2.1. Experimental Setup

Individuals diagnosed with RA (ACR/EULAR 2010 criteria [2]) and healthy controls were included in the study. Patient participants were recruited from the Internal Medicine 3—Rheumatology and Immunology outpatient clinics. Exclusion criteria for healthy and patient participants comprised of fracture of hand and finger bones in the five years before entry in the study and distinct destruction of the finger joints. Disease activity was assessed using the erythrocyte sedimentation rate (ESR), see [45], C-reactive protein (CRP), tender/swollen joint count 78/76, and Disease-activity-score (DAS)-28, see [46]. Patient reported disease activity was recorded using the visual analog scale (VAS) for global disease activity and HAQ. The assessment of clinical hand function included three components. First of all, isometric grip strength was measured in pounds (lbs) using a hand dynamometer (Lafayette Instrument, Lafayette, IN, USA). After a familiarization trial, three measurements of grip strength were performed, starting with the dominant hand and alternating between hands. The highest measured force for each hand was included in the data analysis. Secondly, fine motor skills were assessed using the MPUT [7] which is a validated test procedure for inflammatory diseases [47]. Briefly, subjects are asked to pick up twelve small items and drop them into a box as fast as possible while the time to complete the task is recorded. With each hand two repetitions of the test were completed starting with the dominant hand. The fastest trial for each hand was included in the analysis. Thirdly, subjective hand function was measured using the MHQ, a patient reported outcome measure that scores hand function of the left and right hand [48,49] and has previously been used successfully in RA [5].

Hand segment kinematics were recorded with a 29 retroreflective spherical marker layout, described in [16], with diameters of 8 mm and 14 mm at a frame-rate of 100 Hz. Reduced marker layouts, such as [50,51] have been presented in literature. However, the analysis required of their outputs requires higher efforts to compute joint angles, which far outweighs the effort to use a 29 marker layout. Furthermore, since the measurements were performed for a high number of subjects for a pilot study, it was desirable to begin with acquiring measurements with highest possible accuracy, before progressing to reduced layouts. The markers were tracked by synchronized and calibrated high-resolution and high-speed infrared cameras (eight Oqus7+ cameras and one Oqus5+ camera, Qualisys AB, Sweden). They were placed on the hand dorsum using double-sided hypoallergenic adhesive tape, as shown in Figure 1a. As a variation from [16], the thumb metacarpal (MC) was tracked using a T-cluster with 3 markers [52], and a marker each on the interphalangeal (IP) joint and the thumb tip was used. The thumb markers were labeled as T1 (MC base), T2 (MC head), T4 (IP joint) and T5 (thumb tip), see Figure 1c. The third marker in the T-cluster was named T3. Furthermore, since RA deformities are primarily observed in the finger MC heads, along with their small relative motion in precision grasping, 14 mm markers were used to allow for better placement and tracking, as compared to the setup in [16].

Marker setup showing all 29 markers on the hand in (a) with the hand in open position and displaying the marker labels and in (b) with the hand in the first flat reference posture and in (c) with the hand in the second reference posture displaying thumb marker labeling.

The radar measurement setup used in this study to measure a simple finger movement described in Section 2.2.3 consisted of a 24 GHz CW radar, which was originally designed as a vital signal radar, one absorber mat as well as one absorber wall to reduce disturbing reflections of the incident radar wave, see Figure 2a. An optical marker was placed on the respective fingertip (see Figure 1a I5 and L5) to generate reference data with the OMS, see Table 1 set E. Both systems were synchronized using a common trigger signal. The distance between the absorber mat and the monostatic radar module (RSM2650, B+B Thermo-Technik GmbH) was approximately 60 cm. The respective finger was positioned within the 3 dB beam width, which describes the angle between the points of half power of the radar main lobe, and moved along the line of sight of the radar. The horizontal and vertical beam widths were 80° and 35° respectively. The placement of the optical fingertip marker within the 3dB beam width can be seen in Figure 2b. As a first step of the analog signal processing, the received radar signal was down-converted using an In-phase and Quadrature (IQ) demodulator. The analog signal processing also included a low-pass-filter with a cutoff frequency fLP= 50 kHz to get the baseband signal and to filter out unwanted signal components, which arise due to the IQ demodulation, as well as a high-pass-filter with a cutoff frequency fHP= 160 mHz that eliminated the DC offset and signal components from stationary targets. After filtering, the radar data was sampled at 13 kHz with an analog-to-digital converter. This was followed by the digital signal processing in Section 2.3.3, which e.g., included further filtering processes, to extract the fingertip speed of the recorded signals.

Radar measurement setup: 1—CW radar, 2—absorber wall, 3—absorber mat on table, 4—optical marker placed on fingertip to collect reference data.

Overview of the complete testing procedure for one patient/hand.

Further insight into the RA effects on the hand musculotendon network was obtained by the measurement of muscle activity with two surface EMG sensors (Noraxon Desktop DTS dual electrodes) while patients performed the different activities in sets C and D in Table 1. The electrodes were placed on the flexor carpi radialis and extensor carpi radialis muscles on the dorsal and ventral sides of the forearm, respectively [53], as seen in Figure 3a. The EMG system measures muscle activation in voltage units, specifically, mV. When the measurements will be used for analysis later, the activation will be normalized with respect to the maximum voluntary contraction, see [54], performed during the grip strength test.

The hand postures for the different recordings, listed in Table 1, with the respective marker set-up. In (a), the 25 marker set, along with the surface EMG sensors, as described in Section 2.1.2, for the MPUT. The participants are instructed to lift and place 12 objects in the nearby container. In (b), finger tipping motion is shown between the thumb and the index finger with a 29 marker set. In (c), the fist posture showing the full flexion capacity of the hand is demonstrated.