Abstract
In everyday muscle action or exercises, a stretch-shortening cycle (SSC) is performed under different levels of intensity. Thereby, compared to a pure shortening contraction, the shortening phase in a SSC shows increased force, work, and power. One mechanism to explain this performance enhancement in the SSC shortening phase is, besides others, referred to the phenomenon of stretch-induced increase in muscle force (known as residual force enhancement; rFE). It is unclear to what extent the intensity of muscle action influences the contribution of rFE to the SSC performance enhancement. Therefore, we examined the knee torque, knee kinematics, m. vastus lateralis fascicle length, and pennation angle changes of 30 healthy adults during isometric, shortening (CON) and stretch-shortening (SSC) conditions of the quadriceps femoris. We conducted maximal voluntary contractions (MVC) and submaximal electrically stimulated contractions at 20%, 35%, and 50% of MVC. Isometric trials were performed at 20° knee flexion (straight leg: 0°), and dynamic trials followed dynamometer-driven ramp profiles of 80°–20° (CON) and 20°–80°–20° (SSC), at an angular velocity set to 60°/s. Joint mechanical work during shortening was significantly (p < 0.05) enhanced by up to 21% for all SSC conditions compared to pure CON contractions at the same intensity. Regarding the steady-state torque after the dynamic phase, we found significant torque depression for all submaximal SSCs compared to the isometric reference contractions. There was no difference in the steady-state torque after the shortening phases between CON and SSC conditions at all submaximal intensities, indicating no stretch-induced rFE that persisted throughout the shortening. In contrast, during MVC efforts, the steady-state torque after SSC was significantly less depressed compared to the steady-state torque after the CON condition (p = 0.034), without significant differences in the m. vastus lateralis fascicle length and pennation angle. From these results, we concluded that the contribution of the potential enhancing factors in SSCs of the m. quadriceps femoris is dependent on the contraction intensity and the type of activation.
Highlights
A stretch-shortening cycle (SSC) is a muscle action that often occurs in everyday movements or sporting exercises
A t-test revealed no statistical difference between maximal voluntary contractions (MVC) at the beginning (92.5 ± 29.9 Nm) and at the end of the test protocol (91.3 ± 29.2 Nm), indicating no fatigue
For the intensities of 35, 50, and 100%, the torque after the stretch phase (T1) in SSCs was significantly higher compared to the isometric pre-activation in the CON conditions at the same activation level and the corresponding dynamometer angle (35%: p = 0.013; 50%: p = 0.001, and 100%: p = 0.032)
Summary
A stretch-shortening cycle (SSC) is a muscle action that often occurs in everyday movements or sporting exercises. During a SSC, a lengthening contraction is immediately followed by a shortening contraction. This results in increased performance during the shortening phase compared to pure shortening contractions (“SSC-effect”) (Komi and Gollhofer, 1997; Komi, 2000). The mechanisms attributed to the enhanced force or work during the concentric phase of the SSC are the stretch-reflex (Dietz et al, 1979), the release of stored passive-elastic energy (Finni et al, 2001; Kawakami et al, 2002) and the pre-activation of muscles (Bobbert and Casius, 2005). An additional SSC mechanism that has been under discussion in recent literature is related to stretch-induced force enhancing effects within the contractile element of muscles (Seiberl et al, 2015b; Hahn and Riedel, 2018; Tomolka et al, 2020)
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