Background: The relative physical load (% VO2max) is the quotient of oxygen uptake (Vo2) during physical effort and maximum oxygen uptake (VO2max) by the human body. For this purpose the stress test must be performed. The relative load shows a high correlation with minute ventilation, cardiac output, heart rate, stroke volume, increased concentrations of catecholamines in the blood, inner temperature, weight, height and human body surface area. The relative load is a criterion for the maximum workloads admissible for healthy and sick workers. Besides, the classification of effort can be more precise when based on the relative load than on the energy output. Material and Methods: Based on our own and international empirical evidence and the laws of heat transfer and fluid mechanics, a model of temperature control system has been developed, involving the elements of human cardiovascular and respiratory systems. Using this model, we have been able to develop our own methods of determining the relative load, applying only the body core temperature (TW) or heart rate within one minute (HR), body mass (m), height (H), and body surface area (AD) instead of VO2max. Results: The values of the relative physical load (% VO2max) obtained by using our own methods do not differ significantly from those obtained by other methods and by other researchers. Conclusions: The developed methods for determining the relative physical load (% VO2max) do not require the exercise test to be performed, therefore, they may be considered (after verification in an experimental study) a feasible alternative to current methods. Med Pr 2014;65(2):189–195