Skeletal muscle mass: what is normal and how much can you add?

Skeletal muscle mass is the muscle tissue attached to your bones that moves you. In men it runs to roughly 40 percent of body weight, in women about 33 percent. How much more can be added is more limited than most gym machines suggest.

There is a handy number for that question: FFMI.

I use it mostly as a reality check. Not to hold anyone back, but because an impossible expectation always ends in disappointment or in a bottle.

What is skeletal muscle mass exactly?

It is one of three muscle types in your body, alongside cardiac and smooth muscle. Only skeletal muscle is under conscious control and trainable. On an InBody or DEXA result it appears as its own line, separate from your total fat-free mass.

Those two get confused constantly.

Fat-free mass is everything that is not fat: muscle, bone, organs, fluid and blood. Skeletal muscle mass is a portion of that, roughly half. Add two kilos of fat-free mass and nowhere near all of it is muscle.

A solid carbohydrate load alone can add a kilo of fat-free mass as bound water. Which is exactly why a weekly measurement says little.

How much skeletal muscle mass is normal?

In an average man skeletal muscle mass sits around 30 to 35 kilos; in an average woman, around 20 to 25 kilos. As a share of body weight that works out near 40 percent in men and 33 percent in women. Trained people sit above it, and that is normal.

Those absolute kilos say little without your height.

Someone of 1.95 metres carrying 38 kilos of muscle is not more muscular than someone of 1.70 metres carrying 32. That is what FFMI is for: your fat-free mass divided by your height in metres squared. The same idea as BMI, with the fat taken out.

That upper bound comes from work by Kouri and colleagues in 1995. They calculated FFMI in users and non-users of anabolic steroids and found the non-users almost all stayed below about 25 (PMID 7496846).

It is a boundary from one sample, not a law of nature. But it has barely moved since.

For women the bands sit roughly four to five points lower, with a comparable ceiling near 20 to 21.

How do you calculate your own muscle mass?

You need two figures: your weight and your body fat percentage. Fat-free mass is your weight minus your fat mass. Divide that by your height in metres squared and you have your FFMI. Run it with a body fat figure you know the origin of.

An example makes it concrete.

Take someone of 82 kilos at 1.80 metres with 15 percent body fat. Fat mass is 12.3 kilos, fat-free mass 69.7 kilos. Divided by 1.80 squared, so 3.24, that gives an FFMI of 21.5. Far advanced, with room left to the ceiling.

The weak link is that body fat figure. Measure 15 percent on a scale that is five percent off and your FFMI is more than a point out. Which method misses by how much is in body fat percentage: what is healthy and how to measure it.

Why is the muscle gain on your gym machine often wrong?

Because bioimpedance infers muscle from water. More glycogen and more fluid in your muscles reads as more muscle mass, even with no new tissue. After a week of eating and training well the reading can sit one to two kilos higher.

It feels great and it is largely water.

There is also a measurement problem specific to trained people. Brewer and colleagues found multifrequency BIA consistently underestimated fat-free mass in the arms and legs of Division I athletes (PMID 31469766). So the segmental chart gyms point at is the least dependable part of the sheet.

How to read the rest of that sheet is in the InBody measurement.

A realistic rate guards against false hope. A trained lifter doing everything right builds a few hundred grams of muscle a month sooner than a kilo a week. The IOC working group around Ackland also noted that every field method inherits the assumptions of its reference method (PMID 22303996).

What if your skeletal muscle mass drops?

Look first at your energy intake and then at your training, in that order. Muscle loss while training stays the same often comes from persistently low energy availability, and with age sarcopenia plays its part. Continued loss while you train normally is a reason to look past the scale.

Sustained low energy availability is associated with disturbances in bone formation, hormones and performance, in men as well as women (PMID 37752011).

The Gezondheidsraad advises muscle and bone strengthening activity at least twice a week in its physical activity guidelines, and that advice gains weight with age rather than losing it. What changes after forty is in sarcopenia, and how much protein belongs with it in protein per day.

My advice: measure muscle mass in kilos, not percentages, and judge the line across three months. If it keeps falling while your training continues, the hormonal and metabolic side is a logical place to look, with values such as testosterone and your thyroid markers in the 360 health panel.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.

References

• Kouri EM, Pope HG Jr, Katz DL, Oliva P. Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clinical Journal of Sport Medicine, 1995;5(4):223-8. PMID 7496846.
• Brewer GJ, Blue MNM, Hirsch KR, Peterjohn AM, Smith-Ryan AE. Appendicular body composition analysis: validity of bioelectrical impedance analysis compared with dual-energy X-ray absorptiometry in Division I college athletes. Journal of Strength and Conditioning Research, 2019;33(11):2920-2925. PMID 31469766.
• Ackland TR, Lohman TG, Sundgot-Borgen J, et al. Current status of body composition assessment in sport. Sports Medicine, 2012;42(3):227-49. PMID 22303996.
• Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine, 2023;57(17):1073-1097. PMID 37752011.
• Gezondheidsraad. Physical activity guidelines.
• RIVM. Leefstijlmonitor, figures on physical activity and overweight.