Movement adaptations to low back pain (LBP) are believed to protect the painful area. Increased trunk stiffness and decreased trunk damping have been shown in people with recurrent LBP. However, no study has examined these properties using external force perturbations to the trunk during acute LBP when protective adaptations might be expected to have most relevance.

Adaptations to an acute painful stimulus via unilateral injection of hypertonic saline into the right longissimus muscle were assessed using a trunk force perturbation paradigm and a mass-spring-damper model to describe effective trunk dynamical properties. Equal weights (15% body weight) were connected to the front and back of the trunk via a cable. Either one was dropped at random to perturb the trunk. Effective trunk dynamical properties were estimated in fourteen males (mean (standard deviation) age 25 (6) years) assuming that trunk movement can be modelled as a second order linear system. Effective trunk dynamical properties were compared before, during and after the experimentally induced painful period.

Estimates of effective trunk stiffness ($K$) decreased and damping ($B$) increased during pain compared to both before ([mean contrast, 95% CI] $K$: −403 [−651 to −155] Nm⁻¹, $B$: 28 [9–50] Nms⁻¹) and after ($K$: −324 [−58 to −591] Nm⁻¹, $B$: 20 [4–33] Nms⁻¹) the experimentally induced painful period. We interpret our results to show that, when challenged by a step force perturbation, a healthy system adapts to noxious input by controlling trunk velocity rather than trunk displacement, in contrast to observations during remission from recurrent clinical LBP.

运动适应于腰痛（LBP）被认为可以保护疼痛区域。LBP复发者的躯干刚度增加且躯干阻尼降低。但是，尚无研究在急性LBP期间使用外力对躯干的扰动来检查这些特性，而预期的保护性适应最重要。

通过使用躯干力摄动范式和质量弹簧阻尼器模型描述有效的躯干动力学特性，评估了通过向右最长肌中单侧注射高渗盐水来适应急性疼痛刺激。通过电缆将等重（体重的15％）连接到行李箱的前部和后部。任一个随意掉落以扰乱行李箱。假设可以将躯干运动建模为二阶线性系统，则对十四名男性（平均年龄（标准差）年龄25（6）岁）中的躯干有效动力学特性进行了估计。在实验诱发的疼痛期之前，之中和之后比较了有效的躯干动力学特性。

有效躯干刚度的估算（$ķ$）减少，阻尼（$乙$）与之前的两个指标相比，疼痛期间的疼痛感有所增加（[平均对比，95％CI] $ķ$：-403 [-651至-155] Nm ^-1，$乙$：28 [9–50] Nms ^-1）及之后（$ķ$：-324 [-58至-591] Nm ^-1，$乙$：20 [4–33] Nms ^-1）实验诱发的疼痛期。我们解释我们的结果表明，与周期性临床LBP缓解期间的观察结果相反，当受到步进力扰动的挑战时，健康的系统通过控制躯干速度而不是躯干移位来适应有害输入。