Define Your Target
What's the goal of the workout you're about to do?
- Increase muscle?
- Just maintain muscle?
- Improve strength or power?
- Burn calories?
Your answer will dictate your decision-making process during the workout.
If your primary objective is muscle building, you'll want to focus on performing fatiguing, close-to-failure sets with exercises that effectively tax the target muscles. If you're training hamstrings, you might do several sets of stiff-leg deadlifts or back extensions for 8-12 reps, taking each set relatively close to momentary muscular failure.
Further, you'll focus more on the mind-muscle connection (the "feel") than how much weight you're using. While progressive overload still applies to hypertrophy training, when push comes to shove, volume takes precedence over intensity. If bar weight needs to be reduced to eke out a final set, do so.
If getting stronger is your training target, you might instead do conventional deadlifts (which permit greater overall loading), focusing more on bar weight, which means fewer reps.
Also, when training for strength, going close to, or to, failure isn't the goal, but rather an unavoidable consequence of striving to lift as much weight as possible.
To Make Decisions, You Must Have a Defined Objective
Over the course of every workout, you're faced with constant decision-making challenges: Should you do another set or move on to the next exercise? Should you add weight, or increase reps, or both? Should you use a slower lifting tempo? Is it okay if your form is a bit loose?
If your training objectives are clear from the start, these decisions are quickly and easily answered. If not, you'll waste time and energy trying to figure out the best course of action and you'll never be confident that you made the right decisions.