## 4 Discussion
Our study included one male and one female PMHS; however, given the small sample size of this feasibility study, we are not proposing that the reported pressure values are representative of the whole population, nor are we able to attribute legitimate differences in cervical IVD pressure responses between the sexes. IVD pressure sensors are capable of recording pressure that can be correlated to load, especially under compressive loading conditions [25, 39]. The pressure sensors in this study were zeroed with the specimen in the \"neutral\" position at test commencement (i.e., Frankfurt horizontal plane parallel to the ground with relatively little external support) to represent an \"authentic\" anatomical posture with the neck supporting the weight of the head. This methodology was chosen to more equally distribute the resultant IVD responses; however, this approach resulted in measuring pressure relative to this neutral position. In pure tension/compression, the subsequent cervical IVD pressure values are relatively easy to appreciate and interpret; though, in flexion/extension motions, pressure values may depend more heavily on the exact position of the sensor in the cervical IVD. It is possible that regional areas of differing and/or opposing force are created within the disc during neck flexion/extension motions (e.g., neck flexion could result in transitory compression forces in the anterior portion of the disc while also creating contralateral tension forces in the posterior disc, and vice versa for neck extension). Pressure sensor placement in a location of the disc experiencing these regional and opposing forces during specific neck motions may explain the adverse IVD pressures as these changes in disc shape could create areas of vacuum within the disc that could read as negative pressures. Both positive and negative pressure values were recorded for many of the motion trials; therefore, the gauge pressures were examined as relative pressures, and results were examined in terms of absolute value to facilitate data comparison with the existing literature. This study employed the use of manually applied forces to generate head/neck motions and, despite being performed by a physiotherapist, may have affected the repeatability and/or magnitude of the IVD pressure response. The applied cranio-cervical kinematic intervention pursued two primary objectives: (1) to achieve the maximum physiological range of motion and (2) to respect the individual movement limits of each PMHS. We considered manual cranio-cervical mobilization to be an appropriate method for the aims of our study, as it allowed us to tailor the range of motion to the specific characteristics of each PMHS, including age, tissue stiffness, degenerative changes, and postural configurations [62]. The assessment of joint motion limits, or \"articular end-feel\", is a standard procedure in physiotherapy and is routinely applied in clinical settings [64]. For this reason, a PhD physiotherapist with extensive expertise in musculoskeletal practice was selected to perform the mobilizations. Throughout the entire battery of tested movements, particular attention was paid to avoid cranio-cervical axial compression during mobilizations, except in trials specifically designed to assess compressive loading. We believe that, under these conditions, the loads transmitted through the intervertebral discs were primarily related to the progressive reduction in slack in capsuloligamentous and myofascial tissues, rather than to externally applied manual axial pressure. It is important to note that, while similar data trends existed between the two PMHSs, this feasibility study's small sample size, paired with continued refinement of the manually induced quasistatic motions throughout the six test trials (three trials per PMHS), most likely affected the results. Marked improvements in both motion timing and head/neck ROM repeatability were noted to occur as the test series progressed.