Trunk muscle activation during sub-maximal extension efforts

Neuromuscular fatigue of the trunk musculature, particularly lumbar paraspinal and abdominal muscles, is important in when evaluating motor control of the trunk. Activation of agonists and antagonists trunk muscles was hypothesized to change during sub-maximal isometric trunk extension efforts. Thirteen women were positioned in 30° of trunk flexion and performed maximal voluntary isometric contraction in trunk extension against an isokinetic dynamometer. One of two sub-maximal efforts (50% and 70%) was performed to induce neuromuscular fatigue on two different days. Surface electromyography of the lumbar paraspinal (LP), rectus abdominis, and external oblique muscles was recorded during each session. Torque output, median frequency of the power density spectrum, and normalized integrated electromyography were analyzed using repeated measures analysis of variance to evaluate trends in the data over time. Paraspinal muscles showed signs of fatigue in both conditions (p < 0.05) Abdominal activity did not increase during the 70% condition, but showed a non-significant trend (p = 0.07), coinciding with the reduced median frequency of LP muscles. The neuromuscular system modulates its motor control strategy to identify the muscle activation levels necessary to maintain force output. This information is necessary in the evaluation of contributing mechanisms to trunk stability in furthering preventative and rehabilitative treatments.