Impact of Muscle and Tendon Dysfunction on the Mechanics of Muscle Contraction and Locomotion Capacity in People With Type 2 Diabetes Mellitus, and Efficacy of a Passive Stretching Training Program to Counteract These Alterations

Diabetes is a chronic-degenerative metabolic disorder that has reached pandemic proportions, mainly because of the increasing incidence and prevalence of type 2 diabetes mellitus (T2D). According to the International Diabetes Federation (IDF, 2017), 425 million people suffer from diabetes worldwide and these may rise to 629 million in 2045 . Within this epidemiological perspective, diabetes emerges as one of the main metabolic disorders with substantial costs for regional and national sanitary systems.

Diabetes hurts cardiovascular function due to chronic hyperinsulinemia and hyperglycemia, along with increased advanced glycation end products (AGEs), pro-inflammatory cytokines, oxidative stress, obesity, dyslipidemia, and physical inactivity, all of which contribute to vascular dysfunction. In particular, several studies have shown that AGEs exert their negative effects through binding to a specific cellular receptor (RAGE), found in several cell systems such as monocytes and endothelial cells. However, little attention has been paid, so far, to alterations in the musculoskeletal system, which may contribute to the decline of the general state of health of diabetic people and may limit the therapeutic use of exercise in these subjects.

Diabetes causes non-enzymatic glycation of soft tissues, including muscle and tendon, leading to an increase in muscle and tendon stiffness. It was observed that Achilles tendon stiffness and skin connective tissue cross-linking are greater in diabetic patients compared to controls and it has been suggested that the elevated tendon stiffness may influence gait parameters. Indeed, during walking, diabetic patients display less Achilles tendon elongation, higher tendon stiffness and higher tendon hysteresis compared to healthy controls. The higher energy cost of walking in diabetic patients could thus be related to an impairment of the Achilles tendon function. The stiffening of the muscle, on the other hand, reduces its capability to change in shape, affecting its potential for modulating the mechanical request during contraction (and locomotion), also increasing the metabolic demands. Therefore, investigating the mechanical alterations caused by an increase in muscle and tendon stiffness could provide new insights into diabetes pathophysiology.

Training strategies able to reduce muscle and tendon stiffness are expected to improve muscle-tendon function and locomotor capability of diabetic patients. Even if strength and endurance training protocols allow to improve both blood glucose and muscle contractile function, they seem ineffective in reducing muscle and tendon stiffness in T2D patients. Notably, these training modalities present a significant dropout in the diabetic population, generally higher than 25%.

Static and dynamic stretching are effective in decreasing muscle and tendon stiffness but, in both cases, the decrease in stiffness is associated with a temporary decrease in muscle and tendon mechanical function.