Physical activity and other modifiers of extracellular matrix dynamics in healthy and diseased tendon tissue: focus on in vivo techniques

Tendons have traditionally been considered largely metabolically inert, and little research has focused on tendon matrix dynamics. In the past few decades, the study of tendon extracellular matrix dynamics in humans has progressed immensely as new methods have been used to investigate long-term tissue remodeling and more acute changes in the turnover of extracellular matrix in tendon tissue. The number of human in vivo trials has increased, and new exciting fields of research in modifiers of tissue dynamics have advanced tendon research and provided new insight. This paper reviews the current knowledge of tendon extracellular matrix dynamics in healthy and diseased tissues. Furthermore, we review the response of various factors such as loading, unloading, exogenous growth factors, and aging. Physical activity and growth factors stimulate protein synthesis in a minor fraction of the adult human tendon, whereas inactivity reduces synthesis and increases breakdown of proteins in tendon. The influence of physical activity level seems to surpass the impact of aging per se on tissue turnover in tendon. A comprehensive understanding of tendon tissue extracellular matrix dynamics and its adaptation to modifiers is crucial for establishing a foundation for tendon injury prevention, treatment, and rehabilitation.