Chapter 9: Biomechanics
By Alexis Gidley, PhD
Learning Objectives
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Define biomechanics and mechanics and differentiate between qualitative and quantitative biomechanics.
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Explain how Newtonian physics principles, such as force and acceleration, are used to analyze and interpret human movement.
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Identify and describe modern technological tools used in quantitative biomechanics, such as accelerometers and force plates.
Chapter Content
What is Biomechanics?
The study of human movement using the concepts and theories of mechanical physics.
What is Mechanics?
Mechanics is really quite simple, it is the laws and theories that help explain the forces that caused something to move, and the resultant movement from those forces. While we cannot see the forces that cause movement, we can see the results: movement. The forces can be measurable, depending on the question and the equipment available, but we can often assess the movements and track backwards to discern what forces caused the movement to occur.
The short video below will give you a bit more background on biomechanics and how it can be used in a sports environment.
How do we use Biomechanics?
There are two “types” of Biomechanics: qualitative and quantitative.
Qualitative Biomechanics, or analysis, is much more than just a binary declaration of “it changed or didn’t.” An individual who uses qualitative analysis of human movement, such as a coach, physical therapist, or personal trainer, should have a vast knowledge of the movement being assessed and will have a “keen eye” for determining the effect of a change. This person will also have to have a deep understanding of the underlying physics of the task, environment and the person, as there will be little to no data to rely on to answer their questions. This type of assessment only uses the power of observation and the superpower of reasoning to solve the problem. This type of analysis is important in many fields, and is a skill that can take years to develop, even within one discipline. To understand the effectiveness of a movement, the effect of a change or how one may have decreased the risk of injury qualitatively takes great skill, care and knowledge. It also takes proper training to truly “know what is right.” Unfortunately, there are many individuals in any given field who rely firstly on the teachings of others and properly assess the information they just acquired from “experts” and who then pass on said information that is outdated at best, and may be incorrect, leading to poorer performance or increased risk of injury, at worst.
Quantitative Biomechanics uses technology (usually expensive technology) to collect some variables, like forces or body positions. Then using theories, thus equations, further variables can be calculated to provide even more information about the movement. This information is usually in the form of numbers, so it is important to understand the underlying theory behind the calculation so one can understand what the number means. Additionally, figures representing the motions of the body or segments of a body or forces acting on the body over a specific time period are used to help define and describe movement patterns, so it is important to gain the skill of describing a figure well. It is important that all students have a strong understanding of Newtonian physics, even if they never use quantitative biomechanics, presented in the context of human movement, so they have the foundational information to think more critically about “traditional” movement understanding, assess the situation within which their patient, client, customer, student, or athlete is moving, and provide better instruction for performance, health and/or rehabilitation. Starting with quantitative Newtonian Physics can help students of Biomechanics and professionals of human motion make Newtonian Physics more tangible, and less abstract. F=ma is definitely understandable as a sentence:
A force on an object (m) causes an acceleration of that object (m).
A force on an object (m) over a given amount of time causes a change in velocity of that object (m).
But when we put numbers to that introductory equation, it becomes easier to “visualize” and it allows for greater discussion and application at even the beginning levels.
Here is a short video that gives you some examples of quantitative biomechanics using equipment that is quite similar to that found in our lab at WOU.
A few words on technology
Many years ago, Quantitative Biomechanics only occurred in research biomechanics labs. This might be on a University Campus, in an industry setting, like at Nike, or in some private, well-funded locations. Today, with modern technology, there is greater opportunity for anyone to have access to the numbers side of Biomechanics. For example, in the early to mid-2020s Apple watch introduced step length and stride rate to their Apple watch features. There are accelerometers in our phones and watches that measure our steps. There is fancier, but relatively inexpensive, technology that collects data while running, allowing for estimates of the force with which runners hit the ground, or the rate of this force (which is often an indication of injury risk to soft tissue in runners). Power meters on bikes are becoming almost commonplace. It is inevitable that you will run into one of these, or others as technology continues to improve, and someone who has the tech will come to you and want you to explain what the number means and do they need to worry about it. You should know how to help them accurately and confidently, so they use that number as best they can. Additionally, if you are considering going into physical therapy or athletic training, there are many well-equipped therapeutic facilities that use pieces of equipment called Force Frames, or Biodex, or a Noraxon system, (no need to know what these are just now), or force plates and motion capture, like you would see in a University Lab, from which numbers magically appear. But it will be your job to interpret them. Therapeutic practices are no longer purely qualitative careers, they are data driven professions that demand a basic knowledge of quantitative biomechanics to best help their patients. And bad news is, most don’t have a biomechanist hanging around who can help interpret the numbers, that’s your job.
Some Final Thoughts
Newtonian Physics defines the context, or the environment, within which a human moves. For example, if I say jump as high as you can, you KNOW you will return to the earth and you have a fairly good sense of how quickly that will happen. That’s because we are constrained by the laws of physics, and we can’t break them. When you are learning a new skill or movement, whether that be as a child learning to walk or an adult learning to play pickleball, the environment has very predictable constraints – Newtonian Laws of Physics. There is, however, another set of constraints we need to be aware of, and those are the internal constraints. These are dictated by our musculoskeletal design. Our body is unique in the animal kingdom due to its upright design, large range of motion in the shoulders, a relatively large head on a relatively small neck, smallish feet leading to a small base of support, etc. But also, the way our muscles connect to our bones and the muscle architecture (pennation angle, fiber lengths, moment arms, lines of action, etc) all combine to constrain our movements. Sure we can do a lot, but there are still limitations and we have only so many choices. Have you ever realized that we all walk relatively the same, and we didn’t have to go to a specific walking class to learn it? The combination of the external constraints (Newtonian Physics) and the internal constraints (human musculoskeletal design) provides a framework within which the brain can learn to solve the problems of our movement patterns. And that is just in a healthy context, when you are dealing with an injury or a musculoskeletal condition, like MS, this adds another layer of constraint that may change your movement pattern.
What does this mean, movement pattern? Human movement, which can be quantified with Newtonian Laws and Theories and mathematical processes, is the result of motor programs sent from the motor structures of the brain, to the muscles necessary to accomplish the task. Muscles then produce force and because they act at a distance from an axis of rotation (the joint) they create torques, moving your limbs. All of this information falls within the realm of motor control, kinesiology (functional anatomy) and exercise physiology. Thus, the details of how the human body overcomes the external world to accomplish the goal will be saved for other courses. Biomechanics is simply the study of movement to understand and define how the brain solves the constraint problem while accomplishing the movement goals. By using Newtonian Physics we can better and more easily understand the world through which we move and the generalized responses of the neuro-muscular-skeletal system to overcome the constraints for successful motion. So, even if we are focusing on quantifying the movements of the human (e.g. joint angular velocities) we are learning how the human overcame the Laws of the environment, and thus we are learning about the effect of the environment on the body.
Try to keep this simplistic view of Biomechanics: the human is a biological creature with internal constraints due to its unique design, that must navigate an environment, the constraints of which is dictated by Newtonian Physics, by learning how to create muscle activity to create successful movement patterns. Oh and by the way these three pieces happen to be three courses you will take – biomechanics, kinesiology and motor behavior/control. I wish you great success as you learn to understand and describe how and why the human moves as it does.
Here are a few fun examples of biomechanics in the sports world:
Usain Bolt and Biomechanics – the Science of the Summer Olympics (5:24)
Biomechanics of Sports: Running, Jumping, and Hitting (5:09)
