Muscle mass: what it means, how it changes with age and how to track it

What does muscle mass actually mean?

People use “muscle mass” for several different things. The European consensus on sarcopenia, written by a group that includes researchers from Southampton and Newcastle, says muscle quantity can be reported as total skeletal muscle mass, as appendicular skeletal muscle mass (the muscle in the arms and legs), or as the cross-sectional area of one muscle group.¹²

That matters when you compare numbers. A figure for the whole body and a figure for arms and legs are not the same thing, and neither is the same as lean mass. The consensus treats skeletal muscle and lean tissue as separate quantities, and a Spanish study of 262 healthy young adults likewise reported skeletal muscle mass and lean body mass as separate results.²¹¹

The same consensus also puts strength ahead of size. It names low muscle strength as the key characteristic of sarcopenia, a muscle disease, and uses low muscle quantity or quality to confirm the diagnosis.¹ So a large muscle number on a screen tells you less than it seems to, and how strong you are in daily life matters at least as much.

This guide is about the number you might see on a scale, a body scan or a report. It does not diagnose anything. For the wider picture of fat, muscle and everything else, see what body composition is.

How much muscle mass is normal?

A tidy table of “normal muscle mass by age and sex” is what many people search for. For British adults, one could not be built from sources read for this guide. No published reference range for lean or muscle mass by age band and sex in UK adults was found in a readable form. A German study of 1,958 adults aged 18 to 97 did find that reference values depend on age and BMI, and proposed continuous ranges rather than fixed bands, but it is not a UK dataset.¹⁴

What does exist are the screening thresholds in the European consensus. They are cut-offs used to pick out people who may have low muscle, not a description of what is normal at each age. The table below sets them out beside the UK figures that were readable.

Notice what the table does not give you: a number to aim for. The European thresholds sit where a person may be at risk, and above them you are simply not flagged. They tell you nothing about whether your own muscle is high, low or typical for your age.

Why muscle mass falls with age, and what slows it

The European consensus describes muscle mass and strength rising into young adulthood, with maximum levels reached by about 40, higher in men than in women. They hold through middle age and then decline.²

From around 50, the UK Chief Medical Officers say, there is a natural decline in muscle mass and bone density. They add that strengthening activities delay it, and that losing muscle strength with age is the primary limiting factor for functional independence.³

The consensus quotes reported losses after 50 of 1 to 2% a year in leg lean mass and 1.5 to 5% a year in strength. It takes them from a 2013 literature review (Keller and Engelhardt), which itself gathers figures from earlier studies that differ widely, so treat them as a rough description of a trend, not a prediction for any one person.²¹⁵

The NHS includes muscles getting weaker as you get older among the causes of falls.⁷ For a closer look at that link, see preventing falls in older adults.

Pulling this together: the decline is gradual, it starts from a peak that differs between people, and the Chief Medical Officers point to strengthening activity as what slows it.²³

How is muscle mass measured?

No method reads muscle directly in everyday life, and the main ones answer slightly different questions.

A Spanish study of 262 healthy young adults compared DXA, body measurements and a bioimpedance device. It found a lack of agreement between methods and formulas for skeletal muscle and lean mass, using DXA as the reference.¹¹ Bioimpedance, in short, is a model of muscle, not a measurement of it. For a side-by-side look at the two methods, see DEXA scan versus smart scale.

What a smart scale can and cannot tell you about muscle

The consensus is direct: bioimpedance equipment does not measure muscle mass directly. It infers an estimate from how electricity passes through the body, using an equation calibrated on DXA lean mass in a specific population. Estimates therefore differ with the brand of device and the reference population.²

The same document says bioimpedance can be influenced by the patient’s hydration, and that DXA can be too.² Yet the Spanish study found hydration status had no significant effect on the differences between methods.¹¹ The sources disagree, so this guide gives no figure for how much a drink of water or a heavy meal moves the muscle number on a scale.

There is one reassuring result. In 31 geriatric patients, averaging 82, a dual-frequency bioimpedance device and DXA could not be used interchangeably, but the authors found the device’s high reproducibility allowed monitoring under standardised conditions.¹³ That supports the idea of a repeated, same-conditions routine rather than a single reading.

Smart scales are tested more often on body fat than on muscle. For the evidence on scale accuracy, read how accurate smart scales are, and for the fat side of the picture, body fat percentage.

People with a pacemaker or another implanted electronic device should not use a bioimpedance scale. If that applies to you, rely on strength checks and ask your GP or care team how to follow your muscle.

A worked example in stones and pounds

These figures are an illustration built from the thresholds above. The heights and weights are invented to show the arithmetic, and are not typical values.

Why weight alone says little about muscle

Take an adult weighing 12 st 8 lb. That is 12 × 14 + 8 = 176 lb, or 79.8 kg. A gap of 1 lb (0.45 kg) is 100 ÷ 176 = 0.57% of that weight. A bioimpedance device that overestimates fat-free mass by 3.15 kg (6.9 lb), the gap seen against DXA in the study of young women, is out by about 3.9% of the 176 lb in this example.¹² To see how a scale’s other estimates are built, read how BMR is calculated and what metabolic age means.

That is a different size of error from a 1 lb daily wobble, and a reason to watch change over time on one scale rather than the absolute figure.

What the height-adjusted threshold means in practice

The consensus threshold divides arm and leg muscle by height squared. For a man 1.75 m tall (5 ft 9 in), 7.0 × 1.75² = 21.4 kg, which is 47.3 lb. For a woman 1.62 m tall (5 ft 4 in), 5.5 × 1.62² = 14.4 kg, which is 31.8 lb.² Two people of the same weight can sit on different sides of these lines, because the lines depend on height and sex and not on weight.

No source read says how a particular scale’s “muscle” figure maps on to arm and leg muscle, so do not compare a scale reading directly with these numbers.

How to keep or build muscle: what UK guidelines recommend

The UK guidance is simple. Adults aged 19 to 64 should do strengthening activities that work all the major muscle groups (legs, hips, back, abdomen, chest, shoulders and arms) on at least 2 days a week, says the NHS.⁴ The UK Chief Medical Officers word it the same way: at least 2 days a week, and any strengthening is better than none.³

The Chief Medical Officers also say strengthening should be considered as important as the 150 minutes of moderate or vigorous activity a week.³ They give bodyweight, free weights, resistance machines and elastic bands as ways to do it, and list everyday tasks such as climbing stairs.³

For benefit, the NHS says to work to the point where you need a short rest before repeating the movement.⁴ At home, the NHS suggests sit-to-stands, mini-squats, wall press-ups and biceps curls with filled water bottles, at least twice a week.⁵

For adults aged 65 and over, the NHS and the Chief Medical Officers recommend strength, balance and flexibility activities on at least 2 days a week.³⁶ The NHS suggests talking to a GP first if you have not exercised for a while.⁶ The Chief Medical Officers add that strength and balance programmes with an evidence base reduce the rate and risk of falls.³

Practical next steps are in our guides on strength training guidelines, strength training at home, how to start strength training and progressive overload.

A four-week routine for tracking your own trend

Because the number is an estimate, the trend is more useful than any one value. This routine is a suggestion built from the points above, not a clinical test.

• Use the same scale in the same place, at the same time of day and in the same state, for example first thing before eating. Standardised conditions are what made repeated readings usable in the geriatric study.¹³
• Take three readings a week and use the weekly average, so one odd morning does not mislead you.
• Follow the change between weekly averages, not the absolute value, because different devices and populations give different estimates.²
• Add a strength check you can repeat, such as how long five chair stands take. The consensus treats more than 15 seconds for five as a flag for low strength.²
• Judge the month by what you can do as well as what the scale says.

Frequently asked questions