APEX PWR  |  Female Fitness

RSMI: The Muscle Number on Your Report, and When It Should Send You to a Bone Scan

There is a number on your DEXA body composition report that most people slide past on their way to body fat percentage. It is labeled RSMI, and it is arguably the most forward-looking number on the page.

It is also the number we are going to be careful with, because the version of this article that would get the most clicks is one we do not believe.

What RSMI Measures

RSMI is relative skeletal muscle index. The arithmetic is straightforward.

A DEXA scan measures lean mass region by region. RSMI takes the lean mass in your four limbs, your arms and your legs, which is where most of your usable skeletal muscle lives, and divides it by your height in metres squared.

Scaling by height is the whole point. Raw pounds of limb muscle will always be higher in a taller person, which tells you nothing about whether either of them is well muscled for their frame. Dividing by height squared puts a 5'2" woman and a 6'0" man on the same axis.

The result gets compared against a reference line that research groups use to flag low muscle mass. The commonly cited cut points sit near 5.5 kg/m² for women and around 7.0 for men, though different consensus groups publish slightly different numbers, which is itself worth knowing.

The Honest Version of the Data

Here is where we part company with most of the fitness internet.

You have probably read that nearly everyone over 40 is quietly losing muscle and should be alarmed. It is an excellent premise for selling things. When we checked it against our own scans, it did not hold up.

Roughly 97 to 98 out of every 100 people we scan are above that line. Our clients, as a group, are well muscled.

Two caveats, both pointing the same direction. People who book a body composition scan are not a random slice of Portland, and height is only on file for a subset of our records, skewed toward recent years. So this describes the people who walk through our door, not the metro area.

We are telling you anyway, because the alternative is running a fear campaign with our own data sitting in a drawer contradicting it.

Which Is Exactly Why a Low Reading Is Worth Something

Follow the logic through. A flag that fires for almost everybody tells you nothing. A flag that fires for one or two people in a hundred is a real signal.

So if your RSMI comes back under that line, it is not a reason to panic and it is not a diagnosis. Sarcopenia is a clinical condition that requires a clinician, and current definitions weigh measured strength and physical function at least as heavily as mass. You can be below a mass cut point and perform like someone well above it.

What a low reading is, is a genuinely uncommon finding that deserves a real look rather than a shrug. Three things are worth doing with it.

1. Check it against your strength and your function. Grip strength, how you get out of a chair, what you can carry up the stairs in one trip. Mass without function is only half the picture, and function is the half that predicts how the next twenty years go.
2. Bring it to your physician, especially if it arrives alongside unintended weight loss, a recent illness or surgery, a long stretch of inactivity, or a medication change. Low limb muscle mass can be downstream of something medical that is worth finding.
3. Find out what your bones are doing. Which is the rest of this article.

Why Low Muscle Points at Bone

Muscle and bone are not separate projects. Bone adapts to the forces put through it, and the largest forces your skeleton experiences in ordinary life come from your own muscles pulling on it. Less muscle pulling means less of the signal that tells bone to stay dense.

The two also tend to decline together, often enough that the medical literature treats the combination as its own entity worth naming and studying. And the risk compounds rather than adding: thinner bone makes a fracture more likely when you fall, and thinner muscle makes the fall more likely in the first place.

For women this lands with extra weight, because bone loss accelerates around the menopause transition, it produces no symptoms whatsoever, and most women find out about it after something breaks.

Your Body Composition Scan Is Not a Bone Density Scan

This is the distinction we most want you to leave with, and it is the one that is least well understood.

A whole-body composition DEXA does report a total bone number. It is tempting to treat that as a bone density result. It is not a substitute for one.

Research in nearly 700 adults compared total body bone density against dedicated site-specific scans. At the group level the two tracked together. At the level of individual people they disagreed about who did and did not have osteoporosis, and the whole-body measure specifically under-detected it at the femoral neck, which is the site of the hip fracture that changes someone's life.

A site-specific bone density scan measures the lumbar spine and the hip directly, which is what clinical guidelines and physicians use to assess and diagnose bone health.

What physician review means here

Our site-specific bone density results are reviewed by the physician on our wellness team, with no additional charge and no separate appointment to book. That is unusual for a cash-pay facility and it is the reason we are comfortable putting this test in front of people rather than only talking about body composition.

We also work with community healthcare providers across the Portland metro to help patients choose between the two scans, because they answer different questions and the wrong one is a waste of your money. If you are not sure which you need, contact the team before you book anything and we will tell you. Sometimes the answer is neither yet.

What Actually Builds Both

The useful thing about muscle and bone travelling together is that one intervention addresses both, and it is the least surprising one in the building.

Load the tissue

Resistance training is the only input that directly defends muscle, and it loads bone at the same time by pulling on it. Research in postmenopausal women with low bone mass has found that high-intensity resistance and impact training, delivered under supervision, can improve bone density at the spine and hip, which is a meaningfully different result from the "walk more and take calcium" advice most women have been given.

Note the qualifiers there, because they matter. High intensity. Supervised. In women with existing low bone mass. That is not an argument for loading up a barbell on your own after reading a blog post. It is an argument for doing the right work with someone watching.

What it looks like in practice:

• Two to three full-body sessions a week. Most of the available benefit lives here. A fourth day is optional.
• Loads that are genuinely hard in the last few repetitions. Three sets of fifteen with a weight you could lift thirty times is not loading anything.
• Compound movements. Squat, hinge, push, pull, carry. These load the spine and hip, which is the point, and single-joint machines do not.
• Progression you write down. The same weights for six months is maintenance, and in this window maintenance is drift.
• Impact, where it is appropriate for you. Hops, skips and landings add a bone signal that lifting alone does not. Appropriateness depends on your joints, your history and your current bone status, which is a conversation rather than a rule.

Our strength programming for women in Tigard and Beaverton is run by Coach Nicole Hinkle, a Certified Menopause Coaching Specialist through Girls Gone Strong, who is navigating this stage of life herself. If you would rather start one to one, the personal training route exists for exactly that.

Feed the tissue

Loading muscle without the raw material to rebuild it is half a plan. Protein is the most common gap we find, and the distribution across the day matters nearly as much as the daily total. Three or four meals in the 30 to 40 gram range beats a light breakfast and a large dinner, even when the totals match.

Our nutrition team is led by Jennie Carolan, MS, and coaching is delivered remotely, so distance is not the obstacle it is for training.

Bone also wants adequate calcium and vitamin D, and those are worth a conversation with your physician rather than a number from us, because the right amount depends on what you already eat and what your labs say.

What To Do Next

1. Find the RSMI on your report. If it is blank, give us your height and it will be there next time.
2. If it clears the line, which it probably does, treat that as a starting position worth defending rather than a result to file away. The job after 50 is protecting it.
3. If it is below the line, check it against your actual strength and function, bring it to your physician, and get your bone numbers rather than guessing.
4. If bone is the question, book the site-specific scan. The body composition scan will not answer it reliably.
5. Either way, lift two to three times a week and eat enough protein. That is the answer to both tissues, and it has worked at every age we have ever tested it on.

Frequently Asked Questions