The Effect of Foot Strike Pattern on Achilles Tendon Load During Running

In this study we compared Achilles tendon loading parameters during barefoot running among females with different foot strike patterns using open-source computer muscle modeling software to provide dynamic simulations of running. Muscle forces of the gastrocnemius and soleus were estimated from experimental data collected in a motion capture laboratory during barefoot running for 11 runners utilizing a rearfoot strike (RFS) and 8 runners utilizing a non-RFS (NRFS) pattern. Our results show that peak Achilles tendon force occurred earlier in stance phase (p = 0.007), which contributed to a 15% increase in average Achilles tendon loading rate among participants adopting a NRFS pattern (p = 0.06). Stance time, step length, and the estimated number of steps per mile were similar between groups. However, runners with a NRFS pattern experienced 11% greater Achilles tendon impulse each step (p = 0.05) and nearly significantly greater Achilles tendon impulse per mile run (p = 0.06). This difference equates to an additional 47.7 body weights for each mile run with a NRFS pattern. Runners considering a NRFS pattern may want to account for these novel stressors and adapt training programs accordingly.     

Role of moderate exercising on Achilles tendon collagen crimping patterns and proteoglycans

Abstract

In this study, the morphological and morphometric changes in the collagen crimping pattern of Achilles tendon and metabolism/expression of tenocytes explanted from tendons of running (RUN) and sedentary (SED) rats were investigated to assess the effects of 12 weeks moderate running exercise. The number, the top angle width and the base length of each crimp in three different regions (proximal, central and distal) of RUN and SED tendons were measured with a polarized light microscope. The most significant morphometric differences in the crimps were detectable in the central region of the RUN tendons. In this region, crimps were fewer, larger and more flattened than those of other regions as a consequence of a functional adaptation of extracellular matrix to running, in order to increase tendon stiffness and force transmission efficiency. Conversely, the top angle width of the crimps reduced in proximal and distal regions of the RUN tendons, suggesting that these crimps might act as more reactive mechanical springs, able to store and improve the release of the stored strain energy in most loaded regions. Tenocytes explanted from Achilles tendons of both RUN and SED groups were cultured. Running influenced tenocytes which showed a significant increase in collagen type-I synthesis and proteoglycans production, suggesting enhancement of the loading transmission efficiency and facilitate inter-fibril and inter-fiber sliding.     

Birefringence and Second Harmonic Generation on Tendon Collagen Following Red Linearly Polarized Laser Irradiation

Regarding the importance of type I collagen in understanding the mechanical properties of a range of tissues, there is still a gap in our knowledge of how proteins perform such work. There is consensus in literature that the mechanical characteristics of a tissue are primarily determined by the organization of its molecules. The purpose of this study was to characterize the organization of non-irradiated and irradiated type I collagen. Irradiation was performed with a linearly polarized HeNe laser (λ = 632.8 nm) and characterization was undertaken using polarized light microscopy to investigate the birefringence and second harmonic generation to analyze nonlinear susceptibility. Rats received laser irradiation (P = 6.0 mW, I = 21.2 mW/cm², E ≈ 0.3 J, ED = 1.0 J/cm²) on their healthy Achilles tendons, which after were extracted to prepare the specimens. Our results show that irradiated samples present higher birefringence and greater non-linear susceptibility than non-irradiated samples. Under studied conditions, we propose that a red laser with polarization direction aligned in parallel to the tendon long axis promotes further alignment on the ordered healthy collagen fibrils towards the electric field incident. Thus, prospects for biomedical applications for laser polarized radiation on type I collagen are encouraging since it supports greater tissue organization.     

The adaptability of tendon to loading differs in men and women

The reason why women sustain more soft tissue injury than men during physical activity is unknown. Connective tissue properties and extracellular matrix adaptability in human tendon were investigated in models that addressed biochemical, physiological and biomechanical aspects of tendon connective tissue in response to mechanical loading. Habitual training resulted in a larger patellar tendon in men but not in women. Following an acute bout of exercise, men had an elevated tendon collagen synthesis rate and this effect was less pronounced or absent in women. Moreover, levels of circulating oestrogen affected the acute exercise-related increase in collagen synthesis. Finally, the mechanical strength of isolated tendon collagen fascicles in men surpassed that of women. Thus, compared to men, women have (i) an attenuated tendon hypertrophy response to habitual training; (ii) a lower tendon collagen synthesis rate following acute exercise; (iii) a rate of tendon collagen synthesis which is further attenuated with elevated estradiol levels; and (iv) a lower mechanical strength of their tendons. These data indicate that tendons in women have a lower rate of new connective tissue formation, respond less to mechanical loading, and have a lower mechanical strength, which may leave the tissue more susceptible to injury.     

Temporal Healing in Rat Achilles Tendon: Ultrasound Correlations

The purpose of this study was to explore whether a new ultrasound-based technique correlates with mechanical and biological metrics that describe the tendon healing. Achilles tendons in 32 rats were unilaterally transected and allowed to heal without repair. At 7, 9, 14, or 29 days post-injury, tendons were collected and examined for healing via ultrasound image analysis, mechanical testing, and immunohistochemistry. Consistent with previous studies, we observe that the healing tendons are mechanically inferior (ultimate stress, ultimate load, and normalized stiffness) and biologically altered (cellular and ECM factors) compared to contralateral controls with an incomplete recovery over healing time. Unique to this study, we report: (1) Echo intensity (defined by gray-scale brightness in the ultrasound image) in the healing tissue is related to stress and normalized stiffness. (2) Elongation to failure is relatively constant so that tissue normalized stiffness is linearly correlated with ultimate stress. Together, 1 and 2 suggest a method to quantify mechanical compromise in healing tendons. (3) The amount and type of collagen in healing tendons associates with their strength and normalized stiffness as well as their ultrasound echo intensity. (4) A significant increase of periostin in the healing tissues suggests an important but unexplored role for this ECM protein in tendon healing.     

Effects of Creep and Cyclic Loading on the Mechanical Properties and Failure of Human Achilles Tendons

The Achilles tendon is one of the most frequently injured tendons in humans, and yet the mechanisms underlying its injury are not well understood. This study examines the ex vivo mechanical behavior of excised human Achilles tendons to elucidate the relationships between mechanical loading and Achilles tendon injury. Eighteen tendons underwent creep testing at constant stresses from 35 to 75 MPa. Another 25 tendons underwent sinusoidal cyclic loading at 1 Hz between a minimum stress of 10 MPa and maximum stresses of 30–80 MPa. For the creep specimens, there was no significant relationship between applied stress and time to failure, but time to failure decreased exponentially with increasing initial strain (strain when target stress is first reached) and decreasing failure strain. For the cyclically loaded specimens, secant modulus decreased and cyclic energy dissipation increased over time. Time and cycles to failure decreased exponentially with increasing applied stress, increasing initial strain (peak strain from first loading cycle), and decreasing failure strain. For both creep and cyclic loading, initial strain was the best predictor of time or cycles to failure, supporting the hypothesis that strain is the primary mechanical parameter governing tendon damage accumulation and injury. The cyclically loaded specimens failed faster than would be expected if only time-dependent damage occurred, suggesting that repetitive loading also contributes to Achilles tendon injuries. © 2003 Biomedical Engineering Society. PAC2003: 8719Rr     

Response of Rabbit Achilles Tendon to Chronic Repetitive Loading

The objective of this work was to assess the response of tendon to chronic repetitive loading. Controlled muscle stimulation was used to load the rabbit Achilles tendon at a frequency of 1.25 Hz for two hours per day, three days per week for a period of 11 weeks. Average peak tendon force was 26 N during the protocol. The loading protocol did not modify the gross morphology of the tissue, nor its water content or cellularity. Increases in mRNA expression of collagen Type III and MMPs were observed, but no signs of injury were detected by histologic examination of tendon and paratenon structures. The lack of a detectable injury response suggests that the tendons were not loaded beyond their capacity for repair. Factors additional to mechanical loading such as aging, illness or stress may be necessary to produce pathology.     

Response of rabbit Achilles tendon to chronic repetitive loading.

The objective of this work was to assess the response of tendon to chronic repetitive loading. Controlled muscle stimulation was used to load the rabbit Achilles tendon at a frequency of 1.25 Hz for two hours per day, three days per week for a period of 11 weeks. Average peak tendon force was 26 N during the protocol. The loading protocol did not modify the gross morphology of the tissue, nor its water content or cellularity. Increases in mRNA expression of collagen Type III and MMPs were observed, but no signs of injury were detected by histologic examination of tendon and paratenon structures. The lack of a detectable injury response suggests that the tendons were not loaded beyond their capacity for repair. Factors additional to mechanical loading such as aging, illness or stress may be necessary to produce pathology.     

HGF inhibits TGF-β1-induced myofibroblast differentiation and ECM deposition via MMP-2 in Achilles tendon in rat

Both myofibroblast differentiation and extracellular matrix (ECM) deposition are essential components of scar formation in tendons, and hepatocyte growth factor (HGF) is reported to prevent fibrogenic responses in tendons. Matrix metalloproteinases-2(MMP-2) is also involved in the healing process in tendons. Whether HGF protects healed Achilles tendons from injury-induced scar formation and the mechanisms are unknown. Daily for 2 weeks after wounding, except for the non-surgical control group, the Achilles tendons in rats were locally injected with HGF (100 ng 50 μl⁻¹ per mouse) or phosphate-buffered saline (PBS). Histological examination showed HGF ameliorated disorganized collagen fibers caused by surgical incisions in rats. After transforming growth factor beta-1 (TGF-β1) induced fibrogenic responses in primary Achilles tendon fibroblasts in rats, HGF treatment for 24 h reduced α-smooth muscle actin (α-SMA) (0.60 ± 0.07-fold, P < 0.05) and type III collagen expression (0.39 ± 0.07-fold, P < 0.05). Moreover, HGF elevated MMP-2 expression (1.23 ± 0.11-fold, P < 0.05). The MMP-2 inhibitor, tissue inhibitors of metalloproteinase-1 (TIMP-1), partially blocked the inhibitory effects of HGF on α-SMA expression (from 0.60 ± 0.07-fold to 0.83 ± 0.07-fold, P < 0.05) and type III collagen expression (from 0.39 ± 0.06-fold to 0.86 ± 0.08-fold, P < 0.05). These results indicate HGF attenuates TGF-β1-induced fibrogenic responses in Achilles tendon, which was mediated by MMP-2. These results will aid in developing effective therapeutic approaches for the dysfunctional repair in Achilles tendons.     

Expression,Content, and Localization of Insulin-Like Growth Factor I in Human Achilles Tendon

In animals insulin-like growth factor I (IGF-I) stimulates collagen production by fibroblasts and is expressed in tendons together with its binding protein 4 (IGFBP-4). However, the presence of IGF-I and IGFBP-4 in human tendon tissue is not described. Tissue IGF-I content was examined by immunoflourometric assay, real-time PCR, and immunohistochemistry used to localize and determine expression of IGF-I and IGFBP-4 in 6 postmortem human Achilles tendons. Tendon tissue concentrations of IGF-I were found to be 0.53 ± 0.10 ng/g. Furthermore, we demonstrated that IGF-I and IGFBP-4 are localized around the tendon fibroblasts and that mRNA for IGF-I and IGFBP-4 can be determined in human tendon tissue. The present study adds support for the roles of IGF-I and IGFBP-4 in the regulation of tendon adaptive responses to mechanical loading.     

An in vitro scratch tendon tissue injury model: effects of high frequency low magnitude loading

The healing process of ruptured tendons is suboptimal, taking months to achieve tissue with inferior properties to healthy tendon. Mechanical loading has been shown to positively influence tendon healing. However, high frequency low magnitude (HFLM) loads, which have shown promise in maintaining healthy tendon properties, have not been studied with in vitro injury models. Here, we present and validate an in vitro scratch tendon tissue injury model to investigate effects of HFLM loading on the properties of injured rat tail tendon fascicles (RTTFs). A longitudinal tendon tear was simulated using a needle aseptically to scratch a defined length along individual RTTFs. Tissue viability, biomechanical, and biochemical parameters were investigated before and 7 days after culture . The effects of static, HFLM (20 Hz), and low frequency (1 Hz) cyclic loading or no load were also investigated. Tendon viability was confirmed in damaged RTTFs after 7 days of culture, and the effects of a 0.77 ± 0.06 cm scratch on the mechanical property (tangent modulus) and tissue metabolism in damaged tendons were consistent, showing significant damage severity compared with intact tendons. Damaged tendon fascicles receiving HFLM (20 Hz) loads displayed significantly higher mean tangent modulus than unloaded damaged tendons (212.7 ± 14.94 v 92.7 ± 15.59 MPa), and damaged tendons receiving static loading (117.9 ± 10.65 MPa). HFLM stimulation maintained metabolic activity in 7-day cultured damaged tendons at similar levels to fresh tendons immediately following damage. Only damaged tendons receiving HFLM loads showed significantly higher metabolism than unloaded damaged tendons (relative fluorescence units —7021 ± 635.9 v 3745.1 ± 641.7). These validation data support the use of the custom-made in vitro injury model for investigating the potential of HFLM loading interventions in treating damaged tendons.     

Diabetes Alters Mechanical Properties and Collagen Fiber Re-Alignment in Multiple Mouse Tendons

Tendons function to transfer load from muscle to bone through their complex composition and hierarchical structure, consisting mainly of type I collagen. Recent evidence suggests that type II diabetes may cause alterations in collagen structure, such as irregular fibril morphology and density, which could play a role in the mechanical function of tendons. Using the db/db mouse model of type II diabetes, the diabetic skin was found to have impaired biomechanical properties when compared to the non-diabetic group. The purpose of this study was to assess the effect of diabetes on biomechanics, collagen fiber re-alignment, and biochemistry in three functionally different tendons (Achilles, supraspinatus, patellar) using the db/db mouse model. Results showed that cross-sectional area and stiffness, but not modulus, were significantly reduced in all three tendons. However, the tendon response to load (transition strain, collagen fiber re-alignment) occurred earlier in the mechanical test, contrary to expectations. In addition, the patellar tendon had an altered response to diabetes when compared to the other two tendons, with no changes in fiber re-alignment and decreased collagen content at the midsubstance of the tendon. Overall, type II diabetes alters tendon mechanical properties and the dynamic response to load.     

Canine tendon studies. II. Biomechanical evaluation of normal and regrown canine tendons.

Some of the mechanical properties of regrown canine tendons are compared to those of normal tendons of young and mature animals. Patellar and Achilles tendons from 12 beagle dogs were removed and studied with their bone origin and insertions. Mechanical tests were performed within 24 hr and test conditions simulated the physiological function of the tendon in vivo at room temperature. Specimens were soaked in Ringers solution and mounted in an Instron testing machine with load deflection curves plotted automatically. The parameters used for analysis were load extension, stress relaxation, elastic limit, and strain rate dependence. The regrown tendons in young animals appeared to quickly adjust in dimension and structure so that their properties were not significantly different from those of normal tendons on a load extension basis. The normal tendons were stiffer than regrown ones but the modulus of elasticity increased with age. The Achilles were stiffer than patellar tendons. Cyclic loading with 25 kg did not affect reconstructed tendon models, although some increase in stiffness was noted. The elastic modulus decreased with an increase in ambient temperature and increasing strain rate.     

Crimp morphology in relaxed and stretched rat Achilles tendon

Fibrous extracellular matrix of tendon is considered to be an inextensible anatomical structure consisting of type I collagen fibrils arranged in parallel bundles. Under polarized light microscopy the collagen fibre bundles appear crimped with alternating dark and light transverse bands. This study describes the ultrastructure of the collagen fibrils in crimps of both relaxed and in vivo stretched rat Achilles tendon. Under polarized light microscopy crimps of relaxed Achilles tendons appear as isosceles or scalene triangles of different size. Tendon crimps observed via SEM and TEM show the single collagen fibrils that suddenly change their direction containing knots. The fibrils appear partially squeezed in the knots, bent on the same plane like bayonets, or twisted and bent. Moreover some of them lose their D-period, revealing their microfibrillar component. These particular aspects of collagen fibrils inside each tendon crimp have been termed 'fibrillar crimps' and may fulfil the same functional role. When tendon is physiologically stretched in vivo the tendon crimps decrease in number (46.7%) (P<0.01) and appear more flattened with an increase in the crimp top angle (165 degrees in stretched tendons vs. 148 degrees in relaxed tendons, P<0.005). Under SEM and TEM, the 'fibrillar crimps' are still present, never losing their structural identity in straightened collagen fibril bundles of stretched tendons even where tendon crimps are not detectable. These data suggest that the 'fibrillar crimp' may be the true structural component of the tendon crimp acting as a shock absorber during physiological stretching of Achilles tendon.     

The effect of dry needling and treadmill running on inducing pathological changes in rat Achilles tendon

Achilles tendinopathy is a common degenerative condition without a definitive treatment. An adequate chronic animal model of Achilles tendinopathy has not yet been developed. The purpose of this study was to evaluate the individual and combined effects of dry needling and treadmill running on the Achilles tendon of rats. Percutaneous dry needling, designed to physically replicate microrupture of collagen fibers in overloaded tendons, was performed on the right Achilles tendon of 80 Sprague–Dawley rats. The rats were randomly divided into two groups: a treadmill group, which included rats that underwent daily uphill treadmill running (n?=?40), and a cage group, which included rats that could move freely within their cages (n?=?40). At the end of weeks 1 and 4, 20 rats from each group were sacrificed, and bilateral Achilles tendons were collected. The harvested tendons were subjected to mechanical testing and histological analysis. Dry needling induced histological and mechanical changes in the Achilles tendons at week 1, and the changes persisted at week 4. The needled Achilles tendons of the treadmill group tended to show more severe histological and mechanical changes than those of the cage group, although these differences were not statistically significant. Dry needling combined with free cage activity or treadmill running produced tendinopathy-like changes in rat Achilles tendons up to 4 weeks after injury. Dry needling is an easy procedure with a short induction period and a high success rate, suggesting it may have relevance in the design of an Achilles tendinopathy model.     

The influence of loading intensity on muscle–tendon unit behavior during maximal knee extensor stretch shortening cycle exercise

Tendon stiffness increases as the magnitude and rate of loading increases, according to its viscoelastic properties. Thus, under some loading conditions tendons should become exceptionally stiff and act almost as rigid force transducers. Nonetheless, observations of tendon behavior during multi-joint sprinting and jumping tasks have shown that tendon strain increases whilst muscle strain decreases as the loading intensity increases. The purpose of the current study was to examine the influence of external loading intensity on muscle–tendon unit (MTU) behavior during a high-speed single-joint, stretch-shortening cycle (SSC) knee extension task. Eighteen men (n = 9) and women (n = 9) performed single-leg, maximum intensity SSC knee extensions at loads of 20, 60 and 90?% of their one repetition maximum. Vastus lateralis fascicle length (L _f) and velocity (v _f) as well as MTU (L _MTU) and tendinous tissue (L _t) length were measured using high-speed ultrasonography (96 Hz). Patellar tendon force (F _t) and rate of force development (RFD_t) were estimated using inverse dynamics. Results showed that as loading intensity increased, concentric joint velocity and shortening v _f decreased whilst F _t and RFD_t increased, but no significant differences were observed in eccentric joint velocity or peak L _MTU or L _f. In addition, the tendon lengthened significantly less at the end of the eccentric phase at heavier loads. This is the first observation that tendon strain decreases significantly during a SSC movement as loading intensity increases in vivo, resulting in a shift in the tendon acting as a power amplifier at light loads to a more rigid force transducer at heavy loads.     

Microstructural stress relaxation mechanics in functionally different tendons

Tendons experience widely varying loading conditions in vivo. They may be categorised by their function as either positional tendons, which are used for intricate movements and experience lower stress, or as energy storage tendons which act as highly stressed springs during locomotion. Structural and compositional differences between tendons are thought to enable an optimisation of their properties to suit their functional environment. However, little is known about structure–function relationships in tendon.This study adopts porcine flexor and extensor tendon fascicles as examples of high stress and low stress tendons, comparing their mechanical behaviour at the micro-level in order to understand their stress relaxation response. Stress-relaxation was shown to occur predominantly through sliding between collagen fibres. However, in the more highly stressed flexor tendon fascicles, more fibre reorganisation was evident when the tissue was exposed to low strains. By contrast, the low load extensor tendon fascicles appears to have less capacity for fibre reorganisation or shearing than the energy storage tendon, relying more heavily on fibril level relaxation. The extensor fascicles were also unable to sustain loads without rapid and complete stress relaxation. These findings highlight the need to optimise tendon repair solutions for specific tendons, and match tendon properties when using grafts in tendon repairs.     

Inflammatory Process Induced by Carrageenan in Adjacent Tissue Triggers the Acute Inflammation in Deep Digital Flexor Tendon of Rats

Tendinopathy is a pathology found mainly in the rotator cuff, patellar, Achilles and flexor tendons. Tendinopathy is a significant impediment to performance in athletes and in workers in the labor market. Some studies have indicated that inflammation in adjacent tissues may affect the rotator cuff and Achilles tendon. In this study alterations were verified in the extracellular matrix (ECM) of the deep digital flexor tendon after two periods (12 and 24 hr) of induction inflammation in rat paw. Wistar rats were divided into three groups: those that received injection of 1% carrageenan; those that received 0.9% NaCl; and those that received no application. The tendon was divided into distal (d), proximal (p), and intermediate (i) regions. Biochemical analyses were performed and included non‐collagenous proteins (NCP), glycosaminoglycans (GAGs), hydroxyproline (HoPro) and metalloproteinases 2 and 9. Tissue sections were stained with toluidine blue, hematoxylin‐eosin, and Ponceau SS and observed under polarization microscopy. Remarkable results were detected that included the presence of MMP‐9, degradation of NCP and GAG and the presence of cellular infiltrate closer to digits in d region. The different concentrations of HoPro, as well as alterations in the organization of the collagen fibers showed the collagenous matrix undergoing some alterations. The results indicated that the induced inflammation in rat paw exhibited characteristics similar to the typical acute inflammatory process observed in tendons. Anat Rec, 2013. © 2013 Wiley Periodicals, Inc.     

Histopathological and biomechanical evaluation of tenocyte seeded allografts on rat Achilles tendon regeneration

Tendon injuries in humans as well as in animals' veterinary medicine are problematic because tendon has poor regenerative capacity and complete regeneration of the ruptured tendon is never achieved. In the last decade there has been an increasing need of treatment methods with different approaches. The aim of the current study was to improve the regeneration process of rat Achilles tendon with tenocyte seeded decellularized tendon matrices. For this purpose, Achilles tendons were harvested, decellularized and seeded as a mixture of three consecutive passages of tenocytes at a density of 1 × 10⁶ cells/ml. Specifically, cells with different passage numbers were compared with respect to growth characteristics, cellular senescence and collagen/tenocyte marker production before seeding process. The viability of reseeded tendon constructs was followed postoperatively up to 6 months in rat Achilles tendon by histopathological and biomechanical analysis. Our results suggests that tenocyte seeded decellularized tendon matrix can significantly improve the histological and biomechanical properties of tendon repair tissue without causing adverse immune reactions. To the best of our knowledge, this is the first long-term study in the literature which was accomplished to prove the use of decellularized matrix in a clinically relevant model of rat Achilles tendon and the method suggested herein might have important implications for translation into the clinic.