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Prospective, single-group, repeated-measures design.To analyze the longissimus thoracis and lumbar multifidi muscle activity with surface electromyography (EMG) during exercises used in back rehabilitation programs.Physical therapists use a variety of exercises when rehabilitating patients with low back pain (LBP). EMG analysis of exercises can provide a measure of muscle activation so a clinician can have a better idea about the effect the exercise may have on the muscle for stabilization, endurance, or strength training.Surface EMG analysis of the muscle activity of the longissimus thoracis and lumbar multifidi was carried out bilaterally on 3 different experimental groups while performing a variety of exercises commonly used in low back rehabilitation programs. Groups 1 and 2 each had 30 subjects and group 2 had 29 subjects, ranging in age from 21 to 35 years. All EMG data during exercises were normalized to percent of the maximum voluntary isometric contraction (MVIC).The lumbar multifidus and longissimus thoracis muscles were most active, with EMG amplitudes of greater than 92% +/- 12% MVIC during prone lumbar extension to end range with resistance applied. Prone lumbar extension to neutral, resisted lumbar extension while sitting, and prone extension with the upper and lower extremities lifted (Superman exercise) produced EMG amplitudes ranging from a mean +/- SD of 77% +/- 13% to 82% +/- 12% MVIC. Exercises that produced EMG amplitudes of less than 50% MVIC were bridging exercises, the side-bridge exercise, and upper and lower extremity raises in either the prone or quadruped positions.The findings from this study may be helpful for physical therapists in selecting exercises when progressing patients with LBP from low-intensity exercises to those that require more muscle activity.