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

Before the number: your scale and the science are not measuring the same thing

This is where almost all the confusion starts. When a study talks about muscle mass it usually means SKELETAL MUSCLE: the muscle that moves your bones. When your bioimpedance scale gives you a percentage, it is almost always talking about lean mass or fat-free mass, which is something else: it includes muscle, but also organs, bone, skin and water. That is why the number on your scale comes out considerably higher than the one in the papers, and why comparing the two makes no sense. Before you go looking for whether your percentage is normal, check what the device that gave it to you is actually measuring.

The figures that really are measured

The reference work is still the one by Janssen and colleagues (J Appl Physiol, 2000): they measured skeletal muscle mass by whole-body magnetic resonance imaging in 468 men and women aged 18 to 88. It matters that it was MRI and not a scale: that is the method that genuinely sees the muscle rather than estimating it from the body's electrical resistance. These are the mean values they published.

The by-decade chart you are looking for does not exist (and that is good news)

Search for "muscle mass percentage by age" and you will find dozens of charts with one row per decade and numbers to the decimal point. Almost none of them says where those numbers came from, and the reason is simple: the reference study does not publish that table. What it did measure, and what can therefore be stated, is the DIRECTION of the change with age: the relative percentage of muscle starts to fall as early as the third decade of life, while the loss of absolute kilos of muscle is not noticeable until the end of the fifth decade — and when it happens it is mostly at the expense of the lower body. That is the useful information: by 30 you are already losing proportion even if the scale has not moved, and the legs go first.

How clinicians decide that muscle mass is low

Here there is a published criterion, and it is not a loose percentage: it is an index compared against the young population. Janssen and colleagues (J Am Geriatr Soc, 2002) analysed 14,818 adults from the NHANES III survey and defined the skeletal muscle mass index as muscle mass divided by body mass, multiplied by 100. From there, you are considered normal if you are above one standard deviation below the mean of young adults aged 18 to 39; between one and two standard deviations below is class I sarcopenia, and below two is class II.

• In people over 60, class I was present in 59% of women and 45% of men
• Class II, the more marked one, in 10% of women and 7% of men
• 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
• Part of that association held after adjusting for age, race, body mass index, health behaviours and other conditions

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

Bioimpedance scales and gym body-composition machines do not measure muscle: they pass an imperceptible current through the body and estimate composition from the resistance they meet. They work, but with a margin worth knowing. One study compared a multifrequency bioimpedance device against DXA in 160 collegiate athletes and found that bioimpedance significantly UNDERESTIMATED fat-free mass: by 3.8 kg on average in the legs and 0.7 kg in the arms. Its authors conclude that this device may not be accurate for measuring a lean, athletic population compared with DXA.

• DXA: the practical standard in research and clinical work. Accurate, but you have to go somewhere for it
• MRI: the most exact method, and the one used for the figures above. Expensive and reserved for research
• Bioimpedance (your scale or the gym's): convenient and cheap; useful for the TREND, not for the absolute value
• Skinfold calipers: they measure fat, not muscle, and depend heavily on the hand holding them

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, protein supplementation combined with resistance training increased fat-free mass and strength; without the training, protein alone does not do the job.

1. 1

   Train strength 2-3 times a week

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

2. 2

   Raise protein to 1.6-2.2 g per kilo

   That is the range the evidence supports for gaining mass. Work out yours with the protein calculator and find the sources in protein-rich foods.

3. 3

   Eat enough, not just well

   Building muscle in an aggressive deficit is rowing against the current. If your goal is to gain, use the muscle-gain calculator and adjust calories before reaching for supplements.

4. 4

   Sleep, and give it time

   Muscle mass is measured in months, not weeks: muscle repairs while you sleep (sleep and your weight) and real change takes a while to show on any device.

5. 5

   Measure the trend, not the day

   Weigh and measure under the same conditions once a week and look at the monthly average. A single bioimpedance reading means almost nothing.

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 is slow and non-linear: a lot at the start, less and less afterwards, and always slower than supplement marketing promises. 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, and the eating plan in the weekly muscle-gain meal plan.

The practical summary: the figure you are looking for exists but is a population average (38.4% of body mass in men and 30.6% in women, measured by MRI), the by-decade chart circulating online has no source, and the number on your scale measures something else and with a margin. 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 day's protein grams without weighing anything.