The passive elastic forces of muscles and tendons and stretch forces determine the stiffness of individual muscle-tendon-fascia units. Stiffness, one of the properties of soft tissue, is defined as the ability to resist external forces or deformations from an initial shape 13. Therefore, an investigation of the intermuscle and intratendon/PF distribution of passive stiffness in vivo would provide more information on the muscle-tendon-fascia biomechanics, allowing clinicians to optimize treatment strategies by, for example, targeting stiffer locations. However, the passive ROM cannot be used to distinguish the different structures crossing the joint, such as the skin, joint capsule, synergistic muscles, tendons, nerves and fasciae. Traditional metrics used to assess passive mechanical properties include the passive ROM and the global joint torques generated by motion resistance. They can lead to reduced joint ranges of motion (ROM) during walking and running 12. Muscle/tendon contractures are the most obvious symptoms and are characterized by decreased muscle lengths/tendon lengths and increased passive stiffness values. Pain caused by Achilles tendinitis and plantar fasciitis can reduce the use intensity of the affected foot, decrease the flexibility of muscle/tendon, and ultimately, lead to muscle/tendon contractures 1, 9, 10, 11. It has been reported that the incidence rate of Achilles tendinitis is 2.35‰ in the general population 7, while the lifetime prevalence of plantar fasciitis is approximately 10% in the general population 8. The passive mechanical properties of muscle/tendon/fascia play an important role in the completion of each movement. This knowledge may greatly improve our ability to distinguish normal regional changes in muscle-tendon-fascia biomechanical properties from those arising due to damage. Therefore, it is necessary to further understand the relationship between muscle-tendon-fascia anatomy and mechanical properties. In other words, these regional structural features (including those of the MG, LG, SOL, AT and PF) have great inherent variability, which may affect the position and severity of damage in an individual 6. Furthermore, the LG is shorter and smaller than the MG, so the MG and LG differ in terms of muscle strength and their contribution to the AT 6. In addition, the superficial layer of the AT is formed by the MG tendons, while the deep layer consists of the LG tendons and SOL tendons 5. These muscle tendons rotate as they descend but do not run parallel to each other 4. The twisted structure of the AT is formed by the MG tendons, LG tendons and SOL tendons. In terms of anatomy, the AT, which is the conjoined tendon of the medial gastrocnemius (MG), the lateral gastrocnemius (LG), and the soleus (SOL), is an important bridge for walking, running and jumping in daily life, and each of the contributing muscles exhibit unique structural features 2, 3. However, because of the complexity of the anatomy and mechanics of the AT and plantar fascia (PF), it can be challenging to identify the region and severity of the damage. For example, eccentric exercises are more effective in the treatment of mid-substance tendinopathy than in the treatment of insertional tendinopathy 1. In the clinic, the type of treatment administered depends in part on the region and intensity of pain. These insights can be used to gain a more intuitive understanding of the relationships between the elastic properties of the muscle-tendon unit and its function.Īchilles tendon (AT) injuries and plantar fasciitis can cause chronic pain and impairment 1. Regardless of the knee and ankle angles, the stiffness of the PF increased in a proximal-to-distal direction (p < 0.001). The stiffnesses of the AT, MG, LG, SOL and the fascia with the knee fully extended were significantly higher than those with the knee flexed to 90° (p AT0cm > AT6cm at PF50° and 0° (p AT3cm > AT6cm at DF25°. Twenty healthy subjects participated in this study in which the passive elastic properties of the medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus muscles (SOL), Achilles tendon (AT, at 0 cm, 3 cm and 6 cm proximal to the calcaneus tubercle, corresponding to AT0cm, AT3cm and AT6cm, respectively) and plantar fascia (PF) were quantified when their knee was fully extended or flexed to 90° using shear wave elastography at 25° of dorsiflexion (DF25°), 0° (neutral position) of flexion, and 50° of plantar flexion (PF50°) of the ankle joint. Stiffness is a valuable indicator of the functional capabilities of muscle-tendon-fascia.
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