The evidence-led answer
NST verdict
No. Being sore the next day is not required for muscle growth, and it does not measure how good a session was. In studies, people who trained in a way that avoided soreness gained just as much muscle and strength as people who got sore. Soreness mostly tells you that you did something new or harder than your body is used to, a new exercise, a return after a break, more lowering phase work, and it fades as you adapt even though the training keeps working. Judge a workout by whether you are progressing over weeks, more weight or more reps, not by how sore you feel afterwards.
Do you have to be sore to build muscle?
No. In an eight week trial, one group used a gradual ramp up that avoided muscle damage and soreness, while another started with a hard, damaging session and got sore. The total work was matched. Both groups gained the same muscle size and the same strength, around 25%. A review of how muscle grows with resistance training reached the same conclusion: muscle damage is not the process that drives growth, and training that does not cause significant damage produces similar gains to training that does. Soreness is a side effect of unfamiliar work, not the mechanism.
Does soreness mean the muscle is growing?
Not directly. In the first sessions of a new programme, a muscle can measure bigger, but that early increase is mostly swelling from the muscle repairing itself, not new tissue. Real growth shows up later, and it is clearest once the damage and swelling have settled. So the soreness and puffiness of week one are signs your body is responding to something new, not a readout of muscle being built.
Why am I not sore anymore, is it still working?
Yes, it is still working. Your body adapts quickly to a given exercise, so the soreness it causes drops sharply after the first few times you do it. This is called the repeated bout effect. You keep making progress on that exercise long after it stops making you sore. Soreness fading is a sign of adaptation, not of a workout that has stopped counting.
Why am I so sore from one workout?
Big soreness usually means the session was unfamiliar or unusually demanding in a specific way: a new exercise or a big change in technique; coming back after time off; lots of loaded lengthening, deep stretch under load, heavy lowering phases; or a large jump in how much you did. None of those necessarily made it a better session for growth. A new movement done at a modest effort can leave you very sore, and a familiar exercise pushed genuinely hard might barely make you sore. Soreness also varies a lot from person to person and between muscles, so it is a poor way to compare sessions, or to compare yourself with someone else.
So how do I know if a workout was good?
Look at the trend over weeks, not the feeling the next morning. Are the weights, reps or quality sets going up over time? Were the working sets genuinely hard, close to the point where form would break down? Are you training consistently? If those are yes, the training is working, sore or not. If progression has stalled for weeks, that is the signal to change something, not a lack of soreness.
When soreness is not a good sign
Soreness that is severe, mostly on one side, or comes with dark urine or marked swelling is not something to chase. It can point to a problem that needs medical attention. The muscle soreness (DOMS) article covers this and when to see a doctor.
What this means in practice
Stop using next day soreness as a scorecard. You do not need to be sore to build muscle, and being sore does not confirm you built any. Expect a lot of soreness when you start something new and much less once you have adapted; both are normal. Judge your training by whether it is progressing over weeks and whether the hard sets are actually hard. If you want less soreness so you can train more comfortably, ramping up new exercises gradually is the simplest tool.
Practical meaning
Do not use next day soreness to judge a workout. You do not need to be sore to build muscle, and soreness does not confirm growth. Expect more soreness when you start something new and less once you have adapted, both are normal. Judge training by progression over weeks (more weight, reps or quality sets) and by whether the hard sets were genuinely hard. To train more comfortably, introduce new exercises gradually. Severe, one sided soreness, or soreness with dark urine or marked swelling, needs medical attention, see the DOMS article.
Limitations and uncertainty
The clearest 'no soreness, same gains' trial is small and used eccentric cycling rather than typical gym training; the broader conclusion rests on a review. Several of these papers come from the same research group. Between person and between muscle variability in soreness for the same session is not well quantified. This is about muscle size and strength, not connective tissue adaptation, skill or sport performance.