Passive Mechanical Properties of Human Medial Gastrocnemius and Soleus Musculotendinous Unit

Abstract

The in vivo characterization of the passive mechanical properties of the human triceps surae musculotendinous unit is important for gaining a deeper understanding of the interactive responses of the tendon and muscle tissues to loading during passive stretching. This study sought to quantify a comprehensive set of passive muscle-tendon properties such as slack length, stiffness, and the stress-strain relationship using a combination of ultrasound imaging and a three-dimensional motion capture system in healthy adults. By measuring tendon length, the cross-section areas of the Achilles tendon subcompartments (i.e., medial gastrocnemius and soleus aspects), and the ankle torque simultaneously, the mechanical properties of each individual compartment can be specifically identified. We found that the medial gastrocnemius (GM) and soleus (SOL) aspects of the Achilles tendon have similar mechanical properties in terms of slack angle (GM: − 10.96 ° ± 3.48 ° ; SOL: − 8.50 ° ± 4.03 ° ), moment arm at 0° of ankle angle (GM: 30.35 ± 6.42 mm; SOL: 31.39 ± 6.42 mm), and stiffness (GM: 23.18 ± 13.46 Nmm-1; SOL: 31.57 ± 13.26 Nmm-1). However, maximal tendon stress in the GM was significantly less than that in SOL (GM: 2.96 ± 1.50 MPa; SOL: 4.90 ± 1.88 MPa, p = 0.024 ), largely due to the higher passive force observed in the soleus compartment (GM: 99.89 ± 39.50 N; SOL: 174.59 ± 79.54 N, p = 0.020 ). Moreover, the tendon contributed to more than half of the total muscle-tendon unit lengthening during the passive stretch. This unequal passive stress between the medial gastrocnemius and the soleus tendon might contribute to the asymmetrical loading and deformation of the Achilles tendon during motion reported in the literature. Such information is relevant to understanding the Achilles tendon function and loading profile in pathological populations in the future.

Tendon compliance and preload must be considered when determining the in vivo force–velocity relationship from the torque–angular velocity relation

“…The GM model was constructed using architectural parameters from Maganaris 7 (an optimal contractile element (CE) length ( ) of 3.78 cm; a pennation angle ( ) at of 32.5°; a maximum isometric force ( ) of 894.7 N) and from Arnold et al 16 (a tendon slack length ( ) of 40.1 cm). The tendon slack length reported by Arnold et al 16 is longer than experimental measurements of the Achilles tendon 34 because it includes both the length of the Achilles tendon and the full length of the aponeurosis and proximal tendon that connects the contractile element of the GM to its origin. As is typical when simulating younger adults 35 , 36 , the maximum shortening velocity was set to be ten times the optimal CE length.…”

EMG-Constrained and Ultrasound-Informed Muscle-Tendon Parameter Estimation in Post-Stroke Hemiparesis

“…In recent years, tendon stiffness in the linear region has been commonly investigated by ultrasonography-based approaches. Our previous study has investigated the stiffness of GAS and SOL aspects of Achilles tendon in able-bodied subjects during passive stretching and found the stiffness of the two sub-compartments were similar [28]. However, due to variant joint configurations and assumptions, incomparable findings were often reported in vivo studies.…”

“…During these contractions, the dynamometer recorded both joint torque and angle measurements simultaneously at 3000 Hz. To ensure that the functional flexion-extension axis of the ankle was well aligned with the flexion-extension axis of the dynamometer, we followed a standardized procedure recommended by previous studies [28]. Specifically, the axis of ankle flexion/extension was aligned with the axis of the dynamometer using a laser-pointing device.…”

Elastography in the assessment of the Achilles tendon: a systematic review of measurement properties

“…With improvements in ultrasound technology, elastography has emerged as a potential measurement instrument offering opportunities to quantitatively investigate the mechanical and material properties [ 15 ] within the tendon. Elastography research has shown rapid growth in the past 10 years and has been used to understand how loading [ 16 ] ageing [ 17 ] and different treatments [ 18 ] are affecting tendon recovery [ 19 , 20 ].…”