Stretch-induced changes in tension generation process and stiffness are not accompanied by alterations in muscle architecture of the middle and distal portions of the two gastrocnemii

The study aimed to evaluate the stretch-induced changes in muscle architecture in different portions of the gastrocnemius medialis (GM) and lateralis (GL) muscles. The reliability and sensitivity of the measurements were also assessed. Fascicle length (FL) and pennation angle (PA) were calculated in the middle and distal portions of GM and GL at 0°, 10° and 20° of ankle dorsiflexion. At the same angles, passive torque (Tpass), peak torque (pT) and myotendinous junction displacement of GM were determined. Stiffness was calculated at muscle-tendon unit (MTU), muscle and tendon level. After static stretching administration, Tpass, pT and MTU stiffness decreased by 22%, 12% and 16%, respectively (p < 0.05). Muscle and tendon stiffness decreased by 15% and 16% (p < 0.05). Nevertheless, no changes in FL and PA occurred. The reliability of the approach was always very high (intraclass correlation coefficient > 0.90), with an adequate level of sensitivity. pT after static stretching was related to decreases in MTU, muscle and tendon stiffness, but not to alterations in muscle architecture.