تركيب الجسم
في اللياقة البدنية، تركيب الجسم body composition، مصطلح يستخدم لوصف النسبة المئوية للدهون، العظام والعضلات في جسم الإنسان. ولأن الأنسجة العضلية تأخذ حيز أقل في أجسامنا عن الأنسجة الدهنية، فإن تركيب جسمنا، بالإضافة إلى وزننا، determines leanness. قد يبدو شخصين متساويين في الوزن والطول، بشكلين مختلفين ويرجع ذلك لاختلال تركيب الجسم.
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قياس تركيب الجسم
Body composition (particularly body fat percentage) can be measured in several ways. The most common method is by using a set of measurement calipers to measure the thickness of subcutaneous fat in multiple places on the body. This includes the abdominal area, the subscapular region, arms, buttocks and thighs. These measurements are then used to estimate total body fat with a margin of error of approximately four percentage points.
Another method is bioelectrical impedance analysis (BIA), which uses the resistance of electrical flow through the body to estimate body fat.
Assessment of somatic (skeletal) protein is typically determined by simple measurements and calculations including mid arm circumference (MAC), mid arm muscle circumference (MAMC), and creatinine height ratio (CHI). Creatinine height ratio is calculated as: 24 hour urine creatinine x 100/ expected 24 hour urine creatinine for height. This calculation results in a percentage which can indicate protein depletion.
A technique for measuring body composition has been developed using the same principles as under water weighing. The technique uses air, as opposed to water and is known as air displacement plethysmography (ADP). Subjects enter a sealed chamber that measures their body volume through the displacement of air in the chamber. Body volume is combined with body weight (mass) in order to determine body density. The technique then estimates the percentage of body fat and lean body mass (LBM) through known equations (for the density of fat and fat free mass).
Body composition measurement with Dual energy X-ray absorptiometry (DEXA) is used increasingly for a variety of clinical and research applications. Total body or estimated total body scans using DEXA give accurate and precise measurements of BMD and body composition, including bone mineral content (BMC), bone mineral density (BMD), lean tissue mass, fat tissue mass, and fractional contribution of fat.[1]
These measurements are extremely reproducible, making them excellent for monitoring pharmaceutical therapy, nutritional or exercise intervention, sports training &/or other body composition altering programs. They are also fast, simple, non-invasive, and expose the subject to a level of x-rays less than that of a cross-country flight. DEXA exams provide both total body and up to 14 regional (trunk, individual arms & legs, android, gynoid, etc.) results.
Body Composition is also estimated using cross-sectional imaging methods like magnetic resonance imaging (MRI) and computed tomography (CT). Since MRI and CT give the most precise body composition measures to-date, many pharmaceutical companies are very interested in this new procedure to estimate body composition measures before and after drug therapy especially in drugs that might change body composition.
Ultrasound has also been used to measure subcutaneous fat thickness, and by using multiple points a measurement of body composition can be made. Ultrasound has the advantage of being able to also directly measure muscle thickness and quantify intramuscular fat.
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المصادر
- ^ et al. (2000) J Clin Densitom 3:35–41