Morphological and mechanical properties of the human triceps surae aponeuroses taken from elderly cadavers: implications for muscle-tendon interactions

The human triceps surae (two gastrocnemii and soleus) has aponeuroses in the proximal and distal aspects, the latter of which insert into the calcaneus by sharing the common Achilles tendon. These tendinous tissues are known to have elasticity and upon muscle contraction the aponeurosis is stretched both longitudinally (along the muscle’s line of action) and transversely. Higher aponeurosis transverse deformability has been documented, but there is a paucity of information on the morphology and mechanical properties of human aponeurosis. This study aimed to identify morphological and mechanical characteristics of the human triceps surae aponeuroses. Twenty-five triceps surae muscle-tendon units were procured from 13 human donors (formalin fixed, 6 males, 7 females) aged 67-91 years. Specimens of aponeuroses were excised from the eight regions (posterior and anterior regions of the gastrocnemius medialis and lateralis, medial and lateral parts of soleus; proximal, middle, and distal sites each, 2-4 cm × 2-4 cm). Aponeurosis thickness was measured using a digital caliper. Uniaxial tensile tests were implemented to determine the mechanical properties of specimens loaded longitudinally (along the muscle’s line of action) and transversely. The aponeurosis thickness showed significant differences between muscles and sites, while Young’s modulus showed direction-dependent (longitudinal vs. transverse) differences within sites. Results show different morphology and mechanical properties of aponeuroses between synergist muscles. The reason for site-dependent differences in stiffness is due to a reduced aponeurosis thickness rather than a reduction in the material property. The anisotropic elastic feature (differences between longitudinal and transverse directions) of the aponeuroses was more pronounced than previous in vivo findings, suggesting inherent material design of the aponeurosis that matches three-dimensional contractile behavior of muscle fibers.

人类小腿三头肌（triceps surae）由腓肠肌（gastrocnemii）内外侧头与比目鱼肌（soleus）构成，其近端与远端区域均分布有腱膜（aponeurosis，复数形式aponeuroses），其中远端腱膜通过共同的跟腱（Achilles tendon）附着于跟骨（calcaneus）。这类腱性组织具备弹性特征，当肌肉收缩时，腱膜会沿肌肉作用线方向发生纵向拉伸，同时伴随横向形变。已有研究证实腱膜具有较高的横向形变能力，但目前关于人类腱膜的形态学与力学特性的相关研究数据仍较为匮乏。本研究旨在阐明人类小腿三头肌腱膜的形态学与力学特性。研究从13名年龄介于67~91岁的人体捐献者（经福尔马林固定，其中男性6名、女性7名）处获取了25个小腿三头肌肌-腱单元标本。研究人员从8个区域切取腱膜标本：包括腓肠肌内侧头与外侧头的后、前区域，比目鱼肌的内侧与外侧部分；每个上述区域均分别采集近端、中端、远端位点的标本，标本尺寸为2~4 cm × 2~4 cm。采用数显游标卡尺测量腱膜厚度。对分别沿肌肉作用线纵向加载与横向加载的标本开展单轴拉伸试验（uniaxial tensile test），以测定其力学特性。腱膜厚度在不同肌肉与不同位点间均存在显著差异，而杨氏模量（Young’s modulus）则在同一位点内表现出与加载方向相关的差异（纵向与横向对比）。研究结果显示，协同肌（synergist muscles）间的腱膜形态学与力学特性存在显著差异。刚度随位点产生差异的原因，源于腱膜厚度的降低，而非材料本征特性的变化。腱膜的各向异性弹性特性（纵向与横向方向间的差异）相较于此前的体内（in vivo）研究结果更为显著，这表明腱膜的材料本征设计与肌纤维的三维收缩行为相匹配。