Author(s): TOWAKO, T., KAJI, M., ITO, K., NAKATA, H., OHTAKA, C., SHIBASAKI, M., Institution: NARA WOMEN'S UNIVERSITY, Country: JAPAN, Abstract-ID: 807

INTRODUCTION:
Well-coordinated control of muscle contraction and relaxation is essential for motor performance. While muscle contraction has been extensively studied and its underlying mechanisms are relatively well understood—such as the neuromuscular control of motor units and force generation, the relationship between muscle force and brain activation, and the modulation of muscle contraction assessed by electromyography—these approaches mainly focus on neural activation and provide limited insight into the mechanical state of the muscle. In contrast, muscle relaxation is often more difficult to control than contraction, potentially reflecting residual mechanical tension within the muscle–tendon unit. Despite its importance, the neuromechanical basis of force modulation during relaxation remains poorly understood. Using electromyography (EMG) and shear wave elastography (SWE), this study examined muscle activity and stiffness during isometric force generation and relaxation at the muscle belly (MB) and motor point (MP) to clarify how measurement location and muscle stiffness relate to force modulation.
METHODS:
Eighteen adult women participated in this study. SWE and EMG were recorded from the MB and MP of the rectus femoris (RF) and vastus lateralis (VL) in the right quadriceps femoris. Data collection was conducted over two separate days to minimize the effects of fatigue. After assessment of maximal voluntary contraction (MVC), participants performed single 30-s isometric knee extension contractions at three intensities (10%, 30%, and 50% MVC). In addition, incremental and decremental force tasks were performed at each stage between 10% and 50% MVC, with force changing in 10% MVC steps every 10 s.
RESULTS:
Integrated EMG (iEMG) increased in an intensity-dependent manner during both single and incremental–decremental tasks. No location-dependent differences in iEMG were observed in the RF, whereas iEMG was consistently higher at the MP than at the MB in the VL. SWE showed no intensity-dependent differences during single contractions. During incremental loading, SWE values in the VL were higher at the MP than at the MB at ≥20% MVC, while no regional differences were observed in the RF. Similar patterns were observed during decremental loading, with higher SWE at the MP in the VL at 40% and 50% MVC. Overall, SWE increased with force intensity but showed smaller differences between intensities than iEMG, particularly around 30–40% MVC.
CONCLUSION:
The differences in SWE responses between the single and incremental loading conditions likely reflect a time-dependent increase in muscle stiffness. Prolonged force holding may progressively elevate muscle stiffness, which could mechanically constrain not only force adjustments but also subsequent muscle relaxation when contraction intensity is reduced. Such stiffness accumulation may therefore impair relaxation control and contribute to difficulties in force modulation following sustained contractions.