Most think they exercise hard by adding more work over time instead of working harder now. Each path leads to different results though. You want to work hard in the least amount of time needed to best reach your goals.
Moments
Given enough time, an ant could move the pyramids from Cairo to Capetown –and in the process he would perform an enormous amount of work; while the intensity of work would never be high enough to measure in meaningful terms. In effect, the “power production” would be extremely low. The mass – the actual size – of a muscle clearly indicates its “power potential”, its ability to produce power; even a very small muscle is easily capable of performing a large amount of work, but its power will be limited by its size.
– Arthur Jones
This means pushing with all your might to get that final 8th rep instead of cutting it and then piling on two more sets. This means running faster for each interval on the stairs, not adding more minutes. This means working hard in those last crucial moments and not diluting it with time.
This allows effectiveness since you challenge your fitness. This allows efficiency which conserves resources for recovery. It also gives you more time for other activities in your life.
Hard work has a clear limit since you fail to work at some high rate. When you squat, you can stop when you fail to complete a full rep with 300 pounds. You then work to get as many full reps as you can.
Volume can keep adjusting toward a lower performance. Walking can go on for days yet does very little for peak fitness. This would also waste raw materials toward very little in the end.
Some argue that training to failure is bad. With good form and the right safety features in place, you just match your effort with the demands. When you can no longer continue you end the exercise.
Effort
Effort only serves as a tool toward progress, but a certain level seems required to reach the threshold of improvement per exercise session. You could lift at a percentage close to your max for one rep yet still quit early, failing to fatigue the fast-twitch fibers enough. You would get better results working as hard as possible for 15 reps, a number many associate with endurance. Would you be weak if you could squat 500 pounds 15 times?
Effort ranks as the most important factor for progress, as long as it takes place within the liberal time range for developing a fitness component. These components include strength and cardiovascular endurance. You would never expect to build much muscle through hard work on your mile time. You need to lift fairly heavy to gain strength, but 10 reps per set could work just as well as 5 reps as long as you added weight over time.
You still need some minimum time frames. You need enough time to improve your cardio, but this turns out to be no more than 2-5 minutes. You need to address all the major muscle groups in your body with strength training, but this requires only three simple exercises. You can perform a single heavy set of any reasonable rep range, with most doing well between 5-15 reps.
Working far below certain time frames though may risk safety, fail to warm you up, or otherwise not meet the criteria required for your goal. You could push against a wall with vigor for 3 seconds yet you would never have any great results in strength and size. Sprint as fast as possible for 5 seconds a day and you will remain horribly out-of-shape.
If you set a goal that allows you to sustain too low a rate then you will run into problems as well. Certainly no one would claim that a 100 rep set done as heavy as possible would feel easy, yet this could overtrain you. Always try to work hard in the least amount of time needed.
The Broad Lesson
Consider applying this to your life as well. Value outcomes, not just the time put into your tasks. Consider two routes for time spent on your career.
One, you work 4-6 true hours everyday but take breaks as needed, enjoy your life, and work attentively on the most important tasks when you do work.
Two, you work 5 days a week for 15 hours each yet waste lots of time and never step away when you lose focus.
Which route would keep you productive and sane?
Everything has an opportunity cost. The opportunity cost is the loss you face from all the other things you avoided because of your current activity. Anything with a low return for the time you invest creates more opportunity costs, which makes you unhappy in the long run.
Work hard and not longer. Efficiency allows the best results because it creates the change you seek while conserving time and resources to the best effect. Apply this principle to exercise and life in general to reap great rewards.