Rationale and Objective
Because lower limb muscles differ in architecture and function, the systemic effects of chronic obstructive pulmonary disease (COPD) and related disuse may result in regional abnormalities. The purpose of this study was to investigate the differences between patients with COPD and healthy controls in three-dimensional shape and size measurements of individual thigh muscles.
Materials and Methods
Twenty patients with COPD and 20 healthy adults (aged 55–79 years) underwent magnetic resonance imaging of the thighs. After manual segmentation of individual knee extensor and flexor muscles, the three-dimensional shape of each muscle was obtained using specialized software. Eight shape descriptors were computed both globally (for the whole muscle) and regionally (for portions of the muscle). A two-tailed t test with a modified Bonferroni correction was used to compare group differences.
Results
Compared to the thigh muscles of healthy subjects, vastus intermedius and semimembranosus showed the most shape abnormalities in the COPD group ( P < .01). Greater regional shape anomalies in the COPD group were found in the middle to proximal regions of all knee extensor muscles and the middle region of the semimembranosus muscle, compared to those of the control group ( P < .01). In the COPD group, more shape abnormalities were found in the knee extensors than in the knee flexors ( P < .01).
Conclusions
A non-uniform distribution of atrophy and size changes was found across knee extensors and flexors in patients with COPD. Further research is required to investigate the underlying mechanisms of regional morphologic abnormalities of the thigh muscles and the increased susceptibility of the knee extensors to atrophy-related anatomic anomalies in COPD.
Skeletal muscle weakness, particularly in the lower extremities, is common in patients with chronic obstructive pulmonary disease (COPD) . In fact, lower limb muscles are typically more adversely affected than respiratory muscles in this patient population, in part because of disuse . COPD-related muscle weakness is also associated with other systemic comorbidities, including abnormal arterial blood gases (hypoxia, hypercapnia), malnutrition, systemic inflammation, oxidative stress, and low testosterone levels . Of interest, the magnitude of skeletal muscle weakness in patients with COPD ranges widely among patients , likely reflecting individual differences in the contribution of factors involved in poor muscle performance.
It has been suggested that the loss of muscle mass (size) is associated with skeletal muscle weakness in patients with COPD . However, the contribution of reduced muscle mass relative to other factors, such as changes in the muscle contractile apparatus and/or neuromuscular activation, is unknown . Comprehensive measures of muscle size and shape are therefore required to more precisely examine the relative contribution of muscle mass reduction to force loss. Magnetic resonance imaging (MRI) can be used to accurately distinguish muscle from bone, connective tissue, nerves, and blood vessels and can therefore provide accurate measures of muscle cross-sectional area (CSA) .
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Materials and methods
Subjects
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Table 1
Characteristics of Patients with COPD and Healthy Older Adults
Characteristic Healthy Older Adults ( n = 20) Patients with COPD ( n = 20) Age (y) 64.4 ± 8.1 68.2 ± 10.0 Women/men 11/9 11/9 Height (m) 1.67 ± 0.13 1.66 ± 0.09 Weight (kg) 69.0 ± 14.4 72.1 ± 14.6 BMI (kg/m 2 ) 24.3 ± 2.2 26.6 ± 4.7Lung function FEV 1 (L) (% predicted) ∗ 2.28 ± 0.72 (81 ± 20) 1.34 ± 0.41 (51 ± 17) FVC (L) (% predicted) ∗ 3.05 ± 1.11 (83 ± 20) 2.58 ± 0.47 (78 ± 14) FEV 1 /FVC (%) ∗ 77 ± 9 52 ± 14
BMI, body mass index; COPD, chronic obstructive pulmonary disease; FEV 1 , forced expiratory volume in 1 second; FVC, forced vital capacity.
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MRI
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Image Segmentation and Interpolation
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Shape Descriptor Computation
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Mean Distance to the Centroid (MDC) and Standard Deviation of Distances to the Centroid (SDC)
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Three-dimensional Moment Invariants
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Surface Area and Volume
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Statistical Analysis
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Table 2
Differences Between 3D Shape Descriptors for Global Analysis in Healthy Subjects Compared to Patients with COPD
Muscle MDC (mm) SDC (mm) Moment 1 (mm 2 ) Moment 2 (mm 4 ) Moment 3 (mm 6 ) Area (mm 2 ) RF Mean (SE) 3.08 (1.33) 1.19 (0.74) 7.68E7 (2.91E7) ∗ 4.78E15 (1.71E15) † 4.18E22 (1.84E22) 2,549.10 (1,523.03) 95% CI 0.39–5.79 0.31–2.70 1.78E7–1.36E8 1.32E15–8.25E15 4.58E21–7.91E22 534.11–5,632.31 VI Mean (SE) 5.31 (1.66) † 3.12 (1.00) † 1.58 E8 (5.52E7) † 1.83E16 (6.42E15) † 4.87E23 (1.81E23) ∗ 7,492.95 (2,364.59) † 95% CI 1.95–8.67 1.08–5.15 4.57E7–2.69E8 5.31E15–3.13E16 1.22E23–8.52E23 2,706.08–12,279.82 VM Mean (SE) 2.12 (1.06) 0.74 (0.79) 8.49E7 (4.15E7) 1.11E16 (5.33E15) 2.83E23 (1.46E23) 4,667.11 (1,770.98) ∗ 95% CI 0.03–4.27 0.87–2.35 9.07E5–1.69E8 3.14E14–2.19E16 1.14E22–5.78E23 1,081.94–8,252.27 BF-SH Mean (SE) 3.78 (1.55) 2.13 (0.855) 3.65E7 (1.39E7) ∗ 7.15 E14 (2.71E14) ∗ 2.84 E21 (1.21E21) 2,277.42 (1,171.59) 95% CI 0.63–6.92 0.40–3.87 8.39E6–6.47E7 1.68E14–1.26E15 3.96E20–5.29E21 94.35–4,649.18 SM Mean (SE) 5.30 (1.56) † 2.47 (0.88) † 7.49E7 (2.36E7) † 3.25E15 (1.46E15) 3.87E22 (1.97E22) 4,516.24 (1,732.28) ∗ 95% CI 2.15–8.45 0.689–4.24 2.71E7–1.23E8 2.99E14–6.21E15 1.21E21–7.86E22 1,009.41–8,023.07
Statistical measures are reported as mean difference (SE) and 95% CI of the difference. Mean difference is defined as the average absolute difference of group means for each shape descriptor (independent values).
BF-SH, biceps femoris–short head; CI, confidence interval; COPD, chronic obstructive pulmonary disease; MDC, mean distance to the centroid; RF, rectus femoris; SDC, standard deviation of distances to the centroid; SE, standard error; SM, semimembranosus; VI, vastus intermedius; VM, vastus medialis.
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Classification Accuracy
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Results
Analysis of Global Muscle Shape Descriptors
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Analysis of Regional Muscle Shape Descriptors
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Table 3
Differences of Three-dimensional Shape Descriptors for Regional Analysis in Healthy Subjects Compared to Patients with COPD
Muscle Shape Descriptor Region ∗ Mean (SE) of Difference_P_ 95% CI of Difference RF SDC (mm) III 0.91(0.32) .004 0.26–1.54 Area (mm 2 ) III 1,280.20 (460.65) .008 347.66–2,212.73 VL Volume (mm 3 ) II 35,101.15 (9,491.81) .001 15,850.87–54,351.44 Volume (mm 3 ) III 31,114.11 (8,840.89) .001 13,200.76–49,027.46 Area (mm 2 ) III 2,373.28 (860.72) .009 629.30–4,117.26 VI Area (mm 2 ) III 2,634.61 (739.64) .001 1,137.29–4,131.94 VM SDC (mm) II 0.87 (0.26) .001 0.35–1.39 SDC (mm) III 1.28 (0.33) .000 0.62–1.94 SDC (mm) IV 0.96 (0.29) .002 0.37–1.56 Area (mm 2 ) II 1,461.19 (475.00) .004 499.60–2,422.79 Area (mm 2 ) III 1,160.76 (313.56) .001 523.54–1,797.99 SM Area (mm 2 ) II 1,441.56 (546.37) .005 335.48–2,547.64 Volume (mm 3 ) II 16,631.14 (5,140.63) .003 6,215.23–27,047.05
The shape descriptors and corresponding regions that revealed significant between group differences in regional analysis are presented in columns 2 and 3, respectively. Statistical measures are reported as mean difference (SE) and 95% CI of the difference. Mean difference is defined as the average absolute difference of group means for each shape descriptor (independent values).
CI, confidence interval; COPD, chronic obstructive pulmonary disease; RF, rectus femoris; SDC, standard deviation of distances to the centroid; SE, standard error; SM, semimembranosus; VI, vastus intermedius; VL, vastus lateralis; VM, vastus medialis.
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Classification Accuracy
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Table 4
SVM Classification Rates
Muscle Groups Muscle Classification Rate Using All Features (%) Knee extensors (quadriceps) Rectus femoris 99.9 Vastus medialis 97.0 Vastus intermedius 99.9 Vastus lateralis 97.5 Knee flexors (hamstrings) Biceps femoris–long head 99.9 Biceps femoris–short head 95.0 Semimembranosus 97.3 Semitendinosus 99.8
SVM, support vector machine.
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Discussion
Differences in MDC, Surface Area, and SDC
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Physiologic Implications of Global and Regional Measurements
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Distribution of Shape Abnormalities and Atrophy Among Thigh Muscles in COPD
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Measurement of Muscle Volume and Comparison to Previous Studies
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Automated Group Classification on the Basis of Shape Measures
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Significance of the Study
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Limitations and Future Work
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Conclusions
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