Admit it. A lot of times you plan your workouts around your major muscle groups and just "throw in" a few sets at the end for your ancillary muscles – calves, abs, and forearms. The problem is that by the end of leg day, you probably don't put much focus and effort into training your abs. That's why a few of us around here have started practicing what we call an "ancillary day."
On a normal "off" day, we sometimes go to the gym anyway and focus on our abs, calves, and forearms–the three muscle groups that often get left out or not trained properly. One way to do this is to use an ancillary circuit, moving from one exercise to another with little, if any rest between sets. Here's an example:
Set #1: Barbell wrist curl, palms facing up, 1 x failure (do one set to failure)
Set #2: Seated calf raise, 1 x failure
Set #3: Slant board sit-ups or Swiss ball crunches holding weight, 1 x failure
Set #4: Dumbbell wrist curl, palms facing down, 1 x failure
Set #5: Standing calf raise, 1 x failure
Set #6: Hanging leg raise, 1 x failure
After a rest, repeat once or twice more.
Rest just long enough to walk/limp from one exercise to the next.
Since the abs, calves and forearms can handle more frequent exposure compared to larger muscle groups, you can do this circuit two to three times per week. Just don't do it on consecutive days.
The rep range is up to you. You may want to fail around 10 to 12 reps for a few weeks then switch to heavier loads where you fail at 4 to 6 reps. Generally speaking, it's better to go heavy on standing calf raises (using low reps) and lighter on sitting calf raises (using higher reps). This is due to the muscle fiber make-up of the calf muscles.
With ab training, some strength experts prefer high reps with little resistance while others prefer fewer reps with greater resistance. The answer? Variety! Just use a combination of both and you'll be sporting a sixer in no time.
Of course, you don't have to train to failure at all if that's not your thing, but with these smaller muscle groups, overtraining isn't that much of an issue.
Feel free to add other exercises to this list or make up your own circuit. For example, some like to add in traditional cardio work or rotator cuff exercises.
Another option, besides training ancillaries on off days, is to use two training sessions per day. You can train ancillaries in the morning and then do your regular workout in the evening.
After a month or two of regular ancillary circuits, we think you'll see a noticeable difference in the size of your forearms and calves. And if your diet is tuned in, then you'll like the results you'll get for your abs as well.