Synek A, Pahr D H
Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria.
Institute of Lightweight Design and Structural Biomechanics, Vienna University of Technology, Getreidemarkt 9, 1060 Vienna, Austria.
J Biomech. 2016 Oct 3;49(14):3423-3429. doi: 10.1016/j.jbiomech.2016.09.004. Epub 2016 Sep 14.
The extensor mechanism is a tendinous network connecting intrinsic and extrinsic muscles of the finger and its function has not yet been fully understood. The goal of this study was to assess the effect of the extensor mechanism on the maximum isometric fingertip forces - a parameter which is essential for grasping. For this purpose, maximum fingertip forces in all directions (i.e. feasible force sets) of two musculoskeletal models of the index finger were compared: the wEM model included a full representation of the extensor mechanism, whereas in the noEM model the extensor mechanism was replaced by a single extensor tendon without connectivity to intrinsic muscles. The feasible force sets were computed in the flexion-extension plane for nine postures. Forces in four predefined directions (palmar, proximal, dorsal, and distal), and the peak resultant forces were evaluated. Averaged forces in all four predefined directions were considerably larger in the wEM model (+187.6%). However, peak resultant forces were slightly lower in the wEM model (-4.3% on average). The general advantage of the wEM model could be explained by co-contraction of intrinsic and extrinsic extensor muscles which allowed reaching larger activation levels of the extrinsic flexors. Only within a narrow range of force directions the co-contraction of intrinsic muscles limited the fingertip forces and lead to lower peak resultant forces in the wEM model. Rather than maximizing peak resultant forces, it appears that the extensor mechanism is a sophisticated tool for increasing maximum fingertip forces over a broad range of postures and force directions - making the finger more versatile during grasping.
伸肌机制是连接手指固有肌和外在肌的腱性网络，其功能尚未完全明确。本研究的目的是评估伸肌机制对最大等长指尖力的影响——这是抓握的一个关键参数。为此，比较了食指的两种肌肉骨骼模型在各个方向（即可行力集）上的最大指尖力：wEM模型完整呈现了伸肌机制，而在noEM模型中，伸肌机制被一条与固有肌无连接的单一伸肌腱替代。在屈伸平面上针对9种姿势计算可行力集。评估了四个预定义方向（掌侧、近端、背侧和远端）的力以及合力峰值。wEM模型中所有四个预定义方向的平均力显著更大（增加了187.6%）。然而，wEM模型中的合力峰值略低（平均降低了4.3%）。wEM模型的总体优势可以用固有伸肌和外在伸肌的共同收缩来解释，这使得外在屈肌能够达到更高的激活水平。只有在狭窄的力方向范围内，固有肌的共同收缩才会限制指尖力，并导致wEM模型中的合力峰值降低。伸肌机制似乎并非使合力峰值最大化，而是一种精密工具，可在广泛的姿势和力方向上增加最大指尖力，从而使手指在抓握过程中更加灵活多变。
