Studies on the hysteresis of trunk muscles-Muscular specificities must be taken into account.

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Hysteresis refers to a physical phenomenon in which the response or state of a system depends on both the input variable and its history. Hysteresis phenomena are also observed in biological systems and have been described for the sensorimotor system. The aim of the present study was to determine whether hysteresis phenomena can also be detected in trunk muscles during isometric load-varying situations. To this end, 40 healthy individuals (19 women) were subjected to isometric tests, where the applied load was systematically altered by complete rotations of the entire body in the Earth's gravitational field. The study was conducted with 25%, 50%, and 75% of the upper body weight. Additionally, we varied the starting point (forward tilt and backward tilt) and the direction of rotation. The activity of a total of six trunk muscles was recorded using surface EMG (sEMG). The sEMG amplitudes were compared between the phases of increasing and decreasing load levels for each test situation. Hysteresis behavior was observed in all examined muscles, with the movement half-phase with increasing load showing higher amplitudes than the half-phase with decreasing load. However, this was consistently verifiable only for the multifidus muscle. For the abdominal muscles, the longissimus, and the iliocostalis muscle, the occurrence of hysteresis depended on the starting position: it could only be demonstrated if the starting point was chosen to correspond with the muscles' main force direction. Thus, only the multifidus muscle exhibits a situation-independent hysteresis, whereas all other examined trunk muscles only show this phenomenon if subjected to load already at a loading situation. This indicates a physiologically determined functional weakness for load impacts on primarily unloaded muscles, posing a potential injury risk.