The squat. The lunge.

They're the meat and potatoes of lower-body strength training, the mainstays of most leg workouts. To the uninitiated, they may seem intimidating -- the type of exercises designed for serious bodybuilders. Actually, they're appropriate for just about anyone who wants to strengthen and tone her legs. And they're practically essential for runners, rowers and other competitive athletes.

They're also safe. Experts have long debated the safety of the squat in particular. However, after reviewing years of research, the National Strength and Conditioning Association has concluded that the squat is not only safe and effective but also is "a significant deterrent to knee injuries." Injuries that result from squat training appear to be caused by poor form and overtraining.

To test the effectiveness of various types of squats and lunges, we hooked up a highly trained subject to an electromyographic (EMG) machine. With electrodes placed on several muscle groups, our subject performed several variations of squats and lunges. The EMG machine transduced the electrical activity generated by the muscular contractions into a graph. The more muscle fibers contracting, the stronger the signal. The results allowed us to determine which muscles were active during each exercise and to estimate how hard they were working.

Compound benefits

Squats and lunges are popular because they involve several joint movements and muscle groups. Such compound exercises are important because specific movements of sports and daily activities usually involve several muscle groups, rather than just one. Compound movements help develop balanced muscle groups around the joints and help prevent the overdevelopment of one muscle group at the expense of another.

Because compound exercises use a larger amount of muscle mass than isolated movements, they bum more calories. They also may increase your balance, coordinate and stability because they require your back and abdominal muscles to stabilize your torso.

Still, don't count out isolation exercises. With light weights, isolation exercises are excellent for beginners, rehabilitation and sports training because they require much less coordination and you can focus on the muscle group you want to work.

If you plan to combine compound and isolation exercises in one workout, start with the compound exercises. They should be performed when your muscles are fresh to avoid compromising your form and risking injury.

The EMG results

For each exercise tested, our subject used less than 50 percent of the maximum weight she could lift and did not perform repetitions to fatigue. If she had lifted heavier weights or performed more repetitions during testing, the squats and lunges would have worked her gluteal and hamstring muscles to a greater extent. If you follow either the strength or endurance/tone program described in the workout schedule, you will strengthen you gluteal and hamstring muscles to a greater extent than our EMG results indicated.

All of the exercises we tested are excellent for strengthening your quadriceps, particularly the vastus medialis, the inner quadriceps muscle, which is most important for stabilizing the knee. If you want to target your outer thighs, giving your legs more sweep, include the curtsy or side lunge in your program. Both exercises work the medialis and lateralis equally. They are advanced exercises requiring coordination and balance.

During the half and quarter squats, the lower-back muscles (the erector spinae) were 85 percent active. However, during the plie squat and all of the lunge variations, the erector spinae were less than 60 percent active. If you have experienced back problems, the plie squat and lunges may pose less of an injury risk that the half and quarter squats.

The front and back lunges were the only exercises tested that showed significant hamstring activity. Both are excellent for runners and cyclists. All of the squat and lunge variations tested showed minimal gluteal activity. To train your glutes, perform isolation exercises such as the hip extension and the side-lying leg raise.