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 (NH(2)-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 NH(2)-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.     

Morpho-functional changes in human tendon tissue

Insertion tissue biopsies of right arm common extensor tendons from 11 patients with chronic lateral epicondylitis were processed for light and electron microscopy. The subjects were aged between 38 and 54 years (only one was 25). The specimens showed a variety of structural changes such as biochemical and spatial alteration of collagen, hyaline degeneration, loss of tenocytes, fibrocartilage metaplasia, calcifying processes, neovascularization and vessel wall modifications. Tissue alterations were evident in limited zones of the tendon fibrocartilage in which the surgical resection was generally visible. The areas where the degenerative processes were localized, were restricted and in spatial contiguity with morphologically normal ones. The observed cases presented histological and electron microscopic findings that characterize lateral epicondylitis as a degenerative phenomenon involving all tendon components.     

Effects of changes in segmental values and timing of both torque and torque reversal in simulated throws.

An overarm throw in the sagittal plane was simulated using a three-segment model representing the upper arm, forearm and hand plus ball. Torque inputs at each joint were turned on at systematically varied times and maintained constant once initiated. All simulations began from identical initial conditions. The aim was to determine the sequence of onset of joint torques which gave the maximal range which the ball would travel and the maximal velocity of the ball irrespective of direction. Best throws proved to be sequential in that joint torques were turned on in a proximal to distal (P-D) temporal sequence. The P-D sequence was also demonstrated by time of peak joint angular velocities. The P-D sequence also proved to be best when segmental constants and joint torques were changed. As this sequence is a common feature of skilled throwing and striking, it is concluded that the linked segmental nature of the limb, irrespective of normal muscle characteristics, primarily predisposes the system to the use of a P-D sequence. The algebraic sign of the shoulder and elbow torques was reversed instantaneously to represent the use of antagonistic muscles. This led to increased output if performed late in the throw and in a P-D sequence. It is concluded that the use of antagonism leads to beneficial redistributions of angular velocity amongst limb segments.     

Acute effects of antagonist stretching on jump height, torque, and electromyography of agonist musculature

Although there has been substantial research on the acute effects of static stretching on subsequent force and power development, the outcome after stretching of the antagonist musculature has not been examined. The purpose of this study was to investigate the effects of static stretching of antagonist musculature on multiple strength and power measures. Sixteen trained men were tested for vertical jump height and isokinetic peak torque production during knee extension at 60°.s (SlowKE) and 300°.s (FastKE). Electromyography was recorded for the vastus lateralis and the biceps femoris muscles during isokinetic knee extension. Subjects performed these tests in a randomized counterbalanced order with and without prior stretching of the antagonist musculature. Paired samples t-tests indicated significantly greater torque production during the FastKE when preceded by stretching of the antagonist musculature vs. the nonstretch trial (102.2 vs. 93.5 N.m; p = 0.032). For SlowKE, torque production was not significantly different between the trials (176.7 vs. 162.9 N.m; p = 0.086). Vertical jump height (59.8 vs. 58.6 cm; p = 0.011) and power (8571 vs. 8487 W; p = 0.005) were significantly higher after the stretching trial vs. the nonstretching trial. Electromyography responses were similar between the trials. These results suggest that static stretching of the antagonist hamstrings before high-speed isokinetic knee extension increases the torque production. Furthermore, stretching the hip flexors (emphasis on single-joint hip flexors) and dorsiflexors, the antagonists of the hip extensors and plantarflexors, may enhance jump height and power, although the effect sizes were small.     

Acute effects of static stretching on peak torque in women

The purpose of this study was to examine the effects of static stretching on concentric, isokinetic leg extension peak torque (PT) at 60 and 240 degrees.s(-1) in the stretched and unstretched limbs. The PT of the dominant (stretched) and nondominant (unstretched) leg extensors were measured on a calibrated Cybex 6000 dynamometer. Following the prestretching PT assessments, the dominant leg extensors were stretched using 1 active and 3 passive stretching exercises. After the stretching, PT was reassessed. The results of the statistical analyses indicated that PT decreased following the static stretching in both limbs and at both velocities (60 and 240 degrees.s(-1)). The present findings suggested that the stretching-induced decreases in PT may be related to changes in the mechanical properties of the muscle, such as an altered length-tension relationship, or a central nervous system inhibitory mechanism. Overall, these findings, in conjunction with previous studies, indicated that static stretching impairs maximal force production. Strength and conditioning professionals should consider this before incorporating static stretching in preperformance activities. Future studies are needed to identify the underlying mechanisms that influence the time course of stretching-induced decreases in maximal force production for athletes and nonathletes across the age span.     

Predictability of in vivo changes in pennation angle of human tibialis anterior muscle from rest to maximum isometric dorsiflexion

The aim of this study was to assess the predictability of in vivo, ultrasound-based changes in human tibialis anterior (TA) pennation angle from rest to maximum isometric dorsiflexion (MVC) using a planimetric model assuming constant thickness between aponeuroses and straight muscle fibres. Sagittal sonographs of TA were taken in six males at ankle angles of -15 degrees (dorsiflexion direction), 0 degrees (neutral position), + 15 (plantarflexion direction) and + 30 degrees both at rest and during dorsiflexor MVC trials performed on an isokinetic dynamometer. At all four ankle angles scans were taken from the TA proximal, central and distal regions. TA architecture did not differ (P > 0.05) neither between its two unipennate parts nor along the scanned regions over its length at a given ankle angle and state of contraction. Comparing MVC with rest at any given ankle angle, pennation angle was larger (62-71%, P < 0.01), fibre length smaller (37-40%, P < 0.01) and muscle thickness unchanged (P > 0.05). The model used estimated accurately (P > 0.05) changes in TA pennation angle occurring in the transition from rest to MVC and therefore its use is encouraged for estimating the isometric TA ankle moment and force generating capacity using musculoskeletal modelling.     

Structuro-functional organization of the fiber network in the human Achilles tendon

Owing to a complex morphological investigation of the human Achilles tendon, it was possible to distinguish four levels of the structural-functional organization of its fibrous elements and to reveal some regularities of their structure that recur at all the levels. Thus, collagenous molecules, microfibrillae, fibrillae and fibers have a wavy-spiral conformation. This spatial form is stabilized by a complex or a system of transversal connections corresponding to the given level of the organization. In order to maintain integrity (the structural-functional unity) of each level, certain substances of polysaccharide nature take part. Along the course of the long tendinous axis, a re-distribution (branching) of the fibrillar elements is observed at all the levels of the structural-functional organization.     

Training-related changes in the maximal rate of torque development and EMG activity.

This study monitored the effects of a short-term elbow flexor training program on surface electromyographic (SEMG) spike activity. The experimental paradigm consisted of three test sessions separated by 2-week intervals. At the beginning of each session, participants (N=13) performed five maximal effort isometric contractions of the elbow flexors to serve as baseline. After 5 min of rest, the participants then engaged in a 30-trial isometric fatigue protocol during which maximal elbow flexion torque was measured with a load-cell, and the maximal rate of change in the torque (dtau/dt(max)) was obtained from the differentiated torque-time curve. Bipolar electrodes were used to monitor the SEMG spike activity of the biceps brachii. Mean spike amplitude (MSA) and mean spike frequency (MSF) were calculated for the torque development and constant-torque phases of the isometric contraction, termed Segment 1 and Segment 2, respectively. Mean power frequency (MPF) was also calculated for Segment 2. The five baseline contractions of the second and third sessions were compared with those of the first session and analyzed for training-related changes. Training increased dtau/dt(max) but failed to change maximal elbow flexion torque or MSA. However, there was an increase in the MSF during the torque development phase of the contraction (Segment 1). Both MSA and MSF were greatest during the constant-torque phase of the isometric contraction (Segment 2). There was a strong linear correlation (r=0.90, P<0.05) between MSF and MPF during (Segment 2). We hypothesize that the increase in dtau/dt(max) is due to enhanced motor-unit rate-coding. The demonstrated correlation between MSF and MPF measures will allow investigators to use spike analysis to examine the frequency content of the SEMG signal under non-stationary conditions.     

Acute effects of static stretching on maximal eccentric torque production in women

The purpose of this study was to examine the acute effects of static stretching on peak torque (PT) and the joint angle at PT during maximal, voluntary, eccentric isokinetic muscle actions of the leg extensors at 60 and 180 degrees x s(-1) for the stretched and unstretched limbs in women. Thirteen women (mean age +/- SD = 20.8 +/- 0.8 yr; weight +/- SD = 63.3 +/- 9.5 kg; height +/- SD = 165.9 +/- 7.9 cm) volunteered to perform separate maximal, voluntary, eccentric isokinetic muscle actions of the leg extensors with the dominant and nondominant limbs on a Cybex 6000 dynamometer at 60 and 180 degrees x s(-1). PT (Nm) and the joint angle at PT (degrees) were recorded by the dynamometer software. Following the initial isokinetic assessments, the dominant leg extensors were stretched (mean stretching time +/- SD = 21.2 +/- 2.0 minutes) using 1 unassisted and 3 assisted static stretching exercises. After the stretching (4.3 +/- 1.4 minutes), the isokinetic assessments were repeated. The statistical analyses indicated no changes (p > 0.05) from pre- to poststretching for PT or the joint angle at PT. These results indicated that static stretching did not affect PT or the joint angle at PT of the leg extensors during maximal, voluntary, eccentric isokinetic muscle actions at 60 and 180 degrees x s(-1) in the stretched or unstretched limbs in women. In conjunction with previous studies, these findings suggested that static stretching may affect torque production during concentric, but not eccentric, muscle actions.     

Characterization of collagen fiber architecture in the canine diaphragmatic central tendon.

The diaphragmatic central tendon (DCT), a collagenous soft tissue membrane, acts as a mechanical buffer between the costal and crural muscles. Its direction of mechanical anisotropy has been shown to correspond to the collagen fiber preferred directions. These preferred directions were determined by gross histological examination, and were thus qualitative. In this work we quantified the collagen fiber architecture throughout the DCT using small angle light scattering (SALS). Helium-Neon laser light was passed through tendon specimens and the resultant scattered light distribution, which characterized the local collagen fiber architecture, was recorded with a linear array of five photodiodes. Throughout the DCT two distinct collagen fiber populations were consistently found. For each population three parameters were determined: 1) the preferred directions of collagen fibers, 2) the volume fraction (Vf) of fibers, 3) OI, an orientation index, which ranges from 0 percent for a random network to 100 percent for a perfectly oriented network. Vector maps were used to display results from 1) and 2), and showed a primary group (G1) going from the crural to costal muscles and a secondary one (G2) running perpendicular to G1. Comparisons of Vf between G1 and G2 showed that G1 contained about three times as many fibers as G2, a ratio similar to that found for the degree of mechanical anisotropy. OI were found to be about 60 percent, indicating a high degree of orientation, with no significant regional or population differences (p less than 0.05). These quantitative results suggest that throughout the DCT the degree of mechanical anisotropy is controlled exclusively by Vf.     

Strain-induced damage reduces echo intensity changes in tendon during loading

Tendon functionality is related to its mechanical properties. Tendon damage leads to a reduction in mechanical strength and altered biomechanical behavior, and therefore leads to compromised ability to carry out normal functions such as joint movement and stabilization. Damage can also accumulate in the tissue and lead to failure. A noninvasive method with which to measure such damage potentially could quantify structural compromise from tendon injury and track improvement over time. In this study, tendon mechanics are measured before and after damage is induced by "overstretch" (strain exceeding the elastic limit of the tissue) using a traditional mechanical test system while ultrasonic echo intensity (average gray scale brightness in a B-mode image) is recorded using clinical ultrasound. The diffuse damage caused by overstretch lowered the stress at a given strain in the tissue and decreased viscoelastic response. Overstretch also lowered echo intensity changes during stress relaxation and cyclic testing. As the input strain during overstretch increased, stress levels and echo intensity changes decreased. Also, viscoelastic parameters and time-dependent echo intensity changes were reduced.     

Effects of warm-up on peak torque, rate of torque development, and electromyographic and mechanomyographic signals

The purpose of this study was to determine if an active warm-up affects peak torque (PT), rate of torque development (RTD), and the electromyographic (EMG) and mechanomyographic (MMG) signals. Twenty-one men (mean age ± SD: 24.0 ± 2.7 years) visited the exercise physiology laboratory on 2 occasions. During the first visit, they either performed an active warm-up (10 minutes of stationary cycling at 70% of predicted maximum heart rate) or sat quietly (no warm-up). Participants were then tested for isometric and isokinetic (60°, 180°, and 300°·s) PT, and RTD (measured as S-gradient) on an isokinetic dynamometer. Electromyographic and MMG sensors were placed over the vastus lateralis muscle to monitor the electrical and mechanical aspects of muscle contractions, respectively. The testing protocol used for the first visit was repeated for the second visit, but the preexercise treatment (warm-up, no warm-up) not given during the first visit was administered. The results indicated that an active warm-up did not affect PT, RTD, or measures of muscle activation as reflected by EMG amplitude, EMG frequency, or MMG frequency (p > 0.05). However, MMG amplitude at 180°·s was significantly greater in the warm-up condition compared with the no warm-up condition. The isolated increase in MMG amplitude suggested that warm-up may have affected the mechanical properties of muscle by reducing muscular stiffness or decreasing intramuscular fluid pressure, but that it was not sufficient to influence performance.     

Observations of multiscale, stress-induced changes of collagen orientation in tendon by polarized Raman spectroscopy

Collagen is a versatile structural molecule in nature and is used as a building block in many highly organized tissues, such as bone, skin, and cornea. The functionality and performance of these tissues are controlled by their hierarchical organization ranging from the molecular up to macroscopic length scales. In the present study, polarized Raman microspectroscopic and imaging analyses were used to elucidate collagen fibril orientation at various levels of structure in native rat tail tendon under mechanical load. In situ humidity-controlled uniaxial tensile tests have been performed concurrently with Raman confocal microscopy to evaluate strain-induced chemical and structural changes of collagen in tendon. The methodology is based on the sensitivity of specific Raman scattering bands (associated with distinct molecular vibrations, such as the amide I) to the orientation and the polarization direction of the incident laser light. Our results, based on the changing intensity of Raman lines as a function of orientation and polarization, support a model where the crimp and gap regions of collagen hierarchical structure are straightened at the tissue and molecular level, respectively. However, the lack of measurable changes in Raman peak positions throughout the whole range of strains investigated indicates that no significant changes of the collagen backbone occurs with tensing and suggests that deformation is rather redistributed through other levels of the hierarchical structure.     

Perlecan deficiency causes muscle hypertrophy,a decrease in myostatin expression,and changes in muscle fiber composition

Perlecan is a component of the basement membrane that surrounds skeletal muscle. The aim of the present study is to identify the role of perlecan in skeletal muscle hypertrophy and myostatin signaling, with and without mechanical stress, using a mouse model (Hspg2^?/?-Tg) deficient in skeletal muscle perlecan. We found that myosin heavy chain (MHC) type IIb fibers in the tibialis anterior (TA) muscle of Hspg2^?/?-Tg mice had a significantly increased fiber cross-sectional area (CSA) compared to control (WT-Tg) mice. Hspg2^?/?-Tg mice also had an increased number of type IIx fibers in the TA muscle. Myostatin and its type I receptor (ALK4) expression was substantially decreased in the Hspg2^?/?-Tg TA muscle. Myostatin-induced Smad activation was also reduced in a culture of myotubes from the Hspg2^?/?-Tg muscle, suggesting that myostatin expression and its signaling were decreased in the Hspg2^?/?-Tg muscle. To examine the effects of mechanical overload or unload on fast and slow muscles in Hspg2^?/?-Tg mice, we performed tenotomy of the plantaris (fast) muscle and the soleus (slow) muscle. Mechanical overload on the plantaris muscle of Hspg2^?/?-Tg mice significantly increased wet weights compared to those of control mice, and unloaded plantaris muscles of Hspg2^?/?-Tg mice caused less decrease in wet weights compared to those of control mice. The decrease in myostatin expression was significantly profound in the overloaded plantaris muscle of Hspg2^?/?-Tg mice, compared with that of control mice. In contrast, overloading the soleus muscle caused no changes in either type of muscle. These results suggest that perlecan is critical for maintaining fast muscle mass and fiber composition, and for regulating myostatin signaling.     

Postcatelectrotonic potentials and trace changes in the nerve fiber excitability

When cathode subthreshold impulse was turned off, excitable membranes of isolated nerve fibres and nervous trunk show postelectrotonic depolarisation (PED), that is a slow recovery of membrane potential to the resting level. PED of the single nodes of Ranvier and nervous trunk is registered not only in normal conditions, but also after complete block of sodium channels. The size and duration of nervous trunk PED under subthreshold depolarising current increase along with duration of applied depolarisation: when cathode current 1 ms in duration was used, they were 0.093 +/- +/- 0.004 mV and 7.123 +/- 0.576 ms, respectively; when current was 5 ms in duration, they were 0.189 +/- 0.005 mV and 23.212 +/- 1.186 ms, whereas a 10-ms depolarisation yields values of 0.220 +/- 0.011 mV and 68.721 +/- 3.389 ms. Application of the train of catelectrotonic impulses leads to PED built-up. As PED is found not only in normal conditions but also after complete block of sodium channels, it is reasonable to suggest that the most probable reason for PED is an outward potassium current.