Time course of changes in the human Achilles tendon properties and metabolism during training and detraining in vivo

The purpose of this study was to investigate the time course of changes in human tendon properties and metabolism during resistance training and detraining. Nine men (21-27 years) completed 3 months of isometric plantar flexion training and another 3 months of detraining. At the beginning and on every 1 month of training and detraining periods, the stiffness, blood circulation (blood volume and oxygen saturation), serum procollagen type 1 C-peptide (P1P; reflects synthesis of type 1 collagen), echointensity (reflects collagen content), and MRI signal intensity (reflects collagen structure) of the Achilles tendon were measured. Tendon stiffness did not change until 2 months of training, and the increase (50.3%) reached statistical significance at the end of the training period. After 1 month of detraining, tendon stiffness had already decreased to pre-training level. Blood circulation in the tendon did not change during the experimental period. P1P increased significantly after 2 months of training. Echointensity increased significantly by 9.1% after 2 months of training, and remained high throughout the experiment. MRI signal intensity increased by 24.2% after 2 months and by 21.4% after 3 months of training, but decreased to the pre-training level during the detraining period. These results suggested that the collagen synthesis, content, and structure of human tendons changed at the 2-month point of training period. During detraining, the sudden decrease in tendon stiffness might be related to changes in the structure of collagen fibers within the tendon.