Hu Xiaogang, Rymer William Z, Suresh Nina L
Sensory Motor Performance Program, Rehabilitation Institute of Chicago, Chicago, IL, USA.
J Neural Eng. 2014 Apr;11(2):026015. doi: 10.1088/1741-2560/11/2/026015. Epub 2014 Mar 24.
Muscle force is generated by a combination of motor unit (MU) recruitment and changes in the discharge rate of active MUs. There have been two basic MU recruitment and firing rate paradigms reported in the literature, which describe the control of the MUs during force generation. The first (termed the reverse 'onion skin' profile), exhibits lower firing rates for lower threshold units, with higher firing rates occurring in higher threshold units. The second (termed the 'onion skin' profile), exhibits an inverse arrangement, with lower threshold units reaching higher firing rates.
Using a simulation of the MU activity in a hand muscle, this study examined the force generation capacity and the variability of the muscle force magnitude at different excitation levels of the MU pool under these two different MU control paradigms. We sought to determine which rate/recruitment scheme was more efficient for force generation, and which scheme gave rise to the lowest force variability.
We found that the force output of both firing patterns leads to graded force output at low excitation levels, and that the force generation capacity of the two different paradigms diverged around 50% excitation. In the reverse 'onion skin' pattern, at 100% excitation, the force output reached up to 88% of maximum force, whereas for the 'onion skin' pattern, the force output only reached up to 54% of maximum force at 100% excitation. The force variability was lower at the low to moderate force levels under the 'onion skin' paradigm than with the reverse 'onion skin' firing patterns, but this effect was reversed at high force levels.
This study captures the influence of MU recruitment and firing rate organization on muscle force properties, and our results suggest that the different firing organizations can be beneficial at different levels of voluntary muscle force generation and perhaps for different tasks.
肌肉力量由运动单位（MU）募集和活跃运动单位放电率的变化共同产生。文献中报道了两种基本的运动单位募集和放电率模式，它们描述了力量产生过程中运动单位的控制情况。第一种（称为反向“洋葱皮”模式），较低阈值的单位放电率较低，而较高阈值的单位放电率较高。第二种（称为“洋葱皮”模式），呈现相反的排列，较低阈值的单位达到较高的放电率。
本研究通过对手部肌肉运动单位活动进行模拟，考察了在这两种不同的运动单位控制模式下，运动单位池在不同兴奋水平时的力量产生能力以及肌肉力量大小的变异性。我们试图确定哪种速率/募集方案在力量产生方面更有效，以及哪种方案导致的力量变异性最低。
我们发现，在低兴奋水平时，两种放电模式的力量输出均导致分级力量输出，并且两种不同模式的力量产生能力在约50%兴奋度时出现差异。在反向“洋葱皮”模式中，在100%兴奋度时，力量输出达到最大力量的88%，而对于“洋葱皮”模式，在100%兴奋度时，力量输出仅达到最大力量的54%。在“洋葱皮”模式下，低至中等力量水平时的力量变异性低于反向“洋葱皮”放电模式，但在高力量水平时这种效应则相反。
本研究揭示了运动单位募集和放电率组织对肌肉力量特性的影响，我们的结果表明，不同的放电组织在不同水平的随意肌肉力量产生中可能是有益的，也许适用于不同的任务。
