Your scale will tell you that you did well this week. Your mirror will mostly agree. Neither will tell you that the squat you did for ten reps in March is now a struggle for seven.

That's the quiet problem with losing weight on a GLP-1. Almost every signal you check is built to reward shrinking. Smaller number, smaller jeans, kind words from coworkers. Muscle loss produces no alarm. It just makes the stairs a little harder, and you put that down to tiredness.

There is one signal that does catch it, and it costs a notebook.

Your body keeps what it's asked to use

Muscle is expensive tissue. When you eat much less, as most people on semaglutide or tirzepatide do, the body is under pressure to shed whatever it isn't clearly using. Trials of these drugs have found that a meaningful share of the weight lost can be lean mass, not only fat. The exact share varies by study, drug and person, and it is one reason clinicians have started talking about muscle preservation at all.

The lever you control is mechanical tension. Muscle responds to being asked to do something it finds difficult. When the demand stays the same, it has little reason to adapt. When the demand rises, even slightly, it gets a reason to stay.

This principle is called progressive overload. It is not a gym-bro idea. It is the foundation of resistance training science: the stimulus has to be challenging enough, and it has to keep being challenging as you adapt.

On a GLP-1, the goal shifts a little. You are often not trying to build much. You are trying to keep what you have while your body is in a deficit. But the signal you need to send is the same one. This is hard. Keep this.

Why feeling is a bad judge of progress

Here is the trap. Without a record, "I trained hard" is a feeling, and feelings drift.

If you're under-eating, your sessions will feel harder for reasons that have nothing to do with effort. A set of ten that felt like a solid seven out of ten in effort last month might feel like a nine now, because you're low on fuel. You might conclude you're training hard. In reality you may be training with lighter weights than before, working fewer reps, and calling it intense.

The opposite happens too. Some weeks you feel strong and push hard, but without a record you can't tell whether that was a real step forward or a one-off.

A log removes the guessing. It doesn't care how you felt. It says: last Tuesday, goblet squat, 20 lb, 9 reps. Today, goblet squat, 20 lb, 9 reps. Same. Time to change something.

The method: double progression

You don't need a periodised programme. The simplest approach that works for most beginners and returning lifters is called double progression.

Pick a rep range for each exercise, say 8 to 12. Choose a weight that makes you work near the top end of your effort, leaving perhaps one to three reps in reserve. Then:

  1. Each session, try to do more reps with the same weight.
  2. When you can hit the top of the range (12) on every set, increase the weight slightly.
  3. After the increase you'll drop back toward the bottom of the range (8 or 9). Build back up.

That's it. Two ways to progress, reps first, then load. A log is the only thing that makes it possible, because you can't remember whether you did 9 or 10 reps on the third set nine days ago.

The research supports the flexibility here. Reviews of resistance training, including meta-analyses by Brad Schoenfeld and colleagues, have found that muscle growth is similar across a wide range of loads, from fairly light to heavy, provided sets are taken close to failure. So you don't need heavy weights to send the signal. You need honest effort and a record that shows the effort is climbing.

What counts as progress when you're in a deficit

This is where people get discouraged, so it's worth being clear.

When you're eating less than you burn, getting stronger every week isn't a realistic bar, especially if you've trained for years. For many people the honest goal is holding steady. A flat line on a log, week after week, while the scale falls, is a good result. It suggests your muscle is getting the message.

For true beginners it's different. The early strength gains in a new lifter come largely from the nervous system learning the movement, not from new muscle tissue. This is well established in exercise science. It means beginners can often improve on a log even while eating little. That improvement is real and motivating, but it's worth knowing that it says more about coordination than about muscle mass.

So read your log like this:

  • Rising: great, keep going.
  • Flat: acceptable in a deficit. This is maintenance, and maintenance is the job.
  • Falling across two or three sessions in a row, on several lifts: a flag. Look at sleep, protein, total food, hydration, and whether you recently went up a dose. Talk to your clinician if it persists.
  • Falling on one lift, one day: noise. Ignore it.

Notice what the log does here. It turns a vague dread ("am I losing muscle?") into a pattern you can actually read.

What to write down

Keep it almost embarrassingly simple. If logging takes more than two minutes, you'll quit by week three.

For each exercise: the weight, the reps per set, and a rough note on how many reps you had left. Then one line at the top of the page: sleep, appetite, and where you are in your injection week. Many people on a GLP-1 feel noticeably worse for a day or two after a shot, and seeing that pattern on paper stops you from reading a bad Thursday as a failure.

That last line is the hidden value. Over a few weeks the log becomes a map of how the medication interacts with your body, which is more useful than any single workout.

Your next moves

  • Pick three to five movements today, such as a squat or sit-to-stand, a push (wall or incline push-up), a row, and a hip hinge. Don't add more. You can build from there.
  • Do one baseline session this week. Choose a weight where you finish each set with one to three reps left. Write down weight and reps for every set.
  • Set a rep range for each lift (8 to 12 works for most) and write it at the top of the page. Your only rule: beat last time by one rep, or add a small amount of weight once you top the range.
  • Add one line of context to every entry: sleep, appetite, injection day. Review the pattern after three weeks.
  • Define your flag in advance. Decide now that if your numbers drop on most lifts for two or three sessions in a row, you will check your protein and food intake that week and raise it with your prescriber if it continues. Deciding ahead removes the temptation to explain it away.

Smaller on purpose, stronger on paper

A GLP-1 does one thing very well: it makes eating less feel easy. It doesn't tell you what you're losing along the way, and no one around you can see it either. A log is how you check. Lean was built around that same idea, pairing protein targets with a record of your strength so that you can see whether the weight coming off is the weight you meant to lose. If you'd like a place to keep those numbers, you can look at Lean. A pencil and notebook will also do the job, as long as you write the numbers down.