Abstract
Passive muscle stiffness is positively associated with explosive performance. Drop jump training may be a strategy to increase passive muscle stiffness in the lower limb muscles. Therefore, the purpose of this study was to examine the effect of 8-week drop jump training on the passive stiffness in the plantar flexor muscles and the association between training-induced changes in passive muscle stiffness and explosive performance. This study was a randomized controlled trial. Twenty-four healthy young men were divided into two groups, control and training. The participants in the training group performed drop jumps (five sets of 20 repetitions each) 3days per week for 8weeks. As an index of passive muscle stiffness, the shear moduli of the medial gastrocnemius and soleus were measured by shear wave elastography before and after the intervention. The participants performed maximal voluntary isometric plantar flexion at an ankle joint angle of 0° and maximal drop jumps from a 15cm high box. The rate of torque development during isometric contraction was calculated. The shear modulus of the medial gastrocnemius decreased for the training group (before: 13.5±2.1kPa, after: 10.6±2.1kPa); however, such a reduction was not observed in the control group. There was no significant group (control and training groups)×time (before and after the intervention) interaction for the shear modulus of the soleus. The drop jump performance for the training group improved, while the rate of torque development did not change. Relative changes in these measurements were not correlated with each other in the training group. These results suggest that drop jump training decreases the passive stiffness in the medial gastrocnemius, and training-induced improvement in explosive performance cannot be attributed to change in passive muscle stiffness.
Highlights
Passive muscle stiffness could be an important determinant in sports performance because it has been found to be associated with explosive performance (Takahashi et al, 2018; Ando and Suzuki, 2019; Miyamoto et al, 2019; Ando et al, 2021)
The medial gastrocnemius (MG) shear modulus increased from the baseline to 8 weeks later for the control group (p = 0.001, d = 0.86), whereas it decreased for the training group (p = 0.001, d = 1.38)
No significant interactions or main effects were found for the rate of torque development (RTD), electromechanical delay (EMD), and rate of electromyogram rise (RER) of the MG and SOL
Summary
Passive muscle stiffness (muscle stiffness at rest) could be an important determinant in sports performance because it has been found to be associated with explosive performance (Takahashi et al, 2018; Ando and Suzuki, 2019; Miyamoto et al, 2019; Ando et al, 2021). Miyamoto et al (2019) showed that passive vastus lateralis stiffness positively related to the sprinting ability of sprinters. Fouré et al (2009, 2011) indicated a possibility that 8 weeks of jump training increases the passive stiffness in the gastrocnemius of humans They regarded the ankle joint stiffness calculated from the passive plantar flexor torque during dorsiflexion as passive muscle stiffness. Ultrasound shear wave elastography was used to directly measure the longitudinal elasticity of muscle in vivo, and the results showed similar relationships between passive muscle stiffness and explosive performance as described above (Ando and Suzuki, 2019; Miyamoto et al, 2019; Ando et al, 2021). Ultrasound shear wave elastography must be employed to clarify the effect of jump training on muscle stiffness
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