Muscle mass percentage: what is normal for your age and sex

Muscle mass chart by age and sex

The chart by age at the top of this article comes from Janssen and colleagues (J Appl Physiol, 2000), measured with the reference method: whole-body magnetic resonance imaging in 468 adults aged 18 to 88, with the percentage and the kilos for each decade in Table 1 of their paper. That is 468 people; to see the same thing across tens of thousands there are large studies using bioimpedance or DXA, which do not report a percentage but an index: kilos of muscle divided by height squared (kg/m²), so that stature does not distort the comparison. This second chart puts together rows from those studies, and each row says what was measured. Compare your own number only with a row of the same kind: a percentage with the MRI percentage, and an index with another index from the same study.

• Skeletal muscle mass in women: 21.0 kg on average by MRI, 30.6% of body weight; in men, 33.0 kg and 38.4% (Janssen, 2000)
• In the Norwegian study, women and men aged 60 or older had the lowest muscle values of all the age groups
• In the British study, the index in men was already falling from the age of 40, while in most women it barely changed between 40 and 69
• In the Swiss DXA study, the loss of arm and leg muscle accelerated after 60, in both men and women

What each figure measures: skeletal muscle versus the "muscle mass" on your scale

This is where almost all the confusion starts. The studies in the chart talk about SKELETAL MUSCLE: the muscle that moves your bones. Many bioimpedance scales, on the other hand, show a figure that includes a lot more: lean mass or fat-free mass, meaning everything that is not fat (muscle, but also bone, organs and water). Check the manual of yours, because the difference is not small. In the reference by Kyle and colleagues (Nutrition, 2003), with 5,635 healthy people aged 15 to 98, normal body fat ranged from 24.6 to 33.2% in women and from 13.4 to 21.7% in men. Doing the subtraction, fat-free mass makes up between 67 and 75% of body weight in women and between 78 and 87% in men. If your scale gives you figures of that order, you do not have twice the average muscle: it is showing you something else.

Muscle mass percentage in women and men: the reference figure

The reference work is still the one by Janssen and colleagues. It matters that it was MRI and not a scale: MRI makes it possible to measure the volume of the muscles directly in the image, whereas bioimpedance calculates it from the electrical resistance of the body. In that study, men averaged 38.4% of their body mass as skeletal muscle and women 30.6%. The difference between the sexes was larger in the upper body than in the lower body, and muscle rose in a straight line with height but not with weight: the heavier the person, the less muscle each extra kilo contributed.

How muscle mass changes with age

Search for "muscle mass percentage by age" and you will find charts with one row per decade and percentages to the decimal point. Almost none of them says which study it came from. The one at the top of this article does: it is Table 1 of the Janssen study (2000), by MRI, and what it shows is the DIRECTION of change. The muscle percentage starts to fall as early as the third decade of life, while the loss of actual kilos is not noticeable until the end of the fifth, and it happens mostly in the lower body. The European consensus on sarcopenia (Cruz-Jentoft et al., Age Ageing, 2019) sums it up like this: muscle mass and strength peak in young adulthood, up to roughly the age of 40, and beyond 50 losses of leg muscle mass of 1 to 2% per year and of strength of 1.5 to 5% per year have been reported.

• In that study, muscle fell by about 1.9 kg per decade in men and 1.1 kg in women, and the percentage dropped more than twice as fast in men: about 0.19 points a year against 0.08
• The percentage falling before the kilos has a simple reading: if body weight goes up over the years while muscle stays the same, muscle makes up a smaller share of the total
• Men start with more muscle and, in the British study, began to lose it earlier than most women
• There are DXA reference tables by age, such as the one by Clark and colleagues in more than 9,500 healthy people in Mexico, but they report kilos of lean mass and indices, not a percentage of muscle

How to estimate your muscle mass from weight, height, age and sex

If you have no access to an MRI scanner, there is an equation validated against one. Lee and colleagues (Am J Clin Nutr, 2000) measured 244 adults without obesity by MRI and arrived at this formula: skeletal muscle (kg) = 0.244 × weight + 7.80 × height in metres + 6.6 if you are male − 0.098 × age − 3.3. It explains 86% of the differences between people, its standard error is 2.8 kg, and it was only validated for a BMI below 30. The original also adjusts by a few kilos for ancestry (for example, −1.2 kg for Asian and +1.4 kg for African American participants). It is the equation behind the muscle mass calculator, which does the maths for you and compares the result with the Janssen average; to complete the picture with your body fat, there is the body fat calculator.

And with a bioimpedance scale? A different scale altogether

If your figure comes from a scale, MRI reference values are not the right yardstick, because each device calculates muscle with its own equation. One published example: De Rosa and colleagues (Nutrition, 2015) used bioimpedance to calculate the skeletal muscle percentage of 500 young adults from southern Italy (100 men and 400 women, aged 18 to 40) in order to set cut-off points. With that reference, a percentage between 28.8 and 35.6% in men, or between 23.1 and 28.4% in women, already counted as moderate sarcopenia, and anything below those values as severe. These are cut-offs from one specific group and one specific method: they show that the scale shifts with the device, and they are not a way to diagnose yourself with your bathroom scale.

When muscle mass counts as low: the published thresholds

Here there are published criteria, and they are not a loose percentage. Janssen and colleagues (J Am Geriatr Soc, 2002) used data from 14,818 adults in the NHANES III survey, with muscle mass estimated by bioimpedance, and defined the skeletal muscle mass index as muscle mass divided by body mass, multiplied by 100. An index was considered normal if it was above one standard deviation below the mean of young adults aged 18 to 39; between one and two standard deviations below was class I sarcopenia, and below two was class II. Two years later the same group (Am J Epidemiol, 2004) proposed cut-offs in kg/m² associated with a higher risk of physical disability in people over 60. Today the reference in Europe is the EWGSOP2 consensus (Cruz-Jentoft et al., 2019), which starts with strength and only then looks at the amount of muscle.

• In people over 60, Janssen class I was present in 59% of women and 45% of men, and class II in 10% and 7% respectively
• The likelihood of functional impairment or disability was roughly twice as high in men and three times as high in women with class II sarcopenia compared with those with a normal index
• High disability risk in the 2004 study: 5.75 kg/m² or less in women and 8.50 kg/m² or less in men; moderate risk: from 5.76 to 6.75 kg/m² in women and from 8.51 to 10.75 kg/m² in men
• Do not compare these indices with the chart above: they come from different studies, populations and devices

Why the number on your scale is not the one in the studies

Bioimpedance scales and gym body-composition machines do not see muscle: they pass an imperceptible current through the body and estimate composition from the resistance they meet, using their own equations. The European consensus itself points out that their estimates change with the brand of device and with the population it was calibrated on. They work, but with a margin worth knowing about. Brewer and colleagues (J Strength Cond Res, 2019) compared a multifrequency bioimpedance device against DXA in 160 collegiate athletes: bioimpedance significantly UNDERESTIMATED fat-free mass, by 3.8 kg on average in the legs and 0.7 kg in the arms. In the UK Biobank, by contrast, the segmental scale used stayed close to DXA: it overestimated muscle by 2.5% in women and 1.9% in men. It depends on the device and the population, which is why two machines can give you different figures on the same day.

• MRI: measures the volume of the muscles in the image and is the basis of the Janssen figures. Hard to access outside research
• DXA: widely used in research and clinical work; in an analysis of 4,753 people from the UK Biobank, its lean tissue measurement agreed with MRI with a correlation of 0.97
• Bioimpedance (your scale or the one at the gym): convenient and cheap; useful for following the TREND with the same device, not for comparing yourself with a chart made with another method
• Skinfold calipers: they estimate the fat under the skin, not muscle

What to do if you think your muscle mass is low

The answer does not change much with the exact figure, and that is the reassuring part: what builds muscle is always the same. Strength training as the stimulus, enough protein as the raw material, and time. In the meta-analysis by Morton and colleagues (Br J Sports Med, 2018), which pooled 49 studies and 1,863 participants doing resistance training for at least 6 weeks, adding protein increased fat-free mass (by 0.30 kg on average) and strength (by 2.49 kg in the one-repetition maximum) a little more. The benefit of the extra protein was smaller at older ages, and above roughly 1.6 g per kilo per day no further gain in fat-free mass was seen. Every trial included training: the meta-analysis does not evaluate protein on its own.

1. 1

   Train strength every week

   It is the stimulus, and there is no substitute for it. If you are starting from zero, the plan is in the gym routine for beginners and the no-equipment version is in the home workout.

2. 2

   Reach about 1.6 g of protein per kilo

   That is the amount above which the Morton meta-analysis saw no further gain in fat-free mass. Work out yours with the protein calculator, find good sources in protein-rich foods and, if you are over 60, read protein and sarcopenia.

3. 3

   Eat enough, not just well

   Building muscle in an aggressive calorie deficit is rowing against the current. If your goal is to gain, use the muscle-gain calculator and adjust your calories before you start thinking about supplements.

4. 4

   Sleep, and give it time

   Changes in muscle mass are measured in months, not weeks, and rest is part of the plan (sleep and your weight). Do not expect to see real differences on any device within a few days.

5. 5

   Measure the trend, not the day

   Measure under the same conditions once a week and look at the monthly average. A single bioimpedance reading says very little; for body fat, the body fat calculator gives you another reference point.

What to eat to maintain or build muscle mass

First, the order of importance. What builds muscle is strength training; food supplies the raw material. The International Society of Sports Nutrition position stand on protein and exercise (Jäger and colleagues, J Int Soc Sports Nutr, 2017) puts it this way: resistance exercise and protein each stimulate muscle protein synthesis, and together they reinforce each other. As the Morton meta-analysis already showed, with training in every one of its trials, without training there is no basis for expecting more muscle just from eating more protein. With that clear, three general reference points, which are not a prescription for you: EFSA sets 0.83 g of protein per kilo of body weight per day for adults in general; for most people who train, the ISSN gives a range of 1.4-2.0 g per kilo per day (above roughly 1.6 g, Morton saw no further gain in fat-free mass), and per meal it suggests servings of about 20-40 g spread every 3-4 hours. The ISSN itself acknowledges that this can be reached with ordinary food, without supplements. The table gives an idea of what a normal serving provides: meat and fish servings come close to or pass 20 g on their own, while pulses, dairy and plant proteins fall short, so they go further combined on the same plate.

Ask it the other way round: how much muscle can you gain

That is the other half of the search, and it deserves realistic expectations. Gaining muscle takes months, and the average effects in trials are modest: in the Morton meta-analysis, adding protein to training produced on average 0.30 kg more fat-free mass than training without that extra protein. What matters is not how much you gain in month one but that you are still training in month twelve. The detail on combining fat loss with muscle gain is in body recomposition, the eating plan is in the weekly muscle-gain meal plan, and other ways to track your progress are in measuring progress beyond the scale.

The practical summary: measured by MRI, the average is 30.6% skeletal muscle in women and 38.4% in men; by age, the same MRI study goes from 34.1% (18-29 years) to 27.3% (60-69) in women and from 42.3% to 33.8% in men, and the kg/m² indices from the large studies also fall over the years, earlier in men, with a loss that speeds up after 60; and the "muscle mass" on many scales is a different measurement, closer to fat-free mass, so it cannot be compared with these figures. Use your own number as a baseline and watch its trend. And since protein is the raw material that decides whether the training pays off, knowing how much you are actually getting matters more than the exact percentage: photographing your meals with Renzy shows you the protein grams of the day without weighing anything.