Higher intensity, which doesn't have to come from heavier weight per say, but increased power, work/time meaning total work, higher weight up or time down. HII

Higher intensity, which doesn't have to come from heavier weight per say, but increased power, work/time meaning total work, higher weight up or time down. HIIT/interval/crossfit style are prime examples of in my opinion the most efficient way to achieve "exhausting" workouts (lowest time commitment, and with group gym style, lowest mental effort to formulate workout plans; it's all left to the trainer).