Estimation of Muscle Mass

Methods 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:CardiacSmoothSkeletal

Represents 30% of body weight in female and 40% in male.

Estimation of Muscle Mass

ApplicationsMonitor changes in relation to growth and

development in infants and childrenEstimate muscle mass and relate it to aerobic,

anaerobic, and physical training on performance.

Evaluate the progression of catabolic disease and possible intervention.

Anthropometric Indicators

A 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

The 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:In adult body weights ranging from 60 to

120% of ideal body weights, predictive error was 7-8%.

In adult body weights above 150% of ideal body weight, predictive error was >50%.

Gender specific equation have been derived to account for errors.

Muscle Metabolites

Two metabolites specific to skeletal muscles include:Creatinine.3-Methylhistidine

Creatinine

The precursor of creatine found in the liver and kidney.

Related to FFM and skeletal muscle mass.One gram of creatinine excreted was = 18 kg

- 20 kg of muscle mass.

Creatinine

Limitations:There is a large individual variability in daily

creatinine excretion because of dietary intake.Other factors that may invalidate the results

include age, gender, maturity, physical training, and metabolic state.

3-Methylhistidine

3-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

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

3-MH

The 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.

The 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

Although 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

Several radiographic methods have been used to estimate muscle mass:

Computed tomography.Magnetic resonance imaging.Dual X-ray Absorptiometry.

Limitations of CT

Cost per study limits the percentage of healthy people that can be studied.

Exposure to radiation.Validity studies have yet to be completed.

Limitations of MRI

Time: 45 minutes for whole-body test.Validity studies have yet to be completed.

Limitations of DEXA

CostRadiation exposure.DEXA cannot distinguish between intra- and

extra-cellular fluid. Ingestion of fluid and/or regional

accumulation of water and salts will change the readings.

BIA

BIA is another approach for assessing the regional muscle mass.

BIA

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

BIA

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

Summary

Techniques with the best precision cost more.Selection of a method may depend on the

resources available and the purpose of the test.

Summary

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

Alter Metabolism?

Many people believe that since muscle burns more calories than fat, building muscle by weight lifting will noticeably increase the body’s metabolism.

This response is greatly exaggerated.

Metabolism

Weight lifting has virtually no effect on resting metabolism.

Any added muscle is minuscule compared with the total amount of skeletal muscle in the body.

Metabolism

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

Metabolism

Skeletal muscle burns about 13 kcals per kg of body weight over 24 hours when a person is at rest.

A typical man who weighs 70 kg (154 lbs), has about 28 kgs of skeletal muscle.

Metabolism

His muscles, when at rest, burn about 22% of the calories his body uses.

The brain and the liver use about the same number of calories.

Metabolism

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

Metabolism

The average amount of muscle that men gain after lifting weights for 12 weeks is 2 kg.

Women will gain less.

Body Weight

A corollary to this hypothesis is that by adding muscle you can noticeably change you body weight.

The 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

That holds a grain of truth, because muscle is more dense than fat.

The 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

The 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.