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Bi-helical yarns when made by single yarns with different diameters, helix angles and tensile strengths, present a dynamic behaviour that results in a significant difference in the relationship between the transversal and longitudinal variation, that is, in the Poisson´s ratio. The longitudinal and transversal measurement of a bi-helical yarn in a dynamic test is a complex process, especially when dealing with multifilament yarns. For this reason, a methodology was specially developed using image analysis. To demonstrate the dynamic behaviour of the bi-helical yarns under tensile loads, the results obtained for four different samples using polyamide and polyester multifilament yarns are presented and analysed.