Farming is an arduous occupation with several adverse health impacts on farm workers that increase their risk for chronic musculoskeletal pain. Although there are many epidemiologic studies reporting on acute injuries in agricultural populations [Lewis, et al. 1998; Lyman, et al. 1999; Meyers 1998], few studies have investigated the relationships between chronic musculoskeletal pain and cumulative exposure to physical stress, in particular, stress associated with working posture. In California, 24% of farm workers reported at least one musculoskeletal problem in 2003–2004, and the number of cases has increased each year since 1999 [Carroll, et al. 2005]. In the California Agricultural Workers Health Study (CAWHS), the most commonly reported musculoskeletal problems were back pain (22%), knee pain (15%), foot pain (15%), and hand pain (14%) [Villarejo, et al. 2000]. Among Kansas farm workers, the 12-month period prevalence of musculoskeletal pain was 37.5% for low back pain, 25.6% for shoulder pain, 23.6% for knee pain, and 22.4% for neck pain [Rosecrance, et al. 2006]. Park and colleagues reported a significantly higher prevalence of low back pain compared to the general working population [Park, et al. 2001]. Holmberg and colleagues reported similar results as well among farmer and rural non-farmers [Holmberg, et al. 2003]. Osborne and colleagues pointed out that musculoskeletal disorders were more common in farmers working longer hours among Irish farmers and noted the relative paucity of published work addressing the anatomical regional distribution of musculoskeletal disorders [Osborne, et al. 2010].

Musculoskeletal pain develops from repeated exposures to a stressor among farm workers [Davis and Kotowski 2007]. Cumulative forces may cause soft tissue damage, leading to inflammatory response and pain [Fathallah 2004; Rempel, et al. 1992]. Long-term exposure to heavy physical work, heavy lifting and carrying, whole-body vibration, and work in awkward postures with trunk flexion may be risk factors for low back pain [Davis and Kotowski 2007; Holmberg, et al. 2003; Meyers, et al. 2004; Rosecrance, et al. 2006]. Kneeling, bending, heavy lifting and carrying, vibrations from tractor driving and work in uncomfortable postures were found to be associated with hip pain and knee pain [Davis and Kotowski 2007; Holmberg, et al. 2003; Osborne, et al. 2010]. Risk factors for upper extremity pain include poor wrist postures, repetitive gripping, and high hand forces [Davis and Kotowski 2007]. Neck pain may occur when farm workers pick fruit, prune grapes, or perform other tasks above their head [Davis and Kotowski 2007]. Stooping is a major factor for low back pain among crop farm workers [Kirkhorn, et al. 2010].

Latino workers in the U.S. industry numbered 30.3 million in 2010 according to the Bureau of Labor Statistics. In 2004, 30.2% of U.S. agriculture workers were Mexican, and 90% of California farm workers were immigrants [DHHS 2004; Schenker 2010]. California agriculture is heavily dependent on hired Latino farm workers, who provide approximately 85% of farm labor [Schenker 1996]. These farm workers come predominantly from Mexico or Central/South America [Schenker 1996; Schenker 2010]. Despite the increased importance of hired Latino labor in California agriculture, there are few reports in the literature on the health status, including occupational health, of this population [NAWS 2005; Schenker 1996; Schenker 2010; Villarejo 2003; Villarejo and Baron 1999; Villarejo and McCurdy 2008]. Understanding the magnitude of musculoskeletal problems and associated risk factors (particularly working postures) in this population is critical for educating farmers and farm workers, informing policy makers, and implementing effective public health interventions to reduce musculoskeletal pain.

The purpose of the present study is to characterize the association between agricultural work and chronic musculoskeletal pain among a community-based sample of both male and female Latino farm workers in California. This analysis examines common work positions, such as kneeling, stooping, carrying heavy loads, driving tractors and other heavy equipment and repetitive motion, which may increase the risk for chronic musculoskeletal pain.

The goal of this study was to assess whether agriculture work is associated with chronic musculoskeletal pain among Latino farm workers. The sample of both men and women was substantial enough to support stratified analysis to assess differences by gender in both work postures and reported musculoskeletal pain. We found that chronic musculoskeletal pain, especially involving the back, knee, and hip, was common among farm workers, increased for women and with age, and was associated with common work positions and years working in agriculture. The strongest associations were found between stooping/bending with hip pain among both men and women and between kneeling and both knee and back pain in women. Women reported a higher prevalence than did men for chronic musculoskeletal pain at all sites except for the knee. These differences were statistically significant for hip and hand pain.

Our results are consistent with those reported among other populations of agricultural workers. Among Swedish pig farmers, women had significantly greater risk than men for aching fingers and wrists [Stal and Englund 2005]. Women were consistently more likely to report chronic pain compared to men, which was also observed in a study of Swedish dairy workers [Gustafsson, et al. 1994]. This may reflect an actual increased risk for injury, possibly related to smaller body size and muscle mass compared with men. In addition, women may have a lower pain threshold, perhaps due to hormonal differences [Arendt-Nielsen, et al. 2004]. Social and psychological factors also may play a role [Myers, et al. 2003]. Men and women maintain different postures according to their different jobs, and these results are consistent with our knowledge of the types of work tasks in which male and female farm workers engage.

In men, driving tractors or other heavy farm equipment for more than 60 hours/week was associated with chronic hip pain. We did not observe a similar result among women, but power to detect any association was limited since only nine women drove tractors in this sample. Other researchers have noted that tractor driving is associated with hip osteoarthritis in farmers, and chronic hip pain can result from hip osteoarthritis. [Croft, et al. 1992; Kirkhorn, et al. 2003; Thelin 1990; Thelin, et al. 1997].

Stooping/bending more than 30 hours/week increased the risk of chronic hip pain among both men and women. Allen et al. reported that bending/twisting/reaching was associated with symptomatic hip osteoarthritis, of which the major symptom is pain, among a community-based sample in rural North Carolina [Allen, et al. 2010]. Kirkhorn et al. also reported that bending was a risk factor for hip osteoarthritis [Kirkhorn, et al. 2003; Walker-Bone and Palmer 2002]. Thelin et al. reported that farmers with swine confinement operations had an increased risk of developing osteoarthritis of the hip, and those who milked more than 40 cows daily had an increased risk compared to those who did not work in dairy production [Thelin, et al. 2004]. This is consistent with our study results because swine handling and milking requires frequent stooping or bending, which was associated with hip pain in our participants.

We observed a strong and biologically plausible association between kneeling and chronic knee pain among women. Allen et al. reported that crawling was associated with increased odds of symptomatic knee osteoarthritis [Allen, et al. 2010]. This supports our findings as crawling includes kneeling, and pain is the major symptom of osteoarthritis. Thus, we would expect that avoiding prolonged kneeling should reduce the risk of chronic knee pain. Knee pads can also be used to protect against knee pain [Coggon, et al. 2000].

Back pain was the most frequently reported chronic pain in this sample, which is consistent with results from other studies [Rautiainen and Reynolds 2002]. The prevalence of back pain has been reported to range from 26.2% to 41% in other agricultural worker populations [Gomez, et al. 2003; Rosecrance, et al. 2006; Xiang, et al. 1999]. Agricultural work often requires a static forward-bent posture while working at or near ground level (e.g., pruning, weeding, and harvesting crops) and puts a significant physical load and strain on the lower body, which may explain the high prevalence of low back pain [Fathallah 2004; Pinzke 2003].

In this study, kneeling or crawling was strongly associated with back pain among women but not men. This is consistent with findings from other studies showing an association between awkward postures and back pain [Wickstrom and Pentti 1998]. Surprisingly, we did not observe significant associations between back pain and stooping/bending or walking while carrying more than 25 pounds, factors commonly reported by other studies to cause back pain.

Standing at a counter or at a machine was associated with reduced odds of chronic back pain. This was similar to the association between repetitive use of manual tools and reduced odds of finger pain. The underlying mechanism for protective associations is unclear. The questionnaire did not address whether subjects engaged in the posture or other work postures prior to onset of pain or if they changed to the current job and postures as a result of pain. Moreover, it is possible that those without pain were more likely to remain in a job associated with uncomfortable posture, leading to an artificially low prevalence of pain in that group consistent with the “healthy worker effect” [Hartvigsen, et al. 2001].

We observed significant associations only at the highest level of exposure. These results suggest that one prevention strategy for addressing risks of chronic musculoskeletal pain might include reducing the number of hours spent in the same posture. This could be accomplished through job rotation and/or using protective equipment [Rempel, et al. 1992]. For example, a mandatory 10 minutes break every 2 hours working in the positive of kneeling or stooping. Effective interventions likely need to be targeted toward specific types of agricultural tasks. Modifications of the work process offer another possibility for preventing chronic musculoskeletal pain. For example, elevating planting beds would reduce stooping and bending over. Modifying wheelbarrow with adjustable handles and 3 wheels would reduce load on low back [Kirkhorn, et al. 2010].

This study has several important strengths. It focuses on musculoskeletal pain as a common occupational health problem responsible for significant morbidity and productivity loss in a large and relatively understudied group critical to one of California’s largest industries. Additionally the study presents data on both men and women, the latter representing a group often not sampled in sufficient numbers to support gender specific analysis. The cross-sectional design is efficient in terms of time and cost and allowed us to focus on multiple outcomes and potentially important exposures. The study used some standardized questions from NHANES for assessing outcome variables, allowing national comparisons and to other populations having used these instruments. Participation was maximized by our study team’s careful and long-standing integration into the population, and interviewers were native-Spanish speakers drawn from and trusted by the community itself. Finally, we employed careful quality control and data monitoring to maximize validity and precision.

There were also limitations to this study. First, we could not assess causality because temporality is not clear in cross-sectional studies. Second, assessing exposure and establishing category cut points for level of exposure is challenging in the absence of biological information on which to base categorization. Moreover, the relationship between exposures and log odds of chronic pain were not linear, indicating that it was inappropriate to analyze exposures as simple continuous variables. Therefore, we decided to categorize data into three ordinal groups (low/medium/high) based on the observed distribution of exposures so that we could examine dose response without requiring an assumption of linearity. This approach limited power for detecting true differences, especially for sub-groups with smaller cell sizes. Third, we summarized exposures over the entire agriculture career rather than season-to-season data over the participant’s entire career, which would have been impractical in this setting. However, it is our experience that farm workers tend to specialize in crops and associated activities over long periods. Because the duration of farm work (median 11 years) is not excessively long, it is likely that the aggregated responses reasonably reflect the experience of the population, although there will likely be some individuals with unusually varied careers for whom this method is inadequate. Fourth, study data were based on self-report [Kirkhorn, et al. 2010], which we were unable to validate, and there is the potential for recall and interview bias. Finally, this study sampled community-dwelling Latino farm workers in California, and the results may not be applicable to other types of agriculture work or other ethnic groups.

The results of this study show that chronic musculoskeletal pain is common among farm workers, is influenced by work positions, and prevention interventions may require gender specific approaches. The findings may be useful for guiding prevention and management of chronic musculoskeletal pain. Recommendations for engineering changes for equipment and work environment modifications to reduce physical stresses are possible prevention strategies. In particular reducing the number of hours spent in the same posture through job rotation and use of protective equipment or modification of the work process. There is also a need to establish more accurate data on underserved migrant and seasonal farm workers for appropriate public health intervention.