Biomechanical, Healing and Therapeutic Effects of Stretching: A Comprehensive Review

Abstract

Characterized in biomedical terms, stretching exercises have been defined as movements applied by external and/or internal forces to increase muscle and joint flexibility, decrease muscle stiffness, elevate the joint range of motion (ROM), increase the length of the “muscle–tendon” morpho-functional unit, and improve joint, muscle, and tendon movements, contraction, and relaxation. The present review examines and summarizes the initial and recent literature data related to the biomechanical, physiological, and therapeutic effects of static stretching (SS) on flexibility and other physiological characteristics of the main structure and the “joint–ligament–tendon–muscle” functional unit. The healing and therapeutic effects of SS, combined with other rehabilitation techniques (massage, foam rolling with and without vibrations, hot/cold therapy, etc.), are discussed in relation to the creation of individual (patient-specific) or group programs for the treatment and prevention of joint injuries, as well as for the improvement of performance in sports. From a theoretical point of view, the role of SS in positively affecting the composition of the connective tissue matrix is pointed out: types I–III collagen syntheses, hyaluronic acid, and glycosaminoglycan (GAG) turnover under the influence of the transforming growth factor beta-1 (TGF-β-1). Different variables, such as collagen type, biochemistry, elongation, and elasticity, are used as molecular biomarkers. Recent studies have indicated that static progressive stretching therapy can prevent/reduce the development of arthrogenic contractures, joint capsule fibrosis, and muscle stiffness and requires new clinical applications. Combined stretching techniques have been proposed and applied in medicine and sports, depending on their long- and short-term effects on variables, such as the ROM, EMG activity, and muscle stiffness. The results obtained are of theoretical and practical interest for the development of new experimental, mathematical, and computational models and the creation of efficient therapeutic programs. The healing effects of SS on the main structural and functional unit—“joint–ligament–tendon–muscle”—need further investigation, which can clarify and evaluate the benefits of SS in prophylaxis and the treatment of joint injuries in healthy and ill individuals and in older adults, compared to young, active, and well-trained persons, as well as compared to professional athletes.

1. Introduction

In biomechanical terms, stretching has been characterized by Weerapong et al. as a movement applied by an external and/or internal force in order to increase muscle flexibility and to improve the joint range of motion (ROM) [1]. The aim of stretching in physical exercise is to increase the muscle–tendon unit length and to improve joint flexibility, as well as to decrease the risk of soft-tissue injuries [2,3,4,5,6,7].

For the purposes of the present review, the trusted and respected databases of PubMed and Web of Science were used. We aimed to prioritize the latest publications in the field of stretching over the last decade. We examined and summarized literature data related to the biomechanical characteristics and therapeutic effects of stretching on the main structural and functional unit, the “muscle–tendon–ligament–joint”.

2. Topics and Results

2.1. Biomechanical Parameters, Healing, and Therapeutic Effects of Stretching

Interesting results arose from numerous recent investigations and various stretching programs applied in medical practice and sports. The biomechanical parameters and therapeutic effectiveness of stretching applications could modify joint, tendon, and muscle flexibility. For this purpose, different variables, such as collagen and elastin syntheses, fiber elongation and elasticity, energy absorption, etc., could be used as mechanobiological, cellular, and molecular biomarkers.

Many retrospective and prospective studies have been performed and stratified on the acute and chronic effects of stretching, both under physiological conditions and in pathological states. Progressive static stretching is effective during the prophylaxis of injuries in sports and exercise training [8,9,10]. The healing properties of stretching are of importance in the prophylaxis and treatment of joint injuries when also combined with other rehabilitation procedures (massage, heat/cold, warming up, etc.) [7,11,12,13,14]. Recent studies have reported a high effectiveness of stretching in the treatment and prevention of contractures and fasciitis, as well as useful methods for application in routine orthopedic and traumatological practice [5,15,16,17,18,19].

As a rehabilitation method, stretching has been applied to improve the biomechanical parameters of muscles, tendons, ligaments, fascia, and joints [4,6,7,9,20,21].

The viscoelastic responses of muscles, tendons, ligaments, fascia, and joints to slow stretching exercises could result in less passive tension, compared to faster procedures [22,23]. The faster the stretch, the higher the muscle stiffness will be [6]. Most stretching techniques (static, dynamic, ballistic, etc.) have been successfully implemented in clinical practice [2,4,24,25].

The effects of stretching on muscle and joint flexibility are closely related to the joint range of motion (ROM), whereby the increased range of motion induces the analgesic effects of stretching.

Various stretching techniques have been compared. Unfortunately, the current results of the chronic effects of static stretching (SS) exercises on the muscle strength, flexibility, joint ROM, and muscle power are still controversial [5,18,26].

2.2. 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 hyaluronan, glycosaminoglycans (GAGs), and albumin) indicate cellular mechanisms of stretching and a role of the intercellular matrix as a selective molecular filter. The specific rheological properties of joint synovial fluid are altered in traumatic and post-traumatic pathological states (different arthroses, 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]. Zhang et al. examined the effect of stretching combined with ultrashort wave diathermy on joint functions and clarified its cellular mechanisms in a rabbit knee contracture model [38]. Wang L. 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-β-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. Structural and mechanical variables were evaluated for joints and muscle–tendon units: dynamic stretching, static stretching, flexibility, stiffness, mechanical joint properties, muscle morphology and functional activities, changes in the tendon characteristics, proprioceptive neuromuscular facilitation, etc. [6]. Adaptations to chronic stretching protocols shorter than 8 weeks seemed to occur mostly at the sensory level [6].

2.3. 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 Magnusson 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. Cotofana 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. Herberthold 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 [28,52,53,54,55,56] 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 [28,52,53,56]. 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, cm²)—were measured from the ultrasound pictures (Figure 1). They defined the two displacements (mm) and the area (cm²) between the intra-articular femur and tibia cartilage surfaces.

The results obtained for the change in the intra-articular geometry under a load and under stretching could serve as a quantitative assessment of the internal joint kinematics and might determine the joint mobility of individual participants in the stretching exercises [28,52,53,56].

The accuracy of the ultrasound pictures and measurements in our experimental model was limited by three main components (Figure 1) [56]. The first was related to the transducer accuracy characteristics. The second was the accuracy of the identity of the transducer–knee joint image position reproductions. The third component was the researcher’s skill at obtaining scanning pictures. The present preliminary experimental model accuracy was defined as the sum of the three components cited and was lower than 30%.

2.4. Biomechanical and Biological (Cellular and Molecular) Mechanisms of Stretching

The cellular and molecular mechanisms underlying the changes in joint flexibility, muscle strength, and power are not well clarified in medicine and cell biology, and thus, further investigations are needed.

The additional effects of individual training status, age, sex, and different pathological states that moderate the influences of stretching exercises on the joints, muscles, tendons, and ligaments can be characterized as indirect [6,18,22,24,35].

Stretching modulates the synthesis, deposition, concentration, degradation, and distribution of collagen and glucosoaminoglycans (GAGs), affecting the remodeling of the extracellular matrix [57]. The data obtained encourage the therapeutic application of stretches and stretching physiotherapy at the cellular and molecular levels—preliminary in the treatment and management of arthritic joints [58].

The contributions of Bouffard et al. [59] and Wang et al. [7] demonstrate the importance of transforming growth factor β-1 (TGF β-1) in collagen synthesis and extracellular matrix remodeling after a brief static stretching (SS) application. Simultaneously, stretching also modulates the aggrecan concentrations in the matrix. Xiong et al. [60] examined the expression of TGF β-1 as a tissue inhibitor of metalloproteinases.

TGF β-1 is one of the more important cytokines regulating fibroblast responses in connective tissue. In health and diseases, collagen is a major protein in the extracellular matrix, and aggrecan is the main proteoglycan in articular cartilage. Stretching can be used to enhance and engineer the connective tissue’s extracellular matrix with desirable collagen/elastin concentrations, improved elastic properties, and regular mesenchymal cell (fibroblast/chondroblast) functions [61].

Moreover, the biomechanical aspects underlying the different influences of active and passive stretches on joint, tendon, ligament, and muscle flexibility at rest are yet to be identified [5,26,62]. The recent results in medical and sports scientific literature indicate that chronic SS exercises have the potential to improve muscle strength and power [5,20]. Further investigations could examine the benefits of chronic SS exercises in old, healthy, and ill individuals, as compared to young, active, and well-trained persons, as well as compared to professional athletes [20,30,63,64,65,66,67].

2.5. Stretching Is an Integral Component of Mind–Body Exercises Such as Yoga (Mainly Hatha Yoga), Tai Chi, and Gingong

Gothe, McAuley et al. [65,67] compared the functional benefits of stretching and yoga exercises. Four standard fitness tests assessing balance, strength, flexibility, and mobility were administered [64,65]. The experimental stretching protocols varied with combinations of the functional parameters of exercise duration, intensity, frequency, and whole-body posture [66]. Patel and colleagues (2012) concluded that yoga practice and stretching exercises led to improvements in strength, flexibility, mobility, and quality of life in older adults [64]. Considering the benefits of stretching and yoga exercises, the special sports programs in functional fitness could be adapted for healthy individuals in the elderly population, as well as for people with socially important diseases (arthritis, diabetes mellitus, chronic inflammation, etc.). Gothe and co-authors [65,67] recommended the effects of stretching and/or yoga exercises for improving functional fitness outcomes in health and diseases, as well as for improving sports performance and health-related quality of life.

The potential preventive and therapeutic effects of static and yoga stretching (SS and YS) were examined in relation to different pathological states, such as chronic inflammation, wound healing, tumor growth, etc. [15,66,68,69,70].

The biological, cellular, and molecular mechanisms underlying the anti-inflammatory and anti-cancer properties of stretching, yoga, and TCC were very well summarized and presented in the review of Kròl et al. [16]. These physical exercises could enhance the immune state, change the IL-6 and IL-10 levels, and improve the health-related quality of life in older individuals [71,72,73]. On the other hand, chronic inflammation could contribute to the initiation, progression, and development of tumorigenesis [74]. In these and other pathological states, stretching exercises are recommended and included in programs for diabetes mellitus type 2 (DM-2) patients and in stroke and malignant disease rehabilitation and treatment [75,76]. Stretching may serve as a method of connective tissue healing [15,77]. Ferreti and colleagues (2006) [77] examined mechanical signals as having strong anti-inflammatory effects and recommended the use of mechanical forces of an appropriate intensity in the rehabilitation of knee meniscus cartilage. Further studies are required to understand the role of collagen and aggrecan in the remodeling and destruction of the articular cartilage’s extracellular matrix (e.g., in osteoarthritis of various etiologies). Collagen and the impaired synthesis of proteoglycan/aggrecan is related to diseases such as local and systemic (disseminated) scleroses [16]. The same authors pointed out that daily stretching may be a part of therapy in patients with systemic sclerosis (SSc). Similar conclusions could be valid for a local Dupuytren contracture (Morbus Dupuytren) [25]. Guissard and Duchateau studied the effects of static stretching training on the characteristics of the plantar–flexor muscles in 12 subjects. An improved muscle flexibility was associated (r² = 0.88; p < 0.001) with a decrease in passive muscle stiffness. Although the changes in the flexibility and passive stiffness were partially maintained 1 month after the end of the training program, the reflex activities had already returned to control levels. It was concluded that the increased flexibility resulted mainly from the reduced passive stiffness of the muscle–tendon unit and the tonic reflex activity [25].

In only one study [78], a mouse breast cancer model was presented, and the results showed slower tumor growth (from 2–4 weeks) under the influence of stretching.

By comparing various stretching techniques to study their short- and long-term effects on different parameters (knee joint ROM, hamstring flexibility, muscle electromyographic activity, etc.), the researchers registered significant preliminary results for both the joint ROM and hamstring flexibility parameters [79]. The same authors applied static stretching (SS) as a variant of proprioceptive neuromuscular facilitation–contact–relax (PNF–CR) techniques. The knee range of motion (ROM), hamstring flexibility, and knee flexor muscle electromyographic (EMG) activity were also investigated [3,79]. The results obtained demonstrated an immediate, as well as a long-term, effect on the knee ROM and only a long-term effect on flexibility in the elderly. The aging human population exhibited an increase in muscle stiffness, as well as disturbances in the syntheses of types I–III collagens and an alteration in ROMs and EMG activities, due to the cellular and molecular processes in the elderly [3,25,30,31,63].

On the other hand, the PNF–CR and SS techniques were described as effective for increasing hamstring flexibility in young individuals [31,63,79].

Ferber R. et al. [3] applied three PNF stretching techniques: static stretching (SS), contact relaxation (CR), and agonist contact relaxation (ACR). The purposes of these studies were to characterize the effects of stretching on the ROM of joints and the EMG activity of muscles. The authors concluded that the PNF stretching technique increased the ROM in older adults. However, a paradoxical effect was also observed: PNF stretching might not induce muscular relaxation or reduce muscle stiffness in older adults due to age-related alterations in collagen synthesis and muscle elasticity [3,75,80,81].

Recent studies have examined the combined effects of static and/or dynamic stretching, followed by foam rolling (FR) and other techniques with or without local vibrations [12,32,34,38]. The combination of SS and FR is a very effective and frequently used method in sports programs and platforms to increase the ROM of joints and simultaneously decrease muscle stiffness. In this relationship, it has been reported [3,82] that the cell and tissue changes associated with aging are mainly related to the loss of the joint range of motion (ROM), increased muscle stiffness, and pathological changes in collagen and proteoglycan (GAG) synthesis and metabolism.

3. Conclusions

Stretching therapy and prophylaxis include passive and active stretching techniques and some partner-assisted methods precisely summarized in [5,53,83,84].

In this review paper, we briefly described the experiences of international researchers and our study group with the application of stretching as a very interesting field of theoretical and practical medicine, sports sciences, and biomechanics. The accuracy and limitations of therapeutic stretching techniques were also defined. We paid special attention to the simultaneous biomechanical and healing effects of static and/or active isometric stretching, which were also applied to our in vivo model of the human knee joint. The improvements to the knee joint range of motion (ROM) and flexibility were confirmed by ultrasound measurements. An accurately applied stretching treatment led to efficient short- and long-term results: a high movement quality and the reduced risk of further joint soft-tissue injuries in different pathological states, as well as in the elderly and those engaging in sports practice. Further investigations could continue to examine and compare the healing, therapeutic, and preventive effects of static stretching (SS) exercises in different study groups: ill adult patients, old healthy individuals, young persons (well-trained and physically active), and a group of professional athletes.

From a historical point of view and in the present day, the benefits and main components of mind–body exercises such as yoga (Hatha yoga) and stretching could be successfully applied in special sports platforms and programs for stretching management, prophylaxis, and treatment.

The therapeutic and preventive effects of stretching exercises have also been established in different experimental modeling systems, including animal, computational, and mathematical models, which have significance for human therapies and the prophylaxis of injuries.

In addition to static (passive) stretching, the authors characterized stretching therapy as the combined application of a wide range of techniques (e.g., stretching combined with foam rolling with or without vibration, massage, motion movements, etc.). The conclusions of prevalent studies suggest that when properly applied and combined with other techniques, stretching therapy could improve joint, muscle, fascia, tendon, and ligament health and flexibility and resolve problems associated with joint and muscle stiffness. The prevention of sports injuries in the morpho-functional unit of the joint–ligament–tendon–muscle could improve the health and sports performance of healthy persons and professional athletes. From a long-term perspective, increasing the flexibility and reducing the stiffness of muscles would lead to relaxation and muscle fiber elongation, which is also related to better sports performance.

The synthesis and localization of the main biomechanical variables and biomarkers in the extracellular matrix of the joints and cartilage at the cellular and molecular levels, such as collagen, elastin, hyaluronic acid, and other glucosaminoglycans (GAGs), specific genes, TgF-β1, etc., need further investigation.

The proposed combined stretching techniques could be applied in efficient therapeutic programs in medicine and sports, depending on their short- and long-term healing effects.

Recent studies have determined that static progressive stretching (SPS) therapy, alone or in a combined treatment, is the main way to improve the joint range of motion (ROM) and reduce or prevent the development of arthrogenic contractures, joint capsule fibrosis, and muscle stiffness, thus positively influencing the biological structures, functions, and biomechanics of the joint–ligament–capsule–tendon–muscle unit. The therapeutic and preventive effects of static stretching (SS) need new clinical and sports applications. Further successful retrospective and prospective studies could elucidate the cellular, molecular, and biomechanical mechanisms of the effects of stretching.

Static and/or dynamic stretching (SS and/or DS) applied in sports sciences could improve joint and muscle properties, which is of great importance in prophylaxis and the treatment of sports injuries.

However, the cellular and molecular mechanisms of stretching need further investigation.