Region specific patellar tendon hypertrophy in humans following resistance training

Effect of acute exercise on patella tendon protein synthesis and gene expression

Evidence suggests that habitual loading can result in patellar tendon hypertrophy, especially at the proximal and distal parts of the patellar tendon. The underlying protein kinetic changes and its regulation remains controversial and human data, investigating this topic, are limited. The present study investigated how acute exercise affects growth factor production and collagen fractional synthetic rate in patellar tendon tissue from patients undergoing an anterior cruciate ligament reconstruction operation. The operation was performed by use of the bone-patellar tendon-bone method under spinal anesthesia.

Twelve subjects were randomized to one of two groups: a control group or an exercise group (1-hr unilateral knee-extension 67% of W_max 24 hours before operation). Two hours before the anterior cruciate ligament operation a flooding-dose of L-[1-¹³C]proline was given. Tissue from the most proximal part of the patellar tendon was obtained during the operation. Tendon collagen fractional synthetic rate and mRNA concentrations of TGF-β-1, CTGF, and type I and III collagen were measured.

CTGF and type I collagen expression were higher in the exercise group compared to the control group (p < 0.05). Type III collagen expression (p = 0.11), TGF-β-1 expression (p = 0.34), and collagen fractional synthetic rate (p = 0.26) did not differ between groups.

Although the expression of CTGF and type I collagen were higher, the patellar tendon collagen fractional synthetic rate was not correspondingly higher after exercise. The elevated CTGF expression in the exercise group indicates that the TGF-beta pathway could be an important link between mechanical loading and stimulation of tendon tissue type I collagen expression.

Impact of oral contraceptive use and menstrual phases on patellar tendon morphology, biochemical composition, and biomechanical properties in female athletes

Sex differences exist with regards to ligament and tendon injuries. Lower collagen synthesis has been observed in exercising women vs. men, and in users of oral contraceptives (OC) vs. nonusers, but it is unknown if OC will influence tendon biomechanics of women undergoing regular training. Thirty female athletes (handball players, 18-30 yr) were recruited: 15 long-term users of OC (7.0 ± 0.6 yr) and 15 nonusers (>5 yr). Synchronized values of patellar tendon elongation (obtained by ultrasonography) and tendon force were sampled during ramped isometric knee extensor maximum voluntary contraction to estimate mechanical tendon properties. Furthermore, tendon cross-sectional area and length were measured from MRI images, and tendon biopsies were obtained for analysis of tendon fibril characteristics and collagen cross-linking. Overall, no difference in tendon biomechanical properties, tendon fibril characteristics, or collagen cross-linking was observed between the OC users and nonusers, or between the different phases of the menstrual cycle. In athletes, tendon cross-sectional area in the preferred jumping leg tended to be larger than that in the contralateral leg (P = 0.09), and a greater absolute (P = 0.01) and normalized tendon stiffness (P = 0.02), as well as a lower strain (P = 0.04), were observed in the jumping leg compared with the contralateral leg. The results indicate that long-term OC use or menstrual phases does not influence structure or mechanical properties of the patellar tendon in female team handball athletes.

Efeito dos exercícios de fortalecimento e alongamento sobre a rigidez tecidual passiva

INTRODUÇÃO: Níveis excessivos ou reduzidos de rigidez passiva dos músculos, tendões, ligamentos e fáscias podem estar relacionados à ocorrência de disfunções de movimento e ao desenvolvimento de lesões musculoesqueléticas. O tratamento dessas condições comumente envolve a aplicação de técnicas voltadas para alterar a rigidez, tais como fortalecimento ou alongamento. OBJETIVO: Realizar uma revisão crítica da literatura para investigar os efeitos de exercícios de fortalecimento e alongamento sobre a rigidez tecidual passiva. MATERIAIS E MÉTODOS: Foi realizada consulta aos bancos de dados Medline, Scielo, Lilacs e PEDro. Foram incluídos estudos experimentais realizados em animais ou humanos, sem limite de data. RESULTADOS: Foram selecionados 20 estudos que investigaram o efeito do fortalecimento sobre a rigidez passiva e 13 que pesquisaram o efeito de programas de alongamento sobre a rigidez passiva. CONCLUSÃO: Os estudos sugerem que exercícios de fortalecimento de alta intensidade são capazes de aumentar os níveis de rigidez tecidual tanto em animais quanto em humanos. O aumento da área de secção transversa e modificações na composição dos tecidos são alguns dos mecanismos responsáveis por esse aumento. Em relação ao fortalecimento muscular em posição alongada e ao fortalecimento excêntrico em toda amplitude com carga moderada, os resultados são insuficientes para afirmar sobre o real efeito dessas técnicas em reduzir os níveis de rigidez. Por fim, programas de alongamento estático ou do tipo contrai-relaxa parecem reduzir a rigidez tecidual quando realizados por meio de protocolos de longa duração e/ou alta frequência.

The effects of immobilization on the mechanical properties of the patellar tendon in younger and older men

BACKGROUND

It remains unknown if inactivity changes the mechanical properties of the human patellar tendon in younger and older healthy persons. The purpose was to examine the effects of short-term unilateral immobilization on the structural and mechanical properties of the patellar tendon in older men and younger men, in vivo.

METHODS

Eight older men and eight younger men underwent 14 days of unilateral immobilization. All individuals were assessed on both sides before and after the intervention. MRI was used to assess whole patellar tendon dimensions. The mechanical properties of the patellar tendon were assessed using simultaneous force and ultrasonographic measurements during isometric ramp contractions.

FINDINGS

In older men, tendon stiffness [Pre: mean 2949 (SD 799) vs. Post: mean 2366 (SD 774) N mm(-1), P<0.01] and Young's Modulus [Pre: mean 1.2 (SD 0.3) vs. Post: mean 1.0 (SD 0.3) GPa, P<0.05] declined with immobilization on the immobilized side. On the control side, tendon stiffness [Pre: mean 3340 (SD 1209) vs. Post: mean 2230 (SD 503), P<0.01] and Young's Modulus [Pre: mean 1.5 (SD 0.4) vs. Post: mean 0.9 (SD 0.3) GPa, P<0.05] also decreased with immobilization. In younger men, tendon stiffness [Pre: 3622 (SD 1760) vs. Post: mean 2910 (SD 1528) N mm(-1), P<0.01] and Young's Modulus [Pre: mean 1.7 (SD 1.1) vs. Post: mean 1.4 (SD 0.8) GPa, P<0.05] decreased only on the immobilized side.

INTERPRETATION

Short-term immobilization led to impaired mechanical properties of the patellar tendon on the immobilized side in both younger men and older men, which can influence the function of the muscle-tendon complex.

Biomechanical properties of isolated fascicles of the Iliopsoas and Achilles tendons in African American and Caucasian men

OBJECTIVES

To investigate biomechanical properties of the Iliopsoas and Achilles tendons in young African American (AA) and Caucasian (CC) men, and attempt to clarify whether the difference in Achilles tendon ruptures between AA and CC can be explained by differences in material properties.

METHODS

Tissue from 12 young males (AA, n=6; CC, n=6) was obtained from routine forensic autopsies. Iliopsoas and Achilles tendon samples were obtained from cadavers that were age, height and weight matched. Tendon collagen fascicles were tested micromechanically in a Deben mechanical testing rig.

RESULTS

Peak failure stress in Iliopsoas tendon fascicles was considerably higher (p<0.05) in AA (22.4 ± 7.2MPa) than CC (6.8 ± 2.1MPa) whereas peak strain did not differ (AA: 19.7 ± 5.2%, CC: 18.3 ± 3.5%). Likewise, Young's modulus was greater (p<0.05) in AA (165.3 ± 67.3MPa) than CC (63.6 ± 23.6MPa). In contrast, peak failure stress in Achilles tendon fascicles was similar (p>0.1) in AA (21.9 ± 9.9MPa) and CC (28.1 ± 9.8MPa), and peak strain did not differ (p>0.1) between AA (16.3 ± 3.5%) and CC (13.8 ± 4.4%). Young's modulus was slightly greater in CC (316.8 ± 110MPa) than AA (222.8 ± 84.6MPa), yet not significantly (p>0.1).

CONCLUSIONS

These findings indicate that Iliopsoas tendon fascicles are stronger in young AA compared to CC males, which is suggested to reflect differences in muscle mass and force generating capacity. This could not be confirmed in Achilles tendon fascicles.

Neuromuscular factors influencing the maximum stretch limit of the human plantar flexors

Maximum joint range of motion is an important parameter influencing functional performance and musculoskeletal injury risk. Nonetheless, a complete description of the muscle architectural and tendon changes that occur during stretch and the factors influencing maximum range of motion is lacking. We measured muscle-tendon elongation and fascicle lengthening and rotation sonographically during maximal plantar flexor stretches in 21 healthy men. Electromyogram (EMG) recordings were obtained synchronously with ultrasound and joint moment data, and H-reflex measurements were made with the ankle at neutral (0°) and dorsiflexed (50% maximal passive joint moment) positions; the maximum H amplitude (normalized to maximum M-wave amplitude; M(max)) and H-amplitude elicited at a stimulation intensity that evoked 10% M(max) were obtained. Maximal stretch was accomplished through significant muscle (14.9%; 30 mm) and tendon lengthening (8.4%; 22 mm). There were similar relative changes in fascicle length and angle, but planimetric modeling indicated that the contribution of fascicle rotation to muscle lengthening was small (<4 mm). Subjects with a greater range of motion showed less resistance to stretch and a greater passive joint moment at stretch termination than less flexible subjects (i.e., greater stretch tolerance). Also, greater fascicle rotation accompanied muscle elongation (9.7 vs. 5.9%) and there was a greater tendon length at stretch termination in more flexible subjects. Finally, a moderate correlation between the angle of EMG onset and maximum range of motion was obtained (r = 0.60, P < 0.05), despite there being no difference in H-reflex magnitudes between the groups. Thus clear differences in the neuromuscular responses to stretch were observed between "flexible" and "inflexible" subjects.

Uphill running improves rat Achilles tendon tissue mechanical properties and alters gene expression without inducing pathological changes

Overuse Achilles tendinopathy is a common and challenging problem in sports medicine. Little is known about the etiology of this disorder, and the development of a good animal model for overuse tendinopathy is essential for advancing insight into the disease mechanisms. Our aim was to test a previously proposed rat model for Achilles tendon overuse. Ten adult male Sprague-Dawley rats ran on a treadmill with 10° incline, 1 h/day, 5 days/wk (17-20 m/min) for 12 wk and were compared with 12 control rats. Histological, mechanical, and gene-expression changes were measured on the Achilles tendons after the intervention, and local tendon glucose-uptake was measured before and after the intervention with positron emission tomography. No differences were detected between runners and controls in tissue histology or in glucose uptake, indicating that tendon pathology was not induced. Greater tendon tissue modulus (P < 0.005) and failure stress/body weight (P < 0.02) in runners compared with controls further supported that tendons successfully adapted to uphill running. Several genes of interest were regulated after 12 wk of running. Expression of collagen III and insulin-like growth factor I was increased, while collagen I was unchanged, and decreases were seen in noncollagen matrix components (fibromodulin and biglycan), matrix degrading enzymes, transforming growth factor-β1, and connective tissue growth factor. In conclusion, the tested model could not be validated as a model for Achilles tendinopathy, as the rats were able to adapt to 12 wk of uphill running without any signs of tendinopathy. Improved mechanical properties were observed, as well as changes in gene-expression that were distinctly different from what is seen in tendinopathy and in response to short-term tendon loading.

Eccentric strengthening effect of hip-adductor training with elastic bands in soccer players: a randomised controlled trial

BACKGROUND

Soccer players with weak hip-adductor muscles are at increased risk of sustaining groin injuries. Therefore, a simple hip-adductor strengthening programme for prevention of groin injuries is needed.

OBJECTIVE

We aimed to investigate the effect of an 8-week hip-adductor strengthening programme, including one hip-adduction exercise, on eccentric and isometric hip-adduction strength, using elastic bands as external load.

METHODS

Thirty-four healthy, sub-elite soccer players, mean (±SD) age of 22.1 (±3.3) years, were randomised to either training or control. During the mid-season break, the training group performed 8 weeks of supervised, progressive hip-adduction strength training using elastic bands. The participants performed two training sessions per week (weeks 1-2) with 3×15 repetition maximum loading (RM), three training sessions per week (weeks 3-6) with 3×10 RM and three training sessions per week (weeks 7-8) with 3×8 RM. Eccentric hip-adduction (EHAD), isometric hip-adduction (IHAD) and isometric hip-abduction (IHAB) strength, and the IHAD/IHAB ratio were measured assessor-blinded preintervention and postintervention, using reliable hand-held dynamometry procedures.

RESULTS

In the training group, EHAD strength increased by 30% (p<0.001). In the control group, EHAD strength increased by 17% (p<0.001), but the increase was significantly larger in the training group compared with the control group (p=0.044). No other significant between-group strength-differences in IHAD, IHAB or the IHAD/IHAB ratio existed (p>0.05).

CONCLUSIONS

8 weeks of hip-adduction strength training, using elastic bands, induce a relevant increase in eccentric hip-adduction strength in soccer players, and thus may have implications as a promising approach towards prevention of groin injuries in soccer.

Influence of acetaminophen consumption and exercise on Achilles tendon structural properties in male Wistar rats

Chronic consumption of acetaminophen (APAP) during exercise training leads to a reduction in tendon stiffness and modulus compared with a placebo. We explored whether this effect could be due to a reduction in tendon collagen content or cross-linking. Ten-week-old male Wistar rats ( n = 50) were divided into placebo or APAP groups and into sedentary or treadmill-exercised groups. APAP (200 mg/kg) or saline was administered once daily by oral gavage. Rats in the exercise groups ran on a treadmill 5 days per week for 8 wk with progression to 60 min per day, 20 m/min, and 8° incline. After 8 wk, lyophilized Achilles tendon samples were assayed for the collagen-specific amino acid hydroxyproline and cross-linking [hydroxylyslpyridinoline (HP)] content by high-performance liquid chromatrography. Collagen content was not influenced by exercise or APAP ( P > 0.05). Compared with placebo, tendon water content was 7% ( P = 0.006, main effect) lower in animals consuming APAP (placebo: 54.79 ± 0.8%, APAP: 50.89 ± 1.2%). HP in the Achilles tendon was 36% greater (sedentary: 141 ± 15, exercise: 204 ± 26 mmol/mol collagen) in the exercise-trained rats independent of drug treatment ( P = 0.020, main effect). Independent of exercise, HP content was 33% lower ( P = 0.032, main effect) in the animals consuming APAP (placebo: 195 ± 21, APAP: 140 ± 19 mmol/mol collagen). Our data suggests that chronic consumption of APAP results in a reduction in collagen cross-linking and a loss of tissue water independent of chronic exercise. This reduction in cross-linking and water content could contribute to the decrease in tendon stiffness noted in humans chronically consuming APAP.

The magnitude and character of resistance-training-induced increase in tendon stiffness at old age is gender specific

Human tendon mechanical properties are modified with loading. Moreover, there are indications that the training response in the tendon is gender specific. The aim of the current study was to examine whether in vivo patella tendon stiffness (K) differentially alters with training in older males compared with females. We also aimed to identify which endocrine pathway underlies the responses. Maximal knee extensor forces were also monitored to determine the training effect on muscle function. Fourteen healthy, habitually active older persons (seven males aged 74.0 ± 1.2 years (mean±SEM) and seven females aged 76.7 ± 1.2 years) were tested at baseline and after 12 weeks of weekly, progressive resistance training. With training, percentage increase in quadriceps maximum voluntary isometric force (MVC) was similar in males (2,469.6 ± 168.0 to 3,097.3 ± 261.9 N; +25.3 ± 6.1% (p < 0.01)) and females (1,728.8 ± 136.3 to 2,166.5 ± 135.8 N; +30.4 ± 15.1% (p < 0.05)), respectively. K increased more in males (338.0 ± 26.6 to 616.9 ± 58.7 N/mm; 79.8 ± 4.2% (p < 0.001)) compared to females (338.9 ± 31.0 to 373.2 ± 25.8 N/mm; +13.0 ± 3.7% (p < 0.001)). Interestingly, a pattern was found whereby below ~40% MVC, the females showed their greatest degree of K changes, whereas the males showed their greatest degree of K change above this relative force level. This gender contrast was also true at a standardised force level (1,200 N), with 5.8 ± 0.4% vs. 82.5 ± 1.8% increments in the females (i.e. value change from 380.3 ± 14.1 to 402.4 ± 13.3 N/mm) and the males (i.e. value change from 317.8 ± 13.8 to 580.2 ± 30.9 N/mm), respectively (p < 0.001). While circulating levels of both IGF-I and IL-6 did not alter with training, IGFBP-3 showed a significant training effect (19.1 ± 4.8%, p < 0.001) and only in the male sub-group (p = 0.038). We show here that with training, in vivo older females' tendon is less dramatically modulated than that of males'. We also show that the relative forces, at which the greatest adaptations are exhibited, differ by gender, with a suggestion of endocrine adaptations in males only. We thus propose that both training and rehabilitation regimens should consider gender-specific tendon responsiveness, at least in older persons.

Changes in bone alkaline phosphatase and procollagen type-1 C-peptide after static and dynamic exercises

We investigated the effects of two types of nonweight-bearing exercise on changes in bone-specific alkaline phosphatase (BAP) and procollagen type 1 C-peptide (PIP). BAP is a specific marker of bone synthesis, whereas P1P reflects synthesis of type 1 collagen in other organs as well as bone. Eight participants performed static and dynamic unilateral knee extensions. BAP and PIP were measured before, and at 1, 2, 24, 48, and 72 hr after exercise. PIP increased at 24 hr after a static knee extension exercise, whereas BAP did not change during the experimental period. We found no changes in these markers after dynamic exercise. These results imply that type I collagen synthesis in tendons increases after static exercise.

A wide number of trials is required to achieve acceptable reliability for measurement patellar tendon elongation in vivo

The purpose of the current study was to examine the reproducibility of patellar tendon elongation measurements using brightness-mode ultrasonography (BMU) during isometric knee extension contractions. We hypothesized that the measurement of the patellar tendon elongation during only one maximal voluntary isometric knee extension contractions would not provide reliable results and that a wide number of trials is required to achieve acceptable reliability. Ten participants (eight male and two female) performed 10 isometric knee extension contractions on two separate days (5 trials on each day). Using a modified knee brace, the ultrasound probe was firmly adjusted in the sagittal plane overlying the patellar tendon. The registered ultrasound images were analyzed by three different but equally trained observers. The reproducibility was examined by the calculation of the within-day, between-day and overall coefficient of multiple correlations (wCMC, bCMC and oCMC). The Spearman-Brown prophecy formula was use to estimate the required trials to achieve the desired reliability. The wCMC, bCMC and oCMC were in average 0.824, 0.798 and 0.770, respectively, suggesting a rather moderate reproducibility of patellar tendon elongation measurements. For a high reliability (≥0.95) of tendon elongation measurements 5-6 trials are required. Finally, the results revealed an independence of the measurements from days and observers.

Mechanical properties of human patellar tendon at the hierarchical levels of tendon and fibril

Tendons are strong hierarchical structures, but how tensile forces are transmitted between different levels remains incompletely understood. Collagen fibrils are thought to be primary determinants of whole tendon properties, and therefore we hypothesized that the whole human patellar tendon and its distinct collagen fibrils would display similar mechanical properties. Human patellar tendons ( n = 5) were mechanically tested in vivo by ultrasonography. Biopsies were obtained from each tendon, and individual collagen fibrils were dissected and tested mechanically by atomic force microscopy. The Young's modulus was 2.0 ± 0.5 GPa, and the toe region reached 3.3 ± 1.9% strain in whole patellar tendons. Based on dry cross-sectional area, the Young's modulus of isolated collagen fibrils was 2.8 ± 0.3 GPa, and the toe region reached 0.86 ± 0.08% strain. The measured fibril modulus was insufficient to account for the modulus of the tendon in vivo when fibril content in the tendon was accounted for. Thus, our original hypothesis was not supported, although the in vitro fibril modulus corresponded well with reported in vitro tendon values. This correspondence together with the fibril modulus not being greater than that of tendon supports that fibrillar rather than interfibrillar properties govern the subfailure tendon response, making the fibrillar level a meaningful target of intervention. The lower modulus found in vitro suggests a possible adverse effect of removing the tissue from its natural environment. In addition to the primary work comparing the two hierarchical levels, we also verified the existence of viscoelastic behavior in isolated human collagen fibrils.

Effect of alpine skiing training on tendon mechanical properties in older men and women

Strain is one of the parameters determining tendon adaptation to mechanical stimuli. The aim of this study was to test whether the patellar tendon strain induced during recreational alpine skiing would affect tendon mechanical properties in older individuals. Twenty-two older males and females (67 ± 2 years) were assigned to a 12-week guided skiing programme (IG) and 20 aged-matched volunteers served as controls (CG). Patellar tendon mechanical properties and cross-sectional area (CSA) were measured before and after training, with combined dynamometry and ultrasonography scanning. None of the variables changed significantly in the CG after training. In the IG, tendon stiffness and Young's modulus were increased (respectively, 14% and 12%, P<0.01), without any significant change in tendon CSA. In addition, changes in tendon stiffness were blunted in women (9%) compared with men (19%). Serum IGF-1 concentration tended to be lower in women (-19%, P=0.07). These results demonstrate that the mechanical stimulus induced by alpine skiing is sufficient to elicit adaptive changes in patellar tendon mechanical and material properties in older subjects. Furthermore, the present sex-specific adaptations are consistent with previous reports of lower collagen metabolic responsiveness in women and may be underpinned by anthropometric and metabolic differences.

Influence of aerobic cycle exercise training on patellar tendon cross-sectional area in older women

Nine to 12 weeks of resistance exercise training in young individuals induces quadriceps muscle (∼6%) and region-specific patellar tendon (4-6%) hypertrophy. However, 12 weeks of resistance exercise training (∼1 h total exercise time) in older individuals (60-78 years) induces quadriceps muscle hypertrophy (9%) without impacting patellar tendon size. The current study examined if a different loading paradigm using cycle exercise would promote patellar tendon hypertrophy or alter the internal tendon properties, measured with magnetic resonance imaging signal intensity, in older individuals. Nine women (70 ± 2 years) completed 12 weeks of aerobic upright cycle exercise training (∼28 h total exercise time). Aerobic exercise training increased (P < 0.05) quadriceps muscle size (11 ± 2%) and VO2max (30 ± 9%). Mean patellar tendon cross-sectional area (CSA) (2 ± 1%) and signal intensity (-1 ± 2%) were unchanged (P > 0.05) over the 12 weeks of training. Region-specific CSA was unchanged (P > 0.05) at the proximal (-1 ± 3%) and mid regions (2 ± 2%) of the tendon but tended (P = 0.069) to increase at the distal region (5 ± 3%). Region-specific signal intensity differed along the tendon but was unchanged (P > 0.05) with training. Although more studies are needed, exercise-induced patellar tendon hypertrophy, compared with skeletal muscle, appears to be attenuated in older individuals, while the loading pattern associated with aerobic exercise seems to have more impact than resistance exercise in promoting patellar tendon hypertrophy.

Computation methods affect the reported values of in vivo human tendon stiffness

PURPOSE

Scientific validity is questionable when findings from studies cannot be used to make sense of physiological and/or biomechanical data. In particular, is the case of in vivo determination of tendon stiffness (K). Here, approaches range from taking the gradient (a) throughout the data range of resting to Maximal Voluntary Contraction (MVC), (b) tangents at individual data points, (c) linear regressions at discrete force levels ((b) and (c) being 'reference standard' as they utilise a number of distinct regions of the Force-Elongation Relationship (FER)).

STUDY DESIGN

A mathematical model approach is used to develop simple curvilinear FERs as seen when determining tendon mechanical properties, to allow variable calculations of K.

OBJECTIVES

To compare variability in K estimates using the various approaches currently seen in the literature.

METHODS

Three FER models were developed, representing low, medium and high K. Values of K were determined and compared using the approaches reported in the literature to estimate the magnitude of the difference between values attained of K.

RESULTS

Through mathematical modelling, we demonstrate that the impact on the recorded value of K is substantial: relative to the reference standard methods, computation methods published range from underestimating K by 26% to overestimating it by 51%.

CONCLUSION

This modelling helps by providing a 'scaling factor' through which the between studies variability associated with computational methods differences is minimised. This is especially important where researchers or clinicians require values which are consistent in the context of establishing the 'true' tendon mechanical properties to inform models or materials based on the biological properties of the human tendon.

Mechanical properties of the human Achilles tendon, in vivo

BACKGROUND

Ultrasonography has been widely applied for in vivo measurements of tendon mechanical properties. Assessments of human Achilles tendon mechanical properties have received great interest. Achilles tendon injuries predominantly occur in the tendon region between the Achilles-soleus myotendinous junction and Achilles-calcaneus osteotendinous junction i.e. in the free Achilles tendon. However, there has been no adequate ultrasound based method for quantifying the mechanical properties of the free human Achilles tendon. This study aimed to: 1) examine the mechanical properties of the free human Achilles tendon in vivo by the use of ultrasonography and 2) assess the between-day reproducibility of these measurements.

METHODS

Ten male subjects had the Achilles tendon moment arm length, Achilles tendon cross sectional area and free Achilles tendon length determined. All subjects performed isometric plantarflexion ramp contractions to assess between-day reproducibility on two separate days. Simultaneous ultrasonography based measurements of Achilles-soleus myotendinous junction and Achilles-calcaneus osteotendinous junction displacement together with Achilles tendon force estimates yielded free Achilles tendon mechanical properties.

FINDINGS

Free Achilles tendon maximal force, deformation and stiffness were 1924 (SD 229) N, 2.2 (SD 0.6) mm and 2622 (SD 534) N/mm on day 1. For between-day reproducibility there were no significant differences between days for free Achilles tendon mechanical properties. The between-day correlation coefficient and typical error percent were 0.81 and 5.3% for maximal Achilles tendon force, 0.85 and 11.8% for maximal Achilles tendon deformation and 0.84 and 8.8% for Achilles tendon stiffness respectively. Last, osteotendinous junction proximal displacement on average contributed with 71 (SD 12) % of proximal myotendinous junction displacement.

INTERPRETATION

This study, for the first time, presents an ultrasonography based in vivo method for measurement of free AT mechanical properties. The method is applicable for evaluation of free human Achilles tendon mechanical properties in relation to training, injury and rehabilitation.

Influence of acetaminophen and ibuprofen on in vivo patellar tendon adaptations to knee extensor resistance exercise in older adults

Millions of older individuals consume acetaminophen or ibuprofen daily and these same individuals are encouraged to participate in resistance training. Several in vitro studies suggest that cyclooxygenase-inhibiting drugs can alter tendon metabolism and may influence adaptations to resistance training. Thirty-six individuals were randomly assigned to a placebo (67 ± 2 yr old), acetaminophen (64 ± 1 yr old; 4,000 mg/day), or ibuprofen (64 ± 1 yr old; 1,200 mg/day) group in a double-blind manner and completed 12 wk of knee extensor resistance training. Before and after training in vivo patellar tendon properties were assessed with MRI [cross-sectional area (CSA) and signal intensity] and ultrasonography of patellar tendon deformation coupled with force measurements to obtain stiffness, modulus, stress, and strain. Mean patellar tendon CSA was unchanged (P > 0.05) with training in the placebo group, and this response was not influenced with ibuprofen consumption. Mean tendon CSA increased with training in the acetaminophen group (3%, P < 0.05), primarily due to increases in the mid (7%, P < 0.05) and distal (8%, P < 0.05) tendon regions. Correspondingly, tendon signal intensity increased with training in the acetaminophen group at the mid (13%, P < 0.05) and distal (15%, P = 0.07) regions. When normalized to pretraining force levels, patellar tendon deformation and strain decreased 11% (P < 0.05) and stiffness, modulus, and stress were unchanged (P > 0.05) with training in the placebo group. These responses were generally uninfluenced by ibuprofen consumption. In the acetaminophen group, tendon deformation and strain increased 20% (P < 0.05) and stiffness (-17%, P < 0.05) and modulus (-20%, P < 0.05) decreased with training. These data suggest that 3 mo of knee extensor resistance training in older adults induces modest changes in the mechanical properties of the patellar tendon. Over-the-counter doses of acetaminophen, but not ibuprofen, have a strong influence on tendon mechanical and material property adaptations to resistance training. These findings add to a growing body of evidence that acetaminophen has profound effects on peripheral tissues in humans.

Interleukin-6: a growth factor stimulating collagen synthesis in human tendon

Human connective tissue, e.g., tendon, responds dynamically to physical activity, with collagen synthesis being increased after both acute and prolonged exercise or training. Markers of collagen synthesis and degradation as well as concentration of several potential growth factors have been shown to increase markedly in the peritendinous tissue around the human Achilles tendon following exercise. Of these potential growth factors interleukin-6 (IL-6) showed the largest fold increase, suggesting that IL-6 may be involved in transforming mechanical loading into collagen synthesis in human tendon tissue. In the present study the tissue levels of type I collagen turnover markers [procollagen type I NH(2)-terminal propeptide (PINP) and C-OOH terminal telopeptide of type I collagen (ICTP)] were measured by the use of microdialysis in peritendinous tissue of the Achilles tendon in 14 male volunteers, who had recombinant human IL-6 (rhIL-6) infused into the peritendinous tissue of the Achilles' tendon in one leg, with the other leg serving as control. Subjects were randomly assigned to either a resting group or an exercise group performing a 1-h treadmill run (12 km/h, 2% uphill) before infusion. In addition to IL-6, serum concentrations of collagen turnover markers PINP, ICTP, and COOH-terminal telopeptide of type I collagen (ICTX) were measured. The peritendinous concentration of PINP rose markedly in response to rhIL-6 infusion in both the exercise and the rest group, demonstrating that infusion of IL-6 significantly stimulates collagen synthesis in the peritendinous tissue in humans. Exercise alone did not result in an increased collagen synthesis. This indicates that IL-6 is involved in the collagen synthesis and supports the hypothesis that IL-6 is an important growth factor of the connective tissue in healthy human tendons.

Increased cellular proliferation in rat skeletal muscle and tendon in response to exercise: use of FLT and PET/CT

PURPOSE

The purpose of this study is to investigate exercise-induced cellular proliferation in rat skeletal muscle/tendon with the use of 3'-[F-18]fluoro-3'deoxythymidine (FLT) and to quantitatively study concomitant changes in the proliferation-associated factor, Ki67.

PROCEDURES

Wistar rats (n = 13) performed 3 days of treadmill running. Cellular proliferation was investigated 3 days before and 48 h after the running exercise with the use of FLT and positron emission tomography/computed tomography (PET/CT). Results were compared to a sedentary control group (n = 10). Image-derived standardized uptake values were calculated for Achilles tendons and calf muscles and compared to gene expression and immunohistochemical evaluations of Ki67.

RESULTS

Treadmill running induced increased uptake of FLT uptake in calf muscles (30%; p < 0.001) and in Achilles tendon (21%, p < 0.001). The image-derived results were supported by a correlation in calf muscle to Ki67 (protein and mRNA level), while this coherence was not found in tendon.

CONCLUSION

FLT-PET seems to be a promising tool for imaging of exercise-induced cellular proliferation in musculo-tendinous tissue.

Reliability and responsiveness of musculoskeletal ultrasound in subjects with and without spinal cord injury

Rehabilitation after spinal cord injury (SCI) aims to preserve the integrity of the paralyzed musculoskeletal system. The suitability of ultrasound (US) for delineating training-related muscle/tendon adaptations after SCI is unknown. The purpose of this study was to quantify within- and between-operator reliability for US and to determine its responsiveness to post-training muscle/tendon adaptations in SCI subjects. Two novice operators and one experienced operator obtained sonographic images of the vastus lateralis, patellar tendon, soleus, and Achilles tendon from seven SCI subjects and 16 controls. For control subjects, within-operator concordance (ICC [3,1]) ranged from 0.58 to 0.95 for novice operators and exceeded 0.86 for the experienced operator. Between-operator concordance (ICC [2,1]) ranged from 0.62 to 0.74. Ultrasound detected muscle hypertrophy (p < 0.05) following electrical stimulation training in subjects with SCI (responsiveness) but did not detect differences in tendon thickness. These error estimates support the utility of US in future post-SCI training studies.

Plasticity of human Achilles tendon mechanical and morphological properties in response to cyclic strain

The purpose of the current study in combination with our previous published data (Arampatzis et al., 2007) was to examine the effects of a controlled modulation of strain magnitude and strain frequency applied to the Achilles tendon on the plasticity of tendon mechanical and morphological properties. Eleven male adults (23.9 ± 2.2 yr) participated in the study. The participants exercised one leg at low magnitude tendon strain (2.97 ± 0.47%), and the other leg at high tendon strain magnitude (4.72 ± 1.08%) of similar frequency (0.5 Hz, 1s loading, 1s relaxation) and exercise volume (integral of the plantar flexion moment over time) for 14 weeks, 4 days per week, 5 sets per session. The exercise volume was similar to the intervention of our earlier study (0.17 Hz frequency; 3s loading, 3s relaxation) allowing a direct comparison of the results. Before and after the intervention ankle joint moment has been measured by a dynamometer, tendon-aponeurosis elongation by ultrasound and cross-sectional area of the Achilles tendon by magnet resonance images (MRI). We found a decrease in strain at a given tendon force, an increase in tendon-aponeurosis stiffness and tendon elastic modulus of the Achilles tendon only in the leg exercised at high strain magnitude. The cross-sectional area (CSA) of the Achilles tendon did not show any statistically significant (P > 0.05) differences to the pre-exercise values in both legs. The results indicate a superior improvement in tendon properties (stiffness, elastic modulus and CSA) at the low frequency (0.17 Hz) compared to the high strain frequency (0.5 Hz) protocol. These findings provide evidence that the strain magnitude applied to the Achilles tendon should exceed the value, which occurs during habitual activities to trigger adaptational effects and that higher tendon strain duration per contraction leads to superior tendon adaptational responses.

Plyometric training effects on Achilles tendon stiffness and dissipative properties

The aim of this study was to determine the effects of 14 wk of plyometric training on mechanical properties of the Achilles tendon. Nineteen subjects were randomly assigned to trained or control group. Cross-sectional area (CSA), stiffness, and dissipation coefficient of the Achilles tendon were measured before and after the training period. In the trained group, a decrease in dissipation coefficient (−35.0%; P < 0.05) and an upward trend in stiffness (+24.1%) of the Achilles tendon was found, without any changes in Achilles tendon CSA ( P > 0.05). Plyometric training enhances the muscular tension transmission mainly through a reduction in energy dissipated by the tendon. The lack of changes in the Achilles tendon CSA indicates that changes in mechanical properties would mainly result from a qualitative change in tendinous tissues rather than from changes in the geometry of the Achilles tendon.

Biomechanical responses of different rat tendons to nandrolone decanoate and load exercise

Androgenic-anabolic steroids (AAS) have been associated with an increased incidence of tendon rupture. The aim of this study was to compare the biomechanical properties of the rat calcaneal tendon (CT), superficial flexor tendon (SFT), and deep flexor tendon (DFT), and to determine the effect of jump training in association with AAS. Animals were separated into four groups: sedentary, trained, AAS-treated sedentary rats (AAS), and AAS-treated and trained animals. Mechanical testing showed that the CT differed from the DFT and SFT, which showed similar mechanical properties. Jump caused the CT to exhibit an extended toe region, an increased resistance to tensional load, and a decreased elastic modulus, characteristics of an elastic tendon capable of storing energy. AAS caused the tendons to be less compliant, and the effects were reinforced by simultaneous training. The DFT was the most affected by training, AAS, and the interaction of both, likely because of its involvement in the toe-off step of jumping, which we suggest is related to the rapid transmission of force as opposed to energy storage. In conclusion, tendons are differently adapted to exercise, but responded equally to AAS, showing reduced flexibility, which is suggested to increase the risk of tendon rupture in AAS consumers.

Fibril morphology and tendon mechanical properties in patellar tendinopathy: effects of heavy slow resistance training

BACKGROUND

Patellar tendinopathy is characterized by pathologic abnormalities. Heavy slow resistance training (HSR) is effective in the management of patellar tendinopathy, but the underlying functional mechanisms remain elusive.

PURPOSE

To investigate fibril morphology and mechanical properties in patellar tendinopathy and the effect of HSR on these properties.

STUDY DESIGN

Cohort study; Level of evidence, 2.

METHODS

Eight male patients with patellar tendinopathy completed 12 weeks of HSR. Nine healthy subjects served as controls. Assessments were conducted at baseline and at 12 weeks. Patients assessed symptoms/function and maximal tendon pain during activity. Tendon biopsy samples were analyzed for fibril density, volume fraction, and mean fibril area. Tendon mechanical properties were assessed using force and ultrasonography samplings.

RESULTS

Patients improved in symptoms/function (P = .02) and maximal tendon pain during activity (P = .008). Stiffness and modulus of control and tendinopathy tendons were similar at baseline. Stiffness remained unaffected in control tendons (3487 +/- 392 to 3157 +/- 327 N/mm, P = .57) but declined in tendinopathic tendons at 12 weeks (3185 +/- 187 to 2701 +/- 201 N/mm, P = .04). At baseline, fibril volume fraction was equal, fibril density smaller (P = .03), and mean fibril area tended to be higher in tendinopathy versus controls (P = .07). Fibril morphology remained unchanged in controls but fibril density increased (70% +/- 18%, P = .02) and fibril mean area decreased (-26% +/- 21%, P = .04) in tendinopathic tendons after HSR.

CONCLUSION

Fibril morphology is abnormal in tendinopathy, but tendon mechanical properties are not. Clinical improvements after HSR were associated with changes in fibril morphology toward normal fibril density and mean fibril area. Heavy slow resistance training improved the clinical outcome of patellar tendinopathy, and these improvements were associated with normalization of fibril morphology, most likely due to a production of new fibrils.

Contraction intensity and feeding affect collagen and myofibrillar protein synthesis rates differently in human skeletal muscle

Exercise stimulates muscle protein fractional synthesis rate (FSR), but the importance of contractile intensity and whether it interplays with feeding is not understood. This was investigated following two distinct resistance exercise (RE) contraction intensities using an intrasubject design in the fasted (n = 10) and fed (n = 10) states. RE consisted of 10 sets of knee extensions. One leg worked against light load (LL) at 16% of one-repetition maximum (1RM), the other leg against heavy load (HL) at 70% 1RM, with intensities equalized for total lifted load. Males were infused with [(13)C]leucine, and vastus lateralis biopsies were obtained bilaterally at rest as well as 0.5, 3, and 5.5 h after RE. Western blots were run on muscle lysates and phosphospecific antibodies used to detect phosphorylation status of targets involved in regulation of FSR. The intramuscular collagen FSR was evenly increased following LL- and HL-RE and was not affected by feeding. Myofibrillar FSR was unaffected by LL-RE, whereas HL-RE resulted in a delayed improvement (0.14 +/- 0.02%/h, P < 0.05). Myofibrillar FSR was increased at rest by feeding (P < 0.05) and remained elevated late in the postexercise period compared with the fasting condition. The Rp-s6k-4E-binding protein-1 (BP1) and the mitogen-activated protein kinase (MAPk) pathways were activated by the HL intensity and were suggested to be responsible for regulating myofibrillar FSR in response to adequate contractile activity. Feeding predominantly affected Rp-s6k and eukaryotic elongation factor 2 phosphorylations in correspondence with the observed changes in myofibrillar FSR, whereas 4E-BP1 remained to respond only to the HL contraction intensity. Thus the study design allows us to conclude that the MAPk- and mammalian target of rapamycin-dependent signaling responds to contractile activity, whereas elongation mainly was found to respond to feeding. Furthermore, although functionally linked, the contractile and the supportive matrix structures upregulate their protein synthesis rate quite differently in response to feeding and contractile activity and intensity.

From mechanical loading to collagen synthesis, structural changes and function in human tendon

The adaptive response of connective tissue to loading requires increased synthesis and turnover of matrix proteins, with special emphasis on collagen. Collagen formation and degradation in the tendon increases with both acute and chronic loading, and data suggest that a gender difference exists, in that females respond less than males with regard to an increase in collagen formation after exercise. It is suggested that estrogen may contribute toward a diminished collagen synthesis response in females. Conversely, the stimulation of collagen synthesis by other growth factors can be shown in both animal and human models where insulin-like growth factor 1 (IGF-I) and transforming growth factor-beta-1 (TGF-beta-1) expression increases to accompany or precede an increase in procollagen expression and collagen synthesis. In humans, it can be demonstrated that an increase in the interstitial concentration of TGF-beta, PGE2, IGF-I plus its binding proteins and interleukin-6 takes place after exercise. The increase in IGF-I expression in tendon includes the isoform that has so far been thought only to exist in skeletal muscle (mechano growth factor). The increase in IGF-I and procollagen expression showed a similar response whether the tendon was stimulated by concentric, isometric or eccentric muscle contraction, suggesting that strain rather that stress/torque determines the collagen-synthesis stimulating response seen with exercise. The adaptation time to chronic loading is longer in tendon tissue compared with contractile elements of skeletal muscle or the heart, and only with very prolonged loading are significant changes in gross dimensions of the tendon observed, suggesting that habitual loading is associated with a robust change in the size and mechanical properties of human tendons. An intimate interplay between mechanical signalling and biochemical changes in the matrix is needed in tendon, such that chemical changes can be converted into adaptations in the morphology, structure and material properties.

Effects of plyometric training on passive stiffness of gastrocnemii and the musculo-articular complex of the ankle joint

This study aimed to determine simultaneously the effects of plyometric training on the passive stiffness of the ankle joint musculo-articular complex, the gastrocnemii muscle-tendon complex (MTC) and the Achilles tendon in order to assess possible local adaptations of elastic properties. Seventeen subjects were divided into a trained (TG) group and a control (CG) group. They were tested before and after 8 weeks of a plyometric training period. The ankle joint range of motion (RoM), the global musculo-articular passive stiffness of the ankle joint, the maximal passive stiffness of gastrocnemii and the stiffness of the Achilles tendon during isometric plantar flexion were determined. A significant increase in the jump performances of TG relative to CG was found (squat jumps: +17.6%, P=0.008; reactive jumps: +19.8%, P=0.001). No significant effect of plyometric training was observed in the ankle joint RoM, musculo-articular passive stiffness of the ankle joint or Achilles tendon stiffness (P>0.05). In contrast, the maximal passive stiffness of gastrocnemii of TG increased after plyometric training relative to CG (+33.3%, P=0.001). Thus, a specific adaptation of the gastrocnemii MTC occurred after plyometric training, without affecting the global passive musculo-articular stiffness of the ankle joint.

Behavior of fascicles and the myotendinous junction of human medial gastrocnemius following eccentric strength training

This study is the first in which measurements of thickness, fascicle angle and length, and tendon elongation were combined to examine the impact of eccentric strength training on both muscle architecture and tendinous structures. Eighteen healthy male subjects were divided into an eccentric strength training group (n = 10) and a control group (n = 8). The training program consisted of 18 sessions of eccentric exercises over a 7-week period. All subjects were tested at baseline and after the last training session. Using ultrasound imaging, the fascicle angle and length and thickness of the medial gastrocnemius (MG) were analyzed at rest (i.e., theta(p), Fl(p), and t(p), respectively), at 50% of maximal voluntary contraction (MVC) (i.e., theta(50), Fl(50), and t(50), respectively), and during MVC (i.e., theta(100), Fl(100), and t(100), respectively). Tendon elongation (TE) was measured by tracking the proximal displacement of the myotendinous junction of the MG during ramp isometric contraction. During ramp isometric contraction, the slope of the load-deformation relationship of the gastrocnemius tendon above 50% MVC was defined as an index of stiffness. After training, muscle thickness and fascicle angle increased significantly at rest and during contraction, whereas fascicle length increased at rest and did not change during contraction. Furthermore, the stiffness of the gastrocnemius tendon increased significantly. The results suggest that the behavior of muscle architecture and tendon mechanical properties are affected differently by strength training.

Ultrasonographically detected changes in Achilles tendons and self reported symptoms in elite gymnasts compared with controls--an observational study

There are no published data on the prevalence of Achilles tendinopathy in gymnasts. 40 elite gymnasts and 41 controls of mean (SD) age 16.3 (2.1) years had bilateral AT thickness (at 3 points along the tendon in longitudinal and transverse views) and abnormality measured using ultrasound (US). Data on a history of AT symptoms were gathered using a questionnaire. Among female gymnasts there were 17.5% of Achilles tendons (ATs) with current symptoms, compared with none of the controls (chi(2)=6.2, p<0.05). Similarly, among male gymnasts 12.5% of ATs were currently symptomatic, compared with none of the controls (chi(2)=6.62, p<0.01). US scanning showed gymnasts had significantly (p<0.001 to p<0.05) thicker ATs for 5 out of 6 measures, and had more irregular longitudinal AT thickness (p<0.05). 32.5% of female gymnast ATs had US abnormalities, compared with 6.2% of female control ATs (chi(2)=7.43, p<0.01). Similarly, 35% of male gymnast ATs had US abnormalities, compared with 4% of male control ATs (chi(2)=7.43, p=0<01). There was no relationship between AT symptoms and US-detected abnormalities in the gymnasts. Achilles tendinopathy is a common problem in gymnasts. The poor correlation between signs and symptoms suggests pathology is not always symptomatic.

Mechanical properties and collagen cross-linking of the patellar tendon in old and young men

Age-related loss in muscle mass and strength impairs daily life function in the elderly. However, it remains unknown whether tendon properties also deteriorate with age. Cross-linking of collagen molecules provides structural integrity to the tendon fibrils and has been shown to change with age in animals but has never been examined in humans in vivo. In this study, we examined the mechanical properties and pyridinoline and pentosidine cross-link and collagen concentrations of the patellar tendon in vivo in old (OM) and young men (YM). Seven OM (67 +/- 3 years, 86 +/- 10 kg) and 10 YM (27 +/- 2 years, 81 +/- 8 kg) with a similar physical activity level (OM 5 +/- 6 h/wk, YM 5 +/- 2 h/wk) were examined. MRI was used to assess whole tendon dimensions. Tendon mechanical properties were assessed with the use of simultaneous force and ultrasonographic measurements during ramped isometric contractions. Percutaneous tendon biopsies were taken and analyzed for hydroxylysyl pyridinoline (HP), lysyl pyridinoline (LP), pentosidine, and collagen concentrations. We found no significant differences in the dimensions or mechanical properties of the tendon between OM and YM. Collagen concentrations were lower in OM than in YM (0.49 +/- 0.27 vs. 0.73 +/- 0.14 mg/mg dry wt; P < 0.05). HP concentrations were higher in OM than in YM (898 +/- 172 vs. 645 +/- 183 mmol/mol; P < 0.05). LP concentrations were higher in OM than in YM (49 +/- 38 vs. 16 +/- 8 mmol/mol; P < 0.01), and pentosidine concentrations were higher in OM than in YM (73 +/- 13 vs. 11 +/- 2 mmol/mol; P < 0.01). These cross-sectional data raise the possibility that age may not appreciably influence the dimensions or mechanical properties of the human patellar tendon in vivo. Collagen concentration was reduced, whereas both enzymatic and nonenzymatic cross-linking of concentration was elevated in OM vs. in YM, which may be a mechanism to maintain the mechanical properties of tendon with aging.

Training-induced changes in structural and mechanical properties of the patellar tendon are related to muscle hypertrophy but not to strength gains

To obtain a better understanding of the adaptations of human tendon to chronic overloading, we examined the relationships between these adaptations and the changes in muscle structure and function. Fifteen healthy male subjects (20+/-2 yr) underwent 9 wk of knee extension resistance training. Patellar tendon stiffness and modulus were assessed with ultrasonography, and cross-sectional area (CSA) was determined along the entire length of the tendon by using magnetic resonance imaging. In the quadriceps muscles, architecture and volume measurements were combined to obtain physiological CSA (PCSA), and maximal isometric force was recorded. Following training, muscle force and PCSA increased by 31% (P<0.0001) and 7% (P<0.01), respectively. Tendon CSA increased regionally at 20-30%, 60%, and 90-100% of tendon length (5-6%; P<0.05), and tendon stiffness and modulus increased by 24% (P<0.001) and 20% (P<0.01), respectively. Although none of the tendon adaptations were related to strength gains, we observed a positive correlation between the increase in quadriceps PCSA and the increases in tendon stiffness (r=0.68; P<0.01) and modulus (r=0.75; P<0.01). Unexpectedly, the increase in muscle PCSA was inversely related to the distal and the mean increases in tendon CSA (in both cases, r=-0.64; P<0.05). These data suggest that, following short-term resistance training, changes in tendon mechanical and material properties are more closely related to the overall loading history and that tendon hypertrophy is driven by other mechanisms than those eliciting tendon stiffening.

Corticosteroid injections, eccentric decline squat training and heavy slow resistance training in patellar tendinopathy

A randomized-controlled single-blind trial was conducted to investigate the clinical, structural and functional effects of peritendinous corticosteroid injections (CORT), eccentric decline squat training (ECC) and heavy slow resistance training (HSR) in patellar tendinopathy. Thirty-nine male patients were randomized to CORT, ECC or HSR for 12 weeks. We assessed function and symptoms (VISA-p questionnaire), tendon pain during activity (VAS), treatment satisfaction, tendon swelling, tendon vascularization, tendon mechanical properties and collagen crosslink properties. Assessments were made at 0 weeks, 12 weeks and at follow-up (half-year). All groups improved in VISA-p and VAS from 0 to 12 weeks (P<0.05). VISA-p and VAS improvements were maintained at follow-up in ECC and HSR but deteriorated in CORT (P<0.05). In CORT and HSR, tendon swelling decreased (-13+/-9% and -12+/-13%, P<0.05) and so did vascularization (-52+/-49% and -45+/-23%, P<0.01) at 12 weeks. Tendon mechanical properties were similar in healthy and injured tendons and were unaffected by treatment. HSR yielded an elevated collagen network turnover. At the half-year follow-up, treatment satisfaction differed between groups, with HSR being most satisfied. Conclusively, CORT has good short-term but poor long-term clinical effects, in patellar tendinopathy. HSR has good short- and long-term clinical effects accompanied by pathology improvement and increased collagen turnover.

Early and late rate of force development: differential adaptive responses to resistance training?

The objective of this study is to investigate the potentially opposing influence of qualitative and quantitative muscular adaptations in response to high-intensity resistance training on contractile rate of force development (RFD) in the early (<100 ms) and later phases (>200 ms) of rising muscle force. Fifteen healthy young males participated in a 14-week resistance training intervention for the lower body and 10 matched subjects participated as controls. Maximal muscle strength (MVC) and RFD were measured during maximal voluntary isometric contraction of the quadriceps femoris muscle. Muscle biopsies were obtained from the vastus lateralis. The main findings were that RFD in the late phase of rising muscle force increased in response to resistance training whereas early RFD remained unchanged and early relative RFD (i.e., RFD/MVC) decreased. Quantitatively, muscle fiber cross-sectional area and MVC increased whereas, qualitatively, the relative proportion of type IIX muscle fibers decreased. Multiple regression analysis showed that while increased MVC positively influenced both early and late RFD, decreased-type IIX negatively influenced early RFD only. In conclusion, early and late RFD responded differently to high-intensity resistance training due to differential influences of qualitative and quantitative muscular adaptations on early and later phases of rising muscle force.

Effect of estrogen on tendon collagen synthesis, tendon structural characteristics, and biomechanical properties in postmenopausal women

The knowledge about the effect of estradiol on tendon connective tissue is limited. Therefore, we studied the influence of estradiol on tendon synthesis, structure, and biomechanical properties in postmenopausal women. Nonusers (control, n = 10) or habitual users of oral estradiol replacement therapy (ERT, n = 10) were studied at rest and in response to one-legged resistance exercise. Synthesis of tendon collagen was determined by stable isotope incorporation [fractional synthesis rate (FSR)] and microdialysis technique (NH2-terminal propeptide of type I collagen synthesis). Tendon area and fibril characteristics were determined by MRI and transmission electron microscopy, whereas tendon biomechanical properties were measured during isometric maximal voluntary contraction by ultrasound recording. Tendon FSR was markedly higher in ERT users ( P < 0.001), whereas no group difference was seen in tendon NH2-terminal propeptide of type I collagen synthesis ( P = 0.32). In ERT users, positive correlations between serum estradiol (s-estradiol) and tendon synthesis were observed, whereas change in tendon synthesis from rest to exercise was negatively correlated to s-estradiol. Tendon area, fibril density, fibril volume fraction, and fibril mean area did not differ between groups. However, the percentage of medium-sized fibrils was higher in ERT users ( P < 0.05), whereas the percentage of large fibrils tended to be greater in control ( P = 0.10). A lower Young's modulus (GPa/%) was found in ERT users ( P < 0.05). In conclusion, estradiol administration was associated with higher tendon FSR and a higher relative number of smaller fibrils. Whereas this indicates stimulated collagen turnover in the resting state, collagen responses to exercise were negatively associated with s-estradiol. These results indicate a pivotal role for estradiol in maintaining homeostasis of female connective tissue.

Human patellar tendon stiffness is restored following graft harvest for anterior cruciate ligament surgery

Minimising post-operative donor site morbidity is an important consideration when selecting a graft for surgical reconstruction of the torn anterior cruciate ligament (ACL). One of the most common procedures, the bone-patellar tendon-bone (BPTB) graft involves removal of the central third from the tendon. However, it is unknown whether the mechanical properties of the donor site (patellar tendon) recover. The present study investigated the mechanical properties of the human patellar tendon in 12 males (mean+/-S.D. age: 37+/-14 years) who had undergone surgical reconstruction of the ACL using a BPTB graft between 1 and 10 years before the study (operated knee; OP). The uninjured contralateral knee served as a control (CTRL). Patellar tendon mechanical properties were assessed in vivo combining dynamometry with ultrasound imaging. Patellar tendon stiffness was calculated from the gradient of the tendon's force-elongation curve. Tendon stiffness was normalised to the tendon's dimensions to obtain the tendon's Young's modulus. Cross-sectional area (CSA) of OP patellar tendons was larger by 21% than CTRL tendons (P<0.01). Patellar tendon stiffness was not significantly different between OP and CTRL tendons, but the Young's modulus was lower by 24% in OP tendons (P<0.01). A compensatory enlargement of the patellar tendon CSA, presumably due to scar tissue formation, enabled a recovery of tendon stiffness in the OP tendons. The newly formed tendon tissue had inferior properties as indicated by the reduced tendon Young's modulus, but it increased to a level that enabled recovery of tendon stiffness.

Effects of static and dynamic training on the stiffness and blood volume of tendon in vivo

The purpose of this study was to investigate the effects of static and dynamic training on the stiffness and blood volume of the human tendon in vivo. Ten subjects completed 12 wk (4 days/wk) of a unilateral training program for knee extensors. They performed static training on one side [ST; 70% of maximum voluntary contraction (MVC)] and dynamic training on the other side (DT; 80% of one repetition maximum). Before and after training, MVC, neural activation level (by interpolated twitch), muscle volume (by magnetic resonance imaging), stiffness of tendon-aponeurosis complex and patella tendon (by ultrasonography), and blood volume of patella tendon (by red laser lights) were measured. Both protocols significantly increased MVC (49% for ST, 32% for DT; both P < 0.001), neural activation level (9.5% for ST, 7.6% for DT; both P < 0.01), and muscle volume (4.5% for ST, 5.6% for DT; both P < 0.01). The stiffness of tendon-aponeurosis complex increased significantly after ST (55%; P = 0.003) and DT (30%; P = 0.033), while the stiffness of patella tendon increased significantly after ST (83%; P < 0.001), but not for DT ( P = 0.110). The blood volume of patella tendon increased significantly after DT (47%; P = 0.016), but not for ST ( P = 0.205). These results implied that the changes in the blood volume of tendon would be related to differences in the effects of resistance training on the tendon properties.

Effect of acute resistance exercise and sex on human patellar tendon structural and regulatory mRNA expression

Tendon is mainly composed of collagen and an aqueous matrix of proteoglycans that are regulated by enzymes called matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). Although it is known that resistance exercise (RE) and sex influence tendon metabolism and mechanical properties, it is uncertain what structural and regulatory components contribute to these responses. We measured the mRNA expression of tendon's main fibrillar collagens (type I and type III) and the main proteoglycans (decorin, biglycan, fibromodulin, and versican) and the regulatory enzymes MMP-2, MMP-9, MMP-3, and TIMP-1 at rest and after RE. Patellar tendon biopsy samples were taken from six individuals (3 men and 3 women) before and 4 h after a bout of RE and from a another six individuals (3 men and 3 women) before and 24 h after RE. Resting mRNA expression was used for sex comparisons (6 men and 6 women). Collagen type I, collagen type III, and MMP-2 were downregulated (P < 0.05) 4 h after RE but were unchanged (P > 0.05) 24 h after RE. All other genes remained unchanged (P > 0.05) after RE. Women had higher resting mRNA expression (P < 0.05) of collagen type III and a trend (P = 0.08) toward lower resting expression of MMP-3 than men. All other genes were not influenced (P > 0.05) by sex. Acute RE appears to stimulate a change in collagen type I, collagen type III, and MMP-2 gene regulation in the human patellar tendon. Sex influences the structural and regulatory mRNA expression of tendon.

Effect of unloading followed by reloading on expression of collagen and related growth factors in rat tendon and muscle

Tendon tissue and the extracellular matrix of skeletal muscle respond to mechanical loading by increased collagen expression and synthesis. This response is likely a secondary effect of a mechanically induced expression of growth factors, including transforming growth factor-beta1 (TGF-beta1) and insulin-like growth factor-I (IGF-I). It is not known whether unloading of tendon tissue can reduce the expression of collagen and collagen-inducing growth factors. Furthermore, the coordinated response of tendon and muscle tissue to disuse, followed by reloading, is unclear. Female Sprague-Dawley rats were subjected to hindlimb suspension (HS) for 7 or 14 days, followed by 2, 4, 8, or 16 days of reload (RL) (n = 8 in each group). Age-matched controls were included for day 0, day 14 HS, and day 16 RL (n = 8). mRNA expression levels for collagen I (COL1A1), collagen III (COL3A1), TGF-beta1, connective tissue growth factor (CTGF), myostatin, and IGF-I isoforms were measured by real-time RT-PCR in Achilles tendon and soleus muscle. The tendon mass was unchanged, while the muscle mass was reduced by 50% after HS (P < 0.05) and returned to control levels during RL. Collagen I and III, TGF-beta1, and CTGF mRNA levels were unaltered by HS, although collagen III tended to decrease in muscle at day 7 HS. IGF-I isoforms were significantly induced in tendon after 7 days of HS (P < 0.001), and mechanogrowth factor increased in muscle at day 14 HS (P < 0.05). Reload increased muscle collagen I and III mRNA (>10-fold) (P < 0.001) and growth factor expression (P < 0.05), while the tendon response was limited to a moderate induction of collagen expression (2-fold) (P < 0.05). Unloading of tendon and muscle tissue did not reduce expression of collagen and collagen-inducing growth factors, indicating that the response to unloading is not opposite that of loading. Furthermore, the tendon response was clearly different and less pronounced than the muscle tissue response.

Soleus T reflex modulation in response to spinal and tendinous adaptations to unilateral lower limb suspension in humans

AIM

To investigate the influence of tendinous and synaptic changes induced by unilateral lower limb suspension (ULLS) on the tendon tap reflex.

METHODS

Eight young men underwent a 23-day period of ULLS. Muscle cross-sectional area (CSA), torque and electromyographic (EMG) activity of the plantar flexor muscles (normalized to the M wave), Achilles tendon-aponeurosis mechanical properties, soleus (SOL) H and T reflexes and associated peak twitch torques were measured at baseline, after 14 and 23 days of ULLS, and 1 week after resuming ambulatory activity.

RESULTS

Significant decreases in muscle CSA (-9%), in maximal voluntary torque (-10%) and in the associated SOL EMG activity (-16%) were found after ULLS (P < 0.05). In addition to a 36% (P < 0.01) decrease in tendon-aponeurosis stiffness, normalized H reflex increased by 35% (P < 0.05). An increase in the slope (28%, P < 0.05) and intercept (85%, P < 0.05) of the T reflex recruitment curve pointed to an increase in the gain and to a decrease in the sensitivity of this reflex, possibly resulting from the decrease in the tendon-aponeurosis stiffness at low forces. Following ULLS, changes in tendinous stiffness correlated with changes in neuromuscular efficiency (peak twitch torque to reflex ratio) at higher tendon tap forces.

CONCLUSION

These findings point out the dual and antagonistic influences of spinal and tendinous adaptations upon the tendon tap reflex in humans under conditions of chronic unloading. These observations have potential implications for the sensitivity of the short-latency Ia stretch response involved in rapid compensatory contractions to unexpected postural perturbations.

Impact of sex and chronic resistance training on human patellar tendon dry mass, collagen content, and collagen cross-linking

Collagen content and cross-linking are believed to be major determinants of tendon structural integrity and function. Sex and chronic resistance training have been shown to alter tendon function and may also alter the key structural features of tendon. Patellar tendon biopsies were taken from untrained men [n = 8, 1 repetition maximum (RM) = 53 +/- 3 kg], untrained women (n = 8, 1 RM = 29 +/- 2 kg), and resistance-trained (10 +/- 1 yr of training) men (n = 8, 1 RM = 71 +/- 6 kg). Biopsies were analyzed for dry mass, collagen content, and collagen cross-linking (hydroxylysylpyridinoline). We hypothesized that these elements of tendon structure would be lower in women than men, whereas chronic resistance training would increase these parameters in men. Tendon dry mass was significantly lower in women than men (343 +/- 5 vs. 376 +/- 8 microg dry mass/mg tendon wet wt, P < 0.01) and was not influenced by chronic resistance training (P > 0.05). The lower tendon dry mass in women tended to reduce (P = 0.08) collagen content per tendon wet weight. Collagen content of the tendon dry mass was not influenced by sex or resistance training (P > 0.05). Similarly, cross-linking of collagen was unaltered (P > 0.05) by sex or training. Although sex alters the water content of patellar tendon tissue, any changes in tendon function with sex or chronic resistance training in men do not appear to be explained by alterations in collagen content or cross-linking of collagen within the dry mass component of the tendon.

Influence of aging on the in vivo properties of human patellar tendon

Tendons are important for optimal muscle force transfer to bone and play a key role in functional ability. Changes in tendon properties with aging could contribute to declines in physical function commonly associated with aging. We investigated the in vivo mechanical properties of the patellar tendon in 37 men and women [11 young (27 +/- 1 yr) and 26 old (65 +/- 1 yr)] using ultrasonography and magnetic resonance imaging (MRI). Patella displacement relative to the tibia was monitored with ultrasonography during ramped isometric contractions of the knee extensors, and MRI was used to determine tendon cross-sectional area (CSA) and signal intensity. At peak force, patellar tendon deformation, stress, and strain were 13 (P = 0.05), 19, and 12% less in old compared with young (P < 0.05). Additionally, deformation, stiffness, stress, CSA, and length were 18, 35, 41, 28, and 11% greater (P < 0.05), respectively, in men compared with women. After normalization of mechanical properties to a common force, no age differences were apparent; however, stress and strain were 26 and 22% higher, respectively, in women compared with men (P < 0.05). CSA and signal intensity decreased 12 and 24%, respectively, with aging (P < 0.05) in the midregion of the tendon. These data suggest that differences in patellar tendon in vivo mechanical properties with aging are more related to force output rather than an age effect. In contrast, the decrease in signal intensity indirectly suggests that the internal milieu of the tendon is altered with aging; however, the physiological and functional consequence of this finding requires further study.

Habitual loading results in tendon hypertrophy and increased stiffness of the human patellar tendon

The purpose of this study was to examine patellar tendon (PT) size and mechanical properties in subjects with a side-to-side strength difference of > or =15% due to sport-induced loading. Seven elite fencers and badminton players were included. Cross-sectional area (CSA) of the PT obtained from MRI and ultrasonography-based measurement of tibial and patellar movement together with PT force during isometric contractions were used to estimate mechanical properties of the PT bilaterally. We found that distal tendon and PT, but not mid-tendon, CSA were greater on the lead extremity compared with the nonlead extremity (distal: 139 +/- 11 vs. 116 +/- 7 mm(2); mid-tendon: 85 +/- 5 vs. 77 +/- 3 mm(2); proximal: 106 +/- 7 vs. 83 +/- 4 mm(2); P < 0.05). Distal tendon CSA was greater than proximal and mid-tendon CSA on both the lead and nonlead extremity (P < 0.05). For a given common force, stress was lower on the lead extremity (52.9 +/- 4.8 MPa) compared with the nonlead extremity (66.0 +/- 8.0 MPa; P < 0.05). PT stiffness was also higher in the lead extremity (4,766 +/- 716 N/mm) compared with the nonlead extremity (3,494 +/- 446 N/mm) (P < 0.05), whereas the modulus did not differ (lead 2.27 +/- 0.27 GPa vs. nonlead 2.16 +/- 0.28 GPa) at a common force. These data show that a habitual loading is associated with a significant increase in PT size and mechanical properties.