Graduation Year

2023

Document Type

Thesis

Degree

M.S.P.H.

Degree Name

MS in Public Health (M.S.P.H.)

Degree Granting Department

Public Health

Major Professor

Thomas E. Bernard, Ph.D.

Committee Member

Steve P. Mlynarek, Ph.D.

Committee Member

Candi D. Ashley, Ph.D.

Keywords

heart rate, heat stress, metabolic rate, predictive formula

Abstract

Thermal equilibrium is primarily maintained by the body’s ability to dissipate internal heat generated during muscle contraction. A number of methods have been used to quantify this thermal burden such as charts and observation of workplace activities and behavior, however these have varying levels of validity. ISO 8996 defines a more accurate method of metabolic rate estimation that uses heart rate and maximal work capacity (MWC); however this requires the use of laboratory settings and equipment that are rarely available in the field. Equations such as the ACSM metabolic equation for the total energy expenditure of walking, hereafter referred to as the ACSM walking equation and the USARIEM load carriage decision aid (LCDA) are commonly used alternatives for estimating metabolic rate. The purpose of this paper is to assess the accuracy and precision of these three methods compared to measured oxygen consumption: two equations using treadmill grade and speed, and the International Organization for Standardization (ISO) heart rate method.

A progressive heat protocol was used that began with establishing thermal equilibrium, then incrementally increasing the temperature and humidity at five-minute intervals while heart rate (HR), rectal temperature, and skin temperature were recorded. Treadmill speed and grade were set to represent one of three metabolic rates: Low, Moderate, and High. Metabolic rate was reported as the average of three measurements of oxygen consumption using the Douglas bag method.

The predictive equations used treadmill speed and grade and the demographics of the participants (age, weight, height) to estimate the metabolic rate. In order to analyze the predictive capabilities of the three methods, analysis of variance with a mixed effects model was used. Three one-way mixed effect ANOVAs were then used to determine the relationship between measured data and predicted values. Next Bland-Altman plots were generated for each of the three approaches to analyze the variance between the methods. Finally, a two-way mixed effects ANOVA was used to identify the effect of heat stress level on ΔM (Mpredicted – Mmeasured) estimations presented by the ISO heart rate method. The ACSM method proved to be the most accurate and precise overall (ΔM = -17 W, standard deviation = 55). The Bland-Altman plots revealed that the ACSM method was the most accurate method, with the least bias and smallest standard deviation. The ISO method exhibited the least bias when measured at the Transition stage of heat stress and most at the Compensable stage.

Overall, the ACSM walking equation was the most accurate and precise method of predicting metabolic rate. Because the ISO heart rate method shows the least bias at the Transition stage, it is a beneficial tool for use when monitoring exposures at the occupational exposure limit. Further research is needed to develop accurate predictive formulae that incorporate readily available metrics like heart rate into metabolic rate estimations.

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