The purpose of this study was to estimate the level of muscular activation and muscle fatigue of the low back muscles during the performance of an intermittent prone back extension (PBE) exercise. Forty-one healthy students (24 males and 17 females) lying prone on a bench with the legs fixed performed two maximal voluntary contractions (MVC) in extension, a maximum of 100 repetitions of an intermittent PBE exercise immediately followed by a final MVC in extension. In addition, 12 subjects (11 males and one female) repeated the PBE exercise but with the addition of a 45-N weight on the back. The PBE exercise consisted of a task broken into four 1-s segments, while lying prone on a bench (10 degrees below horizontal): (1) raising the trunk to a horizontal position; (2) holding the trunk in the static phase (10 degrees above horizontal); (3) returning to the original position; and (4) resting on the bench. Electromyography (EMG) was used to measure the level of muscle activity (erector spinae (ES), gluteus maximus (GM), hamstrings (HA)) relative to the maximum voluntary EMG (MVE).
Results: Most of the subjects (34 out of 41) completed the 100 repetitions without excessive muscle fatigue according to the post-exercise MVC values. The intermittent PBE increased fatigue in the lumbar and hip extensor muscles in terms of: (1) a decrease in the MVC; (2) an increase in the level of muscle activation; and (3) a decline of the median frequency (MF). There was no gender difference in all EMG measurements. The level of muscle activation in the hip extensors (GM and HA) was associated with task failure (number of repetitions <100) for some subjects and the addition of a weight of 45 N had more impact on HA than ES. In conclusion, the PBE exercise as performed in the present study (including rest intervals), although not very strenuous for our healthy subjects, seems an adequate exercise to measure and train the aerobic capacity of the back muscles. However, to train specifically the back muscles, the exercise must be adjusted to avoid task failure due to possible hip extensor fatigue.