12-20-2009, 08:41 AM
kap wrote:
Dennis,
I understand the benefits cross-training brings but the longer runs yield none? Then why are we encouraged to do 30-45 minute workout; not just settle for 15 instead? I think as long as one exercises within one's maximum heart rate it should be good.
The theory is that while you may burn calories in 15 minutes of exercise, your body won't get much conditioning or build muscle until you've done a bit more than that.
As for the duration issue, after a half hour of vigorous exercise with your heart rate at about 90% the fitness benefits quickly diminish to almost nil beyond burning calories. At the same time, the odds of sustaining an injury increase significantly after the same half-hour mark.