Our research shows that knee extension and isometric contraction peak torque is significantly higher at 60° flexion compared to 30° and 90° flexion, regardless of the knee joint's rotation or position, which is similar to many previous research findings 14,18,28, which demonstrate the interaction of muscle forces and joint kinematics in a musculoskeletal model of the lower limb. Cavalcante simarly found that a 60° knee flexion position during neuromuscular electrical stimulation (NMES) generated greater torque and current efficiency in the quadriceps, potentially due to enhanced force transmission29. This aligns with the suggestion that a 60° equal length knee extension may improve motor unit recruitment and nerve excitation signals, leading to greater quadriceps force. An other research showed that motor imagery training increased muscle force capacity, potentially through increased cortical descending neural drive and spinal network excitability. Strength training led to changes in motor unit recruitment and rate coding, suggesting that these adaptations may contribute to increased muscle force30. Therefore, our study once again confirmed that a 60 ° isometric knee extension may generate greater force from the quadriceps by improving the efficiency of motor unit recruitment and transmitting more nerve excitation signals.
However, contrary to our expectations, we did not obtain the same results in muscle stiffness testing by MyotonPRO. Although we also observed significant changes in knee joint isometric contraction and hardness relative to muscles in resting positions under all set conditions, there was no significant difference in the muscle stiffness(mechanical property) of the various components of the quadriceps femoris muscle between groups of Isometric angles (30°, 60 °, and 90 °).This may be due to the similar degree of activation on the neuromuscular system at different angles12,31. On the other hand, the torque output is affected by the knee joint angle, with a stable milieu for muscle electrification observed at knee joint angles from 80° to 130°13, also explaining the potential reason for the small difference in muscle hardness. Knee alignment during isometric contraction, muscle activation level and contraction intensity can also affect the muscle activity ratio of the vastus medialis and vastus lateralis10,32. However, the hip and knee joint angles can influence the properties of the quadriceps muscle-tendon unit during maximal voluntary isometric contraction, with higher neuromuscular efficiency and tendon stiffness observed at 60° of knee flexion33. This requires further study considering a range of factors and perspectives. So far, our research can conclude that changes in the hardness of each component of the quadriceps femoris muscle related to angle in healthy individuals cannot explain their changes in isometric force, as there is no difference in the stiffness of each component of the quadriceps femoris muscle at different angles.
Our recent research has shown that joint flexion angle and rotation, as independent factors or interactive effects, can have a significant impact on corresponding muscle hardness and muscle strength34. However, in this study, we found that there was no interaction between joint flexion angle and rotation. The force exerted on the knee joint was influenced by the knee flexion angle, but not by the 10 ° rotation of the knee joint. No significant changes were observed in the stiffness of various components of the quadriceps femoris muscle. Tibial rotation may have limited impact on knee extension torque due to various factors. Bates found that while tibial rotation offsets altered knee kinetics, they did not significantly affect anterior cruciate ligament strain35. In other words, the effect of intra-articular tension caused by tibial rotation is not significant, so there will be no significant difference in muscle force. Another study also supports this point which found that knee prosthesis rotation did not significantly influence tibiofemoral motion during extension tasks36. Our finding suggest that while tibial rotation may have some impact on knee biomechanics, its direct effect on knee extension torque may be limited. Despite these mechanical effects, no corresponding changes in muscle hardness have been observed under different rotational conditions35. Furthermore, the use of a rotating platform in TKA has been shown to reduce tibiofemoral torque and cortical strain, potentially acting as a safeguard in cases of poor bone stock37. Tibial rotation has a significant impact on knee stability and kinematics. Lorenz also found that tibial rotation influences anterior knee stability, with the greatest effect in slightly externally rotated positions38, which suggesting that tibial rotation, particularly in the context of knee surgery, can significantly impact knee stability and kinematics.
In addition, Kärrholm explored the impact of tibial rotation on knee mechanics, with finding no significant changes in quadriceps femoris muscle stiffness. He also observed changes in axial tibial rotation during weight-bearing flexion39, but did not directly link this to quadriceps muscle function. Our study linked axial rotation with knee extension torque of the quadriceps femoris muscle and validated this conclusion. Therefore, it can be considered that tibial rotation within 10 ° does not affect the torque and muscle stiffness of isometric knee extension, but may instead increase the stability of knee extension force,
The correlation between stiffness(Mechanical properties)and force, as well as their significance and mutual inexplicability. Although muscle hardness under various conditions can reflect the state of force exerted, as there is a moderate correlation that may come from the correlation with mechanical and physiological properties, muscle hardness cannot explain the changes in force exerted under various conditions. At present, we are trying to incorporate other physiological indicators in the laboratory. In future research, we will continue to investigate the potential mechanisms underlying the changes in knee extension torque of equal length.
Our research has shown a moderate correlation between muscle stiffness and force exertion, likely due to the influence of mechanical and physiological properties10. However, muscle stiffness alone cannot fully explain changes in force exertion under different conditions39. Other physiological indicators, such as intermuscular coherence, may play a role in this relationship7. Additionally, pre-exercise muscle stiffness has been found to be related to the amount of muscle damage induced by eccentric exercise9. These findings suggest that while muscle stiffness is a significant factor in force exertion, it is not the sole determinant, and other physiological indicators should be considered.
Limitations of the MyotonPRO have been identified, such as the potential impact of muscle tremors and instability on measurements 40. In addition, The accuracy of torque arm length in isometric testing is influenced by various factors. Bauer found that visual feedback and the nature of the isometric contraction can affect force complexity and variability41. Signal processing methods, such as filtering and sample rate, can also impact isometric torque-time parameters27. Although we used the isometric testing system standard of the isokinetic training instrument to record the force output, this is still a possible influencing factor. Some research reports suggest that the strength of skeletal muscles affects the stiffness of tendons. The relationship between tendon stiffness and muscle performance is more complex, with tendon mechanical properties accounting for a significant portion of the variance in rate of torque development19. In the future, it is necessary to measure tendons similar to this study considering the impact of extreme rotation.