Ergonsim - Human Thermal Modelling
Solutions

Distal extremities

Distal extremities play an important role in the human temperature regulation. Hands and feet are characterized by large surface area / tissue volume ratios and are subject to the largest variations in the cutaneous vaso-activity. They act as efficient heat-exchangers capable of removing large amounts of bodily heat to the environment. Dedicated modelling of the distal extremities is therefore critical to a realistic thermophysiological simulation of humans.

distal extremities

Anthropometry, morphology and heat transfer extremity models


In the extended distal extremities (XDX) model, hands are modelled consisting of a dominant compartment, i.e. the hand body, and an explicit representation of individual fingers. The XDX foot model consists of three regions: midfoot, heel and toes. The palmar and sole sides of the distal extremities are considered separately as these regions contain glabrous skin including unique vascular structures that underlie special regulatory regimes. An alternative, extended formulation of the bioheat transfer equation was defined and implemented to simulate the heat and mass transport in the terminal extremities.

hand anthropometry

Arterio-Venous Anastomoses (AVA) model


Distal extremities contain unique vascular structures – the so-called arterio-venous anastomoses (AVA) - which regulatory mechanisms differ from those of the rest of the human body. AVA are direct connections between small subcutaneous arteries and veins. When open, AVA provide low-resistance connections between arteries and veins, shunting warm arterial blood directly into the venous plexuses thus elevating the surface temperature and heat loss of the distal extremities to the environment. The incorporated AVA model contains dedicated algorithms for

temperature regulation and simulation of the dynamic processes of AVA opening and closing.

The XDX model includes scalable anthropometry and morphology representations of distal extremities. The body elements are configured based on local body part dimensions which are either a direct user input or are calculated using scaling algorithms developed based on anthropometric field surveys published over the past seven decades. The scaling processes contain calculations of local surface areas, body part masses and tissue densities that reproduce the results of dedicated field surveys.

Arterio-Venous Anastomoses AVA

Cold-induced Vasodilatation (CIVD)


Under cold stress conditions particularly terminal extremities are susceptible to pronounced, rapid cooling and low skin temperatures. In extreme cold situations, seemingly paradoxical, temporal increases of the local skin temperatures may occur in digits as a cryoprotective response avoiding tissue damage. The short periods of skin warming and cooling may occur in a cyclic fashion which is commonly attributed to AVA vaso-activity characterized by corresponding dynamic behaviours.

AVA blood flow

Variations of the static maximum effective AVA blood perfusion rates predicted by the XDX AVA model for fingers plotted as functions of error signals associated with local skin and mean body temperatures.

sleeping bag temperature

Comparison of the mean skin and two local skin temperatures of the foot as measured [18] and predicated for male subjects lying in a sleeping bag and exposed to an ambient temperature of −6.4°C.

CIVD responses

Two examples [14] of finger skin temperature fluctuations due to CIVD as measured and predicted for subjects in a moderate environment with their fingers immersed in cold water of 4°C (top) and 8°C (bottom).

Non-invasive monitoring system for people in industrial environments

In various industries, workers are exposed to hot environmental conditions causing heat stress associated with an elevated risk of work accidents. In the EU-funded project Protective Responsive Outer Shell for People in Industrial Environments (Prospie) an intelligent workwear clothing system was developed to enable fast identification of physiologically dangerous situations and to alert the workers.


The non-invasive monitoring and warning system (MWS) uses wearable sensors to determine the internal temperature and heat stress levels. MWS measures skin temperatures as a key information to determine non-invasively the body core temperature of workers featuring different personal characteristics by means of numerical simulation using the individualized FPC Model [4,5].

non-invasive body monitoring
core body temperature monitor

Universal Thermal Climate Index (UTCI)

UTCI-Fiala Model

The Universal Thermal Climate Index, UTCI is a temperature-scale index defined as the air temperature of a reference environment (UTCI equivalent temperature) that elicits the same thermophysiological response (strain) as the actual environment [1]. UTCI was developed for use in public weather services, public health systems, precautionary planning, and climate impact research [9].


The European Science Foundation’s COST Action 730 brought together scientists from 23 countries to develop an index that is applicable to all habitable climates including their seasonal changes and is based on the “most advanced” thermophysiological models predicting ‘average-population’ responses [19].

outdoor climate conditions
UTCI-Fiala Model

The Action activities included extensive inter-model comparison and validation tests using measured data [16]. The tests covered the entire spectrum of ambient conditions including exposures to extreme cold and hot, and prolonged field exposures to complex transient scenarios. Following the tests, the UTCI-Fiala model – a variant of the FPC Model – was used to form the basis for developing UTCI [3]. Principal component analyses of the multivariate physiological responses predicted by the model were then employed to constitute the one-dimensional temperature-scale index [1] - UTCI - which has found internationally wide-spread use and acceptance.

Whole-body cryotherapy & personal exposure safety limits

Whole-body and partial heat and cold therapies are rapidly evolving modalities deployed to promote positive health effects in medicine, health and sports domains. Positive physiological outcomes of heat therapies attributed to elevated internal temperatures include e.g. improvements in the metabolic profile and cardiovascular fitness in clinical applications as well as improvements in muscle mass and strength in active groups. Provided a critical level of cooling is achieved, the premise of cold therapies is to cool the injured tissue to achieve analgesia, lower cell metabolism, alleviate inflammation, and facilitate rehabilitation in sporting and post surgical settings.


Whole-body cryotherapy (WBC) is one mode of cold therapy during which semi-nude subjects are exposed for a short period of time to extreme cold air in the range within -90°C and -140°C. Besides the apparent positive outcomes, however, WBC bears health risks if not applied cautiously. While WBC impacts the thermal status of the entire body, skin is the directly affected body region thus including the risk of cold-related tissue damage when skin temperatures of vulnerable body parts fall below critical thresholds. However, there is a considerable inter-individual variability in the skin cooling response to WBC which, beyond the treatment dose, depends on the personal characteristics of the treated individuals thus making a reliable risk assessment difficult [20].

whole body cryotherapy WBC
WBC skin temperature sex

Sex-specific differences in the mean skin temperature response to 2.5-min WBC exposure to -110°C as measured and predicted for males and females featuring respective group-average body characteristics.

The common practice of using “one-size-fits-all” WBC protocols appears to be inadequate regarding both the efficacy and the safety of WBC treatments. The FPC Model, adapted and validated for physiotherapeutic treatments, was deployed to develop algorithms for safe personalized WBC protocols [2]. The work involved detailed numerical simulations using the FPC Model to accurately predict body and local skin temperature responses of test subjects undergoing WBC who featured a wide variety of personal characteristics. The developed new algorithms calculate the duration of the WBC exposure  for individuals depending on their sex, anthropometry, body compositions, and skin type (colour) ensuring the local skin temperature of the shin not falling below defined safety limits.

cryotherapy skin temperature limit
sweat Torso

Course of local shin skin temperatures during a -110°C WBC exposure plotted as group-average and individual response including safety limits for personalized maximum WBC exposure times (t).

muscle tissue temperature

Tissue temperature profile across the radius of the upper leg predicted for a test person undergoing a cooling protocol featuring distinct anthropometric and body composition properties.

Athletics - running

Leading sportswear manufacturers increasingly deploy computer simulation technologies to simulate humans engaged in different types of sports activities to obtain important quantitative information on how the choice of garments and fabric materials influences the athlete. Apparel developers then use the information to design sportwear for optimum performance, comfort and well-being for the target sports use cases.


In an dedicated R&D project, an interactive numerical simulation tool was devised to simulate in detail running activities allowing to translate environmental conditions, personal characteristics, fitness and workout data into thermophysiological and performance metrics. In the project, the FPC Model was adapted and extended to utilize tracking and weather data datasets to predict body temperatures, regulatory, cardiovascular, pulmonary and perceptual responses of running athletes.

Long distance running

The work involved the development and implementation of several sub-models including a cardio-vascular response (CVR) model to predict heart rate-, stroke volume-, blood flow redistribution, and cardiac output responses as a result of exercise intensity, physical fitness, bodily thermal and dehydration states, age, and geographical altitude [13]. A dedicated cardio-pulmonary kinetics (CPK) model simulates the dynamics of cardio-pulmonary and –vascular responses during temporal changes in the exercise intensity. The simulation system provides additional information e.g. if and when “exercise failure” limits related to the performance of the athlete’s cardio-vascular and pulmonary system are reached or thermal risk or tolerance thresholds are to be considered when exercising can’t be maintained due to extreme environmental conditions causing critical hypo- or hyperthermic body states.

pulmonary ventilation oxygen

Incorporated cardio-pulmonary kinetics model predicting individual pulmonary ventilation and oxygen consumption rates measured [7] during a standardized incremental running test on a treadmill.

muscle temperature exercise

Predicted and measured [10] muscle tissue temperatures in the upper leg before, during and after 15 min exercise plotted over the exposure time (top) and as temperature profile over the tissue radius (bottom).

heart rate predict

Comparison of predicted and measured [6] cardio-vascular and body temperature responses of subjects performing exercise in a warm humid environment of 35 °C and 80% relative humidity.

Sweating TORSO simulation-measurement system

A thermophysiological 'human simulator’ device was developed in a research project to enable measurements of clothing properties under physiologically realistic conditions [14]. The single-sector heated sweating cylinder 'Torso’ of the Swiss Empa Institute that enables measurements of the coupled heat and moisture transfer through garments [8] was used to simulate the physical human body. The 'Torso’ device was coupled with a FPC Model version to predict human physiological responses.

sweating Torso

The coupling method is based on real-time data exchange between the simulation model and the device. The 'Torso' instrument adjusts surface temperatures and moisture excretion rates predicted by the simulation model using measured surface heat fluxes which result from the actual exposure conditions, clothing item properties and the thermophysiological body status. The coupled measurement system delivers additional information on the resultant physiological conditions including e.g. core body temperatures and further thermoregulatory responses.

Asymmetric radiant fields & human thermal comfort

Humans are frequently subjected to asymmetric radiant fields. Asymmetric thermal and solar radiation may cause significant discomfort both outdoors and inside buildings, cars, aircraft cabins and other artificial climates. In buildings, occupants are exposed to asymmetric radiation e.g. in the proximity of cold windows, hot radiators, or to solar radiation transmitted through windows or glazed façades causing restrictions in the usability and functionality of spaces, and reducing occupants' performance at the workplace. In cars and aircraft cabins such uncomfortable conditions prolong the reaction times of drivers and pilots. Modelling human responses to asymmetric radiation has thus important implications for the assessment of thermal comfort, human performance and health conditions in complex environmental settings.


The prediction of human thermal and perceptual responses to asymmetric long- and short-wave radiation using predecessor versions of the FPC Model has been subject to several research projects [12,21]. The work involved detailed 3D modelling of the long- and short-wave radiative heat exchange between the human body, radiant enclosures, and high intensity sources [11]. The results have found application in multiple areas of research and technology.

radiation comfort
human projected area factor

Projected area factors predicted for the left lateral face sector of a standing person plotted over solar azimuth and different solar altitude angles [11].

asymmetric radiation body temperatures

Comparison of measured and predicted (Simulation) local skin and body core temperatures for different levels of vertical radiant temperature asymmetry [12].

warm ceiling discomfort

Comparison of observed and predicted percentages dissatisfied due to local discomfort for subjects exposed to asymmetric radiation from a warm ceiling.

Aquatic activities

The environmental heat transfer of humans immersed in water differs notably from air exposures. During immersion, bodily heat losses are dominated by surface convection while the radiative and latent heat exchange components are suppressed. The heat transfer coefficients for free and forced convection are typically multiple times higher in water than in air. As a result, humans lose rapidly significant amounts of heat in cold water which is associated with a notable drop of body temperatures and the corresponding transient thermoregulatory reactions.

dive simulation

Using the FPC Model it is now possible to readily set up and carry out numerical simulations of humans undergoing even complex immersion scenarios including partial and whole-body immersions, sole immersions or sequences of water immersion – air exposure periods. The impact of garments on physiological responses during immersions can be systematically analysed including the role of different fabrics, e.g. water absorbent or repellent materials.

swimm model
water immersion body temperature

References

  1. Bröde P, Fiala D, Blazejczyk K, Holmér I, Jendritzky G, Kampmann B, Tinz B, Havenith G (2012) Deriving the operational procedure for the Universal Thermal Climate Index (UTCI). Intl J Biometeorol, vol 56, pp 481-494.

  2. Broede P, Fiala D, Viroux P, Tiemessen I, Trembley JP (2017) Impact of personal characteristics on whole body cryostimulation settings - a numerical simulation study using the FPC model. Proc.14. Cryogenetics IIR conf. Dresden, Germany, pp. 322-328.

  3. Fiala D, Havenith G, Bröde P, Jendritzky G (2012) UTCI-Fiala multi-node model of human heat transfer and temperature regulation. Intl J Biometeorol 56: 429-441.

  4. Fiala, D, Havenith G (2016) Modelling human heat transfer and temperature regulation. In: Gefen, A., Epstein, Y. (eds) The Mechanobiology and Mechanophysiology of Military-Related Injuries. Studies in Mechanobiology, Tissue Engineering and Biomaterials, vol. 19, pp. 265-302. Springer, Cham.

  5. Fiala D, Havenith G, Daanen H. (2010) Dynamic thermophysiological model of a worker. Prospie EU Project, Report Deliv. 3.1, pp. 1-58.

  6. Havenith G, Inoue Y, Luttikholt V, Kenney WL (1995) Age predicts cardiovascular but not thermoregulatory responses to humid heat stress. Eur J Appl Physiol 70: 88-96.

  7. Heyde Ch, Leutheuser H, Eskofier B, Roecker K, Gollhofer A (2014) Respiratory inductance plethysmography – A rationale for validity during exercise. Med Sci Sports Exerc. 46(3): 488-95.

  8. ISO 18640-1 (2018) Protective clothing for firefighters - Physiological impact. Part 1: Measurement of coupled heat and moisture transfer with the sweating torso.

    Internat. Organization for Standardization, Geneva.

  9. Kenny GP , Reardon FD, Zaleski W, Reardon ML, Haman F, Ducharme MB (2003) Muscle temperature transients before, during, and after exercise measured using an intramuscular multisensor probe.

  10. Kubaha K, Fiala D,. Toftum J,  Taki A (2004) Human projected area factors for detailed direct and diffuse solar radiation analysis. Intl J Biometeorol 49: 113-129.
  11. Kubaha K (2005) Asymmetric Radiant Fields and Human Thermal Comfort. PhD thesis, De Montfort University, Leicester, UK.
  12. Lloyd AB, Fiala D, Heyde C, Havenith G (2022) A mathematical model for predicting cardiovascular responses at rest and during exercise in demanding environmental conditions. J Appl Physiol 133: 247–261.

  13. O’Brien C (2005) Reproducibility of the cold-induced vasodilation response in the human finger. J Appl Physiol 98: 1334 –1340.

  14. Psikuta A, Richards M, Fiala D (2008) Single-sector thermophysiological human simulator. Physiol. Meas. 29, pp 181–192.

  15. Psikuta A, Fiala D, Laschewski G, Jendritzky G, Richards M, Blazejczyk K, Mekjavic I, Rintamäki H, de Dear R, Havenith G (2012) Validation of the Fiala multi-node thermophysiological model for UTCI application. Intl J Biometeorol 56(3): 443-460.

  16. Rees S.J., K.J. Lomas and D. Fiala (2008) Predicting Local Thermal Discomfort Adjacent to Glazing. ASHRAE Transactions, Vol. 114 (1), pp. 431-441.

  17. Song WF, Zhang CJ, Lai DD, Wang FM, Kuklane K (2016) Use of a novel smart heating sleeping bag to improve wearers' local thermal comfort in the feet. Sci Rep 6:19326.

  18. Viroux PF, Fiala D, Trembley J, Tiemessen IJH (2019) Optimizing safety by customization in conjunction with whole-body cryotherapy. Proc. XVIII ICEE conf., Amsterdam.

  19. Yousaf R., D. Fiala, and A. Wagner (2007) Numerical Simulation of Human Radiation Heat Transfer Using a Mathematical Model of Human Physiology and Computational Fluid Dynamics (CFD). In: High Performance Computing in Science and Engineering `07, Ed. W. Nagel, D. Kröner and M. Resch, Transactions of the High Performance Computing Center Stuttgart, Springer Berlin Heidelberg, Part 8, pp. 647-666.