Neck muscle function in individuals with persistent pain and disability after whiplash injury

Background: Neck pain and disability are common after whiplash injury. One year after the accident up to 50 % still have symptoms called whiplash associated disorders (WAD). Despite decades of research the cause of persistent pain and disability are largely unknown and effective treatment and diagnostic tools are lacking. Altered neck muscle function may cause pain and disability, and real-time non-invasive methods that investigate both superficial and deep neck muscle function need to be evaluated. Aim: The general aim of the work presented in this thesis was to investigate mechanical neck muscle function and evaluate effects of three different exercise interventions related to neck muscle function in individuals with persistent pain and disability after whiplash injury. Method: The thesis comprised two studies, reported in four papers. Study A was a prospective randomized controlled trail with 216 participants. The effects of three exercise interventions; neck-specific exercises, neck-specific exercises with behavioral approach and prescription of physical activity were evaluated. Neck muscle endurance, perceived pain following testing, kinesiophobia and satisfaction with treatment were compared between the three groups (paper I). Study B was an experimental case-control study with participants consecutively recruited from the randomized controlled trial. Deformation and deformation rates in the neck muscles were investigated with real-time ultrasound imaging during ten repetitive arm elevations (paper II-IV). To investigate ventral neck muscles, 26 individuals with WAD were compared with 26 healthy controls (paper II). The dorsal neck muscles were investigated in paper III, including 40 individuals with WAD and 40 controls. In total 46 individuals, 23 with WAD and 23 healthy controls were included in paper IV to develop ventral neck muscle interaction models. Results: Paper I: Participants in the two neck-specific exercise groups (with and without behavioral approach) showed increased dorsal neck muscle endurance (p = 0.003), decreased pain intensity following testing (p = 0.04) and were more satisfied with treatment (p < 0.001) than participants in the prescribed physical activity group. Kinesiophobia did not significantly differ between groups (p > 0.12). Paper II: Deformation and deformation rate showed linear positive relationship between ventral muscle pairs in healthy controls, especially between superficial and deep neck muscles. This relationship was weaker or absent in the WAD group. Paper III: The WAD group had higher deformation rates in the deepest dorsal neck muscles during the first and tenth (only women) arm elevations compared to the control group (p < 0.04). Women in the WAD group showed a weaker linear relationship between the two deepest dorsal neck muscles compared to women in the control group. Paper IV: The results revealed two different ventral neck muscle models in individuals with WAD and healthy controls (R2Y = 0.72, Q2Y = 0.59). The models were capable to detect different neck muscle interplay in people with WAD. Conclusion: Neck-specific exercise intervention with or without a behavioral approach appears to improve neck muscle endurance in individuals with persistent WAD. Decreased pain after the neck muscle endurance test also suggests improved tolerance of load in these two groups. Altered mechanical neck muscle function was revealed in individuals with WAD indicating decreased muscular support for maintain a stable cervical spine during repetitive arm elevations. The results show great promise for improved diagnosis of neck muscle function in WAD.The corrections in the errata list has been carried out in the electronic version.