Discussion
The main finding of this one year intervention study is that the largest effects in terms of decrease in musculoskeletal pain and disability was attained during the periods of organized and supervised strength training, irrespective of the time of the season. The study had an effect on pain in Neck, R-shoulder, R-hand and upper and lower back as well as on DASH as a result of the supervised training intervention period. Another important finding is that having participated in such strength training for 20 weeks initially in the study a long-term effect was attained, i.e. the decrease of musculoskeletal pain and disability was maintained at one-year follow-up.
According to questionnaires regular adherence was achieved by 87% of the participants in TG1during the first 20 weeks and by 85% of the participants in TG2 from June 09 to January 10. At the one year follow-up around 40% did not reply to the questionnaire with no significant difference between TG1 (41%) and TG2(38%), so although being a waiting list group motivation and concomitant adherence was not lower in TG2 compared to TG1. However, this also highlights the difficulties in obtaining high follow-up response rates in long-term research studies. To account for the missing replies we used a statistical procedure which inherently accounts for missing values, i.e. the mixed procedure.
There was a significant time effect for pain in all regions and DASH showing significantly lower values after one year follow-up compared to baseline (Figures 2, 3). Thus for TG1 self-administered training did not reduce pain further but pain was kept on the new reduced level probably due to training as without training we would expect an increase in pain in the autumn, due to normal seasonal variation herhaps due to physical activity normally being highest in summer and less in the winter. Accordingly pain and disability index decreased in TG2 in the spring due seasonal variation (Figures 1, 2).
This study adds to the earlier published results on pain in shoulder, neck and forearm as well as DASH by documenting an effect also on R-hand and upper and lower back as well as the long-term maintenance of the positive effects. Beaton et al. showed in a case series design (uncontrolled longitudinal study) that DASH was responsive to treatment, i.e. DASH scores decreased with 12 weeks of treatment in 200 patients with wrist/hand or shoulder disability. The mean overall differences in changes in pain between groups (Figure 3) were up to 0.7 which is less than the minimal perceptible change on an individual basis and much less than the minimal value of 2 for a clinical significant individual change. For cases only the mean changes were higher (1.06 to 1.82; Figure 4). Small effect sizes like these are normal with clinical trials and it may be more informative to examine the distribution of responses between treatment groups. Therefore we looked at the number of subjects having clinical significant reductions (i.e. a decrease of at least 2 on a ten-point scale) in pain (Figure 5) and found significant differences in the distribution of changes between groups with the highest reductions in the actual training groups (Figure 5). For the neck these changes were significant between groups after both intervention periods and for cases as well as for all subjects. For right shoulder and upper and lower back there were significant differences between changes in groups for some combinations of periods/ITT/Cases suggesting clinically significant intervention effects. For right hand there were no significant differences in the clinically significant changes between groups.
These distributions of differences in changes between groups presents additional evidence for the effect of physical training on neck pain as reported in systematic reviews as well as the evidence of a moderate effect of training on low back pain. It also supports a few high quality studies showing effectiveness of training on shoulder symptoms and combined neck/shoulder symptoms. Our study was unique in reporting an effect of strength training on upper back pain among cases. To our knowledge this has not been reported in previous training intervention studies. The shoulder girdle attaches by muscles to the scapula and the back of the thoracic rib cage. These upper back muscles are prone to developing irritation that can be painful. In clinical practice pain complaints from the neck, the shoulder girdle, and part of the shoulder go together. Neck, shoulder, and upper back muscles are all involved during repetitive movements/activity of the arms with a common effect on all three regions. Importantly this study report results on laboratory technicians as opposed to other occupational groups reported in the above mentioned studies.
The small but significant effect on R-hand pain may be due to a change in central pain perception, which is known to be altered in chronic pain conditions. A change in neck pain could result in beneficial changes in overall pain perception and a decreased pain sensitization. A previous study showed central adaptations of pain perception in response to neck/shoulder rehabilitation, i.e. pressure pain threshold increased also in other non-trained parts of the body.
The limited evidence for a clinical effect on R-hand pain and no evidence for an effect on elbow pain could be due to low power as a result of low pain at baseline compared to pain in Neck, Shoulder, Low back and Upper back. Recently we showed an effect of a one year training intervention on pain in right elbow and right hand among office workers. Increasing the training periods for TG1 + TG2 might have resulted in more significant results for pain in R-elbow and R-hand.
Participants in TG1 were allowed to continue training until January 2010 in spite that their supervised training period ended medio 2009, but they did not receive any guidance from the training instructors and were not provided with new training diaries. As a result TG1 may have performed very limited amount of training in the autumn. An important novel finding in this study was that the effect of training on pain in TG1 lasted half a year after the intensive supervised period. This is in line with a previous study on office workers with chronic neck muscle pain, where pain relief as a result of strength training was prolonged and unchanged 10 weeks after the intervention. We therefore cannot rule out if the prolonged pain reduction in TG1 in the autumn was a result of continued training or a prolonged effect of the training in the first intervention period or a combination on both. Lack of supervision may partly explain the lack of further reduction in pain in TG1 in the autumn as supervision seems to be a precondition for a training intervention effect.
The significant intervention effect on DASH although small points to an increased ability to work as a result of strength training and ads to the positive results of the training intervention.
As reported earlier employees with higher pain levels were more interested in participating than those with lower pain levels suggesting that pain per se was no hindrance for participating.
The intention-to-treat analysis, showed only small average changes in pain, but including all employees instead of only subjects with pain reveals the overall impact of the intervention and thus the public health perspective.
The present study has both limitations and strengths. Using a cluster RCT design with a high number of participants increases statistical power of the study. Further, to increase external validity and generalizability we included both public and private sector companies. A limitation is the possible influence of placebo in behavioral interventions. Further, a limitation is that the first intervention group was not monitored for training adherence during the second period where they acted as controls.