Over the past few days I've been working out on things other than racquetball, trying to keep my heart rate up around at least 140 (about 72% according to this). During the day I can peak around 146, but I feel pretty tired (over a continuous 20-30 minute workout). Tonight I was able to sustain 150 (78%) fairly easily for 30 minutes working up a good sweat on an elliptical. Jackie pointed out that I was on a different machine, but I'm going more towards keep my heart rate up and going than what particular aerobic exercise I'm doing.