Tehran University Medical Journal

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It is generally assumed that local muscular fatigue predisposes the individual to injury. Despite the significance of muscular fatigue and endurance, there are not much dynamic endurance studies for trunk muscles in the literature. Most of them have used isokinetic dynamometer which keeps the velocity of movement constant. In this studies with the help of an isoinertial dynamometer we have assessed the effect of repetitive and fatiguing flexion-extension movements patterns and motor output. 38 female subjects with no history of low-back pain for the 6 months volunteered for the study. A triaxial dynamometer was used that simultaneously provided measurement of torque, angular position, velocity impulse, work and power of each axis. Resistance were set independently for each axis by an interfaced computer. The subjects performed trunk flexion and extension movement against a saggital plane resistance equal to 50% of their maximum isometric extension strength in the upright position. The minimum resistance in the coronal and transverse planes were set-up at 5-7 Newton meters. The subjects were asked to perform trunk movement as quickly and as accurately as possible while exerting the maximum efforts until exhaustion. Immediately after dynamic test another isometric test was performed. A paired T-test procedure was performed among the selected parameters of the 1st middle and last 3 repetition cycles. The selected parameters are range of motion, velocity, torque, impulse, work, power, time interval and time of movement. Also, a paired T-test was performed between maximum isometric strength before and after dynamic test. All the selected parameters were significantly reduced in the saggital plane. Subjects displayed significantly less motor control and greater range of motion in the coronal and transverse planes in performing the primary task of flexion and extension. The reduction of the functional capacity of the primary muscles performing the required task is compensated by secondary muscle groups and the spinal structures is located in a more injury prone position, as identified by finite element models. In addition it is suggested that the fatigued muscles would be less able to compensate any perturbation in the load or position of the trunk. The repetitive loading results in a weakening of the viscoelastic passive elements of the spineless structure. The loss of ability to protect these weakened passive elements makes the spine susceptible to industrial and recreational injuries.

Keywords: Isoinertial movements, Motor output, Muscles

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