University School of Physical Education, Department of Neurobiology, 55 Grunwaldzka St., Poznań, Poland.
J Electromyogr Kinesiol. 2010 Aug;20(4):599-607. doi: 10.1016/j.jelekin.2010.01.006. Epub 2010 Feb 24.
The summation of contractile forces of motor units (MUs) was analyzed by comparing the recorded force during parallel stimulation of two and four individual MUs or four groups of MUs to the algebraic sum of their individual forces. Contractions of functionally-isolated single MUs of the medial gastrocnemius muscle were evoked by electrical stimulation of thin filaments of the split L5 or L4 ventral roots of spinal nerves. Additionally, contractions of large groups of MUs were evoked by stimuli delivered to four parts of the divided L5 ventral root. Single twitches, 40Hz unfused tetani, and 150Hz fused maximum tetani were recorded. In these experimental situations the summation was more effective for unfused tetani than for twitches or maximum tetani. The results obtained for pairs of MUs were highly variable (more- or less-than-linear summation), but coactivation of more units led to progressively weaker effects of summation, which were usually less-than-linear in comparison to the algebraic sums of the individual forces. The variability of the results highlights the importance of the structure of the muscle and the architecture of its MUs. Moreover, the simultaneous activity of fast and slow MUs was considerably more effective than that of two fast units.
运动单位（MU）收缩力的总和是通过比较两种和四种个体 MU 或四个 MU 组的记录力与它们各自力的代数和来分析的。通过刺激分裂的 L5 或 L4 脊神经腹根的细纤维来诱发内侧比目鱼肌的功能分离的单个 MU 收缩。此外，还可以通过刺激 L5 腹根的四个部分来诱发大 MU 组的收缩。记录到单抽搐、40Hz 未融合的四抽搐和 150Hz 融合的最大四抽搐。在这些实验情况下，对于未融合的四抽搐，总和比抽搐或最大四抽搐更有效。对于 MU 对的结果变化很大（或多或少是线性总和），但更多的单位的共同激活导致总和的效果逐渐减弱，与个体力的代数和相比，通常是非线性的。结果的可变性突出了肌肉结构和 MU 结构的重要性。此外，快速和慢速 MU 的同时活动比两个快速单位的活动更有效。
