Measurement Of Work And Power

 

Bench Step

 

 The term ergometry refers to the measurement of work output. The word ergometer refers to the appai ratus or device used to measure a specific type of) work. Many types of ergometers are in use today in exercise physiology laboratories ( figure ).

 

 

 

Figure  Illustrations of four different ergometers used in the measurement of human work output and power. (a) A bench step. (b) Friction-braked cycle ergometer. (c) Motordriven treadmill Both the treadmill elevation and the horizontal speed can be adjusted by electronic controls. (d) Arm crank ergometer. Arm crank ergometry can be used to measure work output with the arms and is based on the same principle as cycle ergometry.

 

 A brief introduction to commonly used ergometers follows. One of the earliest ergometers used to measure work capacity in humans was the bench step. This ergometer is still in use today and simply involves the subject stepping up and down on a bench at a specified rate. Calculation of the work performed during bench stepping is very easy. Suppose a 70-kg man steps up and down on a 50-centimeter (0.5 meter) bench for ten minutes at a rate of thirty steps per minute. The amount of work performed during this ten-minute task can be computed as follows:

 

Force = 685.3 N (i.e., 70 Kg X 9.79 N/Kg)

 Distance = 0.5 m * step -I X 30 steps * min I X 10 min

= 150 m

 

Therefore, the total work performed is: 685.3 N X 150 m = 102,795 Joules or 102.8 kilojoules (rounded to nearest 0.1) The power output during this exercise can be calcu-lated as:

Power = 102,795 Joules/600 seconds 171.3 watts

 

Cycle Ergometer The cycle ergometer was developed more than 100 years ago and remains a popular ergometer in exercise physiology laboratories today (see A Look Back Important People in Science). This type of ergometer is a stationary exercise bicycle that permits accurate measurement of the amount of work per-formed. A common type of cycle ergometer is the Monark friction-braked cycle, which incorporates a belt wrapped arounc the wheel (called a flywheel) (figure ).

 

 

The belt can be loosened or tightened to provide a change in resistance. Distance traveled can be determined by computing the distance covered per revolution of the pedals (6 meters per revolution on a standard Monark cycle) times the number of pedal rev-olutions. Consider the following example for the com-putation of work and power using the cycle ergometer. Calculate work given:

 

 

 

 

Duration of exercise = 10 min

 

Resistance against flywheel = 1.5 kg or 14.7 N

Distance traveled per pedal revolution = 6 m

Pedalling speed = 60 rev * min-1

Therefore, the total revolutions in 10 min

= 10 min X 60 rev * min-1

Hence, total work = 14.7 N X (6 m * rev-^1 X 600 rev)

                             = 52,920 Joules or 52.9 kilojoules

 

 

The power output in this example is computed by dividing the work performed by time:

 

 

Power= 52,920 Joules /600 seconds

=88.2 watts

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