**5.1 Cross-sectional area**

The tendons need to adapt based on the ratio of their peak operating stress to their ultimate stress. In this aspect, the modulation in CSA is an adaptive mechanism required for keeping suitable safety factors in response to larger stress levels. Several studies in vivo reported that tendon stiffening after exercise training is accompanied by increases in CSA, which indicated that tendon hypertrophy can also occur with mechanical loading. In a recent systematic review and meta-analysis about human tendon adaptation in response to mechanical loading, it was demonstrated that exercise training induces positive effect for CSA, regardless of type of applied loading regimens. Accumulating evidences from animal and human studies suggest increases in tendon CSA following exercise interventions. In these studies an increase of 20–30% of CSA was noticed in athletes that performed weight-bearing exercise (running and jumping) compared to athletes that did not performed weight-bearing exercise (kayakers) [64, 65]. Interestingly, a tendon CSA increase of 30% in athletes that performed sports where one lower extremity is normally submitted to more loading (leading leg) than the contralateral side was also observed.

In addition, Konsgaard et al. [66] have showed that 12 weeks of heavy RT in healthy young men were efficient to promote increase in both quadriceps tendon CSA in middle and distal regions, as well as in stiffness when compared to other leg that accomplished light RT. Therefore, it is known that loading magnitude could interfere in tendon size. Interestingly, athletes with tendon degeneration cannot present pain symptoms after tendon damage has occurred. It has been demonstrated that around 52% of distance runners will sustain an Achilles tendon injury during their career. In the context, it is extremely essential to determine the difference between tendon remodeling and tendon degeneration. Tendon CSA changes may further indicate positive exercise adaptations or initial degeneration and tendon tissue repair [67].

On the other hand, Wiesinger et al. investigated the tendon structural integrity in athletes engaged in sports with contrasting requirements [68]. Curiously, researchers showed that tendon CSA area normalized to body mass was smaller in water polo players than in other athletes (patellar and Achilles tendon, −28 to −24%) or controls (patellar tendon only, −9%). In contrast, the normalized crosssectional area was larger in runners (patellar tendon only, +26%) and ski jumpers (patellar and Achilles tendon, +21% and + 13%, respectively) than in controls, which indicate that tendon morphological properties can be modulated by functional requirements.

At last, some studies have demonstrated that there are differences in tendon plasticity between men and woman. It was noticed that men's tendons hypertrophy with training, whereas trained women's tendons are the same size as those of untrained women. This work suggests that gender-specific humoral factors may be involved in the training-induced adaptive morphological response of the human tendon. In fact, tendon adaptation inhibition following exercise when levels of estrogen are high was observed [3, 10].

### **5.2 Tendon elongation**

Tendon elongation correlates significantly with clinical outcome, and lengthening is an important cause of morbidity and may produce permanent functional impairment [69]. Increases in tendon stiffness and tensile strength reported following exercise training could conceivably be attributed to tendon elongation. Nevertheless, current literature does not offer conclusive evidence to support this premise. Several research lines about the positive effects of exercise on tendon elongation are controversial [33, 45]. Few studies considered all relevant methodological aspects (e.g., accounting for gravitational forces, axes misalignment of joint and dynamometer, averaging multiple trials to reliably assess tendon elongation, measuring the tendon moment arm directly). Possibly these aspects affect the validity of the applied method. In addition, whenever variations between measured and calculated tendon elongations are observed, it should be standard practice to confirm that there are no shortcomings in the original elongation calculations or the standard stressing procedures.

On the other hand, in tendon injury mechanical theory, tendon tissue overload is blamed for the pathologic process. Sports injuries, such as Achilles tendon rupture, are serious injuries for which the best treatment is still controversial. The main objective of intervention strategies must be to restore normal length, thus obtaining an optimal function [69]. Once tendon lengthening has become permanent, its clinical management is often difficult. The emphasis on exercise programs should be placed on muscle strengthening.

### **6. Tendon molecular signaling in response to exercise training**

In general, the maintenance or changes in tendon CSA, as well as in tendon elongation, are regulated by interaction between synthesis and degradation molecular pathways [70–73]. In the tendon, the molecular adaptations stimulated by different types of exercise occur similarly to the skeletal muscle. Therefore, it is important to notice that in cases where training stimulates muscle hypertrophy and strength increase, the adaptations of muscles and tendons, which are collagen-rich tissues, are essential to maintain muscle-tendon unit integrity [74].

**47**

stimuli.

exercise [79].

**6.1 Collagens fibers and growth factors**

are responsible in the mature collagen synthesis.

and CTGF key mediators for collagen fiber mRNA transcription.

*Exercise and Tendon Remodeling Mechanism DOI: http://dx.doi.org/10.5772/intechopen.79729*

pathway that will permit some ECM gene activation [76].

Tendons are distinct structures from muscles; however, tendon tissue has a direct continuation with the muscle ECM. This characteristic develops an essential mechanism that permits the communication of the mechanical properties of both muscles and tendons, allowing suitable force transmission between them [75]. Based on this communication, the externally applied mechanical load can stimulate ECM components through fibroblasts; however, ECM composition seems to be adapted specifically to changes in load. Therefore, it is possible to understand that mechanical stress can modulate the synthesis of ECM proteins, stimulating paracrine growth factor release, indirectly or directly triggering intracellular signaling

In order to investigate molecular signaling in response to exercise training, different approaches have been used, for instance, microanalysis, incorporation of stable isotope labeled proline into tendon tissue (C-13-proline), and mRNA gene expression of molecules present in the ECM. These approaches have been employed with the goal to analyze the modulations of several molecular mediators responsible for EMC remodeling, as well as molecules that present role key in tendon structure maintenance, such as collagens, proteoglycans, growth factors, as well as collagenases that could response to both endogenous and exogenous

Currently, it is known that exercise induces collagen synthesis in the tendon, but the cellular mechanisms are still unclear. In the same way, growth factors as transforming growth factor-β-1 (TGF-β-1), connective tissue growth factor (CTGF), insulin-like growth factor (IGF), and mediator upstream involved in the collagen synthesis also might be involved in the ECM remodeling [77]. Enzymes involved in collagen processing, such as lysyl oxidase (LOX), in favor to cross-linking of collagen [78], as well as matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) might contribute with tendon remodeling aggravated by

The microanalysis has been employed to measure collagen pro-peptides which

Based on the microanalysis, it was possible to notice, in humans, elevated levels of collagen synthesis in peritendinous tissue in response to mechanical stimuli, in both acute and prolonged exercises [41]. These data corroborate with a previous study that used qPCR to investigate synthesized collagen by mRNA expression, but, in rats. In this study, the rats were submitted to 4 days of concentric, eccentric, or isometric training in the medial gastrocnemius muscle through sciatic nerve stimulation, simulating short-term strength training. Interestingly, in humans, high levels of type I and III collagen mRNA in the Achilles tendon in response in shortterm resistance training were found, but no difference was seen between training types [77]. In this same study, researchers also investigated the regulation of TGF-β

TGF-β family is composed of more than 30 members identified in humans. This family orchestrates several cellular processes as proliferation, differentiation, protein metabolism, and growth and remodeling of the ECM in the tendon [80]. In the tendon, in particular, three TGF-β isoforms are known (TGF-β1, TGF-β2, and TGF-β3), but the most studied and that receive more attention is TGF-β1 isoform. In theory, latent TGF-β1 molecules are stored in ECM, in association with other ECM components such as fibrillin-1 and fibronectin. In cases when there is ECM remodeling necessity, for instance, injury tendon or overload following training,

### *Exercise and Tendon Remodeling Mechanism DOI: http://dx.doi.org/10.5772/intechopen.79729*

*Tendons*

tional requirements.

**5.2 Tendon elongation**

standard stressing procedures.

be placed on muscle strengthening.

estrogen are high was observed [3, 10].

On the other hand, Wiesinger et al. investigated the tendon structural integrity in athletes engaged in sports with contrasting requirements [68]. Curiously, researchers showed that tendon CSA area normalized to body mass was smaller in water polo players than in other athletes (patellar and Achilles tendon, −28 to −24%) or controls (patellar tendon only, −9%). In contrast, the normalized crosssectional area was larger in runners (patellar tendon only, +26%) and ski jumpers (patellar and Achilles tendon, +21% and + 13%, respectively) than in controls, which indicate that tendon morphological properties can be modulated by func-

At last, some studies have demonstrated that there are differences in tendon plasticity between men and woman. It was noticed that men's tendons hypertrophy with training, whereas trained women's tendons are the same size as those of untrained women. This work suggests that gender-specific humoral factors may be involved in the training-induced adaptive morphological response of the human tendon. In fact, tendon adaptation inhibition following exercise when levels of

Tendon elongation correlates significantly with clinical outcome, and lengthening is an important cause of morbidity and may produce permanent functional impairment [69]. Increases in tendon stiffness and tensile strength reported following exercise training could conceivably be attributed to tendon elongation. Nevertheless, current literature does not offer conclusive evidence to support this premise. Several research lines about the positive effects of exercise on tendon elongation are controversial [33, 45]. Few studies considered all relevant methodological aspects (e.g., accounting for gravitational forces, axes misalignment of joint and dynamometer, averaging multiple trials to reliably assess tendon elongation, measuring the tendon moment arm directly). Possibly these aspects affect the validity of the applied method. In addition, whenever variations between measured and calculated tendon elongations are observed, it should be standard practice to confirm that there are no shortcomings in the original elongation calculations or the

On the other hand, in tendon injury mechanical theory, tendon tissue overload is blamed for the pathologic process. Sports injuries, such as Achilles tendon rupture, are serious injuries for which the best treatment is still controversial. The main objective of intervention strategies must be to restore normal length, thus obtaining an optimal function [69]. Once tendon lengthening has become permanent, its clinical management is often difficult. The emphasis on exercise programs should

**6. Tendon molecular signaling in response to exercise training**

elongation, are regulated by interaction between synthesis and degradation molecular pathways [70–73]. In the tendon, the molecular adaptations stimulated by different types of exercise occur similarly to the skeletal muscle. Therefore, it is important to notice that in cases where training stimulates muscle hypertrophy and strength increase, the adaptations of muscles and tendons, which are collagen-rich tissues, are essential to maintain muscle-tendon

In general, the maintenance or changes in tendon CSA, as well as in tendon

**46**

unit integrity [74].

Tendons are distinct structures from muscles; however, tendon tissue has a direct continuation with the muscle ECM. This characteristic develops an essential mechanism that permits the communication of the mechanical properties of both muscles and tendons, allowing suitable force transmission between them [75]. Based on this communication, the externally applied mechanical load can stimulate ECM components through fibroblasts; however, ECM composition seems to be adapted specifically to changes in load. Therefore, it is possible to understand that mechanical stress can modulate the synthesis of ECM proteins, stimulating paracrine growth factor release, indirectly or directly triggering intracellular signaling pathway that will permit some ECM gene activation [76].

In order to investigate molecular signaling in response to exercise training, different approaches have been used, for instance, microanalysis, incorporation of stable isotope labeled proline into tendon tissue (C-13-proline), and mRNA gene expression of molecules present in the ECM. These approaches have been employed with the goal to analyze the modulations of several molecular mediators responsible for EMC remodeling, as well as molecules that present role key in tendon structure maintenance, such as collagens, proteoglycans, growth factors, as well as collagenases that could response to both endogenous and exogenous stimuli.

Currently, it is known that exercise induces collagen synthesis in the tendon, but the cellular mechanisms are still unclear. In the same way, growth factors as transforming growth factor-β-1 (TGF-β-1), connective tissue growth factor (CTGF), insulin-like growth factor (IGF), and mediator upstream involved in the collagen synthesis also might be involved in the ECM remodeling [77]. Enzymes involved in collagen processing, such as lysyl oxidase (LOX), in favor to cross-linking of collagen [78], as well as matrix metalloproteinases (MMPs) and their inhibitors (TIMPs) might contribute with tendon remodeling aggravated by exercise [79].

### **6.1 Collagens fibers and growth factors**

The microanalysis has been employed to measure collagen pro-peptides which are responsible in the mature collagen synthesis.

Based on the microanalysis, it was possible to notice, in humans, elevated levels of collagen synthesis in peritendinous tissue in response to mechanical stimuli, in both acute and prolonged exercises [41]. These data corroborate with a previous study that used qPCR to investigate synthesized collagen by mRNA expression, but, in rats. In this study, the rats were submitted to 4 days of concentric, eccentric, or isometric training in the medial gastrocnemius muscle through sciatic nerve stimulation, simulating short-term strength training. Interestingly, in humans, high levels of type I and III collagen mRNA in the Achilles tendon in response in shortterm resistance training were found, but no difference was seen between training types [77]. In this same study, researchers also investigated the regulation of TGF-β and CTGF key mediators for collagen fiber mRNA transcription.

TGF-β family is composed of more than 30 members identified in humans. This family orchestrates several cellular processes as proliferation, differentiation, protein metabolism, and growth and remodeling of the ECM in the tendon [80]. In the tendon, in particular, three TGF-β isoforms are known (TGF-β1, TGF-β2, and TGF-β3), but the most studied and that receive more attention is TGF-β1 isoform. In theory, latent TGF-β1 molecules are stored in ECM, in association with other ECM components such as fibrillin-1 and fibronectin. In cases when there is ECM remodeling necessity, for instance, injury tendon or overload following training,

### **Figure 5.**

*Overview of TGF-β signaling pathways. Adapted from Gumucio et al. [75].*

active TGF-β can be released of ECM through mechanical force or by matrix proteolytic enzymes as ADAMTS1, MMP-2, and MMP-9 [75]. In the case of mechanical force-mediated TGF-β activation, αvβ6 integrin, transmembrane proteins that connect intracellular cytoskeleton proteins together with ECM, suffers a conformational change that signals to liberate latent TGF-β; now matrix biologically active to binding in surface receptors are found in ECM cell. The binding between ligand (TGF-β) and their receptor permits activation of downstream intracellular signaling pathways, responsible for gene transcription, essential to ECM remodeling (for instance, collagen, MMPs, and TIMPs) (**Figure 5**) [75].

Interestingly, Heinemeyer et al. [77] confirmed in their study that TGF-β could be involved in collagen I and collagen III regulations in different types of training (concentric, eccentric, or isometric). Following 24 hours post training, a TGF-β increased gene expression in all types of training (concentric, eccentric, or isometric) with no difference among training types was noticed. These results are in accordance with previous studies that showed eccentric training is also accompanied by fibroblast proliferation, main cells responsible for synthesizing collagen in response to exercise [81, 82].

About ECM, connective tissue growth factor (CTGF), downstream mediator of TGF-β, in fibroblastic cells, also seems to be responsible for tendon ECM remodeling by exercise. It was noticed in human patellar tendon submitted to 1 hour of unilateral kicking exercise (workload of 67%) with frequency of 35 kicks per min and 2100 concentric contractions that CTGF gene expression total volume was increased, together with COL1A1 mRNA levels, 24 hours postexercise. On the other

**49**

tendon injury [86].

*Exercise and Tendon Remodeling Mechanism DOI: http://dx.doi.org/10.5772/intechopen.79729*

mode, duration, and intensity of the exercise.

tissue injury and exercise training [84].

**6.3 Matrix metalloproteinases**

**6.2 Proteoglycans**

hand, tendon collagen protein synthesis between trained and untrained groups was not modified [83]. Despite that literature is still unclear about the mechanism that links mechanical loading, TGF-β-1, and CTGF, some results have reported that habitual loading is firstly related to stimulating proximal and distal portions of the tendon [40, 66]. However, future studies are necessary to obtain a better understating about the effect of exercise in this gene expression. Another point is inconsistency among loading protocol, so it is possible that some protocols have not reached

Other molecules, such as proteoglycans, are essential for fibrillogenesis regulation and tendon structure maintenance [84]. The proteoglycan regulation from exercise is still not clear on the literature, whereas most studies have observed the exercise effects over collagen and some growth factors responsible for gene expression of those molecules. However, it seems that resistance exercise appears not to induce changes in proteoglycan gene regulation. In the previous study, there were no observed changes in mRNA expression of the proteoglycans: decorin, biglycan, fibromodulin, and versican from resting levels at 4 or 24 hours after resistance training that corresponded to workload of 70% of the subject's concentric maximum repetition [84]. Although, this study hasn't found changes between proteoglycans, it is possible to infer that the regulation of these molecules could be related to

The ECM surrounding tendon provides structural support, protection, and maintenance of the functional integrity. The modulation of ECM function is controlled by matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). The MMPs constitute a large family of endopeptidase zinc dependent that modulates ECM functions, degrading their constituents, such as proteoglycans, collagen, fibronectin, laminin, and other proteins during normal and pathological tissue remodeling. It has been reported that gelatinases (MMP-2 and MMP-9) play an important role in the ECM turnover induced by

In order to compare exercise types (concentric, eccentric, or isometric) over gene expression of MMPs, a previous study noticed that MMP-2 mRNA expression increases moderately in the tendon in concentric and isometric exercises. On the other hand, MMP inhibitor, TIMP-1, and TIMP-2 increased gene expressions in response to all training types [77]. These data suggest a self-regulatory mechanism in attempt to protect the ECM against a high degradation of ECM compounds. In humans, it was found that MMP-2 mRNA decreased significantly 4 hours posteriorly to resistance training but returned to resting levels 24 hours after exercise. The

Interestingly in rodent models, previous data has already shown that acute or chronic exercises upregulate MMP-2 activity in the tendon, which is considered substantial mechanisms to tendon adaptation [85]. In contrast, anabolic-androgenic steroid treatment strongly inhibited this activity. Thus, anabolic-androgenic steroid treatment (AAS) can impair tissue remodeling in animal's tendons undergoing physical exercise by downregulating MMP activity, thus increasing the potential for

mRNA expression of TIMP-1 did not change 24 hours post-acute exercise.

to threshold enough stress-strain to stimulate the CTGF expression.

hand, tendon collagen protein synthesis between trained and untrained groups was not modified [83]. Despite that literature is still unclear about the mechanism that links mechanical loading, TGF-β-1, and CTGF, some results have reported that habitual loading is firstly related to stimulating proximal and distal portions of the tendon [40, 66]. However, future studies are necessary to obtain a better understating about the effect of exercise in this gene expression. Another point is inconsistency among loading protocol, so it is possible that some protocols have not reached to threshold enough stress-strain to stimulate the CTGF expression.
