An article published in the Oct 2011 Medicine & Science in Sports & Exercise journal presents a simple method called the HR Index that can be used to determine the metabolic demands of sustained physical exertions. This new approach may have application in assessing work tasks that require cardiovascular endurance. The tool seems fairly simple to administer and appears to have good predictive validity.
There are many ways to investigate and categorize the physical demands of work. Examples of those familiar to ergonomists include the NIOSH Lifting Equation and the Snook Tables.
- The NIOSH Lifting Equation is a tool used to identify, evaluate and predict the risks of injury associated with a lifting task. It is useful for determining maximum acceptable weights.
- Liberty Mutual Manual Materials Handling Tables. Also known as the “Snook Tables”; these provide estimates of the percentage of the population capable of performing manual material handling tasks without over exertion. These tables can be used for ergonomic assessments of lifting, lowering, pushing, pulling, and carrying tasks.
While these tools are useful for evaluating lifting and other manual handling tasks, the ergonomics practitioner might need to know more about cardiovascular limits to work performance. The practitioner might need to know if specific employees can safely perform the task.
With an aging and increasingly sedentary population we can expect more overweight, deconditioned workers who might also have cardiovascular disease and various other chronic health conditions that reduce work capacity. In these instances it would be useful to be able to measure work demands and worker capabilities for sustained endurance work.
Up until now, measuring cardiovascular endurance meant using expensive laboratory equipment including treadmills, cycle ergometers, and metabolic carts. These tools were required to carefully measure the workload and to calculate the oxygen consumption of the working muscles. The resulting measurement of the volume of oxygen consumed per minute or VO2 showed the cardiovascular workload and at peak showed maximum fitness levels.
Enter The Heart Rate Index:
This new method for estimating cardiovascular demands is called the HR Index, and is based on simple heart rate (HR) measurements from a controlled resting state, plus the steady state work sample.
Prior to this study HR had no established place for estimating energy expenditure. This is due to disruptive influences of fitness, age, gender, environmental factors and common blood pressure medications. These influences limited the ability of HR to accurately predict VO2. Prior attempts to use HR to predict VO2 revealed inaccuracies especially at lower workloads.
The HR Index was statistically derived from a collection of 220 data sets from 60 published exercise studies. It included a cohort of 11,257 individuals representing a diverse range of ages (10-85 yr), and a wide range of health status including; apparently healthy, cardiovascular disease, diabetes, obesity, frailty. Some study participants were in training programs, while others were on beta blocker blood pressure medications.
The study used this data set and compared 3 theoretical models for predicting energy expenditure/ aerobic workload. The analysis looked for the best fit in a regression analysis:
- The HR Absolute model using peak HR data explained 77% of the predicted MET scores.
- The HR Net model using (HR Absolute – HR Rest) explained 98% of the variation, but did result in some over and some under estimated scores in the mid and upper ranges of HR.
- The HR Index model using (HR Absolute/HR Rest) explained 99% of the variation, and improved on the residuals which ended in a more random distribution around the line of best fit.
Because the HR Index is derived from 220 data points that are each group averages, the authors are unable to provide prediction errors for the equation.
How to use this in your practice:
Measuring metabolic demands of steady state work tasks can be a useful method for ensuring that job demands are acceptable for your workers. It can assist in accommodating workers with diminished capacity, and it can ensure that the work is not too taxing for a variety of cases.
- Carefully measure the resting heart rate: Have the subject sit in a quiet area for 5 to 20 minutes then measure the pulse at the wrist or with a monitor. Use a 30 or 60 second count and count twice.
- Your basic formula is (HR Absolute / HR Rest) where Absolute is the steady state working heart rate.
- It may also be helpful to see how close to predicted maximum your subject is working. You can estimate the age predicted maximum heart rate to see how much reserve capacity the subject may have beyond the steady state work. Common formulas include (220-age) and (205- ½ age)
- An alternative to predicting maximum HR is the Borg Scale of Perceived Exertion to see how close to maximum the subject may be when working.
From HR Index you can calculate metabolic equivalents or METs: where METs = (6HR Index – 5). METs are an expression of VO2 and work load. Representative METs chart.
- 1 MET is the metabolic demand from sitting quietly
- 4 METs is considered Light exercise or Moderately Heavy work for an 8-hr shift
- 8 METs Moderate exercise
- 10 METs typical Maximum capacity for untrained 50 year old male
- 12 METs Heavy exercise load
- 14 METs achievable Maximum capacity for trained 50 year old male
- 20+ METs common Maximum capacity for Elite athletes
Ergoweb article: NIOSH Lifting Q & A
Liberty Mutual Manual Handling Tables: Job Evaluator Toolbox™ (JET™)
Gene Kay has a Masters degree in Exercise Science and is a Certified Ergonomics Associate. He has been designing web-based ergonomics programs for 10 years, and owns the ErgoAdvocate Ergonomics Training program. Gene has served as the American Express Global Ergonomics Manager, a Rehab Services Manager, and is Past-President of the Upper Midwest Chapter of HFES.
This article originally appeared in The Ergonomics Report™ on 2011-09-27.