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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Am J Ind Med. Author manuscript; available in PMC 2010 April 1.
Published in final edited form as:
PMCID: PMC2756989
NIHMSID: NIHMS134857

Ergonomic risk factors for low back pain in North Carolina crab pot and gill net commercial fishermen

Kristen L Kucera, PhD, ATC,1,2 Dana Loomis, PhD,2,3,4 Hester J Lipscomb, PhD,1 Stephen W Marshall, PhD,2,5,6 Gary Mirka, PhD,7,8 and Julie Daniels, PhD2

Abstract

Background

The objective of this research was to determine the association between LBP that limited or interrupted fishing work and ergonomic low back stress measured by 1) self-reported task, and 2) two ergonomic assessment methods of low back stress.

Methods

Eligible participants were from a cohort of North Carolina commercial fishermen followed for LBP in regular clinic visits from 1999 to 2001 (n=177). Work history, including crab pot and gill net fishing task frequency, was evaluated in a telephone questionnaire (n=105). Ergonomic exposures were measured in previous study of 25 fishermen using two methods. The occurrence rate of LBP that limited or interrupted fishing work since last visit (severe LBP) was evaluated in a generalized Poisson regression model.

Results

Predictors of severe LBP included fishing with crew members and a previous history of severe LBP. Among crab pot and gill net fishermen (n=89), running pullers or net reels, sorting catch, and unloading catch were associated with an increased rate of LBP. Percent of time in forces >20 lbs while in non-neutral trunk posture, spine compression >3400 Newtons, and National Institute of Occupational Safety and Health lifting indices >3.0 were associated with LBP.

Conclusions

Tasks characterized by higher (unloading boat and sorting catch) and lower (running puller or net reel) ergonomic low back stress were associated with the occurrence of severe LBP. History of LBP, addition of crew members, and self-selection out of tasks were likely important contributors to the patterns of low back stress and outcomes we observed. Based on the results of this study, a participatory ergonomic intervention study is currently being conducted to develop tools and equipment to decrease low back stress in commercial crab pot fishing.

INTRODUCTION

Back pain is a common occupational problem in commercial fishermen. In a cross-sectional study of Swedish deep-sea fishermen, half of fishermen experienced low back symptoms during the last 12 months [Torner et al., 1988]. Low back symptoms were the most common cause of work impairment among a cohort of North Carolina commercial fishermen [Lipscomb et al., 2004]. Risk factors for prevalence of low back symptoms include age, length of time in the occupation, type of fishing and gear, job title, and fishing part-time or working more than one job [Lipscomb et al., 2004, Torner et al., 1988]. However, the importance of these factors from a prevention standpoint is limited by the degree to which they are modifiable. Little is known regarding the relationship of LBP with specific fishing tasks, their frequency, or their duration. It has been previously documented that fishermen perform strenuous tasks [Lipscomb et al., 2004, McDonald et al., 2004, Torner et al., 1988], and ergonomic studies have evaluated biomechanical low back stress for fishing tasks [Fulmer and Buchholz 2002, Kucera et al., 2008, Mirka et al., 2005, Torner et al., 1988]. However, no study has evaluated specific tasks and ergonomic measures as risk factors for low back pain in a population of fishermen.

Previous studies have described characteristics of fishing work such as static, awkward working postures, shoveling and lifting tasks which produce strain to the low back area [Lipscomb et al., 2004, McDonald et al., 2004, Torner et al., 1988]. Ergonomic analyses of commercial fishing crews revealed that work tasks were repetitive and cyclic with high intensity lifts during loading and unloading activities [Fulmer and Buchholz 2002, Kucera et al., 2008, Mirka et al., 2005, Torner et al., 1988]. More specifically, low back stress varied by the type of fishing performed, size of the crew, job, and task performed [Kucera et al., 2008]. While certain job characteristics may produce low back stress, their association with low back pain in fishermen is undetermined.

The objective of this research was to determine the association between low back stress and low back pain that limited or interrupted fishing work. Low back stress was measured by 1) self-reported task, and 2) the percent of time exposed to low back stress (measured with two ergonomic assessment methods). A secondary objective was to examine the influence of other covariates such as previous history of severe LBP, age, and years fishing experience. Our study population was a group of southeastern US commercial fishermen who fished with crab pots and gill nets in small-scale, independent operations on coastal or inland waters.

MATERIALS AND METHODS

Study Population

Participants in this study were members of a cohort of commercial fishermen originally assembled during the period of April 1999 to May 2000 for the purpose of studying exposure to a toxic marine micro-organism [Moe et al., 2001]. This population included licensed commercial fishermen 18-65 years of age who fished on inland rivers and sounds or on the ocean for at least 20 hours per week for at least six months of the year. Individuals completed self-administered questionnaires at baseline and at six month intervals during medical clinic visits for a period of up to two years. Information was gathered on presence of musculoskeletal pain, traumatic injuries, and fishing activities and other exposures. In addition to regular visits, fishermen were encouraged to come in for “trigger” visits defined by conditions relating to exposure to toxic micro-organisms (e.g., skin lesions, memory loss, cognitive impairment) or if they were exposed to diseased fish [Moe et al., 2001]. Fishermen were also interviewed every one to two weeks by phone from August 1999 to May 2002 about work-related injuries, fishing activities, and other exposures of interest. Injury data from clinic visits and follow-up of the cohort have previously been reported [Lipscomb et al., 2004, Marshall et al., 2004].

A Supplemental Questionnaire was administered by telephone in April of 2004 to retrospectively assess more detail on fishing and non-fishing work exposures and whether they performed specific fishing tasks. Of 177 fishermen available for interview, we were unable to reach 60 participants (contact number not available, n=27, and unable to reach, n=33); of those we did reach, 106/117 agreed to participate. Note: we use the term fisherman because that is how the participants, men and women, referred to themselves and to others. The University of North Carolina at Chapel Hill School of Public Health Institutional Review Board approved all study procedures and all subjects provided written informed consent prior to participation.

Low Back Pain

A revised version of the Nordic Musculoskeletal Questionnaire [Kuorinka et al., 1987] was administered in all clinic exams to determine the presence and severity of LBP at baseline and subsequent follow-up visits. Reliability of the instrument ranges from 77% to 100% and validity, compared to clinical history, ranges from 80% to 100%. Information collected included 12-month prevalence of low back pain at baseline and occurrence of LBP since last clinic visit. For both baseline and follow-up clinic visits, participants were asked if this low back pain limited work (reduced work level or tasks) or interfered with work (unable to work for a day or more) and, if so, how long they were unable to work. For this study, severe LBP was defined as any reported LBP that limited or interfered with normal fishing work activity. We could not determine whether reports of LBP at follow up were new or recurrent, therefore we consider all occurrences of LBP in this study.

Fishing Exposure

During the follow-up clinic visits fishermen reported the fishing methods (e.g., pots, gill nets, trawl, dredge) and type of catch (crab, finfish, shrimp, clam, oyster, or other) since last visit. In weekly (April through October) and biweekly (November through March) telephone interviews, the fishermen reported the type of catch, number of days spent on and off the water, and estimated the number of hours they spent on the water for the most recent day fishing. Detailed exposure information was gathered in the supplemental questionnaire for crab pot and gill net fishermen and included the frequency respondents performed specific fishing tasks (e.g., driving the boat, pulling in gear, unloading boat). A Likert rating scale (1 to 5) quantified the frequency of task performance: never, less than half the time but more than never, half the time, more than half the time but less than always, or every time/everyday. This scale was dichotomized for analysis as follows: if fishermen performed a particular task during the study period on average “more than half the time” or “always,” then they were considered exposed to that self-reported task.

Ergonomic Exposure Assessment

In a previous study, ergonomic exposure to low back stress was measured in a purposive sample of 25 commercial crab pot and gill net fishermen using two ergonomic assessment methods appropriate for non-routine work [Kucera et al., 2008]. Researchers observed and video taped fishing work, both on and off the water, for a full day. Video tapes were coded for each fisherman using two different methods.

The first method, a work sampling based method, Posture, Activity, Tools and Handling (PATH) [Buchholz et al., 1996], linked work tasks and activities with posture codes to estimate the percent of time workers spent in various situations stressful to the low back. Cut points were established from previous occupational studies. Non-neutral trunk postures [Burdorf and Sorock 1997, Punnett et al., 1991], lifting 44.5 Newtons (4.5 kg) at least once per minute [Punnett et al., 1991], and material handling tasks [Burdorf and Sorock 1997, Riihimaki 1991] have been identified as risk factors for low back pain. The percent of time fishermen were observed in low back stress was quantified for three PATH measures: percent of time in non-neutral trunk postures (trunk flexion >20 degrees, lateral bend and twist >20 degrees), percent of time handling loads or exerting force >20 lbs (9 kg), and percent of time performing manual materials handling tasks (defined as lifting, lowering, carrying, holding, and pushing or pulling boxes, crates, baskets, etc.). The combination of force >20 lbs in non-neutral trunk postures was examined to capture the multidimensionality of these two measures.

The second method, Continuous Assessment of Back Stress methodology (CABS) [Mirka et al., 2000], utilized three well-established ergonomic assessment methods to evaluate biomechanical stress of occupational activities: the revised National Institute of Occupational Safety and Health Lifting Equation (NIOSHLE) [Waters et al., 1993], the Ohio State University Lumbar Motion Monitor™ (LMM) [Marras et al., 1993], and the University of Michigan Three-Dimensional Static Strength Prediction Program™ (3DSSPP) [Chaffin and Erig 1991, Chaffin et al., 1987]. Low back compression from 3DSSPP, lifting index from NIOSHLE, and the probability of high risk group membership from LMM were measured for defined fishing tasks (e.g., driving the boat, pulling in gear) and combined with the estimated time and frequency fishermen were exposed to these tasks. These values were combined to form time-weighted distributions of low back stress.

Compression values greater than 3400 N has been associated with an increased risk for low back pain among workers [Lavender et al., 1999]. Lifting indices greater than 1.0 have been associated with low back pain, while indices over 3.0 are reported as a potential problem for most workers [Lavender et al., 1999, Waters et al., 1999, Waters et al., 1993]. The percent of time fishermen were exposed to low back stress for these two measures were defined as follows: the percentage of time > 3400 Newtons of spine compression, the percentage of time lifting index > 1.0, and percentage of time lifting index > 3.0. Probability of high risk group membership of 35% or greater has been identified as a problem for industrial workers [Marras et al., 1995]. Because the majority of fishing tasks in this study had greater than 35% probability of high risk group membership [Kucera et al., 2008], we evaluated a higher cut point of greater than 70% probability of high risk group membership. This variable quantifed the percentage of time fishermen were engaged in fishing tasks in the upper 30% probability of high risk.

Exposure assignment

At each follow up period, exposure to low back stress was assigned to participants according to whether they fished with crab pots or gill nets (Table I). If fishermen fished with both methods during the period, they were assigned the fishing task and the higher ergonomic mean by type (crab pot or gill net). If they performed neither crab pot nor gill net fishing during the interval they were assigned a zero.

Table I
Fishing task and ergonomic exposure assignment by fishing type performed during follow up interval for North Carolina commercial fishermen, 1999-2001

Data Analysis

Descriptive statistics were calculated by baseline demographic and work history characteristics as well as by fishing types at follow-up and self-reported job tasks from the supplemental questionnaire.

The occurrence rate of severe LBP was modeled using generalized Poisson regression [Rothman and Greenland 1988] with log person-days at risk included as an offset term. Days at risk were calculated from days between clinic visits. Generalized Estimating Equations (GEE) [Liang and Zeger 1986, Zeger and Liang 1986] were used to account for the statistical dependence between multiple clinic visits and multiple severe LBP occurrences per fisherman. Outcome-covariate rate ratios (RR) and 95% confidence intervals (95% CI) were computed from the model and stratified by previous history of severe LBP. Confidence limit ratios (CLR, calculated as the upper confidence limit divided by the lower confidence limit) were produced to quantify precision for all estimates [Poole 2001]. Non-overlap of stratum-specific confidence intervals indicated heterogeneity by previous severe LBP. Baseline covariates of interest were: gender, age, smoking history, fishing full time (at least 32 hours/week) or year round (at least 9 months of the year). Follow-up covariates included performing more than one type of fishing during the follow-up interval, fishing type and gear, and average hours per day on the water. Variables of interest from the supplemental questionnaire included years of fishing experience and work exposures during the study such as fishing with crew versus alone and working a non-fishing job during follow-up that required any of the following: frequent bending or twisting at the waist; work in awkward postures; frequent lifting (>3 lifts per minute); and lifting > 50 or > 25 pounds.

For crab pot and gill net fishermen who answered the supplemental questionnaire, we modeled the rate of severe LBP by low back stress exposure measured with self-reported fishing task and PATH and CABS methods. PATH and CABS means were modeled with a multi-level mixed linear model accounting for the variability between and within fishing type, crew size within fishing type, and job type within crew size within fishing type [Kucera et al., 2008]. These means were included in Poisson regression models as continuous variables. The exponentiated parameters represent the change in the rate of severe LBP per 1 unit change in mean percent time exposed to low back stress measures. For example, the increase in the rate of severe LBP going from a peak of 29 percent of time to 30 percent of time in non-neutral trunk posture.

RESULTS

Descriptive statistics

The majority of fishermen who answered the supplemental telephone questionnaire (n=105) were male and between the ages of 30 and 59 (Table II). All except one were white, non-Hispanic. Most fished at least 32 hours per week for at least 9 months of the year and owned their own boat. Almost half worked another job not related to fishing. At baseline, 61% reported experiencing any LBP in the last 12 months and 24% experienced LBP that limited or interrupted their work in the past 12 months.

Table II
Baseline demographic and follow up information for North Carolina commercial fishermen who participated in a supplementary questionnaire (n=105), 1999-2001

The 105 fishermen accumulated 58,143 person-days of follow-up during the study. Crab pot and gill net were the most common type of catch and fishing method reported (Table II). Over 40% reported spending on average 4 to 6 hours on the water their most recent day of fishing. Over follow-up, 61% (64/105) of fishermen reported 132 occurrences of any LBP since the last visit and 26% (27/105) of fishermen reported 40 occurrences of severe LBP. Sixty-eight percent of severe LBP occurrences (27/40) interrupted working activity for at least a day: 52% (14/27) interrupted work 1 to 7 days, 33% (9/27) 8 to 30 days, and 15% (4/27) over 30 days. When asked if LBP had ever caused them to change the way they fish, 37% said it had.

Participants began fishing at a young age (Table II). Over half had 20 or more years of experience as a commercial fisherman and most reported being a captain for most of their career. During the study period, the majority of fishermen worked with crewmembers (68%) and fished with others on a boat they owned (61%). Participants who worked a second non-fishing related job during the study reported some form of low back stress in that job. Most were required to twist or bend frequently at the waist or lift >25 pounds; fewer worked in awkward postures, lifted repetitively, or lifted >50 pounds (Table II).

General risk factors for the occurrence of severe LBP

The overall crude rate of severe LBP was 0.69 per 1000 person-days (95% CI: 0.47, 0.90) or 0.25 per person-year. Compared to fishermen 40 years of age and older, fishermen 18 to 29 experienced an increased rate of severe low back pain and fishermen 30 to 39 experienced a decrease in rate. Current smoking, fishing on someone else's boat, fishing types other than crab or finfish, and fishing full-time were associated with severe LBP (Table III). Fishermen who averaged the fewest and the most hours on the water had higher rate of severe LBP compared to fishermen averaging 0 to 6 hours on the water. Fishing year round and performing more than one type of fishing during the follow-up interval were not associated with severe LBP.

Table III
Unadjusted rate ratios and 95% confidence intervals of low back pain occurrences that interrupted or limited work for North Carolina commercial fishermen (n=105, visits=358), 1999 to 2001

The occurrence rate of severe LBP decreased as years of fishing experience increased (Table III). Participants who fished during the study with others experienced an increased rate of severe LBP compared to those who fished alone. Workers with non-fishing related jobs during the study were at decreased rate of severe LBP regardless of whether that job required frequent lifting, twisting or bending frequently, awkward postures or lifting >25 or >50 pounds.

Having a history of severe LBP was strongly associated with subsequent occurrence at follow-up (Table III). Among fishermen with a previous history of severe LBP, smoking, working a non-commercial fishing related job, and fishing full-time were associated with an increased occurrence rate. Among fishermen without a previous history of LBP, increased occurrence rates were observed for finfishing (specifically gill nets) and performing more than one type fishing.

Low back stress measures as risk factors

For those who fished with crab pots and gill nets (n=89), the majority fished alone (crab pots 70% and gill nets 64%). Fishermen reported performing an average of 8.7 (SE 3.7) fishing tasks over half the time (range 1 to 14). In general, over 90% of fishermen reported loading bait and/or supplies, pulling in, emptying, and setting gear, and cleaning the boat more than half the time (Table IV). Few regularly used a dolly or lift to load and unload their boats. A third operated pullers and net reels. The majority of crab pot fishermen reported baiting pots (83%) and the majority of gill net fishermen iced down catch (84%). Thirty-eight percent of crab pot fishermen helped sort their catch at the fish house or point of sale.

Table IV
Percent of fishermen performing fishing tasks overall and stratified by type of fishing for North Carolina commercial fishermen (n=89), 1999 to 2001

Analysis of self-reported tasks with these 89 crab pot and gill net fishermen (313 visits) indicated that running the puller or net reel, sorting catch on the boat, and unloading catch or supplies with or without mechanical assistance more than half the time were each independently associated with an increased occurrence rate of severe LBP compared to those who performed those tasks half the time or less (Table V). Driving the boat, loading bait and supplies with or without mechanical assistance, pulling, emptying or setting gear, cleaning the boat, and maintenance work more than half the time were not associated with severe LBP.

Table V
Crab pot and gill net fishermen: Unadjusted rate ratio and 95% confidence intervals for the occurrence of LBP that interrupted or limited work for self-reported fishing task frequency (n=89, 313 visits)

Little evidence for dose response was observed for the combined number of tasks performed (RR=1.1 95% CI: 0.9, 1.2). Stratifying tasks by potential exposure to low back stress revealed no difference between static tasks and dynamic tasks: number of static tasks including driving the boat, running the puller or net reel, setting gear, sorting catch on the boat or at the fish house, cleaning or maintenance of boat and gear (RR=1.2 95% CI: 0.9, 1.5) versus number of dynamic tasks including loading or unloading the boat, using a dolly or lift for loading/unloading, and pulling in or emptying gear (RR=0.9 95% CI: 0.6, 1.3).

When examining the ergonomic characteristics of the 89 crab pot and gill net fishermen, severe LBP increased with mean percent time exposed to forces >20 lbs in non-neutral trunk postures, >3400 Newtons of spine compression, and lifting index >3.0 (Table VI). The rate of severe LBP was unassociated with non-neutral trunk postures, forces>20 lbs, manual materials handling, lifting index >1.0, and probability of high risk group membership >70%. However, these rate ratios represent an increase in risk per 1 unit increase in the percent of time exposed. An increase from 10% of time to 20% of time (ten unit increase) in the percent of time in non-neutral trunk postures resulted in a RR of 1.40.

Table VI
Crab pot and gill net fishermen: unadjusted rate ratios and 95% confidence intervals for mean percent time exposed to low back stress and low back pain occurrences that interrupted or limited work (n=89, 313 visits)

DISCUSSION

In this cohort of small-scale crab pot and gill net fishermen differences were observed in the occurrence of severe LBP by self-reported fishing task (Tables IV and andV)V) and by ergonomic low back stress assessment (Table VI). Operating pullers and net reels, using a dolly or lift to unload catch and supplies, and sorting catch on the boat were strongly associated with severe LBP. Dose response for task frequency was not observed in this group nor was there a difference observed between static or dynamic tasks. Ergonomic measures associated with the occurrence of severe LBP in this study included forces >20 lbs in non-neutral trunk postures and levels of spinal compression >3400 N and lifting index values>3.

Results for PATH and CABS measures generally supported the independent self-reported task findings. Sorting catch on the boat, a task characterized by static, awkward postures and repetitive motions performed extensively by a mate or third man [Kucera et al., 2008, Mirka et al., 2005], occurred more frequently in larger crew sizes and was associated with severe LBP in this study. Likewise, sorting catch at the fish house, a less stressful task for the low back where fishermen work at tables in upright postures, was not associated with LBP. Unloading catch or supplies, with or without a dolly, was a task characterized by high compression and lifting index values in the ergonomic assessment [Kucera et al., 2008]. We observed an association with severe LBP for this task and high compression and lifting index measures. Previous studies of manual lifting occupations have reported unadjusted associations with any LBP and lifting indices from 1 to 2, 2 to 3, and >3 [Waters et al., 1999].

We did not observe an association for loading activities despite the higher ergonomic low back stress reported by fishermen [McDonald and Kucera 2007] and described in previous work [Kucera et al., 2008]. Similarly, tasks that are not associated with high ergonomic low back stress, running puller or net reel, were also associated with severe LBP. These results likely reflect differences in task performance by fishing type (e.g., gill net fishermen do not use bait; therefore, have less to load). Differences could also be attributed to age and the addition of crew members which could reflect distribution of tasks between captains and mates as well as self-selection into tasks by age or job or previous LBP. Without specific information regarding task-selection and temporality, we were limited in our ability to quantify these potential risks.

We observed age and years of experience were associated with the occurrence of severe LBP. Torner et al. found higher prevalence of LBP for Swedish fishermen age 41 to 50 but prevalence decreased thereafter [Torner et al., 1988]. In addition, fishermen with fewer years experience (20 to 29 years) had more LBP when compared to those who fished over 40 years [Torner et al., 1988]. We observed similar results for years experience in our subset population. Like the ocean-going Swedish fishermen, many in this cohort started fishing at young ages. However, the age participants began fishing in the current study ranged from 5 to 54 years. Those who started their career later, had fewer years experience, and this could explain why we did not see decreasing occurrence rates with increasing age.

Subjective self-reported work-related causes of low back stress have been reported differently by job. Captains have been reported to attribute low back stress to static work postures (driving and running puller) while mates identified dynamic tasks and postures (shoveling and lifting) [Torner et al., 1988]. Interviews with North Carolina commercial fishermen indicated that loading and unloading bait and boxes of catch were stressful for the low back [Lipscomb et al., 2004, McDonald and Kucera 2007, McDonald et al., 2004], and we hypothesized that tasks with higher low back stress measured with PATH and CABS (e.g., loading, unloading, pulling or emptying gear, and sorting on the boat) would be associated with severe LBP. However, we found varying results and suspect this may depend on age, whether crew members were present, or whether other fishing types were performed. These fishermen were largely an independent group of workers and often mediate their exposures in many ways including choice of fishing type, addition of crew, decreasing hours on the water or volume of catch set, or task selection [Lipscomb et al., 2004, McDonald and Kucera 2007, McDonald et al., 2004].

Research such as ours can identify modifiable risk factors and inform interventions to decrease work-related low back stress and ultimately LBP. Modifiable risk factors for fishermen in this study included static, awkward postures for sorting tasks, manual materials handing tasks during loading and unloading activities, and operating pullers and net reels. A participatory ergonomic intervention study is currently being conducted with commercial crab pot fishermen to develop inexpensive tools and equipment to decrease low back stress. Our research indicates that fishermen are willing participants in studies and involving commercial fishermen early on in a participatory capacity is vital to the success of intervention research. This will likely increase adoption of beneficial changes and address recognized worker needs that are cost efficient. This study demonstrated that a multi-disciplinary approach that combined ethnographic techniques and detailed ergonomic assessments with epidemiologic outcome and exposure data can lead to interventions that will hopefully improve the work environment and productivity for commercial fishermen.

Limitations

These findings may reflect a healthy worker effect with those who fished longest having the lowest occurrence of severe LBP. The fishing task results in our study provide some evidence of this self-selection of tasks or addition of crew, because some tasks with higher biomechanical stress values (i.e., loading bait or supplies, pulling in or emptying gear) were not associated with the occurrence of severe LBP. The results obtained when stratifying by history of severe LBP supports the hire of crewmembers to perform the more stressful tasks. However, we could not determine in our data whether fishermen hired crew or selected out of tasks because of previous LBP. Our findings for years of experience are consistent with healthy worker effect reported in other studies of commercial fishermen [Lipscomb et al., 2004, Torner et al., 1988].

The population recruited for the cohort study included licensed commercial fishermen. However, not all fishermen need a license and most mates are not licensed. Therefore a self-defined “mate” in the cohort may not be the same as the “mates” for whom we measured biomechanical stress with PATH and CABS [Kucera et al., 2008]. They were largely unlicensed, young workers employed to help the captains. This should be kept in mind when trying to generalize results.

We did not have complete information on everyone in the cohort, therefore our analysis was limited to the 105 who answered the supplemental questionnaire and supplied information on fishing work history. Our task and ergonomic analyses were restricted to the 89 crab pot and gill net fishermen reducing precision further. Small sample size limited our ability to look at combined effects with a multivariate model in our analyses, illustrated by the wide confidence intervals. Supplemental questionnaire participants reported a higher occurrence of severe LBP compared to the whole cohort which provides some evidence for possible selection bias.

Commercial fishermen are a dispersed workforce and difficult to reach with traditional research methods. Individual exposure assessment was not possible and beyond the scope of this study. Therefore, we used previous PATH and CABS exposure measures from a group of fishermen (n=25) [Kucera et al., 2008] to estimate individual ergonomic stress in crab pot and gill net fishermen who answered the supplemental questionnaire. Group assignment of exposure can lead to misclassification of exposure and potential bias in our estimates.

There were risk factors known to be associated with low back stress and low back pain that we were unable to examine in our study. We did not measure biomechanical stress of other fishing types or non-fishing related work but examined variables to explore these effects. Previous studies have reported boat motion increase musculoskeletal strain for fishermen [Petersen et al., 1989, Torner et al., 1994]. We observed this qualitatively; however, magnitude of motion is affected by weather and self-correction, and we were unable to account for this variable in our analyses.

Strengths

This study had many strengths. We were able to estimate in a unique population of small-scale, independent commercial fishermen the association between the occurrence of severe LBP and crab pot and gill net fishing tasks and biomechanical low back stress. This is the first study to use ergonomic commercial fishing work exposure measures accounting for variation between crew sizes and job types in a predictive model.

A prospective cohort design was employed to assess LBP and fishing types performed over a two year follow-up period. Previous studies of LBP in commercial fishing utilized cross sectional and retrospective designs [Jensen et al., 2005, Norrish and Cryer 1990, Torner et al., 1988]. Use of a prospective cohort design generally decreases the chance of survivor bias.

Detailed interviews with commercial fishermen from the ethnographic study furthered our understanding of the fishing process and informed our ergonomic analysis. Together with the detailed epidemiological data from telephone interviews and clinic visits, this study had a broad and rich context from which to study low back pain associated with commercial fishing work. Most of commercial fishing research has been conducted with large scale fishing operations, but relatively little is known about small-scale fishing operations such as those studied here.

CONCLUSION

Our results demonstrate that neither fishing task frequency nor ergonomic measure alone consistently predict LBP. History of LBP, addition of crew members, and likely self-selection out of tasks were important contributors to low back stress and outcomes. We observed variability in the way fishing work was conducted but were limited in our ability to account for reported differences in our analysis. Possible explanations for this discrepancy are revealed by the fishermen themselves. Fishermen who said they changed the way they fished due to LBP did so by doing less stressful work (e.g., lifting less or work slower), being more careful, using or bending legs when lifting, and lifting with help. Several reported using a puller or net reel, a back brace, anti-fatigue mat, or a longer pole while some adjusted the sorting table height or changed the way they shook the crab pot. One fisherman reported re-outfitting the boat to fish off the port (left) side. We can only speculate as to how these modifications might mediate or prevent severe LBP. Future research should focus on both stressful tasks identified with ergonomic assessments and tasks associated with LBP (e.g., sorting catch, loading and unloading, maintenance work). It is important to know how and why fishermen might adjust their exposures to low back stresses.

ACKNOWLEDGMENTS

This work was supported by NIOSH Grant No. R01 OH004073 and R01 OH008249-02 and NIEHS Training Grant No. P30ES10126. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIOSH or NIEHS.

The parent study was supported through cooperative agreements between the Centers for Disease Control and Prevention and the North Carolina Department of Health and Human Services. We acknowledge the help of Paula Bell, Steve Hutton, Raymond Vickers, Belinda Lee, and Judy Rafson, who participated in data collection.

The authors acknowledge the significant contributions made by Mary Ann McDonald and Josh Levinson for their field work including interviews, photographs, and videotape footage. We acknowledge the help of Scott Fulmer, Allison Marks, Gwansoeb Shin, Micca Pace, Eileen Gregory, and Sue Wolf, who participated in data collection. We thank the commercial fishermen who participated in the cohort study and ethnographic study and especially those fishermen who took us fishing with them.

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