Have you ever wondered what made for the game winning pass in the Super Bowl? What makes one Olympic downhill skier faster than another? How can cyclists in the Tour de France ride so fast? Why do we walk and run the way we do? Why are certain points in a strength training exercise more difficult? Why are hip fractures so common in the elderly? What causes an ACL tear? The answers to these questions and many more are rooted in the field of biomechanics.

Biomechanics is defined as the study of the laws of mechanics as they relate to the structure, function and motion of any biological system. So what is mechanics? Mechanics is the branch of science concerned with motion and forces that produce motion of physical bodies.

NASA's Digital Astronaut Project provides an example of biomechanics in action. "Three biomechanical exercise modules that include joint and muscle modules" by NASA Digital Astronaut Project (Public Domain).

While biomechanics is a distinct field of science it has relationships with many other fields, including anatomy, physiology, neuroscience and motor control. The field of kinesiology has considerable overlap with the field of biomechanics, particularly with aspects of biomechanics that deal with the study of movement as kinesiology is defined as the discipline concerned with the comprehensive and systematic study of human movement. Further, biomechanics and kinesiology have considerable practical application, ranging from describing and quantifying physical performance (often with the goal of improving performance) to understanding the mechanical causes and symptoms associated with various injuries and diseases (often with the goal of improving function or preventing injury). In this course we will focus primarily on the topics of describing and enhancing physical performance and understanding injury mechanisms with the goal of prevention.

Two primary subdivisions of the field of biomechanics are kinematics and kinetics. “Kinematics is defined as a description of motion, including the pattern and speed of movement sequencing by the body segments that often translates into the degree of coordination” (Hall, 2019) or level of performance that an individual displays. Alternatively, kinetics is the study of motion and its causes, particularly forces. While both kinematics and kinetics describe motion, the key distinguishing feature is that kinetics is concerned with forces, whereas kinematics simply describes motion.

PLANES & AXES

One of the first things that we have to consider when describing human motion is that it can be three dimensional (3D). Often, we simplify in biomechanics by separately looking at movement in each of three planes that together make up the 3D space. A plane is a two-dimensional surface that is defined by three points that are not in a straight line. You can think of a plane as if you were looking at a flat projector screen. It is important to note some foundational information about planes and axes:

Axes are always perpendicular (at a 90^o angle) to a plane and can be moved anywhere within the plane
Each plane is only associated with one axis. Always use the relationship between a plane and its’ axis to check your understanding.
To observe, look at the plane (like you would a projector screen), which means that you are looking down the axis (the axis comes out from the flat projector screen). Rotation occurs along the plane and around the axis (there is no movement out from or into the screen).

Before we discuss the specific planes, let’s first talk generally about fundamental movements. Fundamental movements refer to movements from anatomical position that are associated with a specific plane/axis. This means if you were to create the given movement, based on its’ orientation it would be associated with a specific plane/axis. A couple of key points about fundamental movements:

Movement occurs at a joint not a segment e.g. elbow flexion not arm flexion---flexing the arm would involve breaking the bone!
Flexion and extension are movements, not muscle contraction. Both of these movements can be created without any muscle activation! We’ll learn about this later in the course.

As we discuss each specific plane/axis in the up-coming sections, we will also cover the specific fundamental movements associated with each plane/axis.

The anatomical planes:

The three anatomical planes: frontal, sagittal and transverse.

"Anatomical Planes" by CFCF is licensed under CC BY-SA 3.0. Plane titles added from original.

The anatomical axes:

The three anatomical axes: superior-inferior axis (associated with transverse plane), anterior-posterior axis (associated with frontal plane) and medio-lateral axis (associated with sagittal plane).

"Anatomical axes" by Edoarado and Mikael Häggström is licensed under CC BY-SA 3.0 Axis titles changed from original.

Sagittal Plane – Medio-Lateral Axis

The sagittal plane provides a side view of the body by dividing the body into right and left halves. The plane does not necessarily run down the mid-line of the body, but instead can divide the body into any combination of unequal right and left halves. The axis associated with the sagittal plane is the medio-lateral axis. Just like is sounds, the medio-lateral axis runs from medial to lateral, which makes it perpendicular (as it should) to a side view of the body. For example, when observing elbow flexion and extension, we are looking at the side of the body, movement is along the sagittal plane and rotation is around the medio-lateral axis, which is positioned at the elbow joint.

Sagittal plane with medio-lateral axis

"Anatomical Sagittal Plane" by Edoarado is licensed under CC BY-SA 3.0 Titles changed from original.

Sagittal Plane Fundamental Movements

In the sagittal plane, the fundamental movements are flexion, extension, and similar type movements. Remember that all of these fundamental movements are from anatomical position and occur around the medio-lateral axis. The specific sagittal plane fundamental movements include:

Flexion & Extension—flexion is defined as a sagittal plane movement that decreases the joint angle between a segment and its proximal segment, whereas extension is defined as a sagittal plane movement that increases the joint angle between a segment and its proximal segment.
Hyperflexion & Hyperextension—are extensions of flexion/extension movements and refer to flexion/extension movements beyond the plane of the body. (Note: it is okay to use these terms or to simply refer to these movements as flexion and extension).
Plantar flexion & Dorsi-flexion—are specific to the ankle (talo-cural joint) and plantar flexion refers to the toes moving posterior (toward the ground), whereas dorsi-flexion refers to the toes moving toward the shin.
Anterior & Posterior Tilt—are specific to the pelvis and anterior title refers to the anterior aspect (front) of the pelvis tilting anterior (forward and downward), whereas posterior tilt refers to the anterior aspect (front) of the pelvis tilting posterior (backward and up).
Anterior & Posterior Tipping—are specific to the scapula and anterior tipping refers to the anterior aspect (front) of the scapula tipping anterior (forward and downward), whereas posterior tipping refers to the anterior aspect (front) of the scapula tipping posterior (backward and up)

Examples of fundamental movements of the sagittal plane around the medio-lateral axis:

Flexion and extension movements are fundamental movements of the sagittal plane.

"Body Movements" by Tonye Ogele CNX is licensed under CC BY-SA 3.0

Frontal (Coronal) Plane – Anterior-Posterior Axis

The frontal (or coronal) plane provides a front or back view of the body and divides the body into front and back halves. Just like with the sagittal plane, the frontal plane does not necessarily run down the mid-line of the body, but instead can divide the body into any combination of unequal front and back halves. The axis associated with the frontal plane is the anterior-posterior axis, which runs from anterior to posterior making it perpendicular (as it should) to a front or back view of the body. For example, when observing shoulder adduction, we are looking at front view of the body, movement is along the frontal plane and rotation is around the anterior-posterior axis, which is positioned at the shoulder joint.

Frontal plane with anterior-posterior axis

"Anatomical Coronal Plane" by Edoarado is licensed under CC BY-SA 3.0 Titles changed from original.

Frontal Plane Fundamental Movements

In the frontal plane, the fundamental movements are abduction, adduction, and similar type movements. Remember that all of these fundamental movements are from anatomical position and occur around the anterior-posterior axis. The specific frontal plane fundamental movements include:

Abduction & Adduction—abduction is the frontal plane motion of a limb away from the midline of the body, while adduction refers to the frontal plane motion of a limb towards the midline of the body.
Hyperabduction & Hyperadduction—are extensions of abduction/adduction movements and refer to abduction/adduction movements beyond the plane of the body. (Note: it is okay to use these terms or to simply refer to these movements as abduction and adduction).
Right & Left Lateral Flexion—are specific to the trunk and refer to frontal plane rotation to the right in the case of right lateral flexion or left in the case of left lateral flexion (think of the anterior-posterior axis running through the belly button).
Right & Left Lateral Pelvic Tilt—are specific to the pelvis and refer to rotation in which the right side of the pelvis moves superiorly (right lateral pelvic tilt) or the left side of the pelvis moving superiorly (left lateral pelvic tilt)
Radial & Ulnar Deviation—are specific to the wrist (remember palms face anterior in anatomical position) and radial deviation refers to the thumb moving toward the radius and ulnar deviation refers to the pinky moving toward the ulna.
Inversion & Eversion—refers to rotation down the long axis of the foot. Inversion refers to the foot rotating toward the midline of the body and eversion refers to the foot rotating laterally. Note that these movements are often confused with transverse plane movements because they occur around the long axis of a segment (the foot). However, because the foot point anterior in anatomical position, these are considered fundamental frontal plane movements.
Upward & Downward Rotation—are specific to the scapula, with upward rotation corresponding with the acromion process moving superior (upward) and downward rotation corresponding with the acromion process moving inferior (downward).

Examples of fundamental movements of the frontal plane around the anterior-posterior axis:

"Body Movements" by Tonye Ogele CNX is licensed under CC BY-SA 3.0

Transverse Plane – Superior-Inferior Axis

The transverse plane provides a top-down or bottom-up view of the body and divides the body into top and bottom halves. Similar to the sagittal and frontal planes, the transverse plane can divide the body into any combination of unequal top and bottom halves. The axis associated with the transverse plane is the superior-inferior axis, which runs from superior to inferior making it perpendicular (as it should) to a top or bottom view of the body. For example, when observing hip rotation, we are looking at top down view of the body, movement is along the transverse plane and rotation is around the superior-inferior axis, which is positioned down the long axis of the leg. Transverse plane with superior-inferior axis

"Anatomical Transverse Plane" by Edoarado is licensed under CC BY-SA 3.0 Titles changed from original.

Transverse Plane Fundamental Movements

In the transverse plane, the fundamental movements are rotation type movement (note that any movement with rotation in the name is a transverse plane movement). Remember that all of these fundamental movements are from anatomical position and occur around the superior-inferior axis. The transverse sagittal plane fundamental movements with differing names include:

Medial & Lateral Rotation (also referred to as Internal/External and Inward/Outward Rotation)
Right & Left Rotation—are specific to the trunk and refer to rotation where the right side of the body moves anterior (forward) in the case of right rotation or left in the case of left rotation (think of the superior-inferior axis running down the midline of the body).
Pronation & Supination—are specific to the radio-ulnar joint (not the wrist), with pronation referring to the palms turning posteriorly and supination referring to the palms turning anteriorly.
Horizontal Abduction & Adduction—refer to a joint (hip or shoulder) that first moves to a flexed position and then rotates anterior toward the midline of the body (horizontal adduction) or posterior (horizontal abduction).

Examples of fundamental movements of the transverse plane around the superior-inferior axis:

Example of transverse plane movement "Body Movements" by Tonye Ogele CNX is licensed under CC BY-SA 3.0

While we have covered the basics of three orthogonal planes/axes and their associated fundamental movements, there are a few remaining important points:

Circumduction, which involves making a circle with a limb or segment, combines sagittal (flexion, extension) and frontal plane (abduction, adduction) motions.
All movements defined in this slide show are rotational movement, even though we use the word rotation only for transverse plane motions.
Not all joints permit movement in all 3 planes – learn the planes of movement associated with each joint.
Movement terms for each plane vary across joints – learn appropriate terminology for each joint.

The movement of circumduction crosses both the frontal and sagittal planes.

"Body Movements" by Tonye Ogele CNX is licensed under CC BY-SA 3.0

DIRECTIONAL TERMINOLOGY

Superior—toward the head
Inferior—away from the head (toward the feet)
Anterior (ventral)—toward the front of the body
Posterior (dorsal)—toward the back of the body
Medial—toward the midline of the body
Lateral—away from the midline of the body
Proximal—closer to the point of attachment
Distal—away from the point of attachment
Superficial—toward the body surface
Deep—away from the body surface
Contralateral—on the opposite side of the body
Ipsilateral—on the same side of the body

Anatomical directional references in both the lateral and anterior views.

"Directional References" by Blausen.com staff is licensed under CC BY 3.0

FUNCTIONAL JOINT CLASSIFICATION

The morphology of a joint limits the amount of motion that is allowed. Some joints allow movement only in one plane (around a single axis), whereas others allow movement in multiple planes. The functional classification of a joint describes the number of planes within which a joint allows movement.

Uniaxial Joint—allows movement in one plane (e.g. elbow, ankle)
Biaxial Joint —allows movement in two planes (e.g. knee, wrist
Triaxial Joint —allows movement in all three planes (e.g. hip, shoulder)
Nonaxial—no axis (e.g. spine, glide movements of many joints)

Mechanical System

A mechanical system is any system that has moving parts and accomplishes tasks that involves forces and movement. When analyzing human motion, a mechanical system can consist of part of the body, the whole body, or the whole body plus the addition of external tools (e.g. sporting equipment). For example, a bomechanist may choose to analyze the throwing arm during a baseball pitch, the kicking leg in a soccer penalty kick, the trunk during performance of a deadlift, the entire body during performance of a maximal vertical jump, or the entire body and bicycle of a Tour de France cyclist. While the mechanical system can consist of anything ranging from a portion of the body to the full body and external tools, it is generally selected to address some underlying question. For example, if we wanted to understand the forces and motion that leads to the development of tennis elbow, then we would likely choose the arm and possibly the whole upper body in our analysis. It is unlikely that the lower body motion/forces contribute to the development of this overuse injury, so it wouldn’t be particularly relevant to the analysis and hence would be excluded from the mechanical system.

Center of Mass

The Center of Mass (COM), also referred to as the Center of Gravity (COG) of an object is the point around which a body’s weight is equally balanced in all directions. The COM is a simple way to represent a mechanical system.

The COM is located at the geometric center (centroid) of objects that have a uniform density. For example, the COM of a frisbee, which is made of a uniform plastic, is at the center of the disc. The COM may be located outside of the object, for example with a ring disc the COM falls in the open space in the center and similarly with a boomerang the mass is distributed equally top to bottom and left to right leaving the COM in the center of the boomerang.

Center of mass locations of various objects:

Center of mass locations of various objects

The COM of an object is a simple way to represent a mechanical system and this location is important because it is the point where any uniform force can be described as acting on the system. For example, gravity is a uniform force that acts on all components of any system on earth, but we simplify and describe gravity as acting at the COM of the system. Describing uniform forces as acting at the COM simplifies biomechanics problems that require us to describe the motion of oddly-shaped objects or complicated systems, because these complicated structures are treated as a single point mass.

Learn more about the COM, how it is calculated and why it is important: What is center of mass?

Finding the location of the COM in systems with a non-uniform density is a bit more complicated. The human body is an excellent example of a mechanical system that does not have a uniform density. For example, the upper torso is lighter because of the air in the lungs, than the lower body, which is heavy due to the large muscle mass in the legs. Generally, the COM is located at approximately 55% of standing height from the floor, in the midline of the body along the medio-lateral axis (approximately at the location of the umbilicus) and slightly anterior of the midline of the anterior-posterior axis.

Further complicating the location of the COM in the human body is the fact that the human body can move. The location of the COM does not have to be fixed and the human body is a good example of the COM locating shifting with body movement. As we move our segments or add weights to our body, the COM of the entire system will shift in the direction that mass is added or moved toward. For example, when we pick up a suitcase, the COM of our body + the suitcase moves toward the suitcase. Similar to what we observed with uniform objects, the COM of the human body can be located outside of the body’s physical space. Specifically, we can move our segments in such a way that the COM can shift outside the body. (IMAGE An excellent example of the COM moving outside of the body is the high-jump or pole vault. When performed correctly the athlete arches their body when moving over the bar, and in doing so their body moves over the bar and their COM moves under the bar:

Center of mass moves under the bar and body moves over the bar in a successful high jump

Fosbury by Ralf Pfeifer is licensed under CC BY-SA 3.0

Read more on the science of the high jump: Fosbury Flop – The Art of High Jumping.

References:

Hall, S. J. (2019). Basic biomechanics. New York, NY: McGraw-Hill Education.

When no attribution is noted, images are from the Public Domain or the work of the author.