Isolation exercises like the dumbbell curl give you an effective way to work a single muscle in one fluid movement. Using a dumbbell for your curls reduces the isolation a little bit, as a broader range of stabilizing muscles work to give you balance. Because of this, the dumbbell curl can affect muscles you wouldn’t ordinarily expect – such as the back. Generally, the dumbbell curl doesn’t affect the back and should not cause back pain, but when you don't do them right, you may strain your back.
Typically, the dumbbell curl is performed from the standing position in an alternating pattern to help you concentrate on the movement of a single arm at a time. The dumbbell curl is an isolation exercise not because it targets the biceps, but because it involves the movement of a single joint: the elbow. The most common reason for back pain caused by dumbbell curls occurs from the standing position, when you attempt to curl more weight than your biceps can handle. Your natural tendency is to then arch your back and create a slight swinging motion with your arm to generate momentum.
The dumbbell curl primarily targets the biceps, but it also impacts the forearms, the deltoids, the trapezius and your wrist flexors. When curling heavy dumbbells – at a weight at or near your maximum capacity for the curl – your legs and lower back will engage to provide balance through your repetitions; the exercise doesn’t “target” those areas. If your legs and back activate too much during your curls, it can cause injury and it robs the target muscles of complete engagement, which diminishes your gains.
Standing versus Seated
While the standing version of the dumbbell curl may cause back pain if done incorrectly or with too much weight, the seated version can nearly diminish the degree to which the exercise impacts your back. Use a flat-back workout bench, keep your back straight and pressed against the seat, and perform your reps methodically with a steady pace. This will help concentrate the stress of the lift directly on the target muscles.
Back pain may occur during or following dumbbell curls, but the exercise itself may not be the cause. You may simply be overtraining yourself, not giving your body sufficient rest time in between upper-body workouts. You need at least one full day to recover. Also, you may be setting up for your curls incorrectly, especially if you begin with the dumbbells on the floor. Bending incorrectly to pick up the weight prior to your first set, or to put it down following your last set, can strain your lower back. Bend at the knees, not at the waist when picking up weight off the floor, or from a low position.
Bobby R. Goldsmith is a writer and editor with over 12 years of experience in journalism, marketing and academics. His work has been published by the Santa Fe Writers Project, "DASH Literary Journal," the "Inland Valley Daily Bulletin" and WiseGEEK.