Perreault Eric J, Day Scott J, Hulliger Manuel, Heckman C J, Sandercock Thomas G
Department of Physiology, Northwestern University Medical School, Chicago, Illinois 60611, USA.
J Neurophysiol. 2003 Feb;89(2):738-44. doi: 10.1152/jn.00168.2002.
Nearly all muscle models and most motor control concepts assume that forces from individual muscle fibers and motor units sum in an additive manner once effects of in-series tendon compliance are taken into account. Due to the numerous mechanical linkages between individual fibers, though, it is unclear whether this assumption is warranted. This work examined motor unit force summation over a wide range of muscle forces in the cat soleus. Nonadditive summation implies a nonlinear summation of motor unit forces. Summation nonlinearities were quantified during interactions of 10 individual motor units and 4 motor unit bundles containing approximately 10 units each. These protocols allowed motor unit force summation to be examined from approximately 0 to 25% of tetanic muscle force. Nonlinear summation was assessed by comparing the actual forces to the algebraic sum of individual units and bundles stimulated in isolation. Superadditive summation meant that the actual force exceeded the algebraic sum, whereas subadditive summation meant that the actual force was smaller than the algebraic sum. Experiments tested the hypothesis that superadditive summation occurs at low force levels when few motor units are recruited, whereas subadditive summation prevails above 10% of tetanic force. Results were consistent with this hypothesis. As in previous studies, nonlinear summation in the soleus was modest, but a clear transition from predominately superadditive to predominantly subadditive summation occurred in the range of 6-8% of tetanic force. The largest nonlinearities were transient and appeared at the onset of recruitment and derecruitment of groups of motor units. The results are discussed in terms of the mechanical properties of the connective tissue forming the tendon and linking muscle fibers.
几乎所有的肌肉模型和大多数运动控制概念都假定，一旦考虑了串联肌腱顺应性的影响，来自单个肌纤维和运动单位的力会以相加的方式求和。然而，由于单个纤维之间存在众多的机械连接，尚不清楚这一假设是否合理。这项研究在猫的比目鱼肌中，对广泛范围内的肌肉力量下的运动单位力总和进行了研究。非相加总和意味着运动单位力的非线性总和。在10个单个运动单位以及4个每组包含约10个单位的运动单位束的相互作用过程中，对总和非线性进行了量化。这些实验方案使得能够在大约从强直肌力的0到25%的范围内研究运动单位力总和。通过将实际力与单独刺激的单个单位和束的代数和进行比较，来评估非线性总和。超相加总和意味着实际力超过代数和，而次相加总和意味着实际力小于代数和。实验检验了这样一个假设，即当募集的运动单位较少时，在低力量水平会出现超相加总和，而在强直肌力的10%以上则以次相加总和为主。结果与这一假设一致。与之前的研究一样，比目鱼肌中的非线性总和程度适中，但在强直肌力的6 - 8%范围内，明显出现了从主要是超相加总和到主要是次相加总和的转变。最大的非线性是短暂的，出现在运动单位组募集和去募集的开始阶段。将根据构成肌腱并连接肌纤维的结缔组织的力学特性来讨论这些结果。
