The effect of increasing resistant level on reaction time and electromyographic activity of trunk muscles during combined trunk motion

Asymmetric or combined lifting of objects is considered as the most important biomechanical factor in increasing the prevalence of low back pain (LBP) in the working environments. The purpose of this study was to investigate the effect of increasing resistance level on reaction time and electromyographic activity of trunk muscles during combined trunk motion. In this interventional study; after entering cases into the study consecutively, thirty young healthy men aligned with B200 isostation dynamometer in upright standing position randomly performed combined trunk motion against different levels of resistance (minimal resistance, 25, 50, 75% of MVC). The way of test performance was five combined trunk motion trip in two phases of upward and downward movements. The.Rout Mean Square of trunk muscles and temporal parameters of motor control was recorded by the Data Link device. Analysis of variance with repeated measures was used to test the effect of levels of resistance. The results of present study showed that in general, increasing resistance level associated significantly with the amount of RMS of trunk muscles (p=0.001) and motor time (p =0.004). Reaction time has also been increased but it had not any significant difference (p=0.058). Premotor time was not changed significantly (p>0.05). With increasing the level of resistance, trunk controllability reduces and the amount of error in motion system and internal loading will increase. Finally greater stress and strain impose into the annulus fibers, facet joints, soft tissue will increase the risk of injury.