Heart rate is a pretty good proxy for 'effort'. As you get in better shape, your body is more efficient at all sorts of tasks, including fitness challenges, so to get your heart rate higher, you have to increase the intensity of the activity.

I try to avoid "junk" time in the 128-142 zone because the intensity is too high for recovery/endurance benefits, but not high enough to target aerobic threshold or VO2max. When in mediocre shape and trying to do a lower intensity workout, I accidentally end up in that range more easily than when in racing shape. Meanwhile, threshold and VO2max intensities are easier to sustain when better trained, though I can hit similar max heart rates either way. I interpret this as the lactic acid versus heart rate curves have different profiles even with similar endpoints.

I've also seen acute short-term effects. For example, I did a 160-mile backcountry race after a competitive nordic ski season. For about a week after the race, I couldn't get my heart rate above 145 (30 beats below my normal max). During that week, I did VO2max-intensity intervals with a well-matched regular training partner and could match a pace that would normally require me to be at 160 while I was at just 140. The pace felt more comfortable than expected (probably "felt like 150"), but there was no headroom to go faster.

I'm a powerlifter, so that's the perspective that I take. The systems I care about are peak strength across multi-joint compound movements, so I focus on high weight, low reps. Strongman competitions take strength endurance, they emphasize moderate weight, higher rep exercises.

For endurance athletes, systems include respiratory efficiency, VO2max, glycogen storage, among others. Some of those are best trained at low intensity, high volume. Some of those are best trained for at high intensity, low volume. None of those are best trained for at moderate intensity.

When you've reached the level of adaptation where you need to manage your effort/recovery cycle, it becomes important to avoid training efforts that don't optimize for the systems you wish to improve; there's a real recovery cost for little gain. What jedbrown is saying absolutely makes sense. It's also of lesser benefit to anyone who isn't training competitively.

-- http://fastforwardtriathlon.com/wp-content/uploads/2010/08/I...

It's oft-discussed in training plans across endurance sports. Here are a few showing up in a quick search.

http://www.cyclesportcoaching.com/articlejunkmiles.html

http://www.marathon-training-schedule.com/heart-rate-trainin...

http://coachlevi.com/training/junk-miles-what-why-avoid/

Of course if you're training for general fitness rather than to optimize performance, then zone 2 training is fine. It burns calories and you feel like you get a workout without the pain and injury potential of higher intensity training.

This is an accessible review article: http://www.fitnessforlife.org/AcuCustom/Sitename/Documents/D...

On mobile, but Google "junk miles"

It's a staple of sports medicine. Fit people start out with a lower heart rate, and unfit people's heart rate rises much more rapidly with exercise. Get the two to climb a set of stairs - the fit person won't notice it, the unfit person will be slightly out of breath and have a noticeably higher heart rate (in some cases, may now even be able to feel their own pulse).

To reach beats-per-minute X, a fit person starts lower and has a longer ramp-up.

I'm reasonably fit, and I spent the day sitting at my desk. HR right now is 53. I can feel that just fine. Am I broken? ;)

True, but it would also take less effort for them to see improvements at that level of fitness as well. You had to work harder, because your body already handled the easy stuff and wouldn't be challenged.

Side note, most counters estimate your step length based on your height, weight, age, gender, and also allow you to program your step length manually if you like.