Results
The groups were similar in age and BMI, but there were a higher proportion of women in the HC group (Table 2). The NP group had a mean neck pain score of 4.6 on NRS at the day of testing and 7.4 on NRS for worst neck pain last month. A large proportion of the NP patients (74%) stated that their neck pain started more than 6 months ago (Table 2). The NP subjects showed a moderate disability measured by the NDI (mean 31.2; SD 11.6), a moderate kinesiophobia measured by the TSK (mean 24.4; SD 4.3), and a moderate pain catastrophizing measured by the PCS (mean 12.9; SD 8.5). Other characteristics of the NP group are shown in Table 2. Nine NP subjects were excluded due to NRS <3 for neck pain on the day of testing.
Neck Flexibility
NP patients had significantly less maximal cervical ROM in flexion/extension and rotation compared to HC after adjusting for age and gender, while lateral flexion barely fell short of reaching significance (Table 3 and Figure 2A). Summing ROM in the three primary planes in total ROM showed a difference of 33.1° (95% CI; −46.6,-19.5; p < 0.001) between the two groups (Figure 2A). There was no significant gender difference in total cervical ROM or when the primary planes were analyzed separately. Peak velocity during all ROM tests was significantly lower in NP compared to HC, and remained significantly lower also after adjusted for cervical ROM in the primary plane (Table 3). CM in accessory planes during all primary planes motion was significantly smaller in the NP patients compared to HC (Table 3). The differences remained significant after adjusting for maximum ROM in the primary plane. When adjusted for peak velocity CM in flexion/extension and rotation in the NP groups were still significantly smaller compared to HC, but not for CM in lateral flexion (Table 3).
(Enlarge Image)
Figure 2.
Forest plot of the group difference (95 % CI) between neck pain patients and healthy controls. Number in parentheses behind test variables states the analytic model applied. Analysis were adjusted for age and gender (model 1), plus cervical ROM (model 2), plus peak velocity (model 3). All variables in B are adjusted for model 1, except for JPE which is adjusted for model 2. ROM = range of motion. FxEx = flexion/extension. Latflex = lateral flexion. JPE = joint position error. FOE = figure of eight. Fly test 1B = easy pattern, large ROM. Fly test 2B = difficult pattern, large ROM. EO = eyes open. EC = eyes closed. EOB = eyes open balance pad.
Proprioception
There was no significant between group difference (p = 0.11) in relocation error in the JPE test (Table 3).
Head Steadiness
In the low load and high load tests NP patients had markedly lower head angular velocity compared to HC. In the low load test the mean group difference was −0.4 °/s (95% CI; −0.5 to −0.3; p < 0.001) and in the high load test −1.7 °/s (95% CI; −2.0 to −1.4; p < 0.001). Largest effect sizes were found for head steadiness and neck flexibility (Table 5). All subjects in the HC group were able to hold for 60 s in the low load and 30 s in the high load test, whereas 8 subjects in NP group did not manage to hold their head for 30 s in the high load test. Two of these patients had holding time <3 s in the high load test and were therefore excluded from the calculations of the kinematic variables.
Trajectory Movement Control
Table 4 shows that HC subjects departed more from the trajectory pattern in the FOE test than the NP subjects, indicated by the higher point deviation values. The differences were statistical significant for the high speed FOE test with a mean group difference in PD of −0.8 cm (95% CI;-1.3 to −0.2; p < 0.01) and the FOE test in standing (mean difference: −0.5 cm; 95% CI; −0.8 to −0.1; p < 0.05), (Figure 2B). HC also showed more trajectory departure (i.e. higher point deviation) in the Fly test 1A compared to the NP group (mean difference: −0.3 cm; 95 % CI; −0.6 to −0.03; p < 0.05). None of the other movement patterns in the Fly tests revealed any significant group differences in PD between the NP group and HC (Table 4).
Postural Sway
Postural sway during quiet standing with EO and EC did not differ significantly between the groups (Figure 2B and Table 4). The NP group had a significant larger sway area for the EOB test compared to HC (mean difference: 2.9; 95% CI; 1.5 to 4.4; p > 0.01). Contrary, the NP patients had less sway area during the FOE test where subjects had to perform neck motion during the standing balance test, but this difference was not statistically significant (mean difference: −1.6; 95 % CI; −3.5 to 0.3; p = 0.09).
Associations Between Clinical Features and Constructs of Motor Control
Neck flexibility was the only construct that was significantly associated with clinical features, but the associations were weak. Current neck pain was significantly associated with ROM in flexion/extension (r = −0.36; p < 0.01), CM (r = −0.26; p < 0.05), and peak velocity (r = −0.34; p < 0.01) during flexion/extension (Table 5). TSK was significantly correlated with peak velocity in flexion/extension (r = 0.23; p < 0.05). NDI and duration of current neck pain episode were not significantly associated with neck flexibility (Table 6). The Fly test showed a significant correlation with NDI (r = 0.27; p < 0.05), but not for the other clinical features.