Since getting into Cathe, I seem to reach for her workouts 90% of the time, but here are a few that I still throw in here and there because they're almost as challenging as Cathe:
Intense Moves - Gin Miller (Almost easy compared to the Imaxes, but still a challenge.)
Hardcore Kickbox Circuit - Janis Saffell (Nice change of pace to do kickboxing with weight segments in between.)
Breakthru Body Blast - Tracy York and Michelle Dozois (A fairly challenging circuit workout - cardio segments are tougher than the weight segments.)
Step It Strong - Keli Roberts (A pretty advanced circuit workout - weight segments are tougher than the cardio segments, and you get even more upper body weight work and ab work when the circuit part is over.)
Tighter Assets Cardio Blast - Tamilee Webb (An intermediate level hi/lo aerobic workout - good for days when I don't want to do step or just feel like something a little easier.)
Breakthru Core Conditioning Pilates - Tracy York and Michelle Dozois
Prevention Flatten Your Abs with Pilates - Michelle Dozois (These two only run around a half-hour but are good overall Pilates workouts.)
Get Hard Arms and Shoulders - Cory Everson (Somewhat dated, and kind of cheesy when it comes to the music and dialogue, but the exercises can't be argued with. It's a pretty intense half-hour of work for the shoulders, biceps and triceps.)
Some short, but good, "add-ons":
QuickFix Pilates Abs - Keli Roberts (Three 10 minute workouts, but Keli makes every minute count. I do one segment in the evening, three days a week, in addition to the more traditional ab work (such as the abs from Muscle Endurance) that I do in the morning. I've noticed significant strength gains in my abs since combining the two.)
QuickFix Stability Ball - Keli Roberts (A great way to squeeze in some regular stability ball work. The abs section is especially fun.)
I Want Those Abs, I Want Those Buns, I Want Those Arms - Tamilee Webb (Each tape has two 15 minute workouts for the specific area. The arms tape is pretty moderate, but the 2nd workout on the abs tape and both leg workouts are quite challenging. The first workout on each tape is supposed to be the easier of the two, but for some reason I find workout #1 on I Want Those Buns, with it's non-stop 15 minutes of various lunges and squats, to be harder.)
These workouts aren't all that hard, but I pull them out from time to time just because I enjoy them:
Reebok Step, Rhythm and Moves - Petra Kolber
The Step Workout and Step Attack (CIA Videos) - Donna Read
Ultimate Step Workout - Keli Roberts
4 Day Rotation Workout - Donna Richardson
Quick Toning Thighs of Steel - Tamilee Webb
Cardio Options (CIA Video) - Linda McHugh
Step, Kickbox and Funk Workouts (CIA Video) - Janis Saffell
There's also a CIA video with 3 step workouts by Joan Pitt, Dawn Pappas and Lori Saitta. Joan's isn't cued very well, Lori's is pretty easy but enjoyable enough, but Dawn's I like a lot. It's more on the advanced side and has a few intervals thrown in that approach the anaerobic zone. Definitely more athletic than dancy.
Mephie
