundus flex the fingers and hand at the wrist, whereas the extensor digitorum extends the fingers and hand at the wrist

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 muscles contract to form a smile or frown, form sounds or words, or raise 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.

In addition to studying the mechanics of movement, human biomechanics also investigates the effects of different movements on the body. For example, researchers may study the impact of running on the joints and muscles or the effect of lifting heavy objects on the spine. By understanding these effects, biomechanics can develop strategies to reduce the risk of injury and improve performance.

Most likely you are currently in a seated position, hopefully in a comfortable position. Using biomechanics to address the body positioning of a person while working is called ergonomics. Ergonomics strives to allow workers to be optimally efficient at their job while in a safe position. Clearly more physically demanding and dangerous jobs are problematic with people developing acute injuries and research is done on all aspects of that job. As an example: is it more ergonomic for a police officer to wear an equipment belt or vest?

More day-to-day tasks are also studied for ergonomics, from anti-fatigue mats, the correct height of standing desks, and how far away a computer monitor should be from your eyes. Repetitive use injury (RSI) is pain or injury caused by continual repeated movement of the body, like typing on a keyboard. All these ergonomic issues stem from optimal human biomechanics and creating equipment to suit the needs of workers.

Critical Thinking

What are some objects you interact with daily that may have been designed with a biomechanical perspective? Are there any objects that you use that clearly need some redesign?

There are many tools and techniques used in the study of human biomechanics. These include motion analysis systems, which use cameras and other sensors to track the movement of the body, and force plates, which measure the forces acting on the body during movement. Bio mechanists also use computer modeling and simulations to study movement and predict the effects of different forces on the body. The modern smart phone is becoming so powerful and equipped with high quality cameras that many apps can accurately track and record body movement.

Human biomechanics is a complex and multifaceted field that plays a vital role in understanding how the body moves and the factors that influence movement efficiency and performance. By studying the mechanics of movement and the effects of different movements on the body, bio mechanists are able to develop strategies to improve performance and reduce the risk of injury.

If this chapter was of particular interest to you, biomechanics may be the ideal focus of your kinesiology education leading to a career as a biomechanist.

Chapter 9 continues along a similar path as biomechanics but investigates how the mind controls movement of the body.