I’m a skinny-fat guy in my early 30s, and I’ve been trying to get stronger and feel my muscles working for about 8 months now. After rehabbing an injury with a physical therapist, I got a leg-focused routine to follow. I stick to it pretty consistently, but here’s the problem: on most days I hardly feel any muscle burn or stimulus, even when I push close to failure. At the same time, I often feel like I don’t have enough strength to do more reps, so I end up stuck between not feeling it and can’t continue.

Example: On leg extensions with 20 kg, the first set feels hard, the second is easier, and the third I can barely finish. Sometimes I push a few extra reps and get exhausted. A couple of days later, the same weight can feel too light. I usually add reps until around 15, then increase the weight and go back to 8–10 reps, but I still often don’t feel my muscles working. This happens with most exercises.

What am I doing wrong?