Tips » Cardio and Aerobics
Is it safe to do more than 60 minutes of high intensity cardio at one time?
What do you mean by "safe"? In other words, people run marathons that last hours, and they survive. So apparently it is safe as in, you can do it. Is it safe to preserve muscle? That depends less on the time and more upon how you work up to it - going from nothing to 60+ minutes is different that slowly working to 60 minutes and having the right nutrition and recovery.
High intensity? If you can do it for more than 30 minutes, it is not high intensity - unless you are a Norwegian cross-country skiier (these people can keep their heart rate to 80% of the zone for 2 hours or more) ... seriously, in general, if you can go past 30, then increase how hard you are doing it and do it less. With high intensity, more is not better, more intense is better (within reason - don't go out and injure yourself).