Semi-Automatic image analysis method for assessing muscle rigidity in rats

Quantitative imaging techniques are increasingly used to evaluate animal behavior and motor function, yet reliable image-based measures of myotonia and related muscle rigidity remain limited. We developed a semi-automated imaging method to quantify forelimb rigidity in rat models of movement disorders. In this procedure, rats briefly grasp a rod that is then removed, after which the forepaws are photographed. The average distance between the wrist and the tips of three fingers is calculated, with shorter distances indicating reduced extension. Using haloperidol-induced catalepsy as a model, we found that subcutaneous haloperidol (0.3 or 1.0 mg kg -1 ) significantly decreased the wrist–finger distance compared with controls. This reduction persisted for at least six hours, whereas conventional measures, such as the wire-hanging test, no longer detected rigidity at the lower dose. Thus, this imaging method provides higher sensitivity while reducing animal burden and sample size requirements. Beyond catalepsy, this approach may be applicable to other conditions involving muscle stiffness, supporting its utility in preclinical drug testing and its potential for future translation to human assessment.