What this means in real training
What hard enough means
A productive set usually needs to be close enough to failure to recruit and challenge the target muscles.
That does not mean every set must end with form breakdown, missed reps, or recovery debt.
Recent meta-analyses and meta-regressions suggest proximity to failure is a continuum, not an all-or-nothing switch.
Where failure can fit
Failure is more reasonable on stable exercises, isolation movements, or final sets where the cost is lower.
It is riskier and often less useful when fatigue makes technique collapse on heavy compounds.
If the session only works when every set is a grind, the plan is probably too fatiguing to repeat well.
Why failure is not free
Training to failure tends to create more acute fatigue than stopping a bit short, which can affect later sets and recovery.
That extra fatigue cost is one reason “every set” is a bad default even if failure can still be useful sometimes.