# Smart Footwear Insole for Recognition of Foot Pronation and Supination Using Neural Networks
## 2 Data Acquisition and Representation
### Sensors Location  
In the bio-mechanics of the march, the walking cycle is defined as the sequence of events that takes place between two successive repetitions of the contact of the same heel with the ground. This cycle can be divided into two phases: the stance phase (when a foot is on the ground) and the swing phase (when the foot is in the air) which alternate for each lower limb. As detailed in a previous study [11], during the stance phase, three different phases are distinguished according to the area of the foot involved [12]:  
* Impact or Initial Contact: first contact zone of the foot with the ground. It is made with the heel and supports a great tension at the moment of impact.
* Mid-stance: the second phase. The foot rests completely on the floor and the weight of the person is distributed between the heel and the forefoot through the bridge.
* Propulsion or Push off: the last phase. The foot only presents the front part in contact with the ground, and this state corresponds with the moment when it generates the ground reaction force to start the movement again.  
During the march, these three phases (see Figure 1) are constantly repeated alternately between the two feet: while one of them is in phase 3, the other is in phase 1 and vice versa [13]. Moreover, during the second phase, one of the feet is fully supported on the ground while the other is in the air [14; 15]. However, this cycle only remains constant if the person is just walking: if the person is running, there is a moment in which both feet are in the air, but these three phases still apply for each foot.  
To measure the force exerted to the insole and perform a bio-mechanical study, force sensor resistors (FSR) are placed under the insole [16]. FSRs provide information about the maximum pressure point, the impulse and the load forces, among others.  
To select the best position to place the sensors, the anatomy of the foot and the types of footprints (pronator, supinator, neutral) are taken into account. According to [10], the sole can be divided into 15 areas: the heel (areas 1-3), the arch (areas 4-5), the metatarsus (areas 6-10) and the toes (areas 11-15) as shown in Figure 2-left. With this study, a first version of the instrumented footwear insole was developed [11], using eight FSRs (see Figure 2-middle).  
Results obtained in [11] show that the metatarsus area gives more information about the footprints types than the other areas. Moreover, results obtained in other works like [17] reveal that proximal and distal joints to the medial cuneiform, connected with the metatarsus areas, are the main zones involved in pronation and supination movements, together with the heel joints. To have a better view of the footprint, some sensors are usually placed in the part of the foot arch to quickly and easily detect a supinatory walk (sensors 4 and 5 in Figure 2-middle); however, the pressure in this area is more influenced by other factors, such as the curvature of the foot, as explained in work [6].  
So, taking into account these previous works and in order to obtain more accuracy about footprints types classification, more FSRs are placed in the metatarsus area in a second version of the instrumented insole. Due to the hardware limitations, sensors placed in other areas were relocated, obtaining an insole with six sensors in the metatarsus area and one sensor in the heel (see Figure 2-right). In the propulsion phase, only the forefoot is in contact with the ground; so, with the sensors placed in metatarsus area, it is not necessary to place any additional one in the area of the toes [18].  
Once the instrumented footwear insole has been designed, the embedded system is described in the next section.  
Figure 1: Three different phases during the march.  
Figure 2: (**left**) Foot pressure points, (**middle**) Sensors location (v1), (**right**) Sensors location (v2).