Background and aim: The presence of the flexion relaxation phenomenon (FRP) during trunk flexion represents myoelectric  silence consistent with increased load sharing of the posterior discoligamentous passive structures. A number of studies have shown differences in the FRP between patients with chronic low back pain and healthy individuals, Persistent activation of the lumbar erector spinae musculature among patients with back pain may represent the body's attempt to stabilize injured spinal structures via reflexogenic ligamentomuscular activation for protecting them from further injury and avoiding pain.

Materials and methods: Two groups of female subjects ((20 - 40 years old) were participated in this study. First group consisted of 10 subjects with chronic low back pain (CLBP) and second group consisted of 10 healthy ones as control group. Both groups have performed 5 cycles of trunk flexion - extension . The speed of the movement repetition controlled by an electronic metronome . The EMG signals recorded from T12 and L3 paravertebral muscles and bisepse femoris on the right side. The lumbar flexion motion degree has been measured by the digital flexible goniometry.  All subjects have done Sorenson Back Endurance test in prone laying position. The subjects have extended their trunk up to the horizontal position and sustained in this position up to fatigue level .The subjects leave the table and asked to do 5 more cycle of trunk flexion - extension.

Results:  In patients group there is an increment and significant differences in lumbar flexion degree at the time of muscle EMG off  in comparison with healthy subjects after fatigue test (p<0.05). In both groups, the myoelectric silence period showed a significant change with respect to the pre- fatigue (p<0.05). The median frequencies shifted to lower frequencies after fatigue protocol (p<0.05).

Conclusion: Muscle reflexive responses would change following fatigue protocol. Therefore, the muscle activity will increase after the fatigue period. In the other hand, in patient group the role of the muscles as a stabilizer seems to be increased to enhance the stability at the injured segment after fatigue protocol .This protects the segment against pain and disability.

Background and Aim: Different aspects of the motor control parameters of dominant and non-dominant legs are investigated but the effect of dominancy on motor control strategy is still unclear. The aim of the present study is to compare the pre- motor time, motor time and reaction time in dominant and non- dominant leg muscles of the non- professional healthy athletes in response to visual stimulus during forward jumping.

Methods and Materials: Eleven non- professional healthy athletes participated in this cross-sectional study determined by convenience non probability sampling. Surface electromyography were performed in the dominant and non-dominant leg muscles including  gastrosoleous , peroneus longus, peroneus brevis, and tibialis anterior in response to visual stimulus during forward jumping.

Results: There was no significant difference (P > 0.05) between the dominant and no-dominant legs with respect to the pre-motor time, motor time and reaction time of the gastrosoleus, peroneus longus, peroneus brevis and tibialis anterior.

Conclusion: Our findings suggest that limb dominancy is not an important parameter in the motor control studies.

Keywords: Motor control strategy, Visual stimulus, Forward jumping

Background and Aim: Several studies have investigated the effects of angular velocity on mechanical and physiological variables such as joint net torque, muscle force and myoelectrical activity, but so far its effects on the control of the pattern of synergist muscle activation during isokinetic movements have not been studied. In this experiment, a computational method of identifying and analyzing muscle synergies were used which is based on the framework of the uncontrolled manifold hypothesis to determine the variability of muscle synergies following movement velocity alteration.

Materials and Methods: Twelve healthy females without any known neurological or motor disorders participated in the experiment. The participants were positioned on a Biodex dynamometer. They were asked to perform ten continuous knee extension and flexion motions with maximal strength at 45˚/s or 300˚/s. Electromyographic activity of the vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF) were recorded and variance within the UCM (VUCM) and orthogonal to the UCM (VORT) with respect to an appropriate Jacobian matrix was computed. An independent t-test was used to compare VUCM between two velocities.

Results: Across subjects, VUCM was significantly higher than VORT (VUCM>VORT) in both tasks. There was no significant difference in VUCM between two velocities (p>0.05).

Conclusion: Our findings suggest that angular velocity is not an important parameter when a controller of a multi-element system wants to stabilize a particular value of a performance variable.

Keywords: Variability, Angular velocity, Muscle synergie, Externsor muscle.

Background and Aim: The structure and function of the hamstring and quadriceps can contribute to the development and process of patellofemoral pain syndrome. As open and closed kinetic chain exercises have different effects on the knee joint, the aim of the current study was to investigate motor control of knee joint in both kinetic chains according to voluntary response index aspect.

Materials and Methods: 12 healthy knees and 12 injured knees (6 females in each group) were examined. Surface electromyography were performed in knee muscles including vastus medialis oblique, rectus femoris, vastus lateralis and biceps femoris while the subjects were doing flexion-extension of knee joint by isokinetic dynamometer system and one leg semi-squat movement. Then voluntary response index was calculated.

Results: There were no significant difference between the patients and healthy groups (p>0.05). In the patient group, correlation between similarity index and magnitude was seen.

Conclusion: Probably in the low intensity patellofemoral syndromes, voluntary response index could not differentiate between healthy and the patient groups. In the patients with patellofemoral syndrome group, correlation between similarity index and magnitude may represent co-contraction in their muscles.

Keywords: Patellofemoral pain syndrome, Motor control, Voluntary response index, Surface electromyography