Effectiveness of three interventions for secondary prevention of low back pain in the occupational health setting - a randomised controlled trial with a natural course control.

BACKGROUND: We assessed the effectiveness of three interventions that were aimed to reduce non-acute low back pain (LBP) related symptoms in the occupational health setting. METHODS: Based on a survey (n = 2480; response rate 71%) on LBP, we selected a cohort of 193 employees who reported moderate LBP (Visual Analogue Scale VAS > 34 mm) and fulfilled at least one of the following criteria during the past 12 months: sciatica, recurrence of LBP ≥ 2 times, LBP ≥ 2 weeks, or previous sickness absence. A random sample was extracted from the cohort as a control group (Control, n = 50), representing the natural course of LBP. The remaining 143 employees were invited to participate in a randomised controlled trial (RCT) of three 1:1:1 allocated parallel intervention arms: multidisciplinary rehabilitation (Rehab, n = 43); progressive exercises (Physio, n = 43) and self-care advice (Advice, n = 40). Seventeen employees declined participation in the intervention. The primary outcome measures were physical impairment (PHI), LBP intensity (Visual Analogue Scale), health related quality of life (QoL), and accumulated sickness absence days. We imputed missing values with multiple imputation procedure. We assessed all comparisons between the intervention groups and the Control group by analysing questionnaire outcomes at 2 years with ANOVA and sickness absence at 4 years by using negative binomial model with a logarithmic link function. RESULTS: Mean differences between the Rehab and Control groups were - 3 [95% CI -5 to - 1] for PHI, - 13 [- 24 to - 1] for pain intensity, and 0.06 [0.00 to 0.12] for QoL. Mean differences between the Physio and Control groups were - 3 [95% CI -5 to - 1] for PHI, - 13 [- 29 to 2] for pain intensity, and 0.07 [0.01 to 0.13] for QoL. The main effects sizes were from 0.4 to 0.6. The interventions were not effective in reducing sickness absence. CONCLUSIONS: Rehab and Physio interventions improved health related quality of life, decreased low back pain and physical impairment in non-acute, moderate LBP, but we found no differences between the Advice and Control group results. No effectiveness on sickness absence was observed. TRIAL REGISTRATION: Number NCT00908102 Clinicaltrials.gov.

Application of the Theoretical Domains Framework and the Behaviour Change Wheel to Understand Physicians' Behaviors and Behavior Change in Using Temporary Work Modifications for Return to Work: A Qualitative Study.

Purpose Applying the theoretical domains framework (TDF) and the Behaviour Change Wheel (BCW) to understand physicians' behaviors and behavior change in using temporary work modifications (TWMs) for return to work (RTW). Methods Interviews and focus group discussions were conducted with 15 occupational physicians (OPs). Responses were coded using the TDF and the BCW. Results Key behaviors related to applying TWMs were initiating the process with the employee, making recommendations to the workplace, and following up the process. OP behaviors were influenced by several factors related to personal capability and motivation, and opportunities provided by the physical and social environment. Capability comprised relevant knowledge and skills related to applying TWMs, remembering to initiate TWMS and monitor the process, and being accustomed to reflective practice. Opportunity comprised physical resources (e.g., time, predefined procedures, and availability of modified work at companies), and social pressure from stakeholders. Motivation comprised conceptions of a proper OP role, confidence to carry out TWMs, personal RTW-related goals, beliefs about the outcomes of one's actions, feedback received from earlier cases, and feelings related to applying TWMs. OPs' perceived means to target these identified factors were linked to the following BCW intervention functions: education, training, persuasion, environmental restructuring, and enablement. The results suggest that at least these functions should be considered when designing future interventions. Conclusions Our study illustrates how theoretical frameworks TDF and BCW can be utilized in a RTW context to understand which determinants of physicians' behavior need to be targeted, and how, to promote desired behaviors.

Early work-related physical exposures and low back pain in midlife: the Cardiovascular Risk in Young Finns Study.

OBJECTIVE: To examine whether heavy physical workload in young adulthood increases the risk of local and radiating low back pain (LBP) in midlife. METHODS: Longitudinal nationally representative Young Finns Study data among women (n=414) and men (n=324), aged 18-24 years in 1986 (baseline), were used. Physical heaviness of work was reported at baseline and follow-up (2007), and local and radiating LBP at follow-up. Covariates were age, smoking and body mass index. Logistic regression was used to examine the associations between physical heaviness of work and LBP. Additionally, the mediating effect of back pain at baseline was examined (the Sobel test). RESULTS: After adjustment for the covariates, and as compared with sedentary/light physical workload, heavy physical workload was associated with radiating LBP among women (OR 4.09, 95% CI 1.62 to 10.31) and men (OR 2.01, 95% CI 1.06 to 3.82). Among men, early back pain mediated the association (p value from the Sobel test=0.006). Among women, early exposure to physically heavy work showed the most consistent associations, while early and late exposures were associated with radiating and local LBP among men. Persistently heavy physical work was associated with radiating LBP among women and men. CONCLUSIONS: Physically heavy work at a young age can have a long-lasting effect on the risk of LBP, radiating LBP in particular. These results highlight the need to consider early and persistent exposures to prevent the adverse consequences of physical workload for the low back.

Effectiveness of legislative changes obligating notification of prolonged sickness absence and assessment of remaining work ability on return to work and work participation: a natural experiment in Finland.

OBJECTIVES: Policies have been introduced to reduce sickness absence, but their effectiveness is largely unknown. In a natural experiment, we examined effects of legislative changes on return to work and work participation. METHODS: The source population consisted of up to 72,164 Finnish public sector employees with a permanent job contract in 2008-2011 (before) and in 2013-2014 (after). We used employees with a continuous sickness absence of at least 30 calendar-days (n=5708-6393), 60 compensated days (n=1481-1655) and 90 compensated days (n=766-932). We examined sustainable return to work (a minimum of 28 consecutive working days) with survival analysis as well as monthly work participation after a sickness absence, and annual gain in work participation after the intervention, using trajectory analyses. RESULTS: Sustainable return to work after 60 days of sickness absence occurred earlier after the legislative changes (p value 0.017), although the effect reduced towards the end of the follow-up. There were no differences in return to work after a 30 or 90 days of sickness absence. The largest annual gain, postintervention versus preintervention, in monthly work participation was observed among employees with 60 days of sickness absence and was 230.9 person-years/10,000 employees. The corresponding annual gains among those with 30 days and 90 days of sickness absence were 51.8 and 39.6, respectively. CONCLUSIONS: Our findings suggest that the legislative changes, obligating early notification of prolonged sickness absences as well as assessment of remaining work ability and possibilities to continue working, may enhance sustainable return to work in the short term. Other measures will be needed to enhance work participation, especially in the long term.

Cost-effectiveness of providing patients with information on managing mild low-back symptoms in an occupational health setting.

BACKGROUND: Evidence shows that low back specific patient information is effective in sub-acute low back pain (LBP), but effectiveness and cost-effectiveness (CE) of information in early phase symptoms is not clear. We assessed effectiveness and CE of patient information in mild LBP in the occupational health (OH) setting in a quasi-experimental study. METHODS: A cohort of employees (N = 312, aged <57) with non-specific, mild LBP (Visual Analogue Scale between 10-34 mm) was selected from the respondents of an employee survey (N = 2480; response rate 71 %). A random sample, representing the natural course of LBP (NC, N = 83; no intervention), was extracted as a control group. Remaining employees were invited (181 included, 47 declined, one excluded) into a randomised controlled study with two 1:1 allocated parallel intervention arms ("Booklet", N = 92; "Combined", N = 89). All participants received the "Back Book" patient information booklet and the Combined also an individual verbal review of the booklet. Physical impairment (PHI), LBP, health care (HC) utilisation, and all-cause sickness absence (SA) were assessed at two years. CE of the interventions on SA days was analysed by using direct HC costs in one year, two years from baseline. Multiple imputation was used for missing values. RESULTS: Compared to NC, the Booklet reduced HC costs by 196€ and SA by 3.5 days per year. In 81 % of the bootstrapped cases the Booklet was both cost saving and effective on SA. Compared to NC, in the Combined arm, the figures were 107€, 0.4 days, and 54 %, respectively. PHI decreased in both interventions. CONCLUSIONS: Booklet information alone was cost-effective in comparison to natural course of mild LBP. Combined information reduced HC costs. Both interventions reduced physical impairment. Mere booklet information is beneficial for employees who report mild LBP in the OH setting, and is also cost saving for the health care system. TRIAL REGISTRATION: ClinicalTrials.gov NCT00908102.

A mismatch between self-reported physical work load and the HAQ: early identification of rheumatoid arthritis patients at risk for loss of work productivity.

OBJECTIVE: To explore the combination of data on functioning and work load for early identification of patients at risk for diminished work productivity in rheumatoid arthritis (RA). PATIENTS AND METHODS: In the FIN-RACo trial, 162 patients with recent onset RA and available for the workforce were treated with either a combination of disease-modifying antirheumatic drugs (DMARDs) or a single DMARD for 2 years. Otherwise, they received routine care and were followed up for 5 years. Data on their individual income and lost work days came from official registers. Loss of productivity was computed by the human capital approach. Self-reported data on physical work demand (Finnish Institute for Occupational Health Questionnaire) at baseline and on functioning (HAQ) at 6 months were linked according to the International Classification of Functioning, Disability and Health. RESULTS: Data on 112 patients were analyzable at 6 months; 35 (31%) of them had diminished capacity in functions required at paid work. Any mismatch between perceived abilities and requirements predicted future (7 through 60 months) loss of productivity - on average Euro 14,040 (95% confidence interval (CI): 9,143-20,511) per year in patients with the mismatch compared to Euro 3,043 (1,623-5,534) in those without any mismatch - and was associated with RA-related permanent work disability (hazard ratio: 11.6; 95%CI: 4.0-33.4). CONCLUSION: Linking together self-reported data about functioning and work load helps in early identification of the RA patients at risk for loss of working days.

Physical work load factors and carpal tunnel syndrome: a population-based study.

OBJECTIVE: Carpal tunnel syndrome (CTS) is the most common nerve entrapment syndrome. Studies on selected occupational populations suggest an association of CTS with forceful repetitive work and vibration. Only few population-based studies have addressed the role of physical load factors in CTS. The aim of this study was to investigate the relations between exposures to a single or a combination of physical work load factors and CTS. METHODS: The target population consisted of people aged 30 years or older residing in Finland during 2000-2001. Of the 7977 eligible subjects, 6254 (78.4%) were included in the study. Occupational physical load factors were assessed by interview and CTS by physical examination. RESULTS: The prevalence of possible or probable CTS was 2.1% in men and 5.3% in women. Work tasks with vibrating tools (adjusted odds ratio (OR) 1.9, 95% CI 1.2 to 2.9) and handgrip with high forces (OR 1.7, 95% CI 1.2 to 2.5) were related to an increased prevalence of CTS. There were joint effects between work tasks requiring handgrip with high forces and the use of vibrating tools (adjusted OR 3.3, 95% CI 2.0 to 5.4), between forceful activities (handgrip with high forces or handling of loads) and repetitive movements of the hands (OR 2.1, 95% CI 1.5 to 2.9), and between repetitive movements of the hands and the use of vibrating tools (OR 2.8, 95% CI 1.6 to 4.8). Only exposure in the most recent job was associated with CTS. CONCLUSIONS: Work tasks demanding handgrip with high forces or the use of vibrating tools are associated with CTS. The association is stronger if these work tasks are accompanied by repetitive movements of the hand or wrist.

Prospective study on shoulder symptoms among kitchen workers in relation to self-perceived and observed work load.

OBJECTIVES: To study the occurrence of shoulder symptoms among professional kitchen workers, and whether reduction in self-perceived and observed physical work load decreases future symptoms. METHODS: In this prospective study conducted in 2002-2005 in municipal kitchens in Finland, changes during a 1-year follow-up in the physical strenuousness of work tasks were self-assessed by 376 female workers (substudy I). Changes in exposure to manual lifting and awkward upper arm posture during the follow-up were also observed by experts in 69 kitchens with 183 workers (substudy II). Information on shoulder symptoms was collected with questionnaires at baseline and at follow-up. The risk of shoulder symptoms at follow-up related to changes in exposure was estimated with logistic regression. RESULTS: The 3-month prevalence of shoulder pain was 34% at baseline and 41% at follow-up. Reduction during follow-up in work tasks perceived as the most strenuous physically (ie, receiving and storing raw food) led to a significantly reduced risk of future shoulder pain with an adjusted odds ratio (OR) of 0.41 (95% CI 0.17 to 0.98) and trouble caused by the pain with an OR of 0.34 (0.14 to 0.83). The observed reduction in lifting was also associated with a lower risk for future shoulder symptoms. CONCLUSIONS: Reduction in lifting showed beneficial protective effects on the shoulder. Although more risk factor and intervention studies are needed to estimate the health impacts of kitchen work, special attention should be paid to work tasks that include lifting when assessing risk and designing preventive measures.

Physical work and chronic shoulder disorder. Results of a prospective population-based study.

OBJECTIVES: A study of whether occupational physical load predicted subsequent chronic shoulder disorders. METHODS: A comprehensive national survey was carried out among a representative sample (n = 7217) of the Finnish adult population in 1977-80. Twenty years later, 1286 participants from the previous survey were invited to be re-examined, and 909 (71%) participated. After excluding those with diagnosed shoulder disorders at baseline, 883 subjects were available for the analyses. RESULTS: At follow-up, a physician diagnosed chronic shoulder disorders in 63 subjects (7%) using a standardised protocol. Work exposure to repetitive movements and vibration at baseline increased the risk of shoulder disorder: adjusted ORs 2.3 (95% CI 1.3 to 4.1) and 2.5 (1.2 to 5.2), respectively. Exposure to several physical factors increased the risk further, the adjusted OR was nearly 4 for at least three exposures. The adverse effects of physical work were seen even among those older than 75 years at follow-up. The statistically significant risk factors differed between genders: for men vibration and repetitive movements, and for women lifting heavy loads and working in awkward postures. Age and body mass index modified the effects of the physical exposures. The results remained similar after excluding those with any shoulder pain at baseline. CONCLUSIONS: This is the first prospective study in a general population showing that occupational physical loading increases the risk of a subsequent clinical shoulder disorder and the effects seem to be long-term. Early preventive measures at the workplace may have long-lasting health benefits for the shoulder.

A randomised controlled trial on whether a participatory ergonomics intervention could prevent musculoskeletal disorders.

OBJECTIVES: To examine the efficacy of a participatory ergonomics intervention in preventing musculoskeletal disorders among kitchen workers. Participatory ergonomics is commonly recommended to reduce musculoskeletal disorders, but evidence for its effectiveness is sparse. METHODS: A cluster randomised controlled trial among the 504 workers of 119 kitchens in Finland was conducted during 2002-2005. Kitchens were randomised to an intervention (n = 59) and control (n = 60) group. The duration of the intervention that guided the workers to identify strenuous work tasks and to seek solutions for decreasing physical and mental workload, was 11 to 14 months. In total, 402 ergonomic changes were implemented. The main outcome measures were the occurrence of and trouble caused by musculoskeletal pain in seven anatomical sites, local fatigue after work, and sick leave due to musculoskeletal disorders. Individual level data were collected by a questionnaire at baseline and every 3 months during the intervention and 1-year follow-up period. All response rates exceeded 92%. RESULTS: No systematic differences in any outcome variable were found between the intervention and control groups during the intervention or during the 1-year follow-up. CONCLUSIONS: The intervention did not reduce perceived physical work load and no evidence was found for the efficacy of the intervention in preventing musculoskeletal disorders among kitchen workers. It may be that a more comprehensive redesign of work organisation and processes is needed, taking more account of workers' physical and mental resources.

Manual material handling advice and assistive devices for preventing and treating back pain in workers.

BACKGROUND: Training and assistive devices are considered major interventions to prevent back pain among workers exposed to manual material handling (MMH). OBJECTIVES: To determine the effectiveness of MMH advice and training and the provision of assistive devices in preventing and treating back pain. SEARCH STRATEGY: We searched MEDLINE to November 2005, EMBASE to August 2005, and CENTRAL, the Back Group's Trials Register, CINAHL, Nioshtic, CISdoc, Science Citation Index, and PsychLIT to September 2005. SELECTION CRITERIA: We included randomized controlled trials (RCT) and cohort studies with a concurrent control group, aimed at changing human behaviour in MMH and measuring back pain, back pain-related disability or sickness absence. DATA COLLECTION AND ANALYSIS: Two authors independently extracted the data and assessed the methodological quality using the criteria recommended by the Back Review Group for RCTs and MINORS for the cohort studies. One author of an original study supplied additional data for the review. The results and conclusions are based on the primary analysis of RCTs. We conducted a secondary analysis with cohort studies. We compared and contrasted the conclusions from the primary and secondary analyses. MAIN RESULTS: We included six RCTs (17,720 employees) and five cohort studies (772 employees). All studies focused on prevention of back pain. Two RCTs and all cohort studies met the majority of the quality criteria and were labeled high quality. We summarized the strength of the evidence with a qualitative analysis since the lack of data precluded a statistical analysis. There is moderate evidence that MMH advice and training are no more effective at preventing back pain or back pain-related disability than no intervention (four studies) or minor advice (one study). There is limited evidence that MMH advice and training are no more effective than physical exercise or back belt use in preventing back pain (three studies), and that MMH advice plus assistive devices are not more effective than MMH advice alone (one study) or no intervention (one study) in preventing back pain or related disability. The results of the cohort studies were similar to the randomised studies. AUTHORS' CONCLUSIONS: There is limited to moderate evidence that MMH advice and training with or without assistive devices do not prevent back pain, back pain-related disability or reduce sick leave when compared to no intervention or alternative interventions. There is no evidence available for the effectiveness of MMH advice and training or MMH assistive devices for treating back pain.

Leisure-time physical activity and sciatica: A systematic review and meta-analysis.

BACKGROUND AND OBJECTIVE: The role of leisure-time physical activity in sciatica is uncertain. This study aimed to assess the association of leisure-time physical activity with lumbar radicular pain and sciatica. DATABASES AND DATA TREATMENT: Literature searches were conducted in PubMed, Embase, Web of Science, Scopus, Google Scholar and ResearchGate databases from 1964 through August 2015. A random-effects meta-analysis was performed, and heterogeneity and small-study bias were assessed. RESULTS: Ten cohort (N = 82,024 participants), four case-control (N = 9350) and four cross-sectional (N = 10,046) studies qualified for meta-analysis. In comparison with no regular physical activity, high level of physical activity (≥4 times/week) was inversely associated with new onset of lumbar radicular pain or sciatica in a meta-analysis of prospective cohort studies [risk ratio (RR) = 0.88, 95% CI 0.78-0.99, I2  = 0%, 7 studies, N = 78,065]. The association for moderate level of physical activity (1-3 times/week) was weaker (RR = 0.93, CI 0.82-1.05, I2  = 0%, 6 studies, N = 69,049), and there was no association with physical activity for at least once/week (RR = 0.99, CI 0.86-1.13, 9 studies, N = 73,008). In contrast, a meta-analysis of cross-sectional studies showed a higher prevalence of lumbar radicular pain or sciatica in participants who exercised at least once/week [prevalence ratio (PR) = 1.29, CI 1.09-1.53, I2  = 0%, 4 studies, N = 10,046], or 1-3 times/week (PR = 1.34, CI 1.02-1.77, I2  = 0%, N = 7631) than among inactive participants. There was no evidence of small-study bias. CONCLUSIONS: This meta-analysis suggests that moderate to high level of leisure physical activity may have a moderate protective effect against development of lumbar radicular pain. However, a large reduction in risk (>30%) seems unlikely. WHAT DOES THIS REVIEW ADD: Leisure-time physical activity may reduce the risk of developing lumbar radicular pain.

Obesity in early adulthood predicts knee pain and walking difficulties among men: A life course study.

BACKGROUND: Few studies have addressed the role of obesity in young adulthood in knee problems later in life. We assessed the associations of overweight/obesity with knee pain and functional limitations of the knee across the life course. METHODS: Military health records from 1967 to 2000 (baseline) were searched for 18- to 50-year-old Finnish men (n = 1913) who participated in the Health 2000 Study (follow-up). Visits to health care were followed during service. Height and weight were measured at baseline and follow-up and waist circumference at follow-up. Weight was inquired at follow-up for ages of 20, 30, 40 and 50 years, if applicable. Life course body mass index (BMI) was calculated. One-month knee pain and functional limitations (walking difficulties and limping) due to knee problems were enquired with interview at follow-up. Cox regression model, logistic regression and trajectory analysis were applied. RESULTS: Body mass index at the age of 20 increased the risk of unilateral knee pain by 38% and functional limitations by 27% for one standard deviation increment of BMI, respectively. One-unit increment of Z-score of life course BMI increased knee pain by 32%. Development of severe obesity during the follow-up increased the risk of knee pain by 80% and functional limitations by 93%. The effect of obesity on functional limitations was partly mediated by traumatic knee problems during military service. CONCLUSIONS: Reducing overweight already in adolescence and avoiding further weight gain during life course may prevent knee pain and associated disability. WHAT DOES THIS STUDY ADD?: BMI at the age of 20 increases the likelihood of knee pain and functional limitations of the knee later in life. Development of severe obesity in adulthood increases the risk of knee pain by 80% and functional limitations by more than 90%. Both general and abdominal obesity are associated with knee pain, associations being stronger for general obesity.

The effect of excess body mass on the risk of carpal tunnel syndrome: a meta-analysis of 58 studies.

We aimed to estimate the effects of overweight and obesity on carpal tunnel syndrome (CTS), and to assess whether sex modifies the associations. Literature searches were conducted in PubMed, Embase, Web of Science, Scopus, Google Scholar and ResearchGate databases from 1953 to February 2015. Fifty-eight studies consisting of 1,379,372 individuals qualified for a meta-analysis. We used a random-effects meta-analysis, assessed heterogeneity and publication bias, and performed sensitivity analyses. Overweight increased the risk of CTS or carpal tunnel release 1.5-fold (pooled confounder-adjusted odds ratio [OR] = 1.47, 95% CI 1.37-1.57, N = 1,279,546) and obesity twofold (adjusted OR = 2.02, 95% CI 1.92-2.13, N = 1,362,207). Each one-unit increase in body mass index increased the risk of CTS by 7.4% (adjusted OR = 1.074, 95% CI 1.071-1.077, N = 1,258,578). Overweight and obesity had stronger effects on carpal tunnel release than CTS. The associations did not differ between men and women, and they were independent of study design. Moreover, the associations were not due to bias or confounding. Excess body mass markedly increases the risk of CTS. As the prevalence of overweight and obesity is increasing globally, overweight-related CTS is expected to increase. Future studies should investigate whether a square-shaped wrist and exposure to physical workload factors potentiate the adverse effect of obesity on the median nerve.

Neck trouble in machine operating, dynamic physical work and sedentary work: a prospective study on occupational and individual risk factors.

A prospective study on the effects of occupational and individual factors on neck trouble was carried out among 1832 men representing static work with whole-body vibration (machine operators), dynamic physical work (construction carpenters) and sedentary work (office workers). Neck trouble and occupational and individual factors were inquired about via postal questionnaires in 1984 and 1987. Machine operating was associated with an increased risk to contract severe neck trouble, and machine operating and dynamic physical work were associated with persistently severe neck trouble. Other predictors for contracting severe neck trouble were age and current smoking. Physical exercise decreased the risk of persistently severe neck trouble. The results of our prospective study confirm the role of physical factors in neck trouble.

Predictive validity of symptoms and signs in the neck and shoulders.

A standardized examination protocol was developed for the assessment of neck and shoulder disorders in primary health care. We investigated to what extent the symptoms and signs can predict sick leave due to neck and shoulder disorders. Our study involved 474 patients seeking medical advice from an occupational health service. Most of the 20 symptoms and 15 signs of the protocol were associated with the number of sick leave days within the 60-day postexamination period, but only three symptoms (short duration, high pain intensity, and continuous pain) and two signs (pain in the upper limb during rotation of head and pain in the shoulder during abduction of arm) retained their significance in a multivariate model. No unambiguous subset of protocol items can be suggested for general use. The predictive validity should be tested in other populations and settings before a conclusion is drawn about the external validity of the results.

Standardized physical examination protocol for low back disorders: feasibility of use and validity of symptoms and signs.

A standardized examination protocol was developed for the assessment of low back disorders in primary care. The protocol was found feasible in the occupational health service setting. The interexaminer repeatability between an occupational physician and an occupational physiotherapist was good for most items. The predictive validity of different symptoms and signs was investigated with regard to future sick leaves due to low back disorders. Relief of pain when lying, severe trouble at work caused by pain, continuous pain, and pain in the leg or numbness or diminished sensitivity in the foot predicted sick leaves. Of physical signs, pain in the low back or buttock during lateral flexion and a side difference > or = 20 degrees in the straight-leg-raising angle were predictors for sick leaves. The predictive validity of the protocol items should be tested in another patient population before conclusions can be drawn concerning the external validity of our results.

Load-sharing patterns in the shoulder during isometric flexion tasks.

Patterns of load-sharing between the shoulder muscles during isometric flexion tasks were studied by using both a biomechanical shoulder model and electromyographic (EMG) recordings of ten subjects. The effect of changes in several model parameters and shoulder stiffness constraints on the predicted load-sharing patterns were studied, while the arm position, hand load and precision requirements of the tasks were varied. The results calculated using the model were, when compared to the EMG recordings, plausible predicting a high level of synergistic contraction of muscles of the shoulder muscles during flexion tasks. The trends of the model-predicted muscle forces corresponded well to the EMG recordings. At low hand load levels the increasing of the shoulder stiffness strongly increased the muscle force levels, thus increasing also the level of synergistic contraction of muscles. At higher load levels the increase in the muscle forces was not so high, because the model predicted a high level of simultaneous contraction of muscles already at a low level of shoulder stiffness. Cluster analysis of the EMG recordings revealed large inter-individual differences in the load distribution patterns during flexion tasks. The constraint angle of the glenohumeral joint contact force direction was found to be an important model parameter affecting both the predicted forces and the maximum force production ability of the shoulder.

A static shoulder model based on a time-dependent criterion for load sharing between synergistic muscles.

A static shoulder model including a novel muscular synergy principle for computing load sharing between the shoulder muscles is presented. This principle is feasible especially when analysing endurance-type activities. According to the principle, time elapsed from the start of the activity decreases the allowable muscle stress levels on the basis of the stress-endurance time curves of individual muscles. At low load levels the sum of the squared forces is minimized. At higher load levels the increase in the stress levels of individual muscles is counteracted in order to prevent fatigue. In the model the direction of the contact force in the glenohumeral joint is constrained. The stiffness of the shoulder can also be constrained to produce more muscular co-contraction in tasks with high precision demands.