Static stretching (SS) is commonly used to prevent or improve limited joint mobility. However, it is unclear whether the components of the muscle-tendon unit (MTU) are affected by 5 min of SS. This study investigated the acute and prolonged effect of SS on the mechanical properties of the MTU. The subjects comprised 15 male participants (mean age: 21.5 AE 1.6 years). MTU stiffness, muscle stiffness, tendon stiffness, and fascicle length of the gastrocnemius muscle were measured by ultrasonography and a dynamometer while the ankle was passively dorsiflexed. The measurements were performed prior to the 5 min of SS, immediately after the SS, and 10 min after the SS. MTU stiffness and muscle stiffness significantly decreased at both immediately and 10 min after SS, whereas no significant differences in MTU stiffness and muscle stiffness were found between immediately and 10 min after SS. Tendon stiffness immediately after SS was significantly higher than prior to and 10 min after SS. No significant change in the fascicle length occurred after SS. These results suggest that 5 min of SS affects MTU and muscle stiffness both immediately and 10 min after SS, which may be associated with a change in the connective tissue properties. ß

Static stretch is commonly used to prevent contracture and to improve joint mobility. However, it is unclear whether the components of the muscle-tendon unit are affected by a static stretch training program. This study investigated the effect of a four-week static stretch training program on the viscoelastic properties of the muscle-tendon unit and muscle. The subjects comprised 18 male participants (mean age 21.4 ± 1.7 years). The range of motion (ROM), passive torque, myotendinous junction (MTJ) displacement and, muscle fascicle length of the gastrocnemius muscle were assessed using both ultrasonography and a dynamometer while the ankle was passively dorsiflexed. After the initial test, the participants were assigned either to a group that stretched for 4 weeks (N = 9) or to a control group (N = 9). The tests were repeated after the static stretch training program. The ROM and MTJ displacement significantly increased, and the passive torque at 30° significantly decreased, in the stretching group after the study period. However, there was no significant increase in muscle fascicle length. These results suggest that a 4-week static stretch training program changes the flexibility of the overall MTU without causing concomitant changes in muscle fascicle length.

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The minimum time required for Static stretching (SS) to change the passive properties of the muscle-tendon unit (MTU), as well as the association between these passive properties, remains unclear. This study investigated the time course of changes in the passive properties of gastrocnemius MTU during 5 min of SS. The subjects comprised 20 healthy males (22.0 ± 1.8 years). Passive torque as an index of MTU resistance and myotendinous junction (MTJ) displacement as an index of muscle extensibility were assessed using ultrasonography and dynamometer during 5 min of SS. Significant differences before and every 1 min during SS were determined using Scheffé's post hoc test. Relationships between passive torque and MTJ displacement for each subject were determined using Pearson's product-moment correlation coefficient. Although gradual changes in both passive torque and MTJ displacement were demonstrated over every minute, these changes became statistically significant after 2, 3, 4, and 5 min of SS compared with the values before SS. In addition, passive torque after 5 min SS was significantly lower than that after 2 min SS. Similarly, MTJ displacement after 5 min SS was significantly higher than that after 2 min SS. A strong correlation was observed between passive torque and MTJ displacement for each subject (r = -0.886 to -0.991). These results suggest that SS for more than 2 min effectively increases muscle extensibility, which in turn decreases MTU resistance.

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