Introduction: In vivo tests and design of experiments were carried out to determine the influence of experimental factors on the mechanical response of soft tissue. Material and methods: This study proposes an inverse method of analysis to obtain the constitutive equations of a multilayer biological model (cutis, hypodermis, and muscle) through the use of indentation tests in combination with the ¿nite element method. A design of experiments was developed to determine the incidence of the following factors on the mechanical behavior of the skin: room temperature (A), indentation velocity (B), indenter temperature (C), pump pressure (D), muscle activation (E), moisture (F), and diameter indenter (G). Results: The factor D (pump pressure), and factor interactions ADE (temperature - pressure pump - muscle activation), CDE (indenter temperature - pressure pump - muscle activation), and ACDE (temperature - indenter temperature - pressure pump - muscle activation) are statistically significant on skin mechanical response with a p-value lower than 0.05. For each combination of these factors one constitutive equation was established. Discussion/conclusions: Sixteen constitutive equations were obtained for their use in further numerical models where the experimental conditions must be taken into account. Thereon, it can be concluded that there is not a unique equation to represent the mechanical properties of skin.