## 2 Materials and Methods
### Time for Completing 5 TSST  
Each volunteer was seated in the middle of a calf-height chair, as illustrated in Figure 1b, and was instructed to rise to fully stand and then return to a fully seated position as fast as possible 5 times. The time required to complete 5 sets was recorded using the MSD and by the researchers.  
2.2 Second Investigation: Determining the Feasibility and User Experience of Using the MSD for the Post-TKA In-Home Rehabilitation  
A total of 12 patients aged over 60 years who had received TKA using posterior-stabilizing instruments for advanced knee OA were enrolled. Patients were excluded if they (1) received TKA for a pathology other than OA; (2) had a history of knee replacement on the opposite knee; (3) had severe complications after TKA, including symptomatic deep vein thrombosis, fracture, and infection; (4) lived alone without family support for in-home knee exercise training; (5) had a history of limb disorders that might have interfered with the knee angular motion, including severe limb fracture, limb deformity, or neurovascular disorders; and (7) were unwilling to receive an intervention or participate in the trial.  
After the surgical wound had healed in the second week following TKA, patients were allocated into 2 intervention groups: 6 patients were assigned to the HE group and 6 were assigned to the motion sensor-assisted rehabilitation (MAR) group; patients were assigned the MAR group based on their willingness to use and familiarity with mobile apps. The patients in both groups received 2 months of in-home rehabilitation.  
Figure 4: Mobile app for physiotherapists and physicians: (**a**) the daily and weekly ECR of each patient is displayed with patient information; (**b**) doctors assign daily exercises with specified targeted angles, holding times, and numbers of repetitions.  
Before beginning the exercise program, patients in the HE group received comprehensive instructions on knee rehabilitation after TKA from an experienced orthopedic surgeon. The standard protocol of HEs is available in Multimedia Supplementary Materials Files. These patients were instructed to follow the standard HE protocol but individualized modifications were permitted based on health professionals' judgment. An orthopedic surgeon reviewed the progress of each patient every 2 weeks, provided advice, and modified the exercise program based on patient compliance and the standard protocol.  
Before they began the in-home exercise program, all patients in the MAR group received detailed instructions on operating the MSD, and the Patient App was installed on their mobile phones. The orthopedic surgeon used the Patient App to assign daily HEs to each patient based on the standard protocol (see Supplementary Materials Files). Furthermore, through the mobile app, the orthopedic surgeon individually assigned each set of exercises with a corresponding target angle to patients in the MAR group based on the angle that patients could successfully achieve (Figure 3(b)). Achievement of the assigned exercise was recorded as successful completion of the exercise, which was used to calculate the daily ECR on the mobile app (Figure 3(a)).