ASSOCIATION BETWEEN BODY COMPOSITION AND FAT INFILTRATION IN THE LUMBAR MULTIFIDUS IN YOUNG ADULTS

ABSTRACT Introduction: The increase in body fat is a natural and progressive process with aging, allowing fat infiltration in ectopic sites, such as skeletal muscle, which disrupts its function. Objective: To evaluate the association between body composition, fat infiltration into the low back multifidus muscles, and history of low back pain. Methods: This is a transversal and qualitative study that included young adult subjects of both sexes, and excluded individuals with neurological and musculoskeletal disorders and pregnant women. Fat infiltration into the multifidus and cross section area by magnetic resonance imaging; body composition by Dual-energy X-ray absorptiometry (DXA), and physical activity level determined by the International Physical Activity Questionnaire (IPAQ) were evaluated. The sample was divided by sex and. Pearson and Spearman's correlation and stepwise linear regression were performed. For this study, a p<0.05, a level of significance of 5% and confidence interval of 95% were adopted. Results: Thirty-two individuals were evaluated (59.37% women; 40.63% men). There was a correlation between fat percentage and total cross-sectional area (CSAtotal) (r=0.525; p=0.021), in women, and with lean abdominal mass (r= −0.648; p=0.017) and Body Mass Index (BMI) (r= −0.644; p=0.018) in men. There was also an association, in women, between fat percentage and cross section area (R2=0.275; p=0.021; CI=0.364 − 3.925) and, in men, with lean abdominal mass (R2=0.420; p=0.017; CI: −9.981- [-1.235]). Conclusion: There was correlation between fat percentage in the multifidus and CSA in women, and lean abdominal mass and BMI in men. There was also an association between fat percentage and cross section area in women, and lean abdominal mass in men. However, there was no evidence of any correlation between pain and low back dysfunction. Level of evidence I; Diagnostic studies - Investigating a diagnostic test.


INTRODUCTION
The fat deposition in interleaved adipocyte among the muscular fiber feature the fat infiltration, the intramuscular adipose tissue (TAI) 1,2 . Among the factor that can lead to TAI, may be: female, because they have a physiological higher percentage of fat; age, because the misfunction of lipidic metabolism; and the body composition, because the excess of free floating fat 1,3-6 .
The TAI results in decrease of strength production and muscular mass, consequently, function decrease 1,3 , as relation of a transverse section area decrease, although there are controversial researches these alterations are commonly reported in this musculature 1,3,7,8 . At lumbar spine musculature, the lumber multifidus (LM) shows higher predisposition for infiltration and like one of the main stabilization muscle of lumber spine it may result in lumbar pain 2,9 . The studies focus in female elderly, rare among adult population. In this sense, this study aims to evaluate the association between fat infiltration percentage in lumbar muscles, the body composition and history of pain and lumbar spine misfunction in young adults of both sexes.

Study type and Sample
Transversal and quantitative study, developed at Recife/PE-Brazil, from September of 2016 to march of 2017, attached to Universidade Federal de Pernambuco (UFPE) and with the approval of this university Committee of Etic and Research (Advice Number: 1.479.824). The participation of the study was ensured by reading and signing an Informed Consent (IC) There was included subjects young adults from both genders, with and without lumbar pain and excluded the subjects with neurological, musculoskeletal (fractures, abnormalities, trauma history or spine surgery), pregnant, body with metal pieces and claustrophobic history. The sample was composed by 32 volunteers stratified by gender.

Observational Desing:
The evaluation was performed over two days and all of them made over the first day, except the fat infiltration evaluation, performed at the same week of the first gathering.
It was standardized for analysis the images that correspond with the superior edge of L5 and observed with software ImageJ 1.50i (National Institute of Health, USA). It was measured a. the total cross-sectional area (CSA total ), that comprehends the muscle and the fat inlfiltration; b. the fat-free cross-sectional area (CSA fatfree ), with the muscular mass; c. the fat cross-sectional area (CSA fat ), muscle fat infiltration area. Them, it was quantified the multifidus fat percentage (Multifidus fat percentage = CSA fat /CSA total x 100) by Color Threshold 7 method.
Body Composition. It was evaluated the percentage of body and abdominal fat, lean body and abdominal mass and free-fat mass through Double Energy Radiologic Absormetry (DXA) -Lunar Prodigy Primo (GE Medical Systems Lunar, Wisconsin, United States).
Physical activity level. It was used the International Physics Activity Questionnaire (IPAQ), short way, self-applied and referring to the last week 10 . To determine the weekly energy expenditure, it was attributed the value of Metabolic Equivalent Estimate (MET) for each activity (Walk: 3,3 MET; moderate: 4 MET; vigorous: 8 MET) and multiplied by the frequency and duration in minutes 11 . For the weekly caloric expenditure the MET weekly value was multiplied by the weight and divided by 60 min 11 . History of lumbar pain. It was investigated the occurrence of lumbar pain in the last three months and applied the Oswestry Functional Index (IFO) to evaluate the pain and the intensity by the time of gathering 12 .

Statistical Analysis
It was performed the Shapiro-Wilk test to verify the normality of variables. Then, to compare the averages between male and female was used the Independent Samples t-Test for the normal variables and Mann-Whitney for the outside normality, beside the exactly test of Fisher for categoric variables.
It was applied the Pearson Correlation, to normal variables and Spearman, to variables outside normality. The results of correlation were classifieds as weak (<0.4), moderate (0.4 -0.7) and strong (>0.7) 13 . Then, it was applied the stepwise multiple linear regression between multifidus fat percentage and the predicted variables. The collected data were tabulated and processed by the software Statistical Package for the Social Sciences (SPSS), 22.0 version. It was adopted the p-value < 0.05 for all analysis.

RESULTS
The research was made with 32 volunteers of which 59,37% women (n=19), 40,63% men, with characterization between groups for anthropometric measures, body composition, multifidus CSA and physical activity level presented in Table 1.
For statistical analysis of correlation and association there was chosen the left multifidus. Regarding multifidus fat percentage there was noticed a positive moderate correlation (r=0.525; p=0.021) with CSA total of the multifidus for women. A negative moderate correlation between the quantity of lean abdominal mass (r= -0.648; p=0.017) and the body mass index (BMI) (r= -0.644; p=0.018) ( Table 2).
For men, the regression linear pattern correlated 42% of the data, with F test (F=7.967), used to check the model suitability, showed p-value of 0.017 and standard combings of 1.489 to 2.114 (Table 3).
For women, the regression linear pattern correlated 27.5% of data variability, with test F (F=6.456), showing p-value of 0.021 and standard combings of -1.428 to 2.484 (Table 4).

DISCUSSION
There was noticed a moderate correlation and positive association between multifidus fat percentage and CSA in women and negative moderate correlation among BMI and lean abdominal mass in men. However, was not found correlation between pain and disfunction in lumbar spine.
Men got higher averages of anthropometric measures and higher values related to lean mass both body and abdominal and free-fat body mass as observed by another studies [14][15][16][17] . As well as muscle CSA was higher too 18,19 . In women, the percentage of superior body fat match the expected physiological result for this population 20,21 and showed in the National Health and Nutrition Examination Survey (NHANES) study with evaluation of body composition using DXA 19,22 . Like the fat percentage in multifidus that also follow the total body distribution in women 3,15 .
In men, the fat percentage in lumbar multifidus negatively correlated with lean abdominal mass as showed by another studies 14,[22][23][24] , therefore we estimate the lean circumscribed mass to the lumbar multifidus practice higher influence over the fat percentage than the fat related measures and we should evaluate it to investigate fat infiltration.
Although the lumbar multifidus fat (ML) has presented correlation with BMI in different thigh muscles 24,25 and none correlation for paravertebral muscles 25 , in LM we found negative and moderate correlation. As the measure of BMI includes lean mass, we presume it influences the relation between these too variables.
In women there was correlation and association between fat multifidus percentage and CSA total , estimating this relation with the multifidus area is due the higher space available for intramuscular fat deposition. These findings diverge from a study made in elderly that showed decrease of CSA total 26,27 , however, this decrease can be due an natural reduction of muscular mass in elderly 5 .
The limitations of this study refer to the absence of investigation of others factors as protein supplementation and vitamin D concentration, directly attached to adipocyte deposition and muscle function evaluation.

CONCLUSION
It was evidente association between LM fat percentage and CSA in women, suggesting that CSA may increase due the fat infiltration present in LM. In men the LM fat percentage was inversely associated with lean mass, leading to consider it as main factor in young adults fat infiltration presence.
It is suggested that investigations may be performed to make the findings more accessible like adipometer and bioimpedance and ultrasound, even golden pattern, magnetic resonance and DXA are high cost gadgets.
All authors declare no potential conflict of interest related to this article