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Kinematics of Human Motion

Instructor: John Hamilton

John has tutored algebra and SAT Prep and has a B.A. degree with a major in psychology and a minor in mathematics from Christopher Newport University.

Are you fascinated by watching the seemingly endless variety of human motions? In this lesson, we discuss the kinematics of human motion, including translatory and rotational motion, the three major planes of motion, and several anatomical terms of motion. These include flexion versus extension, circumduction, and pronation and supination.

Human Motion

Many of us have been enthralled while watching world-class gymnasts perform seemingly impossible maneuvers on television, or been thrilled at a stellar dance performance. These events involve varying degrees of human motion to execute them.

Gymnast in Motion on the Pommel Horse
pommel

Newton's first law states that, in general, an object at rest stays in rest, while an object in motion stays in motion. This is also known as inertia and definitely applies to the human body and human motion.

The study of human motion is a branch of biomechanics known as kinematics. Kinematics specifically studies just pure motion and not the actual forces which cause the motion. There are several types of motions exhibited by the human body.

Translatory & Rotational Motion

Most human motion involves a combination of two types of motion, translatory motion, also known as linear motion, and rotational motion, also known as angular motion. Think of a long jumper sprinting toward the sandy pit. He is exhibiting translatory motion as he runs in a straight line. Then he takes off and swings his arms to try and stay airborne, which is a form of rotational motion. Together, the combined movements are general motion.

A Long Jumper Exhibits General Motion
athlete

Three Planes of Motion

The human body is three-dimensional (it has height, width, and depth), although it often just moves in two dimensions in what is known as planar motion. While space is three-dimensional, a plane is simply a surface of two-dimensions that extends to infinity (forever.) Therefore, planar motion is motion that involves only two of the three dimensions. The human body moves through three distinct planes:

  • Frontal Plane - Did you ever have to do jumping jacks in gym class? Those were an example of movement in the frontal plane. With jumping jacks, there is both side-to-side movement of the arms and legs and also an up-and-down movement; however, there is no third movement of forward-and-backward.
  • Sagittal Plane - Movement in the sagittal plane involves bending forward and backward along with up and down; however, there is very little side-to side motion. These movements tend to be the most common used when exercising. Lunges and curls are two notable examples.
  • Transverse Plane - Movement in the transverse plane tends to be a little more complex because it often involves rotational movements. A common example would be to do lunges while holding a medicine ball, but this time step diagonally instead of forward. First step diagonally to the left. Then, return the feet to the original position. Then, step diagonally to the right. Do you see how this would involve a twisting, or rotational, motion of the body? Within this plane, the body moves up/down, forward/backward, and side to side.

A Lunge in the Sagittal Plane
lunge

Anatomical Terms for Motion

Now let us expound on some of the different terms for motion. These movements can be at angles, in circles, and from one side to another. They also range from the head and neck to the torso and the limbs.

Flexion versus Extension

Flexion is simply a decreasing of the angle between two given body parts. Extension is the opposite, and thus is the increasing of the angle. During a bicep curl, you would bring your palm and the dumbbell toward your shoulder, which would decrease the angle between your upper arm and your forearm. This is an example of flexion, or more familiarly stated, you are flexing your muscle. When you lower the dumbbell back down, you increase the angle between your upper arm and your lower arm. This would be an example of extension.

A Bicep Curl Exhibits Flexion
weight

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