Lumbar extensor muscle profile of rural milk production workers

| Background: Milk production is a work system that comprises several tasks from cattle handling, feeding and milking to product consumption. Isometric lumbar extension dynamometry is considered a satisfactory instrument for occupational evaluation and monitoring, especially for populations of workers whose activities demand high levels of strength. Objective: To investigate isometric lumbar extension strength among rural milk production workers. Methods: The sample comprised 47 rural milk production workers affiliated with the Technical Assistance and Rural Outreach Company/Southern Credit and Rural Assistance Company (Empresa de Assistência Técnica e Extensão Rural/Associação Sulina de Crédito e Assistência Rural — EMATER/ASCAR), Rio Grande do Sul, Brazil. The participants were analyzed based on a sociodemographic questionnaire and lumbar extensor muscle strength testing by means of an isometric dynamometry system. Results: Participants were 20 men with average age 50.45±9.44 years old and 27 women with average age 46.15±7.56 years old. The isometric lumbar extension strength was 1,169±289.96 N for the men and 571.34±190.36 N for the women. Conclusion: The lumbar extensor muscle profile of rural milk production workers was characterized by higher values for the men, which might be attributed to the fact that men exhibit higher body mass levels, and consequently produce greater force.


INTRODUCTION
Brazil earned an outstanding place among the main milk producing countries in recent years, ranking fifth globally.Brazil produced 35 billion liters of milk in 2015, 35.2% corresponding to the Southern area.There are 5 million rural establishments in the country, 1.3 millions of which are milk producers 1 .While the beef trade business is characterized by a predominance of large livestock producers, milk cattle is most often tended by small and medium-sized producers to become a source of family income as main or secondary activity 2 .
The work of milk producers comprises several tasks, from the handling, feeding and milking of animals to environment and tool cleaning, milk storage and final product consumption.Some such tasks demand considerable physical effort from workers 3 .According to Santos Filho 4 , many workers also perform parallel activities in farms to make profit of resources and spaces and increase their income.As a function of the large number of tasks they perform, such workers do not have time for non-occupational activities, which causes physical exhaustion and consequent impairment of their quality of life 5 .
Agricultural work, milking more in particular, involves lifting and transporting loads, repeated movements and adoption of inadequate body postures.For this reason it is an occupation that poses considerable risk to postural health, including spinal and limb deviations, repeated strain injury and low back pain [4][5][6] .
In the daily work of rural workers, the low back extensor muscles participate in a large number of actions.When the strength of the abdominal muscles is reduced and there is imbalance between the trunk flexor and extensor muscles, low back pain, dysfunction and muscle fatigue might ensue.These conditions might threaten the motor function of the lumbar spine and are a risk factor for chronic low back pain and other disorders with direct impact on the quality of life and work life of rural milk production workers [7][8][9] .
Muscle strength is intimately related to functional capacity.Muscle strength is the ability of muscles to develop or resist isometric, isokinetic or isotonic force.Dynamometry is a method used to measure muscle strength and investigate the characteristics of a given muscle over time [10][11][12] .
Among the varieties of dynamometry, isometric lumbar extension dynamometry (ILED) is considered a satisfactory instrument for occupational evaluation and monitoring especially of populations of workers whose activities demand high levels of strength 13 .In addition, there is also a need to quantify physical factors associated with the activities performed to improve diagnosis and preventive actions 14 .
Given the relevance of milk production and the high physical demands to which rural workers are exposed, the aim of the present study was to investigate the isometric lumbar extension strength of rural milk production workers engaged.

METHODS
The study population included rural workers in the area covered by Alto Jacuí and Northwest Colonial Regional Development Council (Conselho Regional de Desenvolvimento -COREDE) selected by means of convenience sampling and without previous sample size calculation.and UNICRUZ; the latter also made its physical therapy laboratory available.
The following procedures were used for data collection: • Application of the Worker's Questionnaire adapted from Moraes 15 , which comprises open-and closed-ended questions to collect specific, personal and work conditions data; • ILED, which was performed by means of a isometric lumbar extension measurement system developed by Laboratory of Instruments (LABIN), UDESC 16 .This system is based on the one used by Eichinger et al. 14 with an additional support to evaluate subjects at 140º of trunk anterior flexion.The goal of this modification is to avoid interferences with the strength momentum derived from anthropometric factors.
The device comprises a MioTool ® system and a plank coupled to a hip posterior support.This support is adjustable and serves to place subjects in a way to achieve a 140º angle between the trunk and the thighs by adjusting the length of the chain that connects the load cell to the pull handle.Three measurements are previously made with subjects in straight position and the arms relaxed along the trunk, to wit, distance from shoulder to ground, trochanter to ground and fifth metacarpophalangeal joint axis to ground.These measurements are used to calculate the length of the chain needed to achieve a 140º angle.The isometric strength measurement system used is depicted in Figure 1.The dynamometer component that measures strength is a S-beam load cell with four strain gauges configured as a Wheatstone bridge circuit.Deformation of the load cell generates an electrical signal proportional to the strength applied.The dynamometer was calibrated before data collection.The four-channel data acquisition system comprises signal conditioners and a 14-bit analog-to-digital converter.In addition, it includes data acquisition and exploration software specifically designed for system management, which allows adjusting parameters (acquisition frequency, data collection time and visual feedback of curves), real-time acquisition monitoring and recording files, which are easily exported to data analysis software.
The load cell was connected to the MioTool® system, which allowed measuring the variation of lumbar strength as a function of time.
Descriptive analysis included calculation of the mean and standard deviation of the following parameters: peak force, total work, mean power and conventional ratio.Means were analyzed through the t-test (independent samples).All the analyses included calculation of the 95% confidence interval (p<0.05).
Most participants had attended incomplete elementary education; women, 44.44%; men, 50%.About 14.81% of the women and 10% of the men had completed higher education courses (Table 1).
Table 2 shows that the body mass and height were higher among the men compared to the women, however the difference in height was not considerable.Most participants from both sexes -48.15% of the women and 65% of the men -had worked in milk production for more than 20 years.The smallest proportion of participants were away from work or on sick leave -women, 3.70%; men, 5%.
Fifty percent of the men worked 40 hours per week and the other 50% more hours.Among the women, 51.88% worked 40 hours and 44.44% more hours.Only 25.93% of the women and 20% of the men performed physical activity (Table 2).
Table 3 describes the mean peak and mean force values and their comparison per sex; statistically significant difference was detected for both variables (p<0.05).
Tables 4 and 5 describes correlations between ILED peak force and mean force measurements; no statistically significant difference was detected.

DISCUSSION
The spinal extensor muscles seem to be one of the main agents involved in the early manifestation of low back pain.Training protocols targeting these muscles were effective to reduce low back pain and prevent new cases.Thus being, methods to assess the lumbar extensor muscle strength and function are relevant 17 .
Muscle strength is the ability of muscles to generate or resist force, and is considered the most relevant physical asset of human beings, being related to both health and physical performance at various age intervals 12,14 .
Muscle strength might be influenced by intrinsic or extrinsic factors.Among the former, the following stand    out: muscle fiber cross-sectional area, number, contraction velocity and type, coordination, sex and age.In turn, the extrinsic factors include: time of the day, training method, motivation and nutrition 18 .Dynamometry has been recently used for assessment of muscle strength for being a non-invasive, easy-to-apply and low-cost method, in addition to providing reliable muscle strength parameters 12,14 .Some authors asserted that lumbar dynamometry for assessment of the trunk muscle strength has paramount importance in the evaluation of the global physical capacity, particularly in regard to individuals who perform activities involving physical overload 19,20 .
In the present study, we found that the lumbar strength levels were higher among the men compared to women, indeed, 204.67% superior, thus agreeing with the results of the studies by Eichinger et al. 14 and Cardozo et al. 21.This difference might be attributed to the fact that men exhibit larger muscle mass, and thus produce greater force in general compared to women 21 .
In a study with 50 women aged 30 to 50 years old, Guedes et al. 22 compared the effect of different strength training frequencies on the isometric lumbar muscle strength.The average strength was 611.52±167.68N in the group that trained 2 to 3 times per week and 700.11±149.06N in the group that trained 4 to 5 times per week.In the present study, the lumbar muscle strength was lower among the women, 571.34±190.36N.However, this difference might be due to differences in the profile of samples, as the population of the present study included participants who performed or not physical activity, while Guedes et al. 22 exclusively analyzed women who performed strength training.
The maximum isometric lumbar extension strength (N) found in the present study, 1,169.35±289.96N for men and 571.34±190.36N for women, were both superior to the ones reported by Soares et al. 12 , 1,048.6±235.2N and 548.8±156.8N, respectively; Soares et al. 23 , 1,117.2±240.1 N and 475.3±184.24N, respectively; and Silva 16 , 695±229 N and 360±85 N, respectively.These findings suggest that the isometric lumbar traction strength might be higher among individuals who live and work in rural areas compared to the ones who work in urban environments, such as offices, for instance, for performing activities involving greater overload.

CONCLUSION
Milk production depends on individuals who perform tasks involving considerable physical overload and inadequate body posture.According to Kirkhorn et al. 24 and Fathallah et al. 25 , musculoskeletal diseases are the most frequent and high-cost conditions in rural areas.
In the present study, we found that the lumbar extensor muscle profile of rural milk production workers was characterized by higher values for the men compared to the women, 1,169.35±289.96N and 571.34±190.36N, respectively.This difference might be attributed to the fact that men exhibit higher body mass levels, and consequently produce greater force.
By comparison to other studies, we found that rural workers develop higher levels of strength, which might be due to the fact they perform activities that involve greater physical overload compared to urban workers.We collected quantitative data on the isometric lumbar extension strength of individuals engaged in milk production, which represents a contribution to the literature, since it comprises few studies on this population of workers.For this reason, our results might serve as ground for future studies.
The measurement method used proved to be adequate, and thus might be applied to the monitoring of the trunk function of rural workers.In addition, for being a practical and low-cost method, it might be used in practice since the time of hiring employees for a same job, for the purpose of monitoring and characterizing the return-to-work criteria following sick leaves and layoffs.
However, further studies with larger samples and including variation in anthropometric measurements, occupation, sex and age should be performed with instruments similar to the one we used in the present study to corroborate the scientific understanding on the subject of interest and to contribute to decision-making by occupational professionals.

Table 3 .
Mean, standard deviation and correlation per gender of isometric lumbar extension dynamometry results, Rio Grande do Sul, 2017 (n=47).