Author(s): YOSHIDA, H., YAGI, M., TAKAHASHI, K., HIRONO, T., ARIKAWA, K., TATEUCHI, H., ICHIHASHI, N., Institution: KYOTO UNIVERSITY, Country: JAPAN, Abstract-ID: 2754

INTRODUCTION:
During eccentric contractions (ECC), reducing fascicle strain is important for minimizing muscle damage and the risk of muscle strain injuries [1]. It is well known that the compliance of the external tendon contributes to reducing muscle fascicle strain by absorbing elastic energy during ECC [2]. Given that aponeurosis, an intramuscular sheet-like tendon, also has similar properties [3], its compliance may also reduce fascicle strain. However, how aponeurosis plays a role in fascicle mechanics during ECC remains unclear. We aimed to investigate how the mechanical properties of the aponeurosis are associated with fascicle mechanics during ECC.
METHODS:
For twenty-one healthy adults, the shear elastic modulus of superficial and deep aponeuroses along their length of the medial gastrocnemius (MG) was measured at rest using shear wave elastography, as a stiffness index. Then, they performed a maximal isometric preactivation, followed by a maximal ECC while the ankle moved from 20° plantarflexion to 20° dorsiflexion at 30°/s. During contraction, fascicle length and fascicle angle (FA) of the MG were measured with B-mode ultrasonography. The ratio of muscle belly segment strain to fascicle strain was calculated as muscle gear [4]. Relationships between the shear elastic modulus of aponeuroses and fascicle mechanics were examined using Spearman’s rank correlation.
RESULTS:
Contrary to the hypothesis, the shear elastic modulus of aponeuroses was not significantly correlated with fascicle strain during ECC (superficial: ρ=0.206, p=0.369; deep: ρ=0.010, p=0.964). Meanwhile, within observed range of 13.1–32.2 kPa, individuals with a lower shear elastic modulus of the deep aponeurosis tended to show greater change in FA from rest to the initial state of ECC (i.e., at the end of preactivation) (ρ=–0.604, p=0.004), and greater FA at the initial state of ECC was correlated with higher gear during ECC (ρ=0.960, p<0.001). Consequently, individuals with a lower shear elastic modulus of the deep aponeurosis tended to exhibit higher gear during subsequent ECC (ρ=–0.701, p<0.001).
CONCLUSION:
Although aponeurosis stiffness was not directly related to fascicle strain, its compliance was linked to a greater FA at the initial state of ECC. As a greater FA enhanced gear [4], the compliant aponeurosis was associated with the higher gear (i.e., smaller fascicle strain per muscle belly segment strain) during subsequent ECC. These findings indicate that aponeurosis can play a role in reducing relative fascicle strain through modification of the initial state of ECC and subsequent gearing mechanism during ECC, providing novel insights into the functional relevance of the aponeurosis in skeletal muscle. Maintaining compliant aponeurosis might be one of the strategies for reducing the risk of muscle strain injuries, which warrants further investigation.
[1] Guilhem et al., Acta Physiol. 2016 [2] Azizi and Roberts, Proc Biol Sci. 2015 [3] Arellano et al., Proc Biol Sci. 2019 [4] Pinto et al., Biol Open. 2023