*2.1. Subjects*

A total of six healthy subjects, who have never suffered from muscular atrophy or disorders, participated in this study, with an average height, age and weight of 181 ± 3.8 cm, 72.5 ± 6.9 kg and 24.2 ± 1.6 yrs, respectively. All of the subjects enrolled in this study knew the procedure of the experiment, and signed and agreed to participate in the experimental study as the test subjects. The experiment was approved by Nanjing University of Science and Technology (Date granted: June 12, 2019) and performed in accordance with the Declaration of Helsinki.

### *2.2. Experimental Preparation and Protocol*

The unilateral lower limb of the human body contains at least 30 muscles that drive 7 degrees of freedom of the lower limb. Since the knee is a vital joint in the lower limb, we have chosen it as the key aspect in our research. The sEMG signals related to the knee of the unilateral leg were obtained by Trigno wireless sEMG instrument. To obtain better sEMG signals, several muscles that are easily detected were selected, including the vastus lateralis (VL), rectus femoris (RF), vastus medialis (VM), gastrocnemius medialis (GM) and gastrocnemius lateralis (GL). The adhesive positions (shown in Figure 1) of the surface electrodes are the positions recommended by SENIAM [22]. Before the experiment, the skin surfaces, where the electrodes had to be placed, were shaved and then cleaned with alcohol. This was done to reduce the impedance between the measured skin and electrodes and also to improve the sensor–skin contact.

**Figure 1.** The locations of surface electromyography (sEMG) sensors.

On the lateral side of the leg with sEMG sensors, three markers were placed to obtained the kinematic data of the knee, through a 3D motion capture system called Codamotion. As shown in Figure 2, Marker B was placed at the approximate center of the knee joint, on the sagittal skin of the subject, while Marker A and Marker C were respectively placed on the projection line of the thigh femur and calf tibia on the sagittal skin of the subject. The approximate flexion–extension motion angle of the knee joint was obtained by collecting the spatial motion trajectory of Markers A, B and C.

**Figure 2.** The placement of the Codamotion markers.

During the experiment, the subject was asked to walk back and forth nearly 20 times along a straight line about 5 m in length, at his natural or free cadence, as shown in Figure 3. The two Codamotion CX1 units on the right of the subject collected the kinematic data of the markers at a sampling frequency of 100 Hz, while the wireless Trigno system picked up the raw sEMG signals from the five muscles at a frequency of 2000 Hz. The data were then synchronized and transferred to

the Codamotion hub, and finally, transmitted to the PC and stored. If the subject felt uncomfortable while the experiment was in progress, he would rest for 10 minutes to reduce the effects of such factors as fatigue. If he felt better after the rest, the experiment would continue. Otherwise, the experiment would be stopped. In this way, the data of one trail from as many as 8 strides could be collected, which was more than adequate to obtain 4 representative sEMG profiles for each muscle and gait cycle (GC) of the leg. Approximately 80 full GCs and their corresponding sEMG signals were recorded in all for each subject. Since each subject needed to be properly equipped with the sensors, and it required approximately one hour for each subject to undergo the experiment process, including preparation (15 min), resting (30 min) and walking (15 min), the entire experiment lasted 2 days, with one person in the morning and two in the afternoon.

**Figure 3.** The schematic of the experiment.
