Final answer:
Strength gains for untrained individuals are best achieved at 70-80% of 1-RM, while trained individuals benefit from working at 80-100% of 1-RM. These ranges provide the optimal stimulus for muscular adaptations respective to the individual's conditioning level.
Step-by-step explanation:
The question is asking about the optimal percentage of 1-Repetition Maximum (1-RM) to produce the largest strength gains in both untrained and trained adults. The consensus among fitness professionals and based on resistance training studies is that for novice or untrained individuals, working at a range of 70-80% of 1-RM can yield significant strength improvements. For trained individuals, utilizing a range slightly higher, typically between 80-100% of 1-RM, can be most effective for continued strength gains. The rationale behind these percentages is that higher intensities are required to provide adequate stimulus for adaptation in those who are already conditioned, while moderate intensities are enough to promote muscular adaptation in novices.