|Year : 2017 | Volume
| Issue : 1 | Page : 15-20
Psoas major cross-sectional area: A potential marker of cardiorespiratory fitness
Julie Fitzpatrick1, Edward S Chambers2, James R C Parkinson1, Gary Frost2, Jimmy D Bell1, E Louise Thomas1
1 Department of Life Sciences, Research Centre for Optimal Health, University of Westminster, England, London, UK
2 Department of Medicine, Faculty of Medicine, Imperial College London, England, London, UK
|Date of Submission||31-Jan-2017|
|Date of Decision||15-Feb-2017|
|Date of Acceptance||25-Feb-2017|
|Date of Web Publication||9-Mar-2017|
E Louise Thomas
Department of Life Sciences, Faculty of Science and Technology, University of Westminster, 115 New Cavendish Street, London W1W 6UW
Source of Support: None, Conflict of Interest: None
Background and Aim: Cardiorespiratory fitness is an important marker for overall health that significantly correlates with obesity-associated morbidities and mortality. Maximal oxygen uptake (VO2max) recorded during an incremental exercise test is the gold standard assessment for aerobic fitness. However, its cost, chronic illness, and frailty often preclude its application. The cross-sectional area (CSA) of the abdominal psoas major muscle is a predictor of sarcopenia and surgery outcomes and represents a promising biomarker for cardiorespiratory health. Therefore, in the present study, we have planned to assess the relationship between psoas major CSA, anthropometry, and body composition in a UK-based cohort of 210 men and women. Methods: Body mass (kg), height (cm), waist circumference (cm), VO2max, and blood pressure were measured in each participant. The CSA of psoas major, rectus abdominus, and another abdominal muscle of the core muscle group were assessed. Results: Following adjustment for height, psoas major CSA was found to be a significant predictor of percentage body fat (P = 0.02) in men, and body mass index (BMI) in both men (P = 0.015) and women (P = 0.004). We found psoas major CSA correlated more strongly with VO2max (r = 0.74, P < 0.01) than any other study outcome, including age and BMI. Conclusion: Psoas major muscle CSA represents an accurate, reproducible, and time-efficient surrogate for cardiorespiratory fitness and body composition.
Keywords: Cardiorespiratory fitness, magnetic resonance imaging, maximal oxygen uptake, psoas muscle
|How to cite this article:|
Fitzpatrick J, Chambers ES, Parkinson JR, Frost G, Bell JD, Thomas E L. Psoas major cross-sectional area: A potential marker of cardiorespiratory fitness. Int J Clin Exp Physiol 2017;4:15-20
|How to cite this URL:|
Fitzpatrick J, Chambers ES, Parkinson JR, Frost G, Bell JD, Thomas E L. Psoas major cross-sectional area: A potential marker of cardiorespiratory fitness. Int J Clin Exp Physiol [serial online] 2017 [cited 2017 Mar 25];4:15-20. Available from: http://www.ijcep.org/text.asp?2017/4/1/15/201795
| Introduction|| |
Physical inactivity significantly contributes to both morbidity and mortality, with public health organizations now increasingly promoting habitual exercise to reduce the negative impact of a sedentary lifestyle., Improvements in cardiorespiratory fitness are a key target for intervention, with maximal oxygen uptake (VO2max) recorded during an incremental exercise test to exhaustion, being considered the gold standard., Cardiorespiratory fitness is inversely related to fat mass, type 2 diabetes prevalence, and a more reliable predictor of mortality than other established markers, such as blood pressure or circulating cholesterol. However, the time commitment and cost of performing these tests often make them impractical, and chronic illness or frailty in elderly patients precludes their application. Consequently, there is a need for accurate and reproducible biomarkers for use as surrogates of cardiorespiratory fitness.
Morphometric analysis of core muscle cross-sectional area (CSA) is emerging as a strong indicator of health outcomes, with an increase in muscle fiber CSA as the main functional adaptation arising from aerobic and strength training. The psoas major is a large muscle of the abdomen, forming part of the core muscle group, assisting lateral rotation and abduction of the hip joint. Psoas major CSA has been used in a number of studies to predict the total body lean muscle mass, sarcopenia, and surgical outcomes in elderly patients., It therefore represents a potential marker for cardiorespiratory fitness.
In the present study, we would like to characterize how the CSA of psoas major and the rectus abdominus (RA), another abdominal muscle of the core muscle group, vary with age, gender, and BMI in a cross-sectional population. Second, in a subset of our cohort, we have assessed the relationship between these muscles CSA and VO2max to gauge their potential as a surrogate marker for overall physical health.
| Materials and Methods|| |
Written informed consent was acquired from all volunteers. Ethical approval for this study was obtained from the Brent National Research Ethics Committee (Rec: 12/LO/0139). All studies were carried out in accordance with the Declaration of Helsinki. In total, 210 participants were recruited through advertisements in newspapers, websites, academic newsletters, and inviting male and female volunteers of Caucasian ethnicity from the general public. Participants presented with no history of chronic disease or excess alcohol intake were included in the study. Individuals on prescribed medication and pregnant women were excluded from the study.
Anthropometry, blood pressure, and clinical biochemistry
Body mass (kg), height (cm), and waist circumference (cm) were measured in each participant by a single experienced observer. Fasting glucose, total cholesterol, triglycerides, high-density lipoprotein, low-density lipoprotein, and insulin were measured by standard methods by the Department of Chemical Pathology, Imperial College Healthcare National Health Service Trust. Blood pressure of the participants was measured by trained clinician using an automatic sphygmomanometer after 5 min of rest in supine position.
Individuals underwent magnetic resonance imaging (MRI) at 1.5T (Archiva, Philips Medical Systems, The Netherlands) following an overnight fast. Participants were in prone position, and T1-weighted axial images of the whole body were obtained as described previously. During the same scanning session,1 H MR spectra were also acquired at 1.5T. Using a surface coil, intrahepatocellular lipid (IHCL) was measured relative to liver water content.
Psoas major and rectus abdominus
Using the open source image processing program Image-J (NIH, USA), the CSA of the psoas major was manually isolated at lumbar point L3/L4. The CSA of the rectus abdominis (RA), which can also be clearly observed within the same axial slice, was measured from the same MRI images to provide a comparison core muscle. The CSA values for each muscle group in this study correspond to the sum of the CSA of the right- and left-hand sides [Supplementary [Figure 1]. Due to the strong correlation between psoas major muscle and height, values are also presented as CSA/height 2 (mm 2/m 2).
|Figure 1: Gender-specific distribution of psoas and rectus abdominus muscle cross-sectional area/height2 with age, body mass index, and percentage body fat. Cross-sectional area adjusted for height (cross-sectional area/height2) of psoas major (white square/circle) and rectus abdominus (black square/circle) muscles in men (a, c and e) and women (b, d, and f) by age (a and b), body mass index (c and d), and percentage body fat (e and f). Linear regression performed in GraphPad Prism with corresponding r2 and P values. □: Male psoas; Δ: Female psoas; ◾: Male rectus abdominus; ▲: Female rectus abdominus|
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To test the reproducibility of the manual analysis of psoas major and RA CSA, two separate exercises were undertaken. In test 1, left and right muscle CSA [Supplementary [Figure 1] were assessed three times in a row by the same observer; psoas: (Average [standard deviation [SD]) 4165.1 ± 24.37 mm 2, coefficient of variation (CoV): 0.59%; RA: 1826.0 ± 9.2 mm 2, and CoV: 0.50%. In test 2, the same axial slice was measured on three separate occasions, at 1 week intervals; average (SD): 4117.3 ± 61.92 mm 2 and CoV: 1.5%. In test 2, the analysis was repeated three times on a single, randomly chosen image, 1 week apart by the same observer; psoas CoV: 2.9% and RA: 3.7%.
An incremental cycling test to exhaustion  was carried out on the same study day as the MRI scan to obtain VO2max in a subset of the cohort (99 individuals [67 male, 32 female]).
Student's t-test and Spearman's rank correlations were performed on variables; psoas major CSA, RA CSA, and VO2max. Linear regression was performed in GraphPad Prism version 6.0 (GraphPad Software, USA). IHCL values were log transformed after adding + 1 to their values due to the nonnormally distributed nature of the outcome. Correlation was performed in SPSS 23 (IBM SPSS Statistics for Windows, USA) and linear regression in GraphPad Prism. P < 0.05 was considered significant. All data were presented as mean ± SD.
| Results|| |
A total of 210 participants (97F, 113M) took part in the initial study to characterize psoas major and RA muscles, both raw and adjusted for height, are summarized in [Table 1]. Average psoas CSA/height 2 and RA CSA/height 2 measurements for the entire cohort were 942 + 93 and 436 ± 141, respectively. Women presented with significantly smaller psoas and RA when compared to men (psoas CSA/height 2: female 741 + 167 mm 2/m 2, male 1114 + 266 mm 2/h 2, P < 0.001; RA CSA/height 2: female 324 ± 73 mm 2/h 2, male 491 ± 134 mm 2/h 2, P < 0.001).
|Table 1: Characteristics of baseline cohort n = 210 (113 male, 97 female)|
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[Figure 1] shows how muscle CSA, adjusted for height, varied by gender, age, BMI, and percentage body fat. Linear regression analysis revealed a significant inverse relationship between psoas CSA/height 2 and age in men [r = 0.13; P = 0.016, [Figure 1]]a with no effect in women [Figure 1]b. Both psoas major CSA/height 2 (r = 0.28; P = 0.004) and RA CSA/height 2 (r = 0.46; P < 0.001) were significant predictors of BMI in women [Figure 1]d, while only psoas major CSA/height 2 predicted BMI in men (r = 0.22, P = 0.015). Lastly, psoas major CSA/height 2 was a significant inverse predictor of body fat percentage in men [r = 0.22; P = 0.02, [Figure 1]e. Examination of the relationship between psoas and RA with metabolically adverse fat depots, visceral fat and IHCL, can be found in [Figure 2]. Psoas major CSA/height 2 inversely predicted visceral fat in men [r = 0.20; P = 0.02, [Figure 2]a, while RA CSA/height 2 was a significant predictor of IHCL in women [r = 0.22; P = 0.02, [Figure 2]b.
|Figure 2: Gender-specific distribution of psoas major and rectus abdominus muscle cross-sectional area/height2 with visceral fat and intrahepatocellular lipid. Cross-sectional area adjusted for height (cross-sectional area/height2) of psoas major (white square/circle) and rectus abdominus (black square/circle) muscles in men (a and c) and women (b and d) by visceral fat (kg) (a and b) and log intrahepatocellular lipid (c and d). Linear regression performed in GraphPad Prism with corresponding r2 and P values. □: Male psoas; Δ: Female psoas; ◾: Male rectus abdominus; ▲: Female rectus abdominus|
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A comparison of how psoas major and RA muscles (after adjustment for height) correlate with study outcomes is shown in Supplementary [Figure 2]. Both psoas major CSA/height 2 and RA/height 2 were inversely correlated with age (r = −0.49, P < 0.01, r = −0.50, P < 0.01). RA/height 2 was significantly associated with visceral (r = 0.28, P < 0.01) and nonvisceral adipose tissue (r = 0.20, P < 0.05) with no correlation observed with psoas major CSA/height 2 (P = NS).
Further investigation in a smaller, older subset of the cohort for which VO2max was available (n = 105 (72M), age 54.5 ± 8.5 years) was carried out to assess the validity of psoas major and RA muscle CSA as a marker for cardiorespiratory fitness. Baseline characteristics for this cohort are shown in Supplementary [Figure 3]. Average VO2max was 2523 ± 1091 ml/min, with female VO2max lower (1520 ± 332 ml/min) than male (3002 ± 998 ml/min). After correction for weight, male VO2max was 32.2 ± 12.1 ml/kg/min, while female was 19.9 ± 4.3 ml/kg/min.
|Figure 3: Gender-specific distribution of psoas major and rectus abdominus muscle cross-sectional area/height2 with maximal oxygen uptake (ml/kg/min). Cross-sectional area adjusted for height (cross-sectional area/height2) of psoas major (white square/circle) and rectus abdominus (black square/circle) muscles in (a) men and (b) women. Linear regression performed in GraphPad Prism with corresponding r2 and P values. □: Male psoas; Δ: Female psoas; ◾: Male rectus abdominus; ▲: Female rectus abdominus|
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Correlation analysis between VO2max (adjusted for weight) and study outcomes is shown in [Table 2]. Psoas major muscle CSA/height 2 (mm 2/m 2) correlated strongly with VO2max(ml/kg/min) (r = 0.56, P < 0.01), with no association observed with RA CSA/height 2 (r = 0.17, P = NS). Both age (r=-0.64) and height (r = 0.49) correlated strongly with VO2max to a similar degree of significance (P < 0.01). Gender-specific analysis revealed a significant correlation between psoas major CSA/height 2 and VO2max (ml/kg/min) (r = 0.33, P < 0.01) in males. In female volunteers, VO2max (ml/kg/min) correlated strongly with individual adipose compartments but no associations were observed with anthropometric or core muscle group measurements [Table 2].
|Table 2: Correlation analysis of maximal oxygen uptake with study outcomes|
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Gender-specific distribution of psoas major and RA CSA/height 2 with VO2max (ml/kg/min) is shown in [Figure 3]. Linear regression revealed psoas major CSA/height 2 which were significant predictors of VO2max in male participants [P < 0.001, [Figure 3]a, with no effect in women [Figure 3]b. RA muscle was not found to be a significant predictor of VO2max in either men or women [Figure 3].
| Discussion|| |
In the present study, we characterize how the CSA of psoas major and RA muscles varies with age, gender, and BMI in a cross-sectional population. The CSA of the psoas major strongly correlated with and was a significant predictor of VO2max in a male subset of our cohort, with no such relationship observed with RA.
Physical inactivity is a leading cause of most chronic illness and practical methods to determine fitness levels are needed to enable effective assessment of lifestyle interventions and public health planning., The use of MRI and computerized tomography (CT) scans to measure the content and distribution of body fat is increasingly common in both research and clinical fields, with cross-sectional abdominal imaging a common procedure in a diagnostic setting. Postprocessing of abdominal region scans enables an in-depth investigation of tissue morphology, including the CSA of different muscles.
Muscle size represents a quantitative index, reflecting general health and intervention risk. While obtaining whole-body images can be time-consuming and expensive, studies have shown that the CSA of abdominal skeletal muscle provides a reliable surrogate of whole body muscle mass. Within this region lies the psoas major muscle, a component of the core muscle group and a surrogate marker for sarcopenia and surgical outcomes.,, The RA muscle, often referred to as the abdominals, is another component of the core muscle group that lies within the L4 region and was included in our analysis as a comparator. The psoas major is easily identified on axial images in both MRI and CT scan, and analysis of muscle CSA can be easily translated into any research institute where cross-sectional imaging of the abdominal region is available using this simple and straightforward method.
Our data indicate psoas major CSA predicts VO2max, albeit in men only, with no such effect observed with RA. Correlation analyses of RA and psoas major CSA adjusted for height revealed comparable degrees of association for the majority of study outcomes. There was however a significant inverse correlation between psoas major CSA and age, a relationship not observed with RA. In addition, while the psoas was negatively correlated with total, subcutaneous, and subcutaneous abdominal fat, RA also showed a positive correlation with metabolically adverse visceral and internal fat stores. Cardiorespiratory fitness is known to be a significant predictor of fat mass and together these data indicate that of the two core muscles, the psoas major is the more viable marker for metabolic and cardiovascular health.
Ethnic differences exist regarding muscle mass distribution  and to avoid these potentially confounding effects, study recruitment was limited to Caucasians. Further research is therefore warranted to determine the influence of broader participant demographics on the positive associations between psoas major CSA, cardiorespiratory fitness, adiposity, and fat-free mass. As expected, the CSA of both psoas major and RA muscles was significantly smaller in women compared to men, necessitating gender-specific analysis. The inverse relationship observed between psoas and age in men was expected and reflects an established association. However, in women, we failed to see a reduction in either psoas major or RA size as age increased, or indeed any correlation between VO2max and other outcomes. Indeed, it is clear that the significant associations we did observe between VO2max and study outcomes were driven by the relationship in men.
Several factors may have contributed to this; first, the number of women for which VO2max data were available was considerably smaller (n = 33) compared to men (n = 72). Second, the range of VO2max values was more limited in women (511–1175 ml/min) than men (663–2312 ml/min), perhaps reflecting the reduced levels of reported physical activity in women who participated; 24% reported "fit" (corresponding to >5 h exercise per week), compared to 42% of the men. Interventional studies which employ exercise and subsequently measure the effects on VO2max and core muscle size will be required to eliminate the confounding effects of age and determine the efficacy of psoas as a marker of metabolic fitness.
Limitations of the study
Sample size in the present study was less.
| Conclusion|| |
Our findings indicate that psoas major CSA measured at L4 is strongly associated with cardiorespiratory fitness, adiposity, and fat-free mass. Hence, psoas major is a potential marker of cardiorespiratory fitness. Additional work in a larger, racially diverse population with a more expansive range of fitness levels will be required to confirm its utility.
Financial support and sponsorship
GF, JB, and ELT were all funded through the Nutritech study (FP7-KBBE-289511).
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]
[Table 1], [Table 2]