A New Frailty Score for Experimental Animals Based on the Clinical Phenotype: Inactivity as a Model of Frailty.

The development of animal models to study human frailty is important to test interventions to be translated to the clinical practice. The aim of this work was to develop a score for frailty in experimental animals based in the human frailty phenotype. We also tested the effect of physical inactivity in the development of frailty as determined by our score. Male C57Bl/6J mice, individually caged, were randomly assigned to one of two groups: sedentary (inactive) or spontaneous wheel-runners. We compared the sedentary versus the active lifestyle in terms of frailty by evaluating the clinical criteria used in humans: unintentional weight loss; poor endurance (running time); slowness (running speed); weakness (grip strength), and low activity level (motor coordination) at five different ages: 17, 20, 23, 26 and 28 months of age. Each criterion had a designated cut-off point to identify the mice with the lowest performance. Lifelong spontaneous exercise significantly retards frailty. On the contrary sedentary animals become frail as they age. Thus, physical inactivity is a model of frailty in experimental animals. Our frailty score provides a tool to evaluate interventions in mice prior to translating them to clinical practice.