Thirteen asymptomatic female participants (age: 23 ± 3 years; weight: 56 ± 9 kg; height: 163 ± 0,05 cm, BMI: 21 ± 3 kg/m²), free of any neurological or musculoskeletal disease participated in this study. Only female participants were included in this study to avoid any variation of cervical spine mobility that might be related to gender differences.

This study was carried out in accordance with the recommendations of the local “Ethics Committee for Movement Analysis.” All subjects gave written informed consent in accordance with the Declaration of Helsinki.

Three different cervical collars (Figure 2) were used to vary the mobility of the cervical spine passively, in accordance with the study of Hartman et al. (1975), who measured cervical spine range of motion in flexion-extension, side bending, and rotation while wearing five commonly used cervical orthoses:

The participants sat on the stool fixed on the platform with their upper limbs relaxed along their trunk, hips, and knees flexed to approximately 90°, barefoot and feet apart. Adhesive tape was put around the feet outline in order to keep the same positioning for every trial.

The subjects had to perform the “STS” task, which consisted in rising from the stool to reach the standing position as fast as possible, in response to a “Go” signal.

The STS was performed in four experimental conditions varying the mobility of the cervical area using the three above-mentioned collars: R0, no collar; R1, jersey tubular bandage; R2, foam cervical collar; R3, Philadelphia collar.

A training period was used to familiarize the subjects with the paradigm before recording.

Ten 3-s runs were performed in every condition, with a rest period of 30 s between runs and of 1 min between series.

The order of the experimental conditions was randomly assigned to avoid any order effect.

Three parameters were calculated to characterize the APAs and the FM of the STS task along the anterior-posterior axis. These two distinct phases of the task were considered to be separated by the seat-off instant, for which the curve of the pressure sensor begins its fall toward zero (Figure 3).

The different events (beginning of the APAs, seat-off, end of the FM) were based on the visual inspection of the curves, performed by a fully trained experimenter.

Data from the accelerometers were used to ensure that the head and trunk were kept still before the “Go” signal, and to discard trials in which the participants anticipated the instructions of the experimenter. The accelerometric signals were not used to separate the focal and the postural components of the task, because in complex movements such as the STS, some parts of the bony chain might be involved in both phases, with no clear distinction between the focal and the postural chains.

All parameters were calculated using a customized program written in MatLab software (The MathWorks, Inc., Natick, MA, USA).

A one-way repeated measures analysis of variance (ANOVA) was conducted for each dependent variable, with cervical mobility as a within-subjects factor. When statistical significance was reached, the ANOVA was followed by a within-subjects analysis of contrasts to compare the levels of the independent variable. As we presumed a significant difference between a reference condition (R0, no collar) and the three others ones (R1, R2, and R3), we used a simple contrast which compared R0 to each category (R1, R2, and R3). All statistical analysis were performed using the Statistical Package for Social Sciences (SPSS) software V22 (Chicago, IL, USA).

The statistical analysis of the biomechanical parameters revealed that the FM duration of the STS task increased stepwise with the restriction of cervical spine mobility. As the subjects were requested to perform the task as fast as possible, it can be assumed that lower cervical spine mobility results in poorer performance for the STS task. This phenomenon could firstly be related to a lower ability to perform the FM when the articular free play of the cervical spine is restricted, in accordance with literature depicting the mobilization of this area during the focal phase of the STS. Indeed, Nuzik et al. (1986) reported a neck extension after the first 35% of the movement cycle, and Rodrigues-de-Paula Goulart and Valls-Solé (1999) showed that two motor muscles of the neck, namely trapezius and sternocleidomastoideus, present a noticeable EMG activity during the focal phase of the movement.

Secondly, the loss of cervical mobility may also impair the ability to perform efficient APAs, which represent a part of the central motor program that tends to reduce the early perturbations induced by the FM. In the paradigm of STS (Diakhaté et al., 2013) as in gait initiation (Brenière and Do, 1991), which include a shift of the center of mass (CM), APAs also contribute to the generation of the propulsive forces. It is assumed that APAs must be developed for the movement to be performed efficiently (Bouisset and Zattara, 1981). APAs involve postural chain mobility, and restricted articular free play has been shown to induce poorer performance in various paradigms, such as manual pointing or pushing (Bouisset and Le Bozec, 1999). The implication of cervical spine mobility during the anticipatory phase of the STS task is supported by existing literature, with Jones and Hanson (1970) representing a cervical flexion at the beginning of the STS trajectory in accordance with Nuzik et al. (1986). In the study by Rodrigues-de-Paula Goulart and Valls-Solé (1999), early EMG activity of the sternocleidomastoideus prior to the seat-off instant, very close to the onset of tibialis anterior that is the first muscle to be activated, has been reported. As the motor pattern of the APAs, including the motor muscles and the mobilized joints, is task specific (Bouisset and Zattara, 1981), the restriction of cervical spine mobility might not be easily compensated for. Consequently, APAs will become less efficient and require a significant increase of their amplitude and duration to counterbalance the disturbances associated with the FM, or to generate the propulsive forces. This adaptation of APAs programming is in line with a previous study exploring the effect of support base surface on the paradigm of shoulder flexion at maximum velocity while standing (Zattara and Bouisset, 1992). In this study, the authors showed that restricted support surface was associated with longer APAs and a lower performance of the FM (represented by the angular velocity peak).

Beyond its key role as a part of the postural chain whose mobility reduces the disturbance associated with the FM in the STS, the cervical spine may also have other functions due to its upper location along the bony chain and articulation with the head. The early head flexion followed by an extension (Jones and Hanson, 1970), with a similar pattern for the trunk and pelvis (Nuzik et al., 1986), might provide the head and cervical spine with a guide function for STS kinematics. This function was suggested in a series of pictures taken in the burst mode in a few trials, with head mobility apparently starting and guiding the whole body trajectory. These hypotheses require an extended EMG and kinematic study to be assessed.

This specific role of the cervical spine could also be considered from a neurophysiological point of a view, as these vertebrae determines head orientation in space, and may vary visual field, vestibular activity, and cervical proprioception, which all provide substantial input for motor control (Paulus et al., 1984; Roll et al., 1989; Cullen, 2012). In the performance of complex tasks such as jumping, it has been shown that the head is sequentially stabilized in different positions, during several successive time periods (Pozzo et al., 1990). Hence, restricted articular free play in the cervical area might also impair the sensitive and sensory flow used by the CNS to control the STS task.

The analysis of CP displacements according to the seat-off instant showed that APAs duration and magnitude increased stepwise with the restriction of cervical spine mobility, suggesting that the local variations of the articular free play were integrated in the central programming of the task. This fine tuning of APAs is in line with literature demonstrating their adaptability to different biomechanical parameters, such as the size of the base of support (Yiou et al., 2007), or additional loads (Bouisset and Zattara, 1981). It may now be assumed that APAs are also adaptable to spine mobility variations.

To date, little attention has been paid to cervical spine mobility in the STS task, either in a physiological perspective or in treatment and prevention strategies. It is well know that restriction of cervical spine mobility is frequent in the elderly (Lansade et al., 2009) or in neck pain syndroms (Cagnie et al., 2007), and results from this study suggest that it may impair the ability to perform the STS task. As a consequence, preserving or extending the cervical articular free play in these cases, using rehabilitation techniques or adapted physical programs, may be useful to preserve functional autonomy. Indeed, it must be recalled that the STS, which is considered a fundamental prerequisite for daily activities (Boukadida et al., 2015), is performed in mean 60 times per day (Dall and Kerr, 2010).

Elsewhere, the prescription of cervical collars, although they may represent an efficient tool to relieve pain or to favor cervical spine recovery (Muzin et al., 2007), should take into account their negative effect on the STS task, especially for patients presenting a loss of functional autonomy.