Results of physical and clinical parameters that differ between levels of headache-related disability as measured by the 6-item Headache Impact Test questionnaire
| Variables | Little to no disability n=12 | Some disability n=12 | Substantial disability n=11 | Severe disability n=9 |
| F:M | 9:3 | 9:3 | 7:4 | 7:2 |
| Prior MVC (N) | 101.3±29.6 | 94.5±30.4 | 104.6±36.9 | 80.1±19.4 |
| Decrease in MVC (%) | −11.9±7.9 | −6.5±5.9 | −5.6±7.9 | −1.0±8.2 |
| NDI (/50) | 4.4±2.4 | 7.4±3.9 | 6.9±3.3 | 10.6±6.0 |
| Headache frequency (n/month) | 4.9±6.7 | 5.8±4.3 | 9.8±9.1 | 12.5±9.3 |
| Max headache intensity (/10) | 6.5±2.3 | 6.5±1.4 | 7.2±1.1 | 8.6±0.8 |
F, female; M, male; MVC, maximum voluntary contraction; NDI, Neck Disability Index.