Biomechanical terminology

Mechanics is the area of physics concerned with the motion of physical objects. Biomechanics is the application of mechanics to biological systems.

Biomechanics aims to explain how and why the human body moves the way it does. This is done using mechanical principles like forces and motion. As illustrated in the figure below, biomechanics also draws on fields like biology, medicine, and engineering.

As in anatomy, the terminology used in describing human movement is important for ensuring correct communication. Many of the terms come from other disciplines, like physics. We will go over a few basic ones to ensure that everyone is on the same page with vocabulary.

The Subfields of Biomechanics

The statics branch of biomechanics deals with resting bodies, such as when sitting or standing. During rest, the body is in equilibrium due to balanced forces. Dynamics is concerned with bodies in motion, such as when running or climbing. This motion results from unbalanced forces. For example, you can start running by leaning forwards.

Kinematics is the description of motion; it is sometimes called the “geometry of motion”. Kinetics is the cause of the motion, meaning what causes the body to move the way it does. Simply put, kinematics you can see, kinetics you can’t. We will look at both of these more closely in a later section.

Qualitative vs Quantitative

Biomechanics uses both qualitative (descriptive) and quantitative (numerical) descriptors. Examples of qualitative descriptors are “good”, “slow”, and “smooth”. Examples of quantitative values are 8 m/s (meters per second), 67° (degrees), and 53 N (Newton).

In most cases, one would relate qualitative to quantitative descriptors. However, there are no one-to-one relationships. What is considered “slow” or “fast” usually depends on the context.

Variable Types

Many biomechanical parameters can either be represented as scalar or vector quantities. Scalars only have a magnitude, while vectors have both a magnitude and a direction. Consider the difference between speed and velocity. Speed tells how fast you are moving in any direction. Velocity, on the other hand, tells how fast you covered a specific distance.

A vector can be either linear or angular in nature. Linear values describe motion from one position to another. Angular values describe the rotation about an axis.

Considering a bike rider, like in the above picture, there are three different types of values:

• Scalar: The bicycle moves at a speed of 30 km/hr
• Linear Vector: The bicycle is moving at a velocity of 30 km/hr, heading 45 ° northeast
• Angular Vector: The wheel of the bicycle is rotating with a velocity of 270 °/s

Reference Frame

Just as for the anatomical planes, we also need a reference frame when studying human motion. This can be either global or local. When working with a motion capture system in a lab, we often define the coordinate system relative to the lab space. This can be the centre of the room, from which all other positions are calculated.

Sometimes it makes more sense to use a local reference frame. For example, it is irrelevant to know the exact position in space if one is interested in how the arm moves. Then it is more relevant to study elbow flexion and extension as the angle between the arm and forearm.

Looking for a textbook on biomechanics? We can recommend these:

• Basic Biomechanics is a good introductory text
• Comprehensive Textbook of Clinical Biomechanics and Orthopaedic Biomechanics also include a focus on the musculoskeletal system