Investigated muscles and electrode positions.

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.迟滞效应（Hysteresis）是指系统的响应或状态同时取决于输入变量及其作用历史的一种物理现象。迟滞效应同样存在于生物系统中，且已在感觉运动系统（sensorimotor system）中被报道。本研究旨在探究在等长负荷动态变化的场景中，躯干肌肉是否同样存在迟滞效应。为此，研究招募了40名健康受试者（其中19名女性），开展等长测试：测试过程中，通过受试者全身在地球重力场中的完整旋转，系统性地调整施加的负荷。本次测试分别以上半身重量的25%、50%和75%作为负荷水平开展。此外，研究还调整了测试起始姿态（前倾与后倾）以及旋转方向。研究采用表面肌电图（surface EMG, sEMG）记录了共计6块躯干肌肉的肌电活动。针对每种测试场景，分别对比负荷上升阶段与负荷下降阶段的表面肌电振幅。所有受试肌肉均表现出迟滞效应特征：负荷上升半阶段的肌电振幅高于负荷下降半阶段。但该特征仅在多裂肌（multifidus muscle）中得到了一致验证。而对于腹肌、最长肌（longissimus）以及髂肋肌（iliocostalis muscle）而言，迟滞效应的出现与否取决于起始姿态：仅当起始姿态与肌肉的主要发力方向一致时，才能观测到迟滞效应。由此可见，仅多裂肌展现出与测试场景无关的迟滞效应；其余所有受试躯干肌肉仅在预先承受负荷的负荷场景中，才可观测到迟滞现象。这表明，对于原本未承受负荷的肌肉而言，负荷冲击存在生理学层面的功能性缺陷，这可能带来潜在的运动损伤风险。