# Biomechanical, Healing and Therapeutic Effects of Stretching: A Comprehensive Review
## 2 Topics and Results
### Animal, Mathematical, and Computational Models of Stretching  
More scientific information is needed for the creation of new, successful mathematical and computational models of stretching [27; 28]. The mathematical and rheological models of the joint cavity capsule and intra-articular synovial fluid turnover (viscosity and permeation of hyaluron, glycosaminoglycans (GAGs), and albumin) indicate cellular mechanisms of stretching and a role of the inter-cellular matrix as a selective molecular filter. The specific rheological properties of joint synovial fluid are altered in traumatic and post-traumatic pathological states (different arthoses, rheumatoid arthritis, osteoarthritis etc.) [29]. In the treatment and prevention of sports injuries, as well as in the development of improved sports programs for injury prevention, static stretching (SS) is very important for the efficient rehabilitation of joints [3; 30; 31; 32; 33; 34]. Based on the latest scientific findings, especially on the biomechanical contributions in this field, new preventive and therapeutic measures for avoiding stiffness and motion impairment in the joints can be adopted during the early stages of diseases [2; 9; 15]. The therapeutic effects of stretching have been established in a great number of experimental animal models (e.g., post-traumatic knee contractures in rat and rabbit models,which have significance for humans) [7; 35; 36; 37; 38]. Thus, it is possible to evaluate important data on cellular functions and the intracellular matrix components of joint cartilage, as well as information on the morphological structures and functions of joint capsules, both in healthy controls and in joints that have been modified in the processes of contractures (post-traumatic, myogenic, arthrogenic, etc.) [7; 11; 36; 37; 38; 39]. [PERSON] et al. examined the effect of stretching combined with ultrashort wave diathemy on joint functions and clarified its cellular mechanisms in a rabbit knee contracture model [38]. [PERSON] et al. [7] studied the effects of different static progressive stretching durations on the knee joint's range of motion, collagen- and alpha-actin expressions in fibroblasts, inflammatory cell number, and fibrotic changes in the joint capsule (as the result of different static progressive stretching durations applied to a post-traumatic knee contracture in a rat model). The authors concluded that static progressive stretching could improve post-traumatic knee contractures by increasing the knee joint mobility. Numerous animal models that simulate a \"knee flexion contracture\" and a few models of a \"knee extension contracture\" have been proposed [11]. The authors determined that the \"aggravation of contractures\" was correlated with the degree of \"fibrosis response\" of the joints, which is related to the activation of type I and type III collagen syntheses, as well as to the stimulation of pro-fibrotic gene expression in fibroblasts and chondroblasts. A proteomic analysis of the muscles and joint capsule was performed by the same study group [11]. The expression of transforming growth factor beta-1 (TGF-8-1) was also examined as a significant biomarker of changes in the synthesis and distribution of different collagen types (I-III) in the intercellular matrix. An important fact of clinical relevance is that \"extension contracture models\" better mimic fractures and the bed-associated immobilization of patients in traumatology than \"flexion contracture models\". The main question related to stretching biomechanics is: \"Could chronic stretching change the joint-ligament-tendon-muscle mechanical properties?\" The effects of stretching were reported for joint resistance and muscle and tendon stiffness, but a large heterogeneity was seen for most of the variables obtained [4]. The same authors analyzed 26 papers regarding longitudinal stretching (static, dynamic, and/or PNF) in humans of any age and with different health statuses.