[Back muscle activity during flexions/extensions in a second group of normal subjects]

Natural trunk movements and back muscle electromyographic (EMG) activity are seldom studied but are pertinent to daily life activities, which can lead to low-back pain. In this study, ten normal subjects performed trunk flexion/extensions (F/E) without any restraining apparatus. Duration of the F/E was either free, or at 3, 2.25 and 1.5 s. A fatiguing task consisted of continuous F/E movements accomplished at 1.5 s intervals during 90 s. Photodiodes were placed on the skin to obtain kinematics of the trunk. EMG signals of the back were recorded with 10 pairs of surface electrodes located at 3 levels of the erector spinae. It was found that the F/E movement duration chosen by the subjects varied between 4.07 and 2.07 s and the flexion amplitude varied between 55 degrees and 118 degrees. Similar variations in the amplitude of flexions were found for the tasks realized at fixed periods. The level of fatigue induced in the fatigue task was evaluated by comparing the energy of its EMG signals with those of the 1.5 s task. With this index, fatigue was detected in a few subjects only. Due to the length of the fatigue task (90 s long), it would seem that most of the subjects became adapted to the movements and could produced them more effectively (i.e. with less EMG) than during the 1.5 s task which was repeated only for few seconds.