Torsional flexibility of the thoracic spine is superior to that of the lumbar spine in cats: Implications for the falling cat problem

Cats are renowned for their air-righting behavior, effectively reorienting themselves in midair to land on their feet. However, how cats move their bodies during air-righting remains unclear. Complex motor factors, such as trunk flexion and axial torsion, may be involved; however, to obtain a definitive answer, quantitative data on the flexibility of spinal axial torsion and how this flexibility differs between spine regions are needed. In this study, we measured the mechanical properties of the spine under axial torsion and examined their relationship to air-righting behavior. Destructive failure testing was conducted on five cat cadavers to assess the maximum torque (strength), range of motion, neutral zone, and stiffness of the thoracic and lumbar spine regions separately. Additionally, trunk motion during air-righting behavior was analyzed in two live cats. The thoracic and lumbar spine regions exhibited marked differences in torsional mechanical properties. The thoracic region exhibited a larger range of motion, a larger neutral zone, and lower stiffness than the lumbar region, indicating greater torsional flexibility. It also showed lower maximum torque, indicating lower torsional strength. Notably, the thoracic region had a neutral zone of 47°, whereas the lumbar region had no neutral zone. During air-righting, anterior trunk rotation was completed earlier than posterior trunk rotation. These results suggest that trunk rotation during air-righting in cats occurs sequentially, with the anterior trunk rotating first followed by the posterior trunk, and that their flexible thoracic spine and rigid lumbar spine in axial torsion are suited for this behavior.