2.1. The Wearable System

The system was conceived to detect pure flexions in the user’s sagittal plane. To this end, conventional shoulder straps made of an elastic fabric were modified, so as to integrate in their lower section two rectangular silicone-based capacitive stretch sensors (Courtesy of Parker Hannifin, Mayfield Heights, OH, USA), as shown in Figure 1. The sensors had at rest (i.e., prior to their pre-stretching, induced while securing the straps) a length of 100 mm, a width of 15 mm, and a total thickness of 1 mm (including the insulating coatings of the electrodes). Their sensitivity, expressed as the ratio between the percentage variation of capacitance and the percentage variation of length (strain), was ~1.

Picture of the assembled wearable system, based on two capacitive stretch sensors.

The sensors’ upper ends were attached to a plastic box, which contained wireless electronics and was located between the shoulder blades, whilst the lower ends were clamped to the user’s trousers (Figure 1).

Each volunteer involved in this study (as detailed below) was asked to wear the same type of trousers (gym pants), consisting of an elastic textile, adherent to the body. They were used to minimize the risk of unpredictable displacements, which could change the sensors’ stretching for analogous flexions of any given user.

The arrangement of the sensors was such that they were not parallel to the spine (Figure 1). This configuration, as compared to a possible alternative use of a single sensor aligned with the spine, was motivated by the need to minimize the effect of unintentional asymmetric movements, especially in terms of torsions and lateral bending. In order to limit inaccuracies due to such unintended movements, the two capacitance values were averaged (as described in the following).