Concepts:

1. Units
The bases for the SI (System International) unit system are the Kilogram, Meter, and Second for Mass length and Time.

A coherent unit system is one in which 1 unit of the new quantity is defined by one unit of all the component quantities. For example:

FORCE : Newton's second Law: F = m*a
1 unit of force (the Newton) = 1 unit of mass (the kilogram) * 1 unit of acceleration (1 meter per second squared).

CURRENT Ohm's Law: I= V/R
One amp of current will flow when a potential difference of 1 volt exists across a resistance of 1 ohm

PRESSURE Pressure = Force/Area
The pressure (normal stress) will be 1 Pascal when a force of 1 Newton is distributed over on square meter.

2. Prefixes and Abbreviations

Table 1 shows the prefixes, units and their abbreviations that you should be familiar with. Note that abbreviations and prefixes are always case sensitive.

Table 1: Important quantities and their units

        
 Quantities

        
Units

        
 Mass

        
kilogram (kg)

        
 Length

        
meter (m)

        
 Time

        
second (s)

        
 Frequency

        
hertz (Hz)

        
 Force

        
 newton (N)

        
 Moment

        
 newton meter (Nm)

        
 Pressure

        
 pascal (Pa)

        
 Work

        
 joule (J)

        
 Energy

        
 joule (J)

        
 Power

        
 watt (W)
 
 

3. Familiarization with Frequently
Used Quantities

Table 2 gives some examples of the magnitude of important quantities often measured in biomechanics and motor control that you should learn in order to get a feel for reported values when you encounter them in the literature.

Table 2: The magnitudes of frequently encountered quantities.

      
 Quantity

      
 Context

      
 Typical Magnitude

      
 EMG
 The peak
      to peak magnitude of surface EMG during MVC

      
 5 millivolts (5mV)

      
 EEG
The peak to
      peak magnitude of alpha wave activity

      
 10 microvolts (10mV)

      
 Ground reaction force (GFR)
Peak vertical component of GRF during running

      
 3 kilo newtons (3kN)

      
 Plantar Pressure
Peak
      pressure under the normal forefoot during barefoot walking

      
 350 kilo pascals (kPa)

      
 Time
The delay
      between stimulus and monosynaptic reflex response

      
 30 milliseconds (ms)

      
 Body Mass
The total
      body mass of a 50th percentile year 2000 Japanese female

      
 51.5 kg



      

      
 Body Weight
The
      1G weight of a 50th percentile year 2000 Japanese female

      
 505.2 N

      
 Body Segment mass
The
      mass of one thigh of a typical adult male

      
 8 kilograms (kg)

      
 Moment of Inertia
Moment
      of inertia of the shank about an axis through the center of mass
      and perpendicular to the sagittal plane

      
 64 * 10-3 kg.m2

      
 Muscle moments
Peak
      early support moment at the knee joint during slow running

      
 200 N.m