Estimation of Muscle Mass ßMethods for the assessment of human body composition emphasize the estimation of body fat with limited availability of approaches and techniques to assess muscle mass.

Estimation of Muscle Mass Muscles: ßCardiac ßSmooth ßSkeletal ßRepresents 30% of body weight in female and 40% in male.

Estimation of Muscle Mass Applications ßMonitor changes in relation to growth and development in infants and children ßEstimate muscle mass and relate it to aerobic, anaerobic, and physical training on performance. ßEvaluate the progression of catabolic disease and possible intervention.

Anthropometric Indicators ßA site-specific physical measurement will reflect the mass of that muscle, and the mass of the estimated muscle group is directly proportional to the whole-body skeletal muscle mass.

Estimation of Regional Muscle Mass ßThe common variables for the upper arm include arm circumference corrected for subcutaneous adipose tissue thickness, and muscle cross-sectional area estimated from the corrected circumference.

Estimation of Regional Muscle Mass Validity: ßIn adult body weights ranging from 60 to 120% of ideal body weights, predictive error was 7-8%. ßIn adult body weights above 150% of ideal body weight, predictive error was >50%. ßGender specific equation have been derived to account for errors.

Muscle Metabolites ßTwo metabolites specific to skeletal muscles include: ßCreatinine. ß3-Methylhistidine

Creatinine ßThe precursor of creatine found in the liver and kidney. ßRelated to FFM and skeletal muscle mass. ßOne gram of creatinine excreted was = 18 kg - 20 kg of muscle mass.

Creatinine Limitations: ßThere is a large individual variability in daily creatinine excretion because of dietary intake. ßOther factors that may invalidate the results include age, gender, maturity, physical training, and metabolic state.

3-Methylhistidine ß3-Methylhistidine (3-MH) is an amino acid that has been suggested as a measure of muscle protein breakdown.The concentration of 3-MH in human muscle is relatively constant between the ages of 4 to 65, but decreases with urinary excretion.

3-MH ßTherefore it is reasonable to suggest that the reduction in 3-MH output might reflect a decreased muscle mass with age.

3-MH ßThe use of 3-MH as a marker of muscle mass has been criticized because of the potential influence of non-skeletal protein turnover on its excretion rate. ßThe general use of urinary 3-MH excretion may be reasonable in conditions of physical trauma in which accelerated rates of protein degradation occur, especially in skeletal muscle.

Limitation to the use of Metabolites ßAlthough creatinine and 3-MH arise primarily from muscle, their relationship to skeletal muscle mass needs further examination with respect to factors that affect their pool sizes and turnover rates.

Radiographic Methods ßSeveral radiographic methods have been used to estimate muscle mass: ßComputed tomography. ßMagnetic resonance imaging. ßDual X-ray Absorptiometry.

Limitations of CT ßCost per study limits the percentage of healthy people that can be studied. ßExposure to radiation. ßValidity studies have yet to be completed.

Limitations of MRI ßTime: 45 minutes for whole-body test. ßValidity studies have yet to be completed.

Limitations of DEXA ßCost ßRadiation exposure. ßDEXA cannot distinguish between intra- and extra-cellular fluid. ßIngestion of fluid and/or regional accumulation of water and salts will change the readings.

BIA ßBIA is another approach for assessing the regional muscle mass.

BIA ßThe areas of adipose tissue and muscle in the upper arm for CT are related to those estimated by BIA.

BIA ßThe type of electrodes and their placements have not been defined in reference to minimizing the error of this technique.

Summary ßTechniques with the best precision cost more. ßSelection of a method may depend on the resources available and the purpose of the test.

Summary ßThe various components found in muscle (water, protein, nerve, vascular tissue, etc) make it difficult to measure accurately

Alter Metabolism? ßMany people believe that since muscle burns more calories than fat, building muscle by weight lifting will noticeably increase the body’s metabolism. ßThis response is greatly exaggerated.

Metabolism ßWeight lifting has virtually no effect on resting metabolism. ßAny added muscle is minuscule compared with the total amount of skeletal muscle in the body.

Metabolism ßAnd, muscle actually has a very low metabolic rate when it is at rest, which is most of the time.

Metabolism ßSkeletal muscle burns about 13 kcals per kg of body weight over 24 hours when a person is at rest. ßA typical man who weighs 70 kg (154 lbs), has about 28 kgs of skeletal muscle.

Metabolism ßHis muscles, when at rest, burn about 22% of the calories his body uses. ßThe brain and the liver use about the same number of calories.

Metabolism ßIf the man lifts weights and gains 2 kg (4.4 lbs) of muscle, his metabolic rate would increase by 24 kcals per day.

Metabolism ßThe average amount of muscle that men gain after lifting weights for 12 weeks is 2 kg. ßWomen will gain less.

Body Weight ßA corollary to this hypothesis is that by adding muscle you can noticeably change you body weight. ßThe idea is that when you do resistance training you may actually be thinner yet weigh the same or a little more, because muscle is heavier than fat.

Body Weight ßThat holds a grain of truth, because muscle is more dense than fat. ßThe problem is that few people put on enough muscle in proportion to their total body mass to make a noticeable difference in their weight.

Body Weight ßThe idea that you will weigh the same or more, but you really are thinner may be true if you work hard at weight lifting for many months, otherwise, it is another myth.