*2.1. Participants*

Fifteen cervical non-disabled participants (NDP) (3 females, 12 males) and 9 cervical disabled patients (DP) (4 females, 5 males) were recruited from students in University hospital and among researchers' patients to participate in this study, see Table 1. Inclusion criteria for NDP were the absence of neck pain episodes in the last 6 months and a neck disability index (NDI) [29] score of less than or equal to 8%. Inclusion criteria for the DP were a numeric pain rating scale (NPRS) equal to or greater than 3/10 [30] and an NDI > 8%. Exclusion criteria were for NDP and DP: impaired cognition, blindness, deafness, dizziness, or vestibular disorders diagnosed by a physician. Participants and patients did not exhibit any neuromusculoskeletal or neurologic disorder that could influence the performance of head rotation in the horizontal plane. The participants signed informed consent and gave permission to publish their case details. The study was approved by the local ethics committee

(Comité d'Ethique Hospitalo-Facultaire Saint-Luc-UCL (IRB 00001530)) and conducted in accordance with the declaration of Helsinki.


**Table 1.** Characteristics of the participants in NDP and DP groups. Data are given either under mean ± SD or median [Q1–Q3] form.

#### *2.2. The DidRen Laser Test*

The DidRen Laser Test [26] was used to assess neck mobility through standardized axial rotations of the head in NDP and DP.

After watching an explanatory video, participants sat on a chair with backrest, without armrests, placed at 90 cm from a vertical panel equipped with 3 targets (LEDs) arranged horizontally and located 52 cm apart (Figure 1). Participants wore an adjustable helmet with a laser beam attached on the top was worn by the participant (Figure 1). The experimenter (RH) adjusted the helmet so that the laser hit the central target while the participant was in a neutral position before the test began. The instructions were the same for all participants: "You must reach the targets as fast as you can and perform the head movement without moving your shoulders". The targets were then turned on in a predefined sequence and the participant's task was to rotate his/her head so that the laser beam hit the target as quickly as possible. When the laser beam was stabilized by the participant on a target for at least 0.5 s, the target LED lit up and an audible signal was emitted. A complete test was composed of 5 cycles of cervical axial rotation to the right and left sides respectively.

**Figure 1.** (**A**) DidRen Laser Test installation device. (**B**) Schematic view from above. The passage from one target to another induces an axial rotation of the head of 30◦ either to the left or to the right sides of the bodyline. (**C**) The Helmet worn by the participant with the Laser on the top. The DYSKIMOT sensor can be seen (red circle) at the front of the helmet.

A first test was carried out to familiarize the participant and a second for data recording and analyses [26].

<sup>1</sup>SD = Standard Deviation, BMI = Body Mass Index, Q1 = First Quartile, Q3 = Third Quartile, NDI = Neck Disability Index. NPRS = Numeric Pain Rating Scale, NDP = Non-Disabled Participants. DP = disabled participants.

#### *2.3. Motion Sensors*

#### 2.3.1. Elite System (BTS)

An optoelectronic system composed of 8 infra-red cameras (ELITE, BTS, Milan, Italy) (Figure 2A) with sampling frequency of *f* = 200 Hz test carried out the three-dimensional recording of the markers on the helmet during the DidRen Laser. A kinematic model composed of 3 markers on a helmet and fixed during all experimentations representing the head was used and adapted from [31] (Figure 2B,C). Helmet markers were positioned such that one was just aside the top of the head (Top H) and positioned next to the laser, and two on each side of the head (R.H and L.H) (Figure 2C). Real time detection of head rotation markers was executed around a coordinate system such that the axis of rotation for head axial rotations was X (inferior-superior axis). The Y-axis was aligned with participant's mediolateral axis at the beginning of the test and the Z-axis was aligned with the antero-posterior axis. This is illustrated in Figure 2B. The system was previously calibrated within the infra-red camera's field of view [31] and the instantaneous X, Y, and Z coordinates of the three markers were recorded, leading to → *XTop H*, → *XL*.*H*, and → *XR*.*H*. The vector →*u* = →*XTop H* − →*XL*.*H*+<sup>→</sup>*XRH* 2 gives the orientation of antero-posterior axis (coinciding with that of the laser beam).

**Figure 2.** (**A**) Infra-red cameras (ELITE, BTS, Milan, Italy). (**B**) Head axis of rotation is denoted as X. (**C**) Placement of the reflective markers on the head.

The angular displacement time series of the head, θ*i*, has been computed from the coordinates of the markers as described in details in [32]: θ*i* = *cos*<sup>−</sup><sup>1</sup>- →*ui*. →*u*0 ||<sup>→</sup>*ui*||||<sup>→</sup>*u*0||. The index *i* denotes the vector at time *i* Δ*t*, Δ*t* = 1/ *f*. The angular velocity was then computed as ω*i* = θ*i*+*n*<sup>−</sup>θ*i*−*<sup>n</sup>* 2.*n*.Δ*t* with *n* = 5 and, similarly, the angular acceleration has been computed as α*i* = <sup>ω</sup>*i*+*n*<sup>−</sup>ω*i*−*<sup>n</sup>* 2.*n*.Δ*t* . The choice *n* = 5 guaranteed an optimal smoothness of the curves both for Elite and DYSKIMOT time series (assessed by visual inspection).
