In order to investigate various aspects of physical activity, such as its benefits, the effectiveness of particular programs, and to conduct scientific studies, we require accurate methods for measuring the the amount of physical activity in which we participate. As individuals, these methods prove valuable because once we know where we are, we can figure out what we have to do to improve our physical fitness.

Physical activity is usually described by four parameters, known as the "FITT principle:"

All physical activity involves energy expenditure. This is difficult to measure because there are so many different types of physical activity: leisure time, occupational, transportation, and housework may all have physical components and we must search for a standard of measurement. The total amount of physical activity depends on the frequency, intensity, type, and time spent in various activities. We measure this total in kilocalories.

Before discussing different ways in which we can monitor physical activity, we should first take a look at the criteria by which we rate these methods, as each has unique strengths and weaknesses:

Reliability: how consistent and repeatable the method is. The ability of a test yield consistent scorers.

Validity: the accuracy of the method. The ability of a test to accurately measure what it is supposed to under different circumstances, such as different seasons. This is the most important criteria.

Objectivity: whether different testers will provide similar results for a tested individual.

Feasibility: whether the method costs an individual in terms of comfort, time, and effort. May also include the cost of equipment.

Non-reactivity: weather is the method influences a person's behavior (it should not).

There are two main types of measurement methods: direct monitoring and self reporting.
Direct monitoring methods
These methods measure physical activity through an instrument or objective observer. these methods are often used to establish the validity of self-report methods.
Doubly labeled water: an extremely accurate method for measuring the metabolic rate, or total energy expenditure of an individual over several days. However it cannot give data pertaining to specific activities, and is expensive.

Pedometers: measure the number of steps taken. You can calculate total distance by measuring stride length. Is not an accurate assessor of speed.

Accelerometers: are either unidimensional or triaxial. Unidimensional accelerometers can measure vertical movement of the trunk. Triaxial accelerometers can measure vertical, horizontal, and lateral movement. Neither provide effective assessments of biking, swimming, or skating.

Heart rate monitors: are effective for measuring vigorous physical activity. They are less effective for light and moderate intensity activities because heart rate may rise due to other factors, such as stress.

Third-party observation: is where an observer watches and records another individual's physical activity and is most effectively used with children. While accurate, this method can be time-consuming and expensive.

Self-report methods
There are many different types of self report measures where individuals record their own activity. These may suffer from a lack in reliability and validity. Most of these methods are similar, with time frame often being the biggest differentiating factor, which may range from a single day to a year or longer. Shorter time frames are generally more accurate regarding the physical activity completed, but is less likely to be representative of an individuals year-round activity level. Longer time frames are generally the opposite, with less accuracy pertaining to individual activities but providing a clearer picture of an individuals lifestyle.

It is generally most effective to combine a number of the aforementioned methods. The more information we have about our level of fitness and our physical activity habits, the easier it will be to implement additions or changes to our physical activity regimen