LAUSR

We would like to thank Xia et al. for their comments about our paper. We are excited that this paper can help generate discussion and cast some much-needed attention on the muscle interactions and risk of sarcopenia and spinal surgery. We acknowledge that the assessment of psoas cross sectional area is subject to interpretation, and that fatty infiltration may greatly overestimate the muscle strength and contribution to sagittal balance. The measurement of sarcopenia and muscle wasting was used as a measurement of frailty, more so than this psoas muscles actual contribution to balance and prevention of PJK. The muscle wasting that we see occurs in all muscles with frailty, and we used the psoas as it is easily visualized in a lumbar MRI as a surrogate. The multifidus and erector spinae muscles are important and are also able to be visualized, but in the setting of revision surgery, or deformity their size can be difficult to determine accurately. We believe that what is needed are functional assessments of muscle activity and sarcopenia to better characterize strength, and contribution to frailty. Functional sit to stand, walking, and grip strength tests are often used to characterize frailty. This would help address the issues of fatty infiltration or an injury to a singular muscle that may have occurred with prior surgery and would contribute to measurement error. Again, we appreciate your interest in our paper and enjoy the discussion.