**3. Results**

The present study consisted of 46 participants (CP group: *n* = 23; Control group: *n* = 23), 61% of whom (*n* = 28) were female. Their average age was 8.9 years with a standard deviation of 3.2 years. The GMFCS showed that 47.8% of the CP subjects were classified as level I, 17.4% as level II, 4.4% as level III, and 30.4% as level IV. No patient showed a value of 2 or more in any muscle and over 30% of CP subjects had no impairment in muscle tone, according to the MAS. This means that muscle tone suffered, at most, a slight increase in the CP subjects. No study subjects suffered pain or other difficulties when undergoing the complete evaluation. Other basic descriptive characteristics of the groups are given in Table 1.

The correlation analysis between MAS and ROM, assessed by the CROM and the IMU, showed a common trend, with flexor, extensor, and sternocleidomastoid muscles of CP subjects significantly and negatively correlated with rotational ROM (in all cases: the higher the muscle tone, the lower the ROM). Thus, the tone of flexor and extensor muscles correlated with: CROM: rs = −0.504; IMU first assessment on the first day: rs = −0.510; IMU second assessment on the first day: rs = −0.483; IMU assessment on the second day: rs = −0.412. Right and left sternocleidomastoid muscle tone correlated with: CROM: rs = −0.433; IMU first assessment on the first day: rs = −0.437; IMU second assessment on the first day: rs = −0.420; IMU assessment on the second day: rs = −0.410. No correlation was identified in the planes of flexion-extension and side-bending.


**Table 1.** Demographics and clinical characteristics of the subjects.

Quantitative data are expressed as mean (standard deviation). Abbreviations: GMFCS, Gross Motor Function Classification System; BMI, body mass index. \* indicates *p* < 0.05.

No differences were detected between CP subjects and controls in each ROM for any of the assessments, regardless of the method of measurement (*p* > 0.05).

Additionally, as reported in Table S1, the two-way ANOVA of the ROM of the three spatial planes showed a consistent pattern, with neither evaluation-by-group interaction nor Group factor significance, although the Evaluation factor detected statistical differences (*p* ≤ 0.02). The post-hoc analysis of the Evaluation factor showed differences between CROM and the IMU assessments, with no differences among the three IMU assessments. The only exception to this pattern was the post-hoc analysis of the Evaluation factor concerning the rotational plane ROM, with statistical differences, exclusively, between the CROM and IMU assessments on the second day.

## *3.1. Concurrent Validity*

The measurements obtained by the first IMU assessment on the first day correlated highly with the measurements of the CROM for flexion-extension and side-bending ranges in both groups (r > 0.9), although rotation range correlations were smaller (0.6 < r < 0.8). Significant differences between both methods were observed in all ROMs, with the exception of the rotational range in the control group (Table 2).


**Table 2.** Concurrent validity between the first IMU assessment performed on the first day of measurements and CROM by groups.

Abbreviations: IMU, Inertial Measurement Unit; CROM, Cervical Range of Motion; CI, confidence interval; CP, cerebral palsy. Evaluation data are expressed in degrees.

The Bland–Altman plots (Figures 2 and 3) indicated bias below 5º between both measurement systems for craniocervical ROMs, except for the rotation range of the CP group (mean bias: 8.2º). Nevertheless, the distance between LoAs for all ROMs and both groups were over 23.5º, with the exception of flexion-extension range in the control group (distance between LoAs: 12.1º). Finally, some outliers were found on the Bland–Altman plots of the CP group.

**Figure 2.** Bland–Altman plots for craniocervical ranges of CP group measured by IMU first assessment on the first day and the CROM in (**A**) flexion-extension range, (**B**) rotational range, (**C**) side-bending range. The red line indicates the mean bias, whereas the blue lines refers to its upper and lower limits (mean ± 1.96 standard deviation). All plots were developed as follows: the Y axis corresponds to the differences between the paired values of both methods (IMU-CROM), whereas the X axis represents the respective value of the average of both (IMU + CROM/2).

**Figure 3.** Bland–Altman plots for craniocervical ranges of the Control group measured by IMU first assessment on the first day and the CROM in (**A**) flexion-extension range, (**B**) rotational range, (**C**) side-bending range. The red line indicates the mean bias, whereas the blue lines refer to its upper and lower limits (mean ± 1.96 standard deviation). All plots were developed as follows: the Y axis corresponds to the differences between the paired values of both methods (IMU-CROM), whereas the X axis represents the respective value of the average of both (IMU + CROM/2).

## *3.2. Intra-Day and Inter-Day Reliability*

Table 3 shows absolute reliability results. For the intra-day reliability, all ICCs for both groups were from 0.82 to 0.93, with the 95% CI showing a common trend of (upper limit: ICC+0.2, lower limit: ICC−0.2). Nevertheless, absolute reliability data were variable, although all SEM were below 8,5º, and MDC90 between 11.4º (side-bending range of the control group), and 19.4º (rotational range of CP group).


**Table 3.** Intra-day and inter-day reliability of the IMU by groups.

Abbreviations: IMU, Inertial Measurement Unit; ICC, Intraclass Correlation Coefficient; CI, confidence interval; SEM, Standard Error of Measurement; MDC, Minimum Detectable Change. Evaluation data are expressed in degrees.

For the inter-day reliability, all ICC values were higher than 0.8 and the 95% CI also showed a trend of (ICC+0.2; ICC−0.2), except for rotational range of the CP group (ICC = 0.53) with a wide 95% CI. SEM showed higher values compared to the intra-day values in all ROMs and both groups. Furthermore, the SEM and MDC90 were higher than 20º for flexion-extension and rotational ranges in CP subjects, and for flexion-extension range in controls.
