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Kinematics and Kinetics

Biomechanics can be broken into two parts: kinematics and kinetics. Here we will look at some of the principles of both.

As we saw in a previous step, biomechanics can be broken into two parts: kinematics and kinetics. Here we will look at some of the principles of both.

Kinematics

Kinematics uses common physics-based variables to describe motion concerning space and time. These variables can refer to a single point, a body segment, a joint, a limb, or the whole body. The main basic kinematic variables in biomechanics are position, velocity, and acceleration. Each of them can be linear or angular. They can also be either local or global.

The position is the location of an object at a particular time. Therefore, it is a scalar variable and only tells about where the object is located in space. In a three-dimensional motion capture system, the position is usually described in terms of its X, Y, and Z coordinates. It is the calibration of the system that decides how the three coordinates relate to the physical space. We will look more at that shortly.

As can be seen in the figure below, the displacement tells about how far an object has moved in space. It is a vector and is represented by both a location and a direction. The distance travelled is a scalar value describing the total amount of movement.

Graphical Explanation of Position Displacement Distance.

The distance travelled is often longer than the displacement. In fact, if one walks around in a room and ends up in the same spot that one started, the distance travelled can be large while the displacement is zero.

The speed describes how fast an object is moving. It is a scalar quantity calculated from the distance travelled. The velocity is a vector and is calculated from the displacement. It can be used to look at how fast and in what direction a part of the body moves in space.

The acceleration describes the change in velocity of an object. It is beneficial for detecting changes in the movement direction.

Kinetics

Kinetics focuses on how forces make the body move. The musculoskeletal system is responsible for generating forces that move the human body and prevent any unwanted motion.

Force and Moment

As shown in the figure below, a force is the push or pull that alters the state of motion of a body. Without forces being applied, there will be no change in motion. In the musculoskeletal system, the muscles generate force that acts upon bones and leads to motion. Gravity is an example of an external force that affects the human body.

Different types of forces.

When a force rotates a body segment, it is called torque or a moment of force. The moment is defined by the length of the rotating object, the force applied, and the angle between the force vector and the object. In biomechanics, they can tell us what muscle group is dominant, so flexors or extensors perform work.

The force and moment can be used to calculate the power used, defined as the work done over time. We will not dive into this here, but interested readers can check out some of the references below.

Balance

Balance is the ability to control equilibrium or stability. In biomechanics, static balance refers to the ability to control the body when stationary. Dynamic balance is the ability to control the body during motion.

Center of gravity (CoG) is simply the point where the distribution of the weight of the body is equal in all directions. As the name suggests, CoG depends on gravity, which means that the location will change with movement. For example, if a person bends forward, their CoG will shift anteriorly.

Base of support is the region beneath a person connecting points of contact with the supporting surface. If the person is standing, the region would include the feet and the space between.

A person is considered in balance when the line of gravity originating from their center of gravity is within their base of support. The figure below illustrates the relationship between body position, CoG and the resulting line of gravity, base of support, and balance.

Four bodies showing four different lines of gravity with relation to the base of support and balance.

Summing up

We have now looked at some of the core terms and concepts in biomechanics. Some of these you may know from school physics. Others may be new. You don’t need deep knowledge of biomechanics to work with motion capture, but it helps to have heard about some of the concepts. You can always look up the specifics when needed.

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Motion Capture: The Art of Studying Human Activity

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