Quantifying Cervical Rotation Smoothness: Exploring Various Jerk Metrics and Test-Retest Reliability of Jerk, Range of Motion, and Head Repositioning Accuracy.

The assessment of smoothness, range of motion (ROM), and head repositioning accuracy (HRA) has gained attention in identifying sensorimotor impairments. Uncertainty persists on the approach for acquiring reliable measures, including choice of smoothness metric, normalization factors, and the required number of measurements for reliable results. This study aimed to address this uncertainty. Thirty healthy participants were included in this single-session randomized cross-over study. The experiment consisted of two parts. One focused on the test-retest assessment of head ROM into right rotation to the end of range from a neutral position using a self-selected movement speed and the HRA when returning to the start-position. In the other part, participants repeated the previous tasks and performed head rotations at slower and faster speeds than their self-selected pace and to the beat of a metronome. All tasks were repeated ten times. For the test-retest, the inter-class-correlation (ICC) values for ROM were between 0.84-0.91, 0.20-0.31 for HRA, and 0.65-0.90 for jerk for 1-10 repetitions. Normalizing jerk through v and v had similar variability and appeared equally valid for our data. However, normalizing by v ensures desirable properties in the smoothness metric. Lower variability was observed when standardizing movements using a metronome. Based on test-retest findings, three repetitions are recommended, as ICC values show marginal improvement beyond 2-3 repetitions, providing limited additional value.