Part III: Basic Biomechanics: Why You Move
Chapter 7: The Nuts and Bolts of Movement
The Nuts and Bolts of Movement
In This Chapter
▶ Understanding forms of motion
▶ Identifying types of forces
▶ Discovering the laws of movement
The world is continually moving. Every day you can look around and notice people doing things like walking, running, cycling, and swimming.
You probably haven’t given much thought to what it takes to perform these skills. Few people actually sit and analyze how to walk, run, cycle, or swim, let alone how to do any of these activities better — unless they’ve been forced to through an injury that immobilized them or they are in a field where performing at peak ability is vital.
Movement and the factors that affect motion are quite complex and require an understanding of several principles related to physics. For example, everything that you encounter is being acted on by forces. Regardless of how something may be moving (or not) — whether it’s spinning, flying through the air, or sitting perfectly still — invisible forces are affecting it. This chapter provides insights into what is behind getting stronger, throwing a ball, and running faster, among other things. You’ll never again think the same about how you move!
Biomechanics: The Study of Movement
You’ve no doubt heard of Newton’s laws, forces, and vectors in high school science or physics courses. When you learned about these principles, chances are you did so in relation to nonhuman examples. Well, the field of biomechanics takes those principles and applies them to human movement.
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Biomechanics is the investigation of how forces act on the human body from a mechanical perspective. The mechanics part of the term represents the study of physical actions, and the bio part refers to living organisms (in this case, humans). Examining the effects of forces, their impacts on the human body, and how the human body creates motion either internally or externally is a fascinating and very dynamic area of investigation.
By studying biomechanics, you can answer questions like, “Why do some baseball pitchers hurt their elbows?” “How can I make myself run faster (or jump higher)?” “Why don’t I fall off my bike when I lean really far over as I go around a corner?” and “How come the Olympic gymnasts can do such amaz- ing acrobatic things and look so graceful?” — a question I (Brian) ask myself repeatedly as my daughter tries to teach me how to do a cartwheel.
The role of the biomechanist
We all appreciate the grace and power that Olympians and professional ath- letes exhibit. But these athletes aren’t as good as they are because they were born that way. Sure, genetics has something to do with their abilities, but they’ve also studied their skills and investigated how to improve. In a sense, they have been their own biomechanists.
Shooting a basketball, walking, jumping, and skipping all sound like very simple tasks, but they are all actually quite complex skills. Executing any task involves the interplay among multiple forces and body positioning. How far you should bend your elbow or knees, how big your steps should be, and which foot goes first are the kinds of things you start to ponder when you learn these skills.
By understanding the types of forces and motion in relation to the structure and function of the human body, you can look for ways to enhance
performance and prevent injury.
The biomechanist’s problem-solving process
To assess any type of activity or task, biomechanists follow a process.
Typically, this process starts with understanding the nature of the task, followed by a deliberate observation of the task in action. From this observation, data is collected and used to evaluate the performance, which ultimately leads to feedback and, if necessary, an intervention. The next sections take you through this process.
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Understanding the nature and objective of the task
The first requirement in assessing a movement is to understand the task being completed. You must have this prerequisite information to fully address what, in fact, is occurring as part of the movement. For example, if you want to jump, you know that you first need to bend at your knees, which causes your ankles and hips to also bend and puts you in a sort of squatting position. From this position, you then push up as hard as you can, a move- ment that makes your body rise and, if you have enough force behind the push, lifts you off the ground.
When you know how the joints move during the activity, you can start asking more complex questions like, “How do I jump higher?” This type of ques- tion requires not only an understanding of joint motions but also knowledge about which muscles are involved and how they’re activated.
In addition to having an understanding of the task being performed, the biomechanist must understand the intent of the activity. For example, analyzing a pitcher’s throwing motion can only be done with an appreciation of what the pitcher is trying to accomplish.
Observing the task and collecting data
Biomechanists must be able to collect the necessary data. Necessary data is any data that provide substantive and defendable information that is key to answering the question.
Suppose a pitcher comes into the athletic training room because of a sore elbow. The athletic trainer must collect the data necessary to answer why the pitcher’s elbow hurts. Key information includes things like how often the pitcher throws, what type of pitches he throws, and whether he was throwing long toss or off the pitching mound. The answers to these kinds of questions help the athletic trainer fully understand the situation being assessed.
Evaluating the data and making a diagnosis
After observing the task and collecting data, biomechanists use the data to make comparisons of the current situation to others like it. Everyone walks or runs differently, but certain things have consistently been proven to affect the success of the task. In this step, biomechanists compare the specific data they gathered from a subject to what they know should be occurring. Based on this information, they can recognize flaws and identify areas of improvement.
The assessment is a critical aspect to answering your question. When you per- form such an assessment, you must do so objectively, taking the information (data) as it is and using your findings to answer the question.
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Sharing the findings with the athlete: Intervention and feedback
Concluding the process, biomechanists share their findings with their patients/athletes. By sharing the information soon after the activity and in a way that identifies the key components, a plan then can be developed to improve performance and, in many cases, avoid injury.
Kinematics: A Compass Telling You Where You Are
To assess and evaluate motion, you must understand the underlying com- ponents of human structure and the various physical and environmental components that affect everyone. Kinematics is the study of movement and includes considerations of form, velocity, direction, time, acceleration, pat- tern, displacement, and sequencing.
Many refer to this area of study as the descriptive, or qualitative, portion of motion analysis because values aren’t quantified. Kinematic references are made in relation to things like direction and how fast someone may be going, or what movement is happening first and what follows. In kinematics, you don’t consider the forces within the movement, which makes kinematics dif- ferent from kinetics, a topic that we introduce in the later section “Studying Kinetics: May the Force Be with You!”
Looking at body systems
Activities of daily living, like brushing your teeth, rising from a chair, or pour- ing a glass of water, are necessary to normal, everyday living. Although they are often taken for granted, especially by healthy, active people, each task is actually very complex. In fact, any movement you make, even the “ordi- nary” ones, requires coordination and communication between your body’s muscular, skeletal, and nervous systems:
✓ The muscular system: This system involves the soft tissue structures that are contractile in nature; that is, they have the ability to create ten- sion by contracting. Muscles are connected to bones by tendons. When a muscle is contracted (see Chapter 11), it contributes to or produces an end result: Often muscle contraction results in joint movement. Other times, muscle contraction results in no motion but helps to support the body and increases stability.
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Assessment in action: How do I run faster?
Figuring out how to run faster isn’t as easy as you may think. Using the process outlined in the section “The biomechanist’s problem-solving process,” you would approach this problem in this way:
1. Understand the nature and objective of the activity and amass the necessary prerequisite information.
To more deeply understand the act of running, you need to ask yourself what actually occurs during this activity. Clearly, you have to move your ankles, knees, and hips. But what other components are involved? Breaking down the running (gait) pattern can help. What about the type of running — is your client a distance runner or a sprinter?
2. Observe the task and collect data.
Typically, the gait cycle is broken down into two phases: stance and swing (refer to Chapter 3). The stance phase begins with your foot striking the ground and continues as your body weight is transferred forward. It ends when you push off at your toes to propel yourself. While one foot is in the stance phase, the other is in the swing phase.
The gait cycle specific to running varies from that of walking in that the heel strike doesn’t exist. Instead, striking occurs at the forefoot (by the toes). Additionally, the weight transition phase is either nonexistent or occurs in a very small window of time. The interesting aspect of running is that, at times, the body is totally airborne, and no contact with the ground occurs — a situation that doesn’t happen when walking.
Understanding the motions that make up the gait cycle is important, but how those motions are achieved is equally valuable information that can help you answer the question, “How do I run faster?” Muscle actions during the gait cycle dictate the amount of push-off during propulsion at the ankle and knee. Additionally, the muscles bring a limb from the swing to the stance phase, prepare for landing, and help main- tain balance throughout.
By understanding the motion and the muscle forces that are needed to com- plete this task, you can begin to address the question. You now know that running speed is dependent on the length of each of your strides, the force exerted during the push-off coupled with the resistance (body weight), and how fast you can bring your leg back after you push off.
3. Evaluate the data and make a diagnosis.
Noticing that someone may be taking strides that are entirely too long or are really bouncy gives you information that you can use to identify flaws. By compar- ing the current task to what you know about how the task should be carried out, you can diagnosis areas for improvement.
4. Provide intervention and feedback.
Given the information that you gathered and the results of the evaluation and diagnosis, you can now provide feedback and suggest an intervention program. This feedback and intervention may be as simple as telling the client to shorten his stride when running or to not bounce so high when he goes from step to step.
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✓ The skeletal system: The skeletal system provides the framework that supports the human body. Your bones are the structural blocks that allow you to stand, sit, and walk. Your muscles attach to these bones, and when they contract, the bones are pulled by the tendons, resulting in movement of the joint.
✓ The nervous system: The nervous system, often overlooked and largely taken for granted, involves the brain and subsequent nerve function throughout the body. Absolutely everything that you do is controlled by your nervous system. Your brain initiates movement by deciding the strategy that is needed for the particular motion, and the peripheral nervous system acts as the highway to transmit the signals among your brain, spinal cord, and the rest of your body. The brain helps determine motor patterns (for information on motor control, head to Chapter 3), and this system interprets pain, activates muscles, and controls motion.
Each of these systems has very important individual functions, but it’s their coordinated work that enables you to go about everyday as you do. Consider something as simple as walking. The motor patterns that dictate walking are defined in the brain from past activities and are sent to the local areas (knee, ankle, and hip), where the movement is carried out. Here’s what happens:
1. As soon as you decide that you want to walk, your brain interprets your body position, where you are (walking up a hill, stairs, and so on), and decides how to best move.
2. Each of your muscles is activated to propel your body. While the motor pattern is being sent to the muscles that are active, other muscles are preparing for their role.
At the same time you may be pushing (propelling) yourself forward from your toes, for example, your other foot is preparing for your heel to strike the ground and your same-side hamstring group is readying to slow your leg down.
3. All the while, your skeletal system serves as the source of support and conduit for movement.
Identifying forms of motion
When analyzing motion, biomechanists often divide movement into three subcategories: linear motion, angular motion, and general motion. Figure 7-1 shows the different types of motion:
Linear motion
In linear motion, all parts of an object move in the same direction at the same velocity (speed and direction). Often this type of motion is referred to as translation. Linear motion can occur in two ways, rectilinear and curvilinear:
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✓ Rectilinear motion occurs when the object in its entirety is moving in a straight line. A passenger sitting still on a train that is going straight is a good example of moving in a rectilinear fashion.
✓ Curvilinear motion results in a uniformly curved pattern of movement.
Curvilinear motion exists when a stunt driver, for example, takes his car and jumps over a flaming bonfire. Because of the momentum generated while going up the ramp, the car goes up and over the fire and lands on the other side. As long as the car doesn’t spin as it goes over the fire, the motion is curvilinear.
Figure 7-1:
Types of motion:
rectilinear, curvilinear, angular, and general.
Angular motion
Moving objects don’t all travel in a uniform direction. Instead, they often involve angular motion as well. Angular motion is the movement of an object around an axis of rotation or an imaginary line.
Illustration by Wiley, Composition Services Graphics
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Nearly every motion that occurs within the body is an example of angular motion. When muscles pull on the bones, they cause the bones or limb seg- ments to bend or rotate at their joints. A figure skater performing a spin on the ice is an example of angular motion. The skater’s entire body is spinning in relation to the axis of rotation while she balances on the ice.
General motion
A majority of the motion that occurs everyday falls into the general motion category. General motion exists when linear and angular motions are com- bined. A ball thrown by a pitcher is an example of both angular and curvilin- ear motion: The spin of the ball is angular motion, while its trajectory as it approaches the batter is curvilinear (an arc).
Human motion is almost always general motion. As you walk down the street, your joints experience angular motion (because they’re swinging in an arc) while your body as a whole is moving in a straight line (translating) down the street in rectilinear fashion.
Defining key terms
To understand and break down the movements that exist in the world, bio- mechanists must have command of the vocabulary. When referring to areas of the body or movements, you do so in reference to their positioning, called anatomical position. Anatomical position is an upright standing position that has the feet separated shoulder width apart, the arms hanging at the sides, and the palms facing forward (see Figure 7-2).
Using directional terminology
To describe how the body moves and the relationship of an object to the body, you must be able to use the following terminology:
✓ Superior: Closer to the head, or “above.” Your shoulder is superior to your hip.
✓ Inferior: Closer to the feet or “below.” Your knee is inferior to your hip.
✓ Anterior: Toward the front of the body. Your nose is anterior to the back of your head.
✓ Posterior: Toward the back of the body. Your heel is posterior to your big toe.
✓ Medial: Toward the middle of the body. Your nose is medial to your ear.
✓ Lateral: Away from the middle of the body. Your ear is lateral to your mouth.
✓ Proximal: Closer to your trunk. Your elbow is proximal to your wrist.
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✓ Distal: Further from your trunk. Your foot is distal to your knee.
✓ Superficial: Closer to the surface of the body. Your sternum is superficial to your heart.
✓ Deep: Away from the surface of the body. Your intestines are deep to your abdominal muscles.
✓ Axial: Along the longitudinal axis of the body. An acorn falling onto the top of your head is an axially directed force.
Figure 7-2:
Anatomical position.
Illustration by Kathryn Born, MA
138 Part III: Basic Biomechanics: Why You Move the Way You Do Planes of motion
When standing in anatomical position, the body is bisected by three cardinal reference planes, shown in Figure 7-3. A plane is an imaginary flat surface that divides the body into equal halves. The three planes (sagittal, transverse, and frontal) all intersect at the point considered as the center of mass. The planes are used as a reference in relation to the human body. When the body moves, the planes also move with it. The planes are
✓ The sagittal plane: This plane travels down the body and divides it into left and right halves.
✓ The transverse plane: This plane separates the body into top and bottom portions.
✓ The frontal plane: This plane travels down and separates the body into front and back portions.
References to body motion are often based on the plane within which the motion occurs. Motion that moves in-line with a particular plane or parallel to it is typically referred to as motion occurring in that plane.
To determine in which plane the motion occurs, imagine that the motion was completed standing next to mirrors representing the planes of motion. The mirror(s) not broken after the movement would be considered the plane that the motion occurred in.
Figure 7-3:
Cardinal planes of motion and axes of rotation.
Illustration by Kathryn Born, MA