Background Strength training has shown effects in reducing neck pain. As neck pain is highly prevalent in tension-type headache (TTH), it is relevant to examine the effect of strength training of the shoulder muscles on TTH patients. Aim To examine the effect of strength training of the shoulder/neck muscles on TTH frequency and duration. Methods Sixty patients with TTH were randomised into strength training or a control group. The strength training group trained ten weeks with elastic resistance bands. The control group performed ergonomic and posture correction. Efficacy was evaluated at follow-up after 19-22 weeks. Results Twenty-three patients completed strength training and 21 completed ergonomic and posture correction (per-protocol). No between-group effect was detected, but within groups numerical reductions were noted in both groups from baseline to follow-up. Frequency of TTH in the strength training group decreased by 11% ( P = 0.041) and duration decreased by10% ( P = 0.036), while the ergonomic and posture correction group showed a significant reduction in frequency of 24% ( P = 0.0033) and a decrease in duration of 27% ( P = 0.041). Conclusion No significant difference between the groups was found and the within-group effects did not reach clinical significance. Combining all the elements into a multifaceted intervention could prove more useful and should be further explored in future studies. Clinical trials registration number NCT02984826.