Abstract details
| Abstract-ID: | 1928 |
| Title of the paper: | Activation level-controlled stretching influences the following shortening and isometric torque in in-vivo human knee extensors |
| Authors: | JO, I., Lee, H. |
| Institution: | Yonsei University |
| Department: | Physical education |
| Country: | Korea, South |
| Abstract text | INTRODUCTION: Stretch-shortening cycle (SSC), a natural muscle behavior that occurs during various human movements, has been known to enhance muscle performance (1). Currently, the effect of SSC is accounted for by activation dynamics, stretch reflex, utilizing elastic energy, as well as residual force enhancement (RFE) (2). Nevertheless, it is unclear how SSC influences human performance in vivo due to the complexity of neural control and muscle-tendon interaction. Therefore, this study investigated how the level of muscle activation during the stretching phase influences the following shortening and isometric contraction. METHODS: Healthy young male (n = 6, age: 25.7±2.8, height: 174.4±4.8, weight: 78.7±6.3) and female participants (n = 2, age: 26±2.8, height: 166.0±1.6, weight: 72±5.4) performed knee extensor SSC on a dynamometer (CON-TREX MJ); activation-controlled eccentric knee extension (40, 80, 100% MVC, 20° to 80°, 60°/s, 0° for the fully-extended knee angle) followed by concentric (80° to 20°, 120°/s) – steady-state isometric (20°) contraction at maximum effort. Electromyography feedback from the vastus lateralis (VL) was provided to gradually reach the target eccentric activation level during the eccentric phase. As references, isometric contraction at 20° and 80° and shortening-isometric contraction was measured. The average knee extension torque and work and muscle activation were obtained during the concentric phase of contractions. Residual force depression (RFD) was assessed during the isometric phase. Data were normalized to body weight and analyzed using One-way ANOVA. RESULTS: In the SSC trials, both 80% (0.19±0.03 Nm/BW) and 100% (0.20±0.03 Nm/BW) resulted in higher normalized peak and average torque during the concentric phase compared to trials without a preceding eccentric contraction (0.13±0.03 Nm/BW). Specifically, at 100%, the torque was greater than at 40% (0.15±0.03 Nm/BW). Similarly, work during the concentric phase was higher at 100% and 80% compared to the shortening contraction only, while torque at 40% was lower than at 100%. Force enhancement relative to the shortening-isometric contraction increased at 100% (0.07±0.04%), whereas it was 0.02±0.03% at 40%. Average VL activation during the concentric phase was higher at 80% (86.22±15.95%) and 100% (85.96±16.52%) compared to the no stretching phase (62.43±11.06%). However, there was no difference in RFD during the steady-state isometric phase between trials. CONCLUSION: This study showed that concentric knee extension performance was enhanced when the eccentric contraction was preceded, suggesting influences of the preceding eccentric contraction-induced increase in muscle activation and force enhancement. The observed results suggest that RFE plays a crucial role in enhancing the SSC during in vivo human movements, such as the countermovement jump. REFERENCES: 1) Komi, P. V, Journal of biomechanics, 2000 2) Seiberl, Wolfgang, et al. Frontiers in physiology, 2021 |
| Topic: | Biomechanics |
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