Multiple outcome measures were obtained at three intervals: pre-treatment, the next day following the completion of the 30 visits, and at 3-months follow-up after the 10-weeks of treatment re-evaluation. The order of measurements was the same for all participants. Radiographic cervical sagittal alignment of AHT and ARA C2–C7 was the primary treatment outcome whereas, neurophysiological variables were the secondary measures.

Standing lateral cervical radiographs were assessed for AHT distance and cervical lordotic angle (ARA C2–C7) via the method investigated by Harrison et al33,34.

The secondary neurophysiological findings included central somatosensory conduction time (N13–N20) and amplitudes of the following potentials: spinal N13, brainstem P14, parietal N20 and P27, and frontal N30. For participant neurophysiological assessment, an electromyogram device (Neuropack S1 MEB-9400K, Nihon Koden, Japan) was used to measure these variables. After the skin was cleaned, the stimulating electrodes were placed on the skin overlying the median nerve 2–3 cm superior relative to the distal crease of the wrist. We used a bearable, painless stimulus intensity set at 3 times above the sensory level; no participant reported this as noxious or pain causing38,39.

We followed previously published protocols and procedures for the neurophysiological assessments; for recording, careful attention was paid to cleaning and scarifying the skin before the attachment of the recording electrodes to the scalp. Using an impedance below 5 kΩ, the recording electrodes were placed superficial to the sixth cervical vertebra spinous process (Cv6) [anterior neck (AC)]. Additional recording electrodes were placed at the frontal and parietal scalp regions contralateral to the side of stimulation (P3, P4, and F3, F4). The reference electrode was placed ipsilateral to the stimulation site at the earlobe38,39.

Each test was repeated at least twice and the summated tracings using two repeatable averages were used for the amplitude and latency measurements. Following previous protocols38,39 the band pass was set to 5–1500 Hz, with an analysis time of 100 ms and a band width of 103 μs. A total of 800 sweeps were averaged using electrical square pulse stimuli of 0.2 ms durations. We identified and measured the following potentials:

The N13 potential40;

The far-field P14 potential40;

The N20 and P27 potentials41;

The N30 potential41,42.

The potential amplitudes were measured from the preceding peak (termed peak-to-peak). There were two different rates of somatosensory evoked potentials (SSEPs), to enable optimal conditions to record the N13, P14, N20, P27 and the N30 SEP peak complexes. The slower rate of 2.47 Hz was optimum for N30 while the faster rate, 4.98 Hz, allowed N13, P14, N20, and P27 to be appropriately quantified.

Using standardized clinical procedures for median nerve stimulation at the wrist, each subject’s central somatosensory conduction time measurement, N13–N20 was determined43,44. Differences in peak latencies between N13 and N20 was measured as central conduction time.

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