Evaluation of the trunk modules in the symmetrical and three-dimensional asymmetrical trunk positions

Modularity (Muscle synergy) is the concept that has been used to answer the question of how the central nervous system (CNS) coordinates the body's high degrees of freedom. This study aimed to investigate the trunk muscle synergies in symmetrical and asymmetrical positions. Fourteen healthy males participated. Electromyographical activities of 16 muscles were recorded during maximum voluntary isometric contraction (MVIC) in six main directions with two repetitions and maximum voluntary isometric extension (MVIE) of the trunk in 23 different three-dimensional trunk positions. Muscle synergies were extracted separately using non-negative matrix factorization during MVIC (with one/two repetitions) and MVIE. The effect of position changes on synergies was investigated using response surface models and the Pearson correlation coefficient. The findings show that 6 synergies for 6 directions MVIC and 2 synergies for MVIE are suitable with the variance accounted for of 99.65 ± 0.65 96 and 94.14 ± 1.59, respectively. Trial repetition does not affect the synergies. In Conclusion, during the same activity in different positions and trials, the synergy of the main activity is preserved. These show the stability of synergies and their dependence on the activity type. This stability may help to determine the main damage caused and provide appropriate treatment protocol for trunk injuries.