Concurrent deficits of soleus and gastrocnemius muscle fascicles and Achilles tendon post stroke

Abstract

Calf muscles and Achilles tendon play important roles in functional activities. However, it is not clear how biomechanical properties of the uniarticular soleus (SOL) and biarticular gastrocnemius muscle and Achilles tendon, including the fascicle length, pennation angle, and stiffness, change concurrently post stroke. Biomechanical properties of the medial gastrocnemius (GM) and soleus muscles were evaluated bilaterally in 10 hemiparetic stroke survivors using combined ultrasonography-biomechanical measurements. Biomechanical properties of the Achilles tendon including the length, cross-sectional area (CSA), stiffness, and Young's modulus were evaluated, together with calf muscle biomechanical properties. Gastrocnemius and SOL contributions were separated using flexed and extended knee positions. The impaired side showed decreased fascicle length (GM: 6%, P = 0.002 and SOL: 9%, P = 0.03, at full knee extension and 0° ankle dorsiflexion) and increased fascicular stiffness (GM: 64%, P = 0.005 and SOL: 19%, P = 0.012, at a common 50 N force level). In contrast, Achilles tendon on the impaired side showed changes in the opposite direction as the muscle fascicles with increased tendon length (5%, P < 0.001), decreased tendon CSA (5%, P = 0.04), decreased tendon stiffness (42%, P < 0.001) and Young's modulus (30%, P < 0.001) compared with the unimpaired side. The fascicle and tendon stiffness changes were correlated negatively to the corresponding fascicle and tendon length changes, and decrease in Achilles tendon stiffness was correlated to the increases of SOL and GM fascicular stiffness (P < 0.05). Characterizations of calf muscle fascicles and Achilles tendon biomechanical properties help us better understand concurrent changes of fascicles and tendon as part of the calf muscle-tendon unit and facilitate development of more effective treatments.

Keywords: muscle fascicles; stiffness; stroke; tendon.

Fig. 1.

Schematic diagram of the experimental setup. Setup was comprised of a custom knee-ankle driving device, a commercial ultrasound imaging system, an EMG system, and a personal computer. Ankle joint torque signal collected by the torque sensor in the knee-ankle device and the EMG signal were recorded by the personal computer and synchronized with the ultrasound image data using a trigger signal.

Fig. 2.

Longitudinal images of gastrocnemius (GM; A) and soleus (SOL; B) muscles using the LOGIQView technique. Selection of fascicle and measurement of fascicle length and pennation angle, as well as some important anatomic landmarks, including muscle belly and distal end, were shown in the images. C: a representative image of GM demonstrating automatic fascicle detection using localized Radon transform. A region around the middle point between muscle belly and muscle-tendon junction (MTJ) was selected, and 3 lines were detected that were superficial and deep aponeuroses and a fascicle. Length of the fascicle and angle between the fascicle and deep aponeurosis were then calculated automatically.

Fig. 3.

GM and SOL fascicle lengths of the impaired (dashed lines) and unimpaired (solid lines) sides at different ankle and knee positions. A: GM fascicles at various ankle joint angles and full knee extension; B: GM fascicles at various ankle joint angles and 90° knee flexion; C: SOL fascicles at full knee extension; D: GM fascicles at 90° knee flexion. Note the error bar represents the standard error of mean, and shaded area indicates significant difference between the impaired and unimpaired sides.

Fig. 4.

GM and SOL pennation angles of the impaired (dashed lines) and unimpaired (solid lines) sides at different ankle and knee positions. A: GM pennation angles at various ankle joint angles and full knee extension; B: GM pennation angles at various ankle joint angles and 90° knee flexion; C: SOL pennation angles at full knee extension; D: SOL pennation angles at 90° knee flexion. Note the error bar represents the standard error of mean, and shaded area indicates significant difference between the impaired and unimpaired sides.

Fig. 5.

SOL (A) and GM (B) fascicle force-length relation curves. Compared with the unimpaired side, both SOL and GM fascicle force-length curves shifted to the left and became steeper, indicating reduced fascicle length and increased fascicular stiffness in the impaired side. Notice that the error bar represents the standard error of mean.

Fig. 6.

Spring model of the SOL and gastrocnemius muscles and Achilles tendon of the impaired (right) and unimpaired (left) sides at comparable ankle position.