Results

INCIDENCE OF SHOULDER PAIN

The 1 year incidence of shoulder pain was 14% (table 2). The results of the preliminary and final multivariate logistic regression models are shown in table 3. The risk of incident shoulder pain increased with age; the risk in the oldest group was nearly threefold higher than in the youngest. Likewise, the risk of incident shoulder pain was positively correlated to the body mass index. Also, mental stress and physically strenuous work increased the risk of incident shoulder pain. Working with a hand above shoulder level, twisting movements of the trunk, working with rotated neck, with the trunk flexed forward, or in a sitting position predicted the risk of incident pain in the preliminary models. Of these work postures only working with the trunk flexed forward remained being a risk factor in the final multivariate model. Of the different types of sports, dancing slightly increased the risk of incident shoulder pain whereas jogging decreased the risk, which is to say that those who practised jogging actively had a significantly lower risk of shoulder pain than those who did not practise jogging (OR 0.3, 95% CI 0.1 to 0.8). Other types of sports or the frequency of physical exercise were not associated with the incidence of shoulder pain.

In the subgroup of those respondents who had no shoulder pain in 1992 and in 1993, the effects of the predictors remained similar, except the effect of age, which decreased and the effect of working with a hand above shoulder level, which slightly increased with time.

PERSISTENCE OF SEVERE SHOULDER PAIN

The 1 year persistence of severe shoulder pain was 55% (table 2). Overload at work predicted the persistent severe pain in our logistic regression models (table 4); the more overload the respondents reported, the higher was the risk. Also, working with a hand above shoulder level increased the risk, but the association was rather weak. Cross country skiing and physical exercise more than 52 times a year decreased the risk of persistent severe pain.

Discussion

The risk factors that explained the incidence of shoulder pain in this longitudinal study did not differ significantly from the risk factors for the prevalence of shoulder pain in our cross sectional study.12 At baseline, shoulder pain was associated with age, previous shoulder injuries, twisting movements of the trunk, working with a hand above shoulder level, mental stress, and playing volleyball.12 In this longitudinal study, the individual risk factors for the incidence of shoulder pain were age and body mass index. Age has also been a strong predictor for shoulder pain in some previous studies,5 15 possibly due to increasing degeneration of the tendons and development of osteoarthrosis in the joints.

In some studies, obesity has been a risk factor for the development of osteoarthrosis in several joints, especially in the knees but also in the hands.16 17 Biomechanic and metabolic factors have been suggested to explain this relation. In our study, body mass index seemed to be an important risk factor for incident shoulder pain.

Of the work related factors, the subjective estimate of physically strenuous work was a risk factor for the incidence of shoulder pain. Physically strenuous work presumably contains activities (for instance lifting, carrying, pushing, or pulling) which load the shoulder and may cause pain. The effect of the most shoulder load specific variable, working with a hand above shoulder level, decreased in the final multivariate model. Working with the trunk flexed forward seemed to be a stronger predictor for incident shoulder pain. This variable does not directly indicate high shoulder loads. However, working with a hand above shoulder level correlated strongly with working with the trunk flexed forward (data not shown). This means that workers with forward flexed trunk postures often had raised arm postures, but the trunk postures being a stronger predictor in the multivariate model might explain the effects of the raised arm postures.

Overload at work (a sum score variable of difficulty and hurry at work) predicted the persistence of severe pain. Hurry at work may indicate too fast a work pace or poor job control whereas difficulty at work may be a sign of psychologically too demanding work. Poor job control and psychologically demanding work have been related to shoulder pain in some previous studies.1

Mental stress was associated with shoulder pain at baseline.12 In this follow up study mental stress predicted the incidence of shoulder pain but not the persistence of severe pain. Mental stress has been related to different musculoskeletal problems in some studies.5 7 18 In a recent prospective study of neck and shoulder disorders, stress was related to neck and shoulder disorder at baseline but was not associated with the development of the disorder.19Presumably a painful condition causes stress, but whether mental stress is a causal factor for musculoskeletal pain is still under discussion. Our results suggest that mental stress may be an independent predictor for incident shoulder pain.

The frequency of physical exercise (according to the general question or the sports activity added score) was not associated with the incidence of shoulder pain. Of the different types of sports, dancing seemed to increase the risk of incident pain. Dancing often contains movements of the upper limbs, which may cause dynamic or static loads on the shoulder. Furthermore, active jogging decreased the risk of incident shoulder pain significantly. Jogging has often been connected to an increased risk of musculoskeletal symptoms in the lower limbs,20 but the preventive effects of jogging on shoulder joints have not been reported before. Also, cross country skiing (especially at a moderate level) decreased the risk of persistent severe shoulder pain. We think that the same type of movement of the shoulder joint could at least partly explain the benefits from these activities. In both sports, the upper arm swings below the horizontal level back and forth repeatedly with none or some external loading (pushing with skiing poles). This pendulum movement increases blood circulation in shoulder tendons but does not expose the tendons or the joints to static tension, abduction, or too heavy external loads. Moreover, these sports activities do not involve a high risk of accidental injuries.

At baseline, the risk of pain among active volleyball players was remarkably higher than among those who did not play the game.12 Volleyball playing did not, however, predict the incidence of shoulder pain. Many active players had reported shoulder pain in 1994, so they were excluded from the incidence study. The follow up time of this study may have been too short to detect new cases of incident shoulder pain among healthy volleyball players. It is possible that some of the players who have had shoulder problems have stopped playing and those who have continued to play have adapted to the loading of volleyball playing. This would explain why volleyball playing did not predict the 1 year incidence although it presumably increases the risk of shoulder problems. This may also hold true for the other sports that actively stress the shoulder—such as swimming and racket sports—but which only had a neutral effect on incident pain in our study. Also, there seems to be a group of active volleyball players who continue playing despite pain; this would explain the strong association found in the cross sectional study. Unfortunately, the size of this group was too small to show any significant effect in the analyses of persistent severe pain.

Earlier studies have shown that a considerable proportion of people forget their lifetime symptoms.21 To diminish this recall error, we used a 12 month questionnaire in this study. For the incidence analyses, the study cohort was defined as those who reported no shoulder pain during the 12 months preceding the baseline questionnaire. However, our data from 1992 and 1993 showed that 26% of those who did not have shoulder pain in 1994 had had shoulder pain earlier. Some of the incident cases may have experienced recurrent shoulder pain (recurrence from 1992 or 1993 or earlier years) and the risk factors for recurrent pain may differ from the risk factors for incident pain. Therefore, as well as the main analyses, we restricted the study population to those who had been healthy for at least 3 consecutive years (since 1992) and investigated the effects of the predictors in this subgroup. The effects remained largely similar, yet the 95% confidence intervals (95% CIs) widened as the size of the study population diminished. This analysis gives support to the fact that our predictors may be true risk factors for the first occurrence of shoulder pain.

The validity of most of the questions about work load has been reported elsewhere.22 The correlations between self assessed and observed estimations were relatively high especially among those workers who were free of musculoskeletal pain. The validity of measuring physical activity with a self administered questionnaire is another complex issue and has been discussed in our previous paper.12 It is possible that the amount of physical activity is overestimated as the benefits of exercising are appreciated and many people hope to do it regularly. Usually, the heavier or more structured the physical exertion is, the easier and more accurate it is recalled.23 In general, self administered questionnaires are reasonably valid and useful tools in assessing physical activity.24 25

Information bias caused by better recall or overestimation of past exposure among those with current symptoms22 was avoided with the prospective study design among painfree subjects in the incidence study. When interpreting the results of this or other similar studies, where the subjects at baseline are symptom free and where the exposure precedes the outcome, there is a tendency to assume a cause-effect relation. But other explanations may exist too. For instance, some factors such as obesity may be indicators of other factors—such as lack of physical fitness. Some people may also be more prone to perceive and report such exposures as physical workloads as well as their symptoms. This may lead to spurious associations. Hence, assumptions of a direct causality between the detected risk factors and the outcome should be made with caution.

One shortcoming of the questionnaire was in the interpretation of the results of persistent pain. The questionnaire did not detect if persistent pain appeared in the same shoulder. However, we assume that the respondents, who reported severe shoulder pain 2 years in a row, had pain on the same side.

In conclusion, the results support the current view that shoulder pain is the result of several factors, including occupational and individual factors. In this longitudinal study, heavy physical work, awkward work postures, mental stress, and obesity were the risk factors, at which preventive measures can be aimed. The role of physical activity either as a preventive or causative factor for shoulder disorders has rarely been studied or even discussed. In our study, during the follow up of 1 year, physical exercise had more protective than impairing effects on the shoulders.