The morning after leg day, you lower yourself onto the toilet seat with one hand braced on the towel rack. Under the wince there's a small glow of pride. That one counted. You can feel it.

A month later you run the same session, hit every set, and wake up feeling fine. Then the doubt starts. Did that even do anything? Should you have gone heavier? Has the program stopped working?

Both mornings you're reading muscle soreness after a workout as a report card, and both times the report is about something else. Soreness mostly tells you how unfamiliar a session was. Below: what causes the ache, why it fades even while you keep getting stronger, when it's a real warning sign, and the few numbers that actually show whether your training is working.

What causes muscle soreness after a workout

Start with the myth that refuses to die: lactic acid. Lactate from hard sets clears from your blood within an hour or so. The ache you're talking about shows up the next day and usually peaks 24 to 72 hours after training. The timing alone rules lactate out.

The research name is delayed onset muscle soreness, or DOMS, and the trigger is load your muscles weren't prepared for. The lowering phase does most of the harm. When a muscle produces force while it lengthens (the descent of a squat, the slow lower of a Romanian deadlift), it takes more strain than when it shortens. That strain causes small-scale disruption in muscle fibers and the connective tissue around them, a local inflammatory cleanup follows, and nearby nerve endings become sensitized. So stairs hurt, and so does sitting down.

Researchers still argue about exactly which tissue the pain comes from. The triggers are well agreed on, though: a new exercise, a bigger dose than usual, extra eccentric work, a longer range of motion. Every one of them describes how new the session was. None of them describes how productive it was.

Is muscle soreness a sign of muscle growth?

The gym-floor logic goes: soreness means damage, damage means rebuilding, rebuilding means growth. Each link is weaker than it looks.

Soreness is a poor gauge of damage. In a 2002 study by Nosaka, Newton and Sacco, people did different amounts of hard eccentric work for the elbow flexors, and how sore they felt didn't line up with creatine kinase, a protein that leaks from damaged muscle into the blood. You can hurt a lot with modest damage, or carry real damage that barely hurts.

Damage is a poor gauge of growth, too. A 2016 study by Damas and colleagues followed young men through a resistance training program and measured muscle protein synthesis along the way. Early on, while damage was high, protein synthesis was high as well, yet it didn't predict how much muscle anyone built. It only tracked growth later, after the damage had settled. The current reading is that damage is mostly a side effect of hard training. Growth comes from muscles working hard against load, session after session, with that load creeping up over months.

The cleanest test came in 2011. Flann and colleagues put two groups through the same eight weeks of eccentric cycling. One group had been eased in over three weeks beforehand; the other went in cold. The cold group reported much more soreness and had creatine kinase levels more than five times higher. Their gains were practically identical: both groups got about 25 percent stronger and added similar muscle volume.

Soreness and growth can show up together, especially in your first months of lifting. One just doesn't tell you much about the other.

Why am I not sore anymore? The repeated bout effect

Your body treats DOMS as a problem to solve, and it solves it fast. Exercise scientists call this the repeated bout effect. Do an unfamiliar, eccentric-heavy session and you get sore. Repeat it a week or two later and the soreness is much smaller. A single session can buy protection that lasts for weeks.

The protection probably comes in layers. Your nervous system gets better at the movement and spreads the load across more fibers, the muscle and its connective tissue remodel to tolerate stretch under load, and the inflammatory response calms down. A 2003 review by Malachy McHugh sorted the candidate explanations into neural, mechanical and cellular changes. The exact mix is still debated. The result is not: the same session, repeated, hurts less.

Follow that through. Getting used to a movement is the whole point of training, and getting used to a movement is what switches the soreness off. So after the first few weeks the signal tends to run backward, and the better a program is working, the less it hurts. If a routine you've run for two months still leaves you wrecked every time, suspect sleep, recovery or too much volume before you credit the program.

This is also how soreness quietly wrecks programs. A routine that stopped hurting looks like a routine that stopped working, so you switch. Week one of the new plan hurts (because it's new), which feels like proof. Repeat that a few times and you've traded several programs that were working for ones that only felt like they were. If that loop sounds familiar, the guide to your first 90 days of working out covers how to pick a plan you can stay on.

Can you work out while sore?

Usually, yes. Ordinary muscle soreness after a workout is not an injury. Strength may be down a little and the first sets will feel stiff and slow, but squatting on legs that complain on the stairs won't hurt you. Give yourself a few extra minutes of a proper warm-up before lifting, expect the early sets to feel rough, and judge once you're warm.

When soreness is a warning sign

• Swelling, weakness far beyond normal, or dark, cola-colored urine after an extreme session. Together these can signal rhabdomyolysis, a rare but serious breakdown of muscle tissue. See a doctor the same day.
• Sharp, pinpoint pain near a joint or tendon, or pain that started in the middle of a rep. DOMS is a dull, spread-out ache through the belly of a muscle. Anything stabbing is a different problem.
• Soreness that is still getting worse after three or four days instead of easing off.

How to tell your workout is working (what to do this week)

If soreness is out as evidence, here is what replaces it. None of it takes more than a few seconds per set.

1. 1.Log three numbers for each main lift: weight, reps, and how many more reps you could have done (reps in reserve, or RIR). A note on your phone works fine.
2. 2.Compare every two to three weeks, same lift, similar conditions. Progress looks like more reps at the same weight, more weight for the same reps, or the same set finishing with more reps left in the tank.
3. 3.Starting a new program or coming back after two weeks off? Do about half your usual sets in week one and stop each set with three or four reps in reserve. You'll still feel it, but you'll skip the hobbling. Go to full volume in week two.
4. 4.On sore days, let your first working set decide. Warm up, then do it. If it moves close to normal, carry on. If it's clearly weaker or your form shifts, cut a set or train another muscle group. For making that call on the fly, see how to adjust a session with RPE and reps in reserve.
5. 5.Stalled for three or four weeks? Change the progression before you change the program: add a set, drop the reps and add weight, or rest a little longer between sets. Keep the exercises.

Over months, add tape measurements and how your clothes fit. Every item on this list measures what you can do. Soreness only measures how unfamiliar the session was.

Progress is quiet, so make it louder

Soreness wins the argument because it's loud. It finds you at breakfast, on the stairs, in every chair. Improvement never taps you on the shoulder to mention that today's eighth rep moved faster than last month's sixth. A good coach fixes that by saying it out loud, at the moment it happens: two more reps than last time at this weight. Whether or not anyone says it for you, the rule holds. Judge your program by your log, not by your legs.

CoachMoach keeps that log for you. Say your rep count out loud when the set ends and it is logged, so the day a weight moves for two more reps than last month, it shows up in your workout history as a number instead of a guess based on how your legs feel. Start your log with three free workouts.