Does it matter which muscle groups you work on first? And beware of overtraining!

Q. Is it true that when you lift weights, you're supposed to work your large muscle groups before your small ones? Why? And how much does the order really matter? My gym is very crowded, and I can't always get to the machines that I want when I want to use them.

A. The order "isn't a major concern," says exercise physiologist Wayne Westcott, Ph.D., fitness research director of the South Shore YMCA in Quincy, Mass. However, there are some general rules of thumb that can make your workouts more effective.

It is indeed a good idea to try to work your larger muscles before your smaller ones, Westcott says. This is because exercises for your big muscles - your chest, lats and glutes - typically require the assistance of nearby smaller muscles. If your smaller muscles are fatigued, they can't provide the necessary help. Let's say you're doing a dumbbell chest press, which primarily works the pecs but also involves the triceps and shoulders. If these smaller muscles are tired, you limit the amount of weight you can use for the exercise, so your chest muscles may not get an optimal workout.

Similarly, perform back exercises, such as the lat pull-down and seated row, before working your biceps. Do squats and lunges - which work the glutes as well as the quadriceps and hamstrings - before performing exercises, such as leg curls and leg extensions, that isolate the quads and hamstrings.

It doesn't matter whether you perform upper- or lower-body exercises first, Westcott says, but he does recommend saving abdominal and lower-back exercises for the end of your workouts. "If you fatigue your core muscles early, they can't function as well in their stabilization role, and that might lead to an injury."

Q. I need to tone my butt and thighs. I bought two books on weight training for women that recommend lower-body strength training five or six days a week. Both say that this does not lead to overtraining because you perform different exercises every day. Are these books right? Should I be working out six days a week?

A. No. Muscles need about 48 hours to recover between workouts. If you work the same muscles on consecutive days - especially on several consecutive days - you're absolutely overtraining them, no matter what exercises you're performing. "Lifting weights creates microscopic muscle tears, which promote growth and improved strength," says Kara Witzke, Ph.D., an associate professor of exercise science at Norfolk State University in Norfolk, Va. "That's a good thing. But if you accumulate microdamage, you end up with large muscle tears. That's not a good thing."

If you're lifting properly - using heavy-enough weights that your muscles fatigue after eight to 12 repetitions - your muscles should feel too fatigued to be worked on consecutive days. If you find that your thigh or butt muscles feel fresh the day after you worked them, this is a sign that you're not working at a high-enough intensity to develop significant muscle tone and strength. Experienced weight lifters tend to work each muscle group just twice a week so that their muscles have sufficient time to rest and recover between workouts.

"It's OK to go to the gym five or six days a week," Witzke says, as long as you don't strength train the same muscles on consecutive days. However, beware of overtraining: You need only two total-body workouts a week to meet fitness requirements. Sometimes less training is more: Your body needs adequate recovery from exercise.