Department of Sport and Health Sciences, Biomechanics in Sports, Technical University of Munich, Munich, Germany.
Department of Human Sciences, Human Movement Science, Bundeswehr University Munich, Neubiberg, Germany.
Physiol Rep. 2021 Aug;9(15):e14944. doi: 10.14814/phy2.14944.
When an active muscle is stretched and kept isometrically active, the resulting force is enhanced compared to a purely isometric reference contraction at the same muscle length and activity; a generally accepted muscle property called residual force enhancement (rFE). Interestingly, studies on voluntary muscle action regularly identify a significant number of participants not showing rFE. Therefore, the aim was to unmask possible confounders for this non-responsive behavior. Ten participants performed maximum voluntary isometric plantarflexion contractions with and without preceding stretch. Contractions were accompanied by the assessment of voluntary activation using the twitch-interpolation technique. The same test protocol was repeated four additional times with a least on day rest in-between. Additionally, at the first and fifth sessions, a submaximal tetanic muscle-stimulation condition was added. At both muscle-stimulation sessions mean rFE higher 10% (p < 0.028) was found. In contrast, during voluntary muscle action, individual participants showed inconsistent rFE across sessions and only one session (#3) had significant rFE (5%; p = 0.023) in group means. As all participants clearly had rFE in electrical stimulation conditions, structural deficits cannot explain the missing rFE in voluntary muscle action. However, we also did not find variability in voluntary activation levels or muscle activity as the confounding characteristics of "non-responders."
当活跃的肌肉被拉伸并保持等长活动时，与相同肌肉长度和活动的纯等长参考收缩相比，产生的力会增强；这是一种被广泛认可的肌肉特性，称为剩余力增强（rFE）。有趣的是，对自愿肌肉活动的研究经常发现相当一部分参与者没有表现出 rFE。因此，本研究旨在揭示这种无反应行为的可能混杂因素。十名参与者进行了最大自主等长跖屈收缩，同时伴有和不伴有预先拉伸。通过抽搐插值技术评估收缩时的自愿激活。该测试方案在至少一天的休息之间重复了四次。此外，在第一和第五次会议上，还增加了一个次最大的强直肌肉刺激条件。在这两个肌肉刺激会议上，平均 rFE 高 10%（p<0.028）。相比之下，在自愿肌肉活动中，个体参与者在各个会议中 rFE 不一致，只有一次会议（#3）在组平均值中具有显著的 rFE（5%；p=0.023）。由于所有参与者在电刺激条件下显然都有 rFE，结构缺陷不能解释自愿肌肉活动中缺失的 rFE。然而，我们也没有发现自愿激活水平或肌肉活动的可变性是“无反应者”的混杂特征。
