Estimation of ground reaction forces and joint moments on the basis on plantar pressure insoles and wearable sensors for joint angle measurement

2.1 Hardware and architecture of the wearable sensor system

The new wearable sensor system weighs 12 kg and consists of plantar pressure insoles with max. two hundred and forty sensors (T&T medilogic Medizintechnik, GmbH Munich, Germany) (Fig. 1c), and wearable sensors for joint angle measurement (Fig. 1a and b). Measurement of angular displacement was performed by means of six 12-bit (0–10V) resolution hall effect absolute encoder Mab28A (MegaMotive, Germany). Transducer signal from the encoders was processed using a DT9800 series measuring card manufactured by Data Translation [8].

The signal was collected and filtered by moving the average filter (at 60 Hz) using LabVIEW software (National Instruments, USA).

2.2 Estimation of ground reaction forces and joint moments

To estimate ground reaction forces and joint moments of the lower limbs, a 23 degree-of-freedom musculoskeletal model and the inverse dynamics technique were used [9]. The structure of the wearable sensor system, including its dimensions and rotation, the ground reaction forces and joint moments are presented in Fig. 2.

OPEN IN VIEWER

The algorithm for estimating the ground reaction forces and joint moments during walking in the sagittal plane (Fig. 3) works in real time [8].

OPEN IN VIEWER

In the first step, angular displacement in joints was performed. Based on the data, a sixth voltage signal representing joint position for the left φ 1 l , φ 2 l , φ l 3 and for the right φ 1 r , φ 2 r , φ 3 r of the lower limb was collected. The ground reaction forces F 1 l , F 2 l , F 1 r , F 2 r were estimated in step II based on signals collected from 240 pressure sensors (Fig. 4).

OPEN IN VIEWER

To estimate ground reaction forces F 1 l , F 2 l , F 1 r , F 2 r , Eq. (1) was used:

F 1 = ∑ i = 1 12 G i ⁢ w i d 4 ,(1) F 2 = ∑ i = 13 18 G i ⁢ w i d 5 ,

where: w i – distance from the axis of rotation to the center of the sensors, G i – sum of forces in i line collected by the sensors.

To estimate the joint moment of the lower limbs in the step III, Eqs (2) and (2.2) were used:

(2)

where:

Z 11 = d 4 + d 1 ⁢ sin ⁡ ( φ 2 + φ 3 ) - d 2 ⁢ sin ⁡ ( φ 3 ) , Z 12 = - d 5 + d 1 ⁢ sin ⁡ ( φ 2 + φ 3 ) - d 2 ⁢ sin ⁡ ( φ 3 ) , Z 21 = d 4 - d 2 ⁢ sin ⁡ ( φ 3 ) , Z 22 = - d 5 - d 2 ⁢ sin ⁡ ( φ 3 ) ,(3) Z 31 = d 4 , Z 32 = - d 5 .

The ground reaction forces and joint moments were normalized to the body mass.

To validate the measurements made by the wearable sensor system and to examine the effectiveness of the proposed method for gait analysis, an experimental study on ten volunteers from Bialystok University of Technology, Poland, was conducted. The exclusion criteria were any other disorders that may impact the subject’s gait. All the subjects received full information about the study before giving their signed consent. The data was recorded at 60 Hz with the use of the new wearable sensor system. For data measurement, the subjects walked a distance of approximately 60 meters at their habitual speed outside of a gait laboratory. The accuracy of measurement of ground reaction forces and joint moments was validated against the results obtained from the reference motion capture system (SMART, BTS, Italy).

2.4 Statistical analysis

An ANOVA for repeated measurements was applied to gait variables to evaluate differences in quantitative walking parameters. The accuracy of estimation of ground reaction forces and joint moment was validated against the results obtained from the reference motion capture system by determining the root mean square error of these maximum values, and Spearman’s rank correlation. Statistical tests were performed using Statistica 13.1 (StatSoft, Poland). The significance level was set at 0.05.