Physical work capacity

1. Physical work capacity How the body does its work. The control systems (the bodies governor) CNS, hormonal and limbic system. Three major systems - Respiratory Circulatory Metabolic

2. Outline Introduction Basis of physical work Key concepts of physical work -the body’s physiological systems; assessment of energy expenditure; and energy requirements at work. Lab

3. Introduction Anthropometric data needs to be considered alongside human work capacity data. Human work capacity data can be divided into four areas: 1.The epidemiological approach 2.The physiological approach 3.The biomechanical approach 4.The psychophysical approach

4. The epidemiological approach This approach is based upon identifying the frequency, distribution and possible control strategies of illness and injury for a given population. The following are a few characteristics of the approach: The examination of trends. This approach often provides the initial indication of a relation between work factors and illness and injury. The epidemiological approach can be expensive, time consuming and subject to confounding variables.

5. The physiological approach The physiological approach is concerned with analysing energy demands of the musculoskeletal system for work activities and reducing the energy expenditure of these work activities.

6. Biomechanical approach This approach is concerned with determining the forces exerted upon the musculoskeletal system during work tasks. The biomechanical approach predicts maximal, low and frequency capacity of individuals.

7. The psychophysical approach The psychophysical approach is concerned with human sensations associated with external stimuli.

8. The physiological approach - characteristics of this approach. The physiological approach seeks to design work activities that would allow the continuous performance of tasks without the accumulation of excessive physical fatigue. When work activities cannot be redesigned, the physiological approach seeks to establish reasonable exposure to the activity. The physiological approach is considered appropriate for many dynamic activities such as frequent materials handling.

9. Assessment of energy expenditure The ability to perform physical work differs in people due to, Gender Age Health Environment and motivation

10. Energy requirements for the task Ergonimists aim to match the person’s work capacity with the requirements of a job by, Knowing the individuals work capacity. Knowing how much the job demands from this capacity. Therefore, to know these things, one measures the workers actually doing the job.

11. Basis of physical work Key concepts for work analysis and design Energy requirements should not exceed one -third of an individuals physical work capacity (PWC) for an eight-hour work day. PWC is defined as the maximum oxygen consumption by working tissues (Vo2 max or maximum aerobic capacity). PWC defines the upper limit of energy expenditure for an individual. Note: I litre of oxygen consumed = 5 kcal

12. Key dependent variables The following are used in assessing the physiological demands of work. Energy expenditure Oxygen consumption Heart rate Pulmonary ventilation Lactic acid accumulation Blood pressure R.P.E.

13. Laboratory PWC is usually measured sub-maximally by ergonomists using regression methods and relying on a strong linear relationship between HR (note HR has psychophysical as well as physical dimensions) and o2 consumption. Therefore, using this linear relationship it is possible to estimate PWC by extrapolating a sub-maximal HR to determine maximum HR. Then certain validated models can also determine oxygen consumption from HR.

14. Max HR estimation Max HR can be estimated using: Males: 205- age/2 Females 220-age (Hellerstein et al., 1973).

15. Sub-maximal test Three successive, increasing workloads that result in a three ordered pairs of data for HR and 02 for a simple linear regression model. Three progressive workloads of 3 minutes each will be used. The age-dependent value of maximum HR serves as an input for the model which then yields the PWC estimate.

16. Example Using a cycle ergonometer a 30 yr old male weighting 70 kg with 12% body fat performed an 3 stage incremental test.

17. Example Determine PWC from collected data PWC (abs) y = a + bx1 b = n ? xy - ? x ? y n? x2 –(? x) 2 a = ? y – b ? x n y = Vo2 x = HR N = number of workloads X1 = estimated max HR y =PWC (Absolute value)

18. Using regression equations b = n ? xy - ? x ? y n? x2 –(? x) 2 b = 3 * 538.9 – (368 * 4.15) = 0.02189 3 * 46,504 – 135,425 a = ? y – b ? x n a = 4.15 - 0.02189 * 368 = 1.30184 3 PWC (abs) y = a + bx1 = 1.30184 + 0.02189 * (190) = 2.858 L/Min PWC body weight = 2.858 * 10000 / 70 = 40.8 mL kg*min

19. Using HR as an indicator of workload Utilse the relationship of: %HR max = (HR ave – RHR) * 100% est. Max HR – RHR %HR max = Percent HR workload criteria HR ave = AV HR during task performance RHR = Resting HR (seated at rest for 5 min) est. Max HR = Age dependant estimation of max HR