SUMMARY: Negative health consequences are associated with prolonged sedentary behaviour. Patients who require total knee replacements tend to be obese and sedentary and often do not increase activity levels post-operatively despite reductions in pain. Objectives: The purpose of this study was to determine the accuracy of ActiGraph and activPAL activity monitors in categorizing posture (e.g. sitting versus standing) in total knee replacement patients. Secondarily we were interested in examining data collected over 7 days as patients went about their normal routines. Methods: Forty patients (50-80 years of age) wore an ActiGraph GT3X+ monitor on their waist and an activPAL monitor on their thigh during their 5-8 week post-operative visit with the surgeon. The patients were directly observed during this visit and all activity and postural changes were recorded for comparison with the activity monitor data. To further characterize the participants, height, weight, knee range of motion and 10m walk speed were also measured. Participants who were interested were then asked to wear an activity monitor over the next 7 days while they went about their normal routine. Results/conclusions: The activPAL had significantly better accuracy in detecting sedentary and upright postures (paired t-tests p<0.05). There was a significant difference in sedentary predictive values (p<0.05) with the activPAL having a better predictive value. No significant difference was found between monitors for upright predictive values. Ten participants spent an average of 73.36% (+/- 10.77%) of monitor wear time in sedentary behaviours. This is more than research has shown in healthy populations.