Laser tracing in chronic neck pain: what held up on repeat testing?

HeadX
Layered paper-cut clinical scene of a seated person wearing a HeadX-style crosshair laser and facing the HeadX Bars I straight-line wallchart while a clinician observes.

Researchers in Tehran asked 30 people with chronic neck pain to follow four paths with a forehead-mounted laser. They repeated the same test 24 hours later. Most measurements were reasonably consistent, but the path made a difference: straight lines gave more stable results than the curved route.

That finding is relevant to clinicians who use laser targets for cervical movement work. It also needs to be kept within the limits of the study. The researchers tested their own single-point laser, camera and tracking software. They did not test HeadX, compare the system with a reference standard or measure change after treatment.

The test used four paths

The participants were adults aged 25 to 65 with neck pain lasting more than three months. Their mean age was 46.4 years and the average pain score was 5.2 out of 10. All were recruited from one physiotherapy clinic.

Each person sat upright with a laser attached to a forehead headband. A screen was placed 90 cm in front of them. They followed a horizontal line, a vertical line, a diagonal line and a curved path labelled A–G, using slow and controlled head movements. Trunk movement and other compensations were discouraged.

The system gave two forms of feedback. Participants could see the laser on the path, and a beep sounded when it moved outside the permitted area. Each path was attempted three times. The same physiotherapist repeated the procedure the following day.

A fixed camera recorded the laser trace. The research team used OpenCV software to calculate speed and what they called “success”. Speed was the time taken relative to the length of the path. Success was the uninterrupted distance completed while the laser stayed within 3 mm of the path boundary.

There is a small reporting inconsistency in the paper. The abstract describes success as a percentage, while the methods and results tables report it as a distance in millimetres. The figures below use the units in the full paper.

The straight paths were more consistent

The abstract reports ICC values between 0.868 and 0.958 for speed and between 0.753 and 0.963 for success when measurements were averaged. These values were described as good to excellent.

The path-by-path figures are more useful. The diagonal line produced the most repeatable success measure: ICC 0.963, with a standard error of measurement of 7.71 mm and a minimal detectable change at 95% confidence (MDC95) of 21.37 mm.

Success on the curved A–G path was less stable. Its ICC was 0.753, with a wider 95% confidence interval of 0.480 to 0.882. The standard error was 19.08 mm and the MDC95 was 52.90 mm.

The difference is plausible. A curved route requires continuous changes in direction and more online correction than a single straight movement. The paper did not isolate which part of that extra demand mattered, however. Pain, fatigue, attention and cognitive load were not controlled.

For repeat testing, a straight path may therefore offer a cleaner starting point. It is simple to reproduce and, in this study, produced less variable measurements. A curved or multi-directional path may still be useful when the clinical aim is to observe control during a harder task. The results do not show that one type of path is better for rehabilitation; they show that the two should not be treated as interchangeable measures.

What the reliability figures mean

An ICC describes how consistently people maintain their relative position within a group when the test is repeated. The study used a two-way mixed-effects, consistency-type ICC. This is worth knowing because a consistency ICC can remain high even if there is a systematic difference between sessions.

SEM and MDC95 provide a different view. They estimate the amount of measurement error in the original unit. In this study, the curved path needed a larger change in success distance before it was likely to exceed that error.

Those MDC values apply to the researchers’ equipment and procedure: their laser, camera, software, 90 cm test distance, path dimensions, audible feedback and 3 mm tolerance. They are not reference values for another device or a routine clinical laser exercise.

This was a visual tracking task

The task should not be confused with a conventional cervical joint-position error test. During laser tracing, participants kept their eyes open, watched the laser and received an audible warning when it left the path. Performance therefore involved visual feedback, attention, movement planning and active correction.

In a typical return-to-neutral joint-position test, the person moves away from a reference position and attempts to return without visual guidance. That examines a different aspect of cervical sensorimotor function. A head-mounted laser can be used for both procedures, but the results describe the procedure being performed, not the laser alone.

How the paper relates to HeadX

The set-up is close to exercises performed with HeadX Kross: a head-mounted laser makes movement visible against a wallchart. Kross projects a crosshair rather than the single point used in the study, so it can also show the orientation of the head and make tilt visible.

The paper provides no performance data for Kross or Duo. Its reliability figures cannot be used as HeadX specifications, clinical thresholds or evidence of treatment effect. The useful connection is practical: keep the distance, path, instructions, feedback and number of trials consistent when comparing one session with another.

For a simple line task, the HeadX Bars I chart provides horizontal and vertical references of different widths. A clinician can select one line and use the same line at reassessment. Moving to a narrower line or a more complex pattern changes the task and should be recorded as a progression rather than compared directly with the earlier result.

Limits of the evidence

The sample was small and came from one clinic. Testing was performed by the same non-blinded examiner on both days, so the study does not tell us whether two clinicians would obtain similar results. The repeat interval was only 24 hours. Longer-term stability is unknown.

The system was not compared with motion capture or another accepted reference measure. There was no group without neck pain, no treatment and no test of responsiveness. The study cannot tell us whether the scores identify an impairment, predict an outcome or change when a patient improves.

The authors reported no specific funding and no conflicts of interest.

Reference and related HeadX resources

This article is intended for qualified clinical professionals. It is educational and does not provide medical advice.

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