# Impact of Load Variation on the Accuracy of Gait Recognition from Surface EMG Signals
## 2 Experimental Method
### Experimental Procedure  
According to the different effects that the lower limb muscles exert in the gait cycle, we selected the four thigh muscles to acquire sEMG signals as described in [4], that is, the tensor fasciae latae (tensor) and the semitendinosus (semitend), which are more active during the stance phase than the swing phase, as well as the adductor longus (adductor) and the vastus medialis (vmo), which play very important roles in the swing phase [4; 18], as shown in Figure 1.  
Before each experiment, the subjects were asked to wear shorts and white shoes in order to show a sharp contrast with the treadmill, for the observation of the gait phase. Their anthropometric measurements--including age, height, and weight--were taken in advance, and then they were asked to become familiar with the experimental procedure, treadmill, and other equipment which would be used in the experiment. Alcohol was used to clean the skin around the target muscle after any hair was shaved off, and electrolyte gel was utilized to reduce any interference produced by the skin, and to improve the contact stability and the electrical conductivity of the electrodes [19].  
Four electrode groups, each electrode group consisting of three Ag/AgCl electrodes (a positive electrode, a negative electrode, and a reference electrode), were attached to the four target muscles, and  
Figure 1: The experimental equipment and the placement of the four-surface electromyogram (sEMG) electrodes.  
a line between the positive and negative electrodes was made exactly in accordance with the direction of the muscle fibers on the midline of the muscle belly [20]. We then marked the exact location on the skin with a marking pen to make sure that the electrodes could be attached to the same location in every experiment, and new electrodes were employed in each trial. Medical bandages were used to fix the wires on the thigh, to reduce the disturbance of shaking wires on the sEMG signal, and the bandage was fixed so that it was not too tight, so as not to interfere with action of the muscle, nor too loose so as to not dislodge the wire.  
After all the equipment was connected as shown in Figure 1, and all signals were normal, every subject was required to perform 50 continuous gait cycles on the treadmill at a constant speed, under four different loads of L0 = 0 (no load), L20 = 20%, L30 = 30%, and L40 = 40% of body weight, respectively [21], at speeds of V3 = 3 km/h, V5 = 5 km/h, and V7 = 7 km/h [22]. This resulted in 6000 data sets (10 subjects \(\times\) 50 gait cycles \(\times\) 4 loads \(\times\) 3 speeds) produced in the experiment. The 50 gait cycles acted as the dividing line to show whether gait cycle number had a significant impact on gait recognition accuracy [4].  
In the experiment, a traditional commercial backpack (U.S. Polo Assn Sport Backpack, Colfax, LA, USA) without a sternum strap or a hip-loading belt was used to carry the load, to suggest a more convincing universal applicability. The backpack load was increased in increments of 0.1, 0.2, 0.5, 1, 2, and 5 kg to match the 20, 30, and 40% of the subject's body weight, and the load was distributed symmetrically on both sides of the spine inside the backpack. The shoulder straps were then adjusted to ensure placement of the backpack above the hips at the low back [23].  
For each trial, when the subjects thought that they were walking steadily on the treadmill at the required speed, they sent instructions to the staff to collect signals. After each trial, the subjects were required to have a 10 min rest to recover from the fatigue caused by the exertion to avoid negative effects on the next experiment.