# Fascial or Muscle Stretching? A Narrative Review
## 3 Results
### Stress/strain Curve  
The stress/strain curve (stress represents the applied force per area, while strain expresses the percentage of elongation beyond resting length) evaluates the mechanical behavior of soft tissues. In stress-relaxation test, a predefined tensile strain is applied, and corresponding stress is followed as a function of time. All biphasic and viscoelastic soft tissues exhibit first the relaxation phase and then the entire load is carried by the solid matrix of a tissue. Consequently, in all the stress/strain curves of musculoskeletal soft tissue we can recognize, at the beginning phase of tension test, the so called the toe region. In this region, the relation between stress and strain is nonlinear and the slope is increasing with increased loading. The reason for the increasing slope is the straightening of the wavy-like collagen fibrils. After the collagen fibrils are completely straightened, the elastic region begins. In this region, the stress and strain are linearly related and the slope of the curve is called the tissue Young's modulus. In the elastic range, all changes of a tissue are still reversible, i.e., if the stress is removed tissue returns to the original strain. It should be also noted that in human musculoskeletal soft tissues, the loading rate affects the slope of the elastic range, i.e., higher loading rate results to steeper slope and higher Young's modulus value. When the stress is further increased from the elastic region, the slope of the curve changes and the plastic region begins. This is called the yield point. After the yield point tissue begins to experience destructive changes, e.g., microfractures in the collagen fibril network. In the plastic region irreversible changes have occurred in a tissue and it does not return to the original strain although the stress would be completely removed. The stress corresponding to such yield point is one characteristic parameter reported for soft tissues under destructive testing. After the plastic region, the sudden failure of the tissue occurs and stress vanishes. The stress corresponding to such breakdown is called the failure point, which is another characteristic parameter reported [35]. In the literature stress-strain curves of many soft tissues have been reported. The tendon and the muscle are the most studied tissues, whilst for the aponeurotic fasciae there is only limited data for the plantar fascia [36] and for the crural fascia [37; 38]. The stress-strain curve of the fascia is non-linear, due to the uncrimping of collagen fibers and elasticity generate by elastin fibers, with the initial portion of stress-strain curve showing a high deformation/low force characteristic. For each increasing increment of applied strain, the fascia answers with a corresponding increase of stress. When all collagen fibers are fully uncrimped and oriented in the direction of the load, the amount of stress is totally governed by the cumulative behavior of tensed collagen fibers. In the experiment of [PERSON] et al. [37], the crural fascia showed a peak strain of about 27% for the specimens of the anterior compartment, and 27.5% for the specimens of the posterior compartment. Besides, it presented also a strong anisotropy due to the specific collagen fibers orientation, being stiffer along the proximal-distal than along medial-lateral direction. In the study of [PERSON] et al. [36], the peak strain was about 12% for the plantar fascia. The mechanical behavior of muscle tissue has been studied in different ways because muscle has a hierarchical structure. This is comprised of micro to macro components of different lengths and structural scales, e.g., a sarcomere, myofibril, fiber, fascicle, whole muscle, and muscle-tendon unit [39]. For example, [PERSON] et al. [40] studied almost intact muscle-tendon units in vivo; thus, their data take in consideration also the stiff membranous materials such as the epimysium and aponeurosis. On the contrary, in the experiments of [PERSON] et al.