Passive repetitive loading of the lumbar tissues influences force output and EMG during maximal efforts.

Loading of the low back tissues induces tension-relaxation in the viscoelastic connective tissues. The extent to which repetitive loading influences the muscle activation and subsequent muscle force production has not been fully explored. The purpose of this study was to examine the myoelectric activity of the trunk muscles during maximal flexion and extension exertions before and after a passive trunk flexion-extension protocol. Nineteen subjects performed three trials of maximal efforts in trunk flexion and extension while seated in an upright position. Surface electromyography (EMG) recordings were collected bilaterally from paraspinal (thoracic, TP, lumbar LP), rectus abdominis (RA), and external oblique muscles. A 10-minute passive trunk flexion-extension protocol was used to repetitively load the lumbar tissues at a rate of 0.17 rad/s, through the subjects' range of motion. The main findings included a significant reduction in moment output during extension efforts (p < 0.05) with significant reductions in the average EMG from the TP and LP muscles during extension (p < 0.05). In flexion, peak and average EMGs were also significantly reduced (p < 0.05). The results indicate a significant reduction in the ability of the trunk extensors to output force, but this may be due to the increased compliance of the connective tissues rather than modified neuromuscular signals to the paraspinal muscles. However, neuromuscular changes were apparent in the TP and RA muscles suggesting a modified control mechanism was present.