# Biomechanical, Healing and Therapeutic Effects of Stretching: A Comprehensive Review
## 2 Topics and Results
### Biomechanical Effects of Static and Active Isometric Stretching Applied to the Human Knee Joint  
The effects of stretching on muscle properties are clearly described in literature and depend on various factors, including stretching techniques, stretching time, retention time, rest time, and the time difference between the intervention and the measurement [28; 40; 41]. Most studies investigated the effects of static stretching on the passive properties of the muscle-tendon unit [42; 43; 44; 45; 46]. In a series of studies by [PERSON] et al. [42; 43; 45; 46; 47], it was shown that static stretching for 90 s over five repetitions reduced muscle resistance, passive stiffness, peak torque, and stress relaxation. Another team of researchers [48; 49] concluded that changes in the viscoelastic properties of the muscle-tendon unit depend more on the duration of stretching than on the number of stretches. An extension of static stretching time (from five to ten minutes) was shown to reduce tendon stiffness, as measured passively by ultrasonography [48; 49]. The reduction in stiffness might be due to a change in the arrangement of collagen fibers in the tendon [48]. Stretching increased the range of motion of the femoral flexion and the outer rotation [21]. Isometric stretching is a type of static stretching associated with the resistance of muscle groups through isometric contractions of the stretched muscles. Due to the fact that this type of muscle stretching works in an isometric mode, the initiated muscle forces will affect the joints around which the muscles are located. The muscle forces produced by this type of stretching trigger processes within the joint itself. [PERSON] et al. [50] demonstrated that the cartilage thickness decreased to 5.2% from a knee load with a force equal to 50% of the body weight. [PERSON] et al. [51] evaluated the deformation at a force load equal to 150% of the body weight. Our experimental model and working hypothesis estimated that, as the result of active isometric stretching of the adjacent locomotor muscles, changes in the distance between the femur and the corresponding end of the tibia could be observed [28]. The changes in the distance between the two bones would, in turn, be conditioned by several factors: the magnitude of the isometric muscle tension during stretching; the duration and direction of the tension applied; the tendon's biomechanical properties; and the biomechanical properties of the knee joint (shape, size, viscosity of the synovial fluid, and mechanical properties of the joint capsule elements). Static investigations of knee joint stability are often directed to stretching exercises [52; 53; 54; 28; 55; 56] and isometric back squats [55]. Our study group's quantitative estimation of the biomechanical processes in human knee joints during active isometric stretching was based on knee joint capsule ultrasound scanning during isometric stretching exercises [52; 53; 28; 53]. During a right-lower-limb pose with a 140-degree femur-tibia angle, the distance between the tibia and femur bones forming the knee joint was measured using ultrasound scanning. Our experimental model included an ultrasound examination of the knee joint after the isometric stretching of healthy men (n = 10). The changes (in millimeters) in the distances between the femur and tibia were measured with a portable ultrasound system (Vinno 6, China; Figure 1). The apparatus was used for the purposes of our study in the musculoskeletal mode and in real time with a scanning frequency for the linear transducer of 8 to 10 MHz. The system was able to work in three different upright positions, all with a femur-tibia angle of 140 degrees at rest. In two of the three upright positions, extra loads of 4 and 8 kg were applied vertically down to the lower right limb to induce isometric stretching. Three quantitative parameters--distance up (Dup), distance down (Down), and area (A, cm\({}^{2}\))--were measured from the ultrasound pictures (Figure 1).