Expand your sets for better results.
If you're over 40, it's no longer enough to make 3 sets of 8 and go home. It may have worked when you were younger and testosterone tiger blood flowed through your veins, but not so much when you splashed a 50/50 mixture of tiger blood and plum juice through your plaques-pierced vessels.  That's why almost every workout should include an extended set, drop set, or finisher, and if you do not end up with an ugly dragon-burned face, you have not worked hard enough.
Make stretcher sets with leg press or Smith squats. Rep out. Pull a plate. Rep out. Pull a plate. Rep out. Pull a plate. Rep out. Collapse into a fetal position.
Try Paul Carter's 1
Or choose a weight that will allow you to do about 10 reps. Look at the wall clock and note the time. Give yourself 5 minutes to do 50 repetitions of the same weight, and take a small chunk of rest between sets to fail. If you actually hit 50, the weight is too low.
Mechanical advantage Barbell curls like these also work well:
- A1. Reverse barbell curls for 6 to 8 repetitions.
- A2. Pull the curls as often as possible.
- A3. Standing barbell curls for as many repetitions as possible.
You have the idea. It sounds unintuitive and it smells of heresy in weightlifting, but you have to train harder than in your youth if you want to stay in the game.
The new rules for lifting over 40
Stop worrying about being over 40