Using vibro-ultrasound method to assess the vastus intermedius stiffness over the entire range of step isometric contraction of knee extensors

Many methods have been developed to quantify skeletal muscle stiffness. One major limitation is their measurement ranges cannot meet the requirement under maximum voluntary contraction (MVC) condition. The muscle stiffness at high intensity contraction is more important but was rarely reported to be assessed. Furthermore, the dependences of gender and age on the relationship between muscle stiffness and contraction intensity were also not studied systematically. To fix these gaps, an improved vibro-ultrasound system was developed and used to assess the shear modulus of vastus intermedius (VI) along the muscle action direction. Forty healthy subjects volunteered to participate and were divided into four groups: young males, young females, elder males and elder females. Measurements were performed over the entire range of isometric contraction, from 0% to 100% MVC (totally 11 levels). The relationship between muscle stiffness and step isometric contraction level, and the gender and age dependences of this relationship were studied. In addition, VI stiffness was compared with the surface electromyography (EMG) collected from vastus lateralis (VL). Our results showed that VI stiffness was positively correlated to the step isometric contraction level over the entire range (p < 0.001). Under relaxed condition there was no significant difference between VI stiffness of different genders (p = 0.156) and age ranges (p = 0.221). However, when performing isometric contraction, VI stiffness of male subjects was larger than that of females (p < 0.001), and that of young subjects was larger than that of elders (p < 0.001). A high correlation was also found between VI stiffness and the neuromuscular activity of VL. These results can provide complementary information of muscle mechanical properties, and help us to further understand the muscle recruitment strategies of the people with different genders and age ranges under different isometric contraction levels.