Harder

Someone recently told me that treadmill running is a lot easier than running outside. He reasoned that, since the treadmill requires a higher speed, and also possibly a steeper incline, to achieve the same calorie burn, the treadmill must be easier. 

This logic is robust on one level, but not on another. One could argue that cycling is easier than running, since bicycles enable us to use mechanical advantage, and this is strictly true. The problem with that argument is that no one uses a bicycle to cover the same distance at the same speed as they would have done running. With the greater mechanical advantage, cyclists tend to ride faster, right up to the point where they're getting just as good a workout on the bike as they would have gotten running. Cycling is different than running, but for most people, it's not inherently "easier."

The same is true of treadmill running. Treadmill running is biomechanically different from running outside on stable ground, it involves slightly different muscles, and so on. In order to achieve a similar calorie burn, one has to approach the treadmill differently. That doesn't mean running on the treadmill is inherently "easier," only that it's different.

There is another issue at play here, however. If we were to relegate ourselves to only those activities which are more difficult than other activities, we'd never become physically fit at all. That's because training your body involves a diverse set of workouts. Some of your workouts must be dedicated to building muscles, some to learning skills, and some to resting and recuperating. If you were to train at 100% effort, 100% of the time, you'd never get anywhere at all.

A good training regimen involves two or three days of muscle-building. For example, the typical marathon training schedule will have you doing interval training twice a week; P90X and similar programs dedicate three days a week to lifting weights. The rest of the time, we train at lower levels of effort to achieve different, but no less important, goals. Runners will use one day a week for a long run and the remaining days running in active recovery mode. P90Xers will find their remaining days dedicated to plyometrics, yoga, and flexibility. Failing to do these "easier" activities means not getting enough rest, or getting too much rest; it means not gaining the benefits of balance exercises and increase flexibility.

As a final dynamic here, note that most training schedules involve relatively easier workouts in the beginning, building to progressively more difficult workouts toward the end, and then culminating in a de-training week or two in order to reduce fatigue and "peak" for a race or final weigh-in. Few would argue that the beginning of a training regimen should be loaded up with the most difficult exercises; that's just a recipe for injury. Fewer still would argue that great athletes should not attempt to plateau for competition; no one would perform at their peak if their muscles were tired and sore.

So, in the big picture, we must utilize more- and less-difficult workouts at different times to achieve long-range results. The immediacy of the comparative difficulty of a thing is really just a minor concern. Every moment in your long-range training regimen serves a purpose in the broader context. Increasing your cognitive time-horizon to include harder workouts as well as easier ones will make you a better athlete. Increasing your cognitive time-horizon alone will make you a better person.