One reason unilateral and bilateral exercises are included in most training programs is that any variation in an exercise potentially changes the emphasis on the muscles involved, as well as the emphasis on the different muscle fibers in those muscles.
By Steven J. Fleck, Ph.D.
Q: Some guys at my gym do a lot of single-arm exercises with dumbbells, and single-leg exercises, such as step-ups. What are the reasons for doing such exercises?
Single-arm or single-leg exercises, such as lunges, step-ups and single-arm dumbbell exercises, are termed unilateral exercises. Exercises performed with both arms or legs at the same time, such as the barbell bench press and back squat, are termed bilateral exercises. Some people do use a lot of unilateral exercises in their training programs, while others do not, although unilateral exercises typically are included in most resistance training programs to some extent. Many lifters use unilateral exercises to correct muscular and/or postural imbalances in their body, giving extra emphasis (and training) to bring up the side of their body that needs extra work.
One reason unilateral and bilateral exercises are included in most training programs is that any variation in an exercise potentially changes the emphasis on the muscles involved, as well as the emphasis on the different muscle fibers in those muscles. A classic example of changing the emphasis on the muscle groups involved in very similar exercises is the use of the flat bench, incline bench and decline bench barbell bench press. The flat bench emphasizes the middle part of the chest musculature, the incline bench emphasizes the upper part and the decline bench emphasizes the lower part of the chest. The use of dumbbells in the same exercises not only changes the emphasis on different portions of the chest musculature, but also changes the emphasis on different muscle fibers within the muscles used during the exercise.
Have you ever compared your 10-repetition maximum in the barbell flat bench press and dumbbell flat bench press? If you have, you found that typically you can use less weight when performing the exercise with dumbbells. This is in large part because when performing the exercise with dumbbells, each arm individually is much freer to move to the left and right when doing the exercise. In anatomical terms, this means each arm is freer to adduct, or move toward the midline of your body and to abduct, or move away from the midline of your body.
In reality, when doing the dumbbell flat bench press your arms tend to move toward the outside of your body, or be abducted, because your arms are not “connected” by the barbell, as in the barbell flat bench press. As a result, you must call upon muscles, such as the chest musculature, to prevent the dumbbell and your arms from moving away from the midline of your body. The muscles that are needed to prevent this movement are not as strong as the muscles that perform the movement of pressing the weight to arm’s length. Therefore, they limit how much weight you can use in the exercise and they become the weak muscle group when performing the exercise. Because they are the weak muscle group, they are the muscle group that’s fatigued first or emphasized during exercise.
If a muscle group, or particular muscle fibers in a muscle, are not recruited or emphasized during an exercise, they will not adapt by becoming stronger and larger. Thus, one major reason for performing both unilateral and bilateral exercises for the same muscle group, such as lunges and the traditional barbell back squat, is to place emphasis on the different muscles and different groups of muscle fibers with the different exercises. This should result in more total development of the muscles involved in the exercises for the same muscle groups.
The bottom line is that even exercises that are very similar, such as a squat with a wide stance and a squat with a narrow stance, emphasize different areas of the muscles involved. This same line of reasoning is true for unilateral exercises such as lunges and dumbbell overhead presses versus bilateral exercises such as the two-legged leg press and barbell overhead press for the same muscle group. From a practical perspective, for total development of a muscle group, not only should you perform different exercises for the same muscle group, but also exercises for the same muscle group in a bilateral and unilateral fashion. Typically, this means using both barbells and dumbbells to perform the same exercise or combinations of exercises such as lunges and back squats.
A sport science study did look at the effects of both unilateral and bilateral exercises on the lower body. In the study, men and women trained two days per week for eight weeks with weights. For five of the eight weeks, they also performed plyometric drills. The people performing bilateral weight training performed predominantly back squats and front squats. The group performing unilateral exercises performed one-legged squats, lunges and step-ups. All the bilateral and unilateral groups started training with three sets of 15 repetitions at 50 percent of each exercise’s one repetition maximum (1RM). As the training progressed, both groups progressed to six sets of five repetitions at 87 percent of each exercise’s 1RM.
Both groups also performed some plyometric, or jumping-type drills. Five-repetition maximum strength testing was performed for a bilateral, or normal, back squat. The predicted 1RM from the five-rep maximum weight showed no significant difference in improvement between the groups that trained bilaterally or unilaterally. Bilateral, or a normal, two-legged vertical jump also showed no significant difference in improvement between the two groups, while unilateral or one-legged vertical jump ability showed a significantly greater improvement in the group that performed unilateral training. Overall, the researchers concluded that unilateral and bilateral training resulted in similar strength and power gains. Are there greater gains in strength, power and muscle size when bilateral and unilateral training are combined in a training program? There is not that much sport science research concerning unilateral versus bilateral exercises, and this question needs further research. (J Strength & Cond Res, 19:9-15)