Stop fearing the number.Start learning the words.

Not quite. On a DXA scan the body is split three ways: bone, fat, and everything else. The "everything else" is called lean mass or lean soft tissue. It includes muscle, but also water, the glycogen stored in muscle and liver, your organs, connective tissue and even fat sitting inside muscle fibres. The scan cannot tell those apart. So "lean mass went down" means that whole bucket got lighter, not that it was all muscle.

Barbell Medicine's review walks through the bucket in detail.

From the 68-week STEP 1 trial of semaglutide, published in the New England Journal of Medicine in 2021. In a sub-group who had DXA scans, roughly 39% of the total weight lost was classified as lean mass. Two things are usually left out when that number travels: it is lean mass, not muscle (see above), and people lost about 15% of their body weight, so most of what went was fat.

The actual contractile tissue: the stuff that shortens and moves you. It is a slice of lean mass, not the whole thing. Consumer scales and most scans estimate it rather than measure it. Barbell Medicine's worked example puts the contractile muscle in a 3 kg DXA "lean loss" at somewhere between 0.2 and 0.5 kg, with water, glycogen and organ tissue making up most of the rest. Treat any single muscle-mass number as a rough estimate.

Dual-energy X-ray absorptiometry. Two low-dose X-ray beams pass through you and the machine works out bone, fat and lean soft tissue by how much each absorbs. It is the best common tool for body composition, and still only three buckets. If you get one, get the next one on the same machine, at the same time of day, similarly hydrated; water alone can move the lean number. Whether you need one is a question for your clinician, not for us.

Sarcopenia is the age-related loss of muscle mass and strength, usually diagnosed from grip strength, walking speed or a chair-stand test rather than a scan. Fast weight loss at any age can nudge you in that direction, which is why clinicians mention it. It is a diagnosis they make, not one you make from a word on a page. This is general information, not medical advice. Talk to your clinician.

Protein targets are written as grams per kilogram of body weight per day. The old 0.8 g/kg RDA was set to prevent deficiency in sedentary adults. For people losing weight and lifting, a 2018 meta-analysis in the British Journal of Sports Medicine found the benefit of extra protein on training results levelled off around 1.6 g/kg. For an 80 kg person that is roughly 130 g a day, which is a lot on a reduced appetite, so spreading it across meals matters. Your own target is between you and your clinician or dietitian; Sturdy lets you set it yourself or turn the goal off.

It is any work where your muscles push or pull against something: a barbell, a band, a chair you stand up from, your own body on the floor. The reason it keeps coming up is that it is the thing with evidence behind it. A 2018 meta-analysis in Nutrients found resistance training prevented about 93% of the lean-mass loss from dieting alone in older adults with obesity, and a 2017 trial found the diet-plus-exercise group lost close to no lean mass while the diet-only group lost around 16% of their weight as lean mass.

Doing a little more over time: another rep, a heavier band, a slower lowering, one more set. Muscle keeps what it is asked to do. The trap on a GLP-1 is that appetite and energy drop, so "more" has to be small and the measure has to be your own last session, not a chart. In Sturdy the difficulty only changes when you choose; it never ramps on its own.

How many more repetitions you could have done before form broke down. "Two in reserve" means stop while two more were still possible. It is a way to train hard enough to keep muscle without grinding to failure every set, which on reduced calories is the quickest way to feel wrecked. Most of the sensible guidance for people on weight-loss medication lands at 2 to 4 reps in reserve.

Sit in a chair, arms crossed, stand fully and sit back down as many times as you can in 30 seconds. It is part of the CDC's STEADI fall-risk toolkit and it is a decent proxy for leg strength, which is the strength that gets you off the floor and up the stairs. Sturdy includes it as an optional check-in; you record the count and conditions and it shows your history without scores, age norms or medical conclusions, on purpose.

Losing fat while keeping or adding muscle, so the scale moves less than the mirror does. It is the realistic goal on a GLP-1 with training and enough protein: you will not add much muscle in a deficit, but you can hold on to most of what you have. Be wary of before-and-after photos as evidence; lighting and water do a lot of work in those.

Because on a reduced appetite the problem is rarely one heroic day; it is the string of days where lunch was a yoghurt. Consistency means how many days you got near your own target, or simply whether each meal had some protein in it. Sturdy lets you log grams from a label, use hand-portion estimates that are labelled as estimates, or switch to meal checkmarks if counting is not for you.