Plyometric Training Effects on Gastrocnemii Muscle and Tendon Passive Stiffness

Plyometric training is commonly used to improve athletic performance but it is unclear how each component of the muscle-tendon complex (MTC) is affected by this intervention. The effects of fourteen weeks of plyometric training on the passive stiffness of the gastrocnemii muscle and Achilles tendon was determined simultaneously in order to assess possible local adaptations of elastic properties. The ankle joint range of motion, passive force of the gastrocnemii MTC and elongation of the gastrocnemii muscle and Achilles tendon were determined using ultrasonography during passive cyclic stretching in nineteen subjects divided into trained (n=9) and control (n=10) groups. A significant increase in vertical jump performances was found for the trained group (P < 0.05). Considering the passive force-length relationship, an upward trend in stiffness of the gastrocnemii MTC P = 0.09) and in the gastrocnemii muscle stiffness (P = 0.07) were found. In contrast, no significant change in gastrocnemii tendon stiffness and in MTC geometry was determined (P > 0.05). The change in intrinsic muscle stiffness might be mainly due to change in intramuscular collagen concentration.