Work physiology Lecture note: IE 665 Applied Industrial Ergonomics

Physiologic response to muscular work ATP molecules are the unit of biologic energy. It can be synthesized aerobically or an-aerobically. Physical work may have both aerobic and anaerobic components. How much ATP is produced to perform the bodily functions is closely coordinated with cardiovascular responses. To match the oxygen demand for work (closely related to the ATP need), adjustment occurs in, HR, SV, A-V difference in capillary blood in O 2 concentration, Blood Redistribution, Blood Pressure, Breathing rate, and Ventilation.

Oxygen debt and repayment

Oxygen consumption (VO 2 ) rate and muscular work intensity VO 2 rate in L/min can be measured using special apparatus during work. VO 2 rate = Diff in O 2 conc. in inspired and expired breath(%) * ventilation (L/min) Also, VO 2 rate = A-V difference (%) * Cardiac output (L/min) –VO 2 rate provides an accurate of ATP production rate, and consequently intensity of physical work can be determined precisely. Higher the work intensity higher the VO 2 consumption rate. –VO 2 is difficult to measure in field. –(VO 2work - VO 2rest )/(VO 2max - VO 2rest ) gives the percent of work capacity being used for a work

Cardiac responses to muscular work Cardiac output (CO), amount of blood pumped per minute, to some extent is proportional to the cellular respiration or ATP need or metabolic demand for a workload. CO = HR (/Min) *Stroke Volume (liters) liters/min HR is effected by:(i) Emotions especially at low metabolic level, (ii) Ambient temperature and (iii) intensity of muscle work Stroke Volume is affected by the intensity of exercise. SV increases with exercise intensity and reaches its maximum level for an exercise intensity of about 40% of ones maximum. As a result of these HR is an excellent predictor of work intensity for moderate to heavy intensity physical work. HR can be easily monitored, and hence used extensively to quantify the work intensity level. (HR work - HR rest )/(HR max - HR rest ) also provides a rough estimate of the percent of work capacity being used for a work. HR max varies with age. Roughly, HR max = Age in years, with a sd= 10 bits per min.

Work Capacity Work capacity or fitness level of an individual can be determined in laboratory by measuring the VO 2 max. There are several exercise protocols, that employ gradually increasing physical work intensity. Using a bicycle ergo-meter or a treadmill, the subject performs exercise with gradually increasing resistance. The rate of O 2 consumption, when the subject reaches his/her limit of exertion is his or her the VO 2 max, which is indicative of his/her fitness level or work capacity of the subject. VO 2 max can also be determined from a sub-maximal exercise, in that case the sub-maximal VO 2 is projected with the help of heart rate data to determine the VO 2 max. In VO 2 max exercises, the cardiac limit can only be reached if large muscle groups are involved during exercise. This is why, the VO 2 max will be lower if a bicycle ergo-meter is used instead of a treadmill to reach the limit. The person will reach his her maximum exhaustion level due to local muscle fatigue, before cardiac limit is reached. VO 2 max is indicative of aerobic fitness level.