This study is a quasi-experimental study collected quantitative data , with a pre-test-post-test design. Pre-intervention, baseline measurements of C5-C6 right facet joint length and width with the head of the treatment plinth downward at 45 and 90 degrees, (relative to the rest of the table) and stiffness at 90 degrees, were taken prior to applying the long-axis thrust manipulation. Post-intervention measures were taken immediately after a long axis thrust manipulation of C5-C6 and again at 24-hours after the manipulation. The baseline pre-intervention measures were then compared to the post-intervention measurements. The investigator performing the manipulation was blinded to the quantitative measurements of the ultrasound images. The ultrasound operator was blinded to the outcome of the manipulation.

The study takes place in Marieb Hall rooms 101 and 103 of Florida Gulf Coast University. After meeting the inclusion and exclusion criteria subjects sign the informed consent form described thoroughly on the following page. The participants undergo three recording sessions on two consecutive days. The first recording session consist of obtaining participant consent and general information, such as height and weight. The participants then has their baseline measures cervical flexion ROM, capsular length and width at both 45 and 90 degrees, and stiffness at 90 degrees, taken. The second and third session consist only of repeating these baseline measures post-manipulation and 24-hours post-manipulation respectively. Each participant will be provided a profile on the ultrasound unit, corresponding to their subject number, with each point in time having an individual exam within the profile. At these sessions, recordings are done in the prone position to measure length of the C5-C6 joint capsule and joint capsule tissue extensibility. Locating the articular pillar was a three-fold process. At the intervention table, the C7 spinous process is identified in upright sitting with palpation, due to its relevance in clinical examination and evaluation, serving as the first landmark. From this position, one of the measuring practitioners moves their finger cranially to the C6 spinous process. The participant's head is moved passively into extension, causing the spinous process to move away from the practitioner's finger, providing further confidence in location accurary. This landmark isthen marked for repeatability. The participant then has their cervical flexion range of motion measured with a fluid-filled inclinometer. A fluid-filled inclinometer utilizes gravity to quantify gross motion. This method is easily repeated and validated for clinical measurements. Each participant is given the instructions, "sit up straight and tuck your chin to your chest." At the ultrasound table, a goniometer is secured with the stationary arm in line with the table, the axis through the pivot point for the head of the table, and the moving line in line with the head of the table.

The participant is then instructed to lie prone on the table, with the head of the table positioned at 45 degrees. Anytime the head position of the table is altered, two researchers confirmed the angle of the goniometer. The 45 degree angle was selected to neutralize the natural lordosis of the lower cervical spine. The table insert is removed for the participant's face to lie within the opening. A piece of tape is placed at the base of the table opening . The participants are given the instructions, "tuck your chin and put the front of your chin on the piece of tape." From this point, the ultrasound probe is placed in a right paramedian sagittal/long-axis view. The probe was angled 15 degrees from lateral-to-medial in the posterior-to-anterior direction.

Beginning at the occipital bone, the probe is moved inferiorly. The articular pillar is known to have a jagged, sawtooth like appearance when observed from this angle. The trough at the bottom of the sawtooth indicates the inferior articular process of the cranial vertebra, while the following peak will be depicting the superior articular process of the following caudal vertebra.

This allowed the practitioners to count the peaks observed and identify the spinal levels visualized on the screen. The mark placed on the C6 spinous process is compared to probe location once the C6 level has been visualized on the screen. After the level is confirmed to be correct, the probe remained stationary on the participant. Once the correct level is visualized, the first screen capture video was taken and saved to the participant exam. The head of the table is then angled fully down towards the floor, measured at 90 degrees with the secured goniometer. By having the head of the table as far down as it allows, the process for achieving full cervical flexion remained standardized. This is also confirmed via palpation of the interspinous space of T1 and by both practitioners audibly confirming the observation of the thoracic spine flexion, indicating full cervical flexion is achieved. While the capsule could theoretically be put under more tension by utilizing multiplanar movements, utilizing only flexion offers a standardized, repeatable method that is less susceptible to confounding variables. As the probe remains stationary with the C5-C6 joint in view, the second screen capture video is taken. While remaining in the fully flexed position, the ultrasound operator start the elastography on the ultrasound unit. Once displayed on screen, a third screen capture video is taken.

The participant then returns to the initial table to receive the intervention. The practitioner performing the HVLA long axis C6-C6 manipulation on the right stands at the head of the plinth, on the right side of the patient positioned in supine. The practitioner's right hand is placed under C6, while the left hand is placed under the chin as the arm cradles the head. Using the left hand and arm, the practitioner passively rotated the head away and side-bent the head toward the targeted side. Once end-range motion is achieved, the practitioner provides a high-velocity low-amplitude long-axis thrust in a superior direction, parallel to the angle of the targeted segment.

Following the manipulation, the participant has their cervical range of motion re-measured, and the ultrasound imaging process is repeated in a new exam. Once the immediate post-intervention measures are taken, the subjects are instructed to come back for a third imaging session at the exact same time the following day. At the 24-hour follow-up, the participants cervical flexion range of motion is repeated, as well as the three ultrasound procedures.

Inclusion Criteria:

• Healthy individuals between 18-35 without neck pain and able to to read and understand English

Exclusion Criteria:

• autoimmune disease, down syndrome, fusion, fracture, spinal cord compression, vascular pathology, cancer, and/or any other conditions that induce severe instability, inflammation, or weakened bone.

only unidentifiable data will be shared.