Here we go. The New York Times getting lazy again. This article isn’t wrong in what it says, it’s just so vague and incomplete that someone who hasn’t ever engaged in regular exercise before but is getting ready to try is likely to make some terrible mistakes. Lets add just a bit of important detail.
First, everything the article says is correct…when talking about aerobic/cardiovascular exercise. You can train multiple days in a row, and such frequency can enhance the quality of your workouts as your practice develops better skills at that particular activity (running, cycling, even power walking have skills that can be improved).
The reason you can perform these activities multiple days in a row is determined by their nature, which is described in the category name AEROBICS. Aerobics refers to both a human energy system and a certain range of physical intensity.
Aerobic type exercise typically falls into a category of intensity between 60% and 85% of a persons theoretical max effort. At these ranges of effort, a person can continuously exercise for 15 or more consecutive minutes before becoming too fatigued to continue. Within this range the entire cardiovascular system becomes healthier and stronger, and that’s great. But it’s only half (some might argue 1/3) your fitness journey.
Anaerobic exercise is the other half of this very important fitness story. Like aerobics, the term anaerobic refers to both a human energy system and a range of physical intensity for the activity. In Anaerobic exercises, the intensity MUST exceed 85% of a persons theoretical maximum effort, causing rapid exhaustion (under 60 seconds, typically, but always under 2minutes). Training in this range improves the tone, and physical strength, of a persons skeletal muscular system, improving the bodies posture, appearance, resistance to physical injuries, and later in life, infirmity and osteoporosis. Can you walk up a flight of stairs when your 80? Can you tie your own shoelaces? Can you carry a bag of groceries or get up out of a cushy sofa? These are matters of physical strength in the geriatric community, and major challenges to millions of Americans.
Training at this intensity level can not be performed on consecutive days, because training at this level actually causes a temporary weakening of the skeletal muscles involved, and the following 24-48 hours are needed for the muscles to recuperate and adapt in order to become stronger.
The Times article correctly states that 72 hours between exercise bouts is too long to wait for any exercise, but fails to caution that certain kinds of exercise require a certain amount of days off before repeating.
I hope this helps. Good luck.
http://nyti.ms/19ztI5l
NYTimes: How Often to Exercise
Fitnessgroan's Blog 
How would this apply to aerobic exercises that have anaerobic components? For example, interval training or cycling where one blast through a hill.
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