A new theoretical study has revealed that, under certain conditions, the mechanical properties of extensile and contractile biological tissues could be indistinguishable. This ambiguity arises when analyzing density fluctuations in these materials, which could have significant implications for understanding biological processes such as embryonic development or wound healing, where distinguishing between these behaviors is crucial.

Active biological tissues, such as those comprising muscles or epithelia, exhibit complex mechanical behaviors driven by molecular motors that consume energy. These motors can generate forces that extend the tissue (extensile behavior) or contract it (contractile behavior). Traditionally, it has been assumed that these two classes of materials behave fundamentally differently and can be easily distinguished by measuring their elastic properties or their response to external perturbations.

However, the current research, based on mean-field models, suggests that this distinction might not be as clear-cut as previously thought. The authors have shown that the signature of density fluctuations, a key measure of the material's response to small variations, can be identical for extensile and contractile tissues in certain regions of the parameter space. This means that experimental measurements of these fluctuations alone might not be sufficient to determine whether a biological tissue is predominantly extending or contracting, posing a challenge for the characterization of these systems. This finding underscores the need to develop more sophisticated characterization methods that can unravel the underlying nature of active forces in biological tissues.