PPS can result from impairments at multiple levels throughout the neuromuscular system . There is increasing evidence that changes in pain processing may enhance sensitivity to noxious stimuli among individuals with chronic pain, compared to pain-free controls [14, 33]. The present study investigated whether physical and psychological domains were related to PPH of the UT in each sex among FWs with nonspecific neck/shoulder MP, because biological differences have been suggested to cause sex differences in pain perception [34–36]. LT strength and dominant painful ipsilateral cervical rotation ROM were characterized as risk factors for PPH of the UT in male and female FWs with nonspecific neck/shoulder MP in adjusted multivariate analyses. Although our interpretations are limited because of the cross-sectional study design, the LT strength and dominant painful ipsilateral cervical rotation ROM identified in the present study could be useful for establishing guidelines for the prevention and management of PPH in FWs with nonspecific neck/shoulder MP.
Concerning LT strength as risk factor for PPH, scapulothoracic muscle imbalances could be cause of impaired biomechanics, postural adaptations, and neck/shoulder pain [37, 38]. These imbalances may occur when the UT becomes tight and the LT becomes weak [39, 40]. Conversely, LT weakness could result in UT overload because of poor scapular mechanics (e.g., increasing scapular elevation and decreasing scapular upward rotation and posterior tilting) [37, 38] and weakly synergistic acceleration of UT overactivation (e.g., involving the SA and LT) . LT strength was significantly different between the ipsilateral (mean ± standard deviation (SD): 21.8 ± 10.0 N) and contralateral sides (mean ± SD: 25.7 ± 11.5 N) in individuals with unilateral neck and shoulder pain . In the current study, LT strength in male FWs with PPH was 31.61 ± 15.58% normalized by body weight. Before LT strength was divided by body weight, LT strength was 23.50 N, which was similar to the results of a previous study involving individuals with neck/shoulder pain . However, Shahidi et al. investigated physical risk factors for chronic neck pain . They found that LT strength was not a risk factor, using a multivariate prediction model that involved cervical active ROM, cervical muscle strength and endurance, and scapular muscle strength. Although this explanation is limited by the cross-sectional study design, LT could be linked to PPH of the UT and could potentially be weaker in terms of PPH of the UT. The process may function in an inverse manner.
Cervical mobility as a risk factor for neck/shoulder pain has been suggested in prospective studies of other populations, but the results have been conflicting. Reduced cervical flexion mobility was more likely to cause neck/shoulder pain in laundry workers (risk ratio: 3.1; 95% CI: 1.2–8.3)  and increased cervical flexion-extension mobility was protective against neck/shoulder pain in office workers (hazard ratio: 0.97; 95% CI: 0.94–0.99) . With respect to dominant painful ipsilateral cervical rotation ROM as a risk factor for PPH, cervical rotation ROM is related to pain intensity in patients with chronic neck/shoulder pain [44, 45]. Moreover, patients with nonspecific neck/shoulder pain show less cervical rotation ROM, compared to asymptomatic controls [46, 47]. Reduced extensibility of upper quadrant neural structures evaluated by the median nerve tension test has been related to decreasing UT length . Furthermore, the presence of PPH of the UT was associated with cervical intervertebral joint dysfunctions . Although interpretations are restricted by the cross-sectional study design, dominant painful side ipsilateral cervical rotation ROM could be linked to PPH of the UT and could potentially cause shortness involving PPH of the UT, or the process could function in an inverse manner. UT length affects ipsilateral cervical rotation ROM and contralateral cervical side-bending ROM because of the muscle attachment locations . Thus, tenderness or PPH of the UT could affect the restriction of cervical ipsilateral rotation ROM. Conversely, reduced UT length could affect PPH by scapular dyskinesis (e.g., scapular elevation during arm lifting) . UT shortness and scapular dyskinesis could generate reduced activity of the SA and/or LT, as well as enhanced activity of the UT, resulting in UT overactivation [51, 52].
The psychological domain (depressed mood), as measured using the BDI, was not significantly different between FWs with PPH of the UT and FWs without PPH of the UT in both men (p = 0.988) and women (p = 0.666). This might have been due to limited statistical power resulting from the small sample size in this study. Psychological depressed mood was reportedly associated with an enhanced risk for neck pain in office workers (OR = 3.36; 95% CI: 1.10–10.31; p = 0.03)  and others [53, 54]. Although it is difficult to directly compare our findings with the results of previous studies, a possible reason for exclusion of the psychological domain from the variable selection process was that the psychological domain could more weakly influence PPH among workers with repetitive and high physical load tasks, compared to white-collar office workers. Furthermore, physical domains of cervical and scapular posture were not significantly different between FWs with PPH of the UT and FWs without PPH of the UT in both men and women. Forward head posture [55, 56] and scapular posture  have been previously associated with neck/shoulder pain. Because cervical and scapular posture are static characteristics, dynamic physical domains could more strongly influence PPH among workers with repetitive and high physical load tasks, rather than static physical domains.
The main limitations of the present study were its small sample size and cross-sectional study design. Furthermore, this study involved a relatively homogeneous sample of FWs. Future studies are necessary to determine whether the risk factors identified in this study can be generalized to other demographic populations and professions. Future studies should also determine whether improvements in LT strength and cervical rotation ROM are effective for reducing PPH among individuals with neck/shoulder MP.