“…Covariates were selected on the basis of previously published associations with readmission risk and/or postoperative ambulation (i.e., age, body mass index, length of stay, comorbidities; [3, 9, 10]). In exploratory analyses, we also considered the role of preoperative activity level, which was assessed prior to surgery using the following question from the National Health Interview Survey: “Outside of your normal work or daily responsibilities, how often do you engage in exercise that at least moderately increases your breathing and heart rate, and makes you sweat, for at least 20 min (such as brisk walking, cycling, swimming, jogging, aerobic dance, stair climbing, rowing, basketball, racquetball, vigorous yard work (gardening), etc.…”