While these high-intensity sessions are believed to be critical to achieving maximal performances, they cannot be performed optimally if intervening basic endur

While these high-intensity sessions are believed to be critical to achieving maximal performances, they cannot be performed optimally if intervening basic endurance sessions are performed at too high an intensity (Bruin et al., 1994). However, high-intensity training sessions appear to be well tolerated when variation in intensity of training is ensured (Lehmann et al., 1991, 1992). Less experienced athletes may tend to train harder than prescribed during low-intensity sessions and not hard enough during prescribed high-intensity sessions (Foster et al., 2001). -- http://fastforwardtriathlon.com/wp-content/uploads/2010/08/Intensity-Distribution-in-Elite-Endurance-Athletes.pdf 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.cyclesportcoaching.com/articlejunkmiles.html http://www.marathon-training-schedule.com/heart-rate-training-zones.html http://www.marathon-training-schedule.com/heart-rate-trainin... http://coachlevi.com/training/junk-miles-what-why-avoid/ 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/DocumentItem/02%20IJSPP_2010_Seiler_0009.pdf http://www.fitnessforlife.org/AcuCustom/Sitename/Documents/D...