Differential adaptations during growth spurt and in young adult rat muscles.

During the post-weaning growth and maturation period (25/90 days after birth), rat limb muscles are submitted to specific adaptations. Our aim was to characterize the mechanical properties of two muscles that are opposite in terms of fibre-type distribution, the soleus and the extensor digitorum longus (EDL) muscles of male Wistar rats. Results showed a fast-to-slow fibre-type transition in soleus while no modification in fibre-type distribution was observed in EDL. A growth-induced increase in muscle force was observed. Soleus underwent an increase in twitch kinetics, but EDL showed no modification. Resistance to fatigue was higher in 90-day-old soleus but not modified in the EDL. Surprisingly, analysis of maximal shortening velocity showed a decrease in both soleus and EDL. Finally, tension/extension curves indicated a growth-induced increase in series elastic stiffness in the two muscles. These results suggest that during this growth period, skeletal muscles are submitted to differential adaptations. Moreover, whereas adaptation of biomechanical properties observed can be explained partly by an adaptation of fibre profile in soleus, this is not the case for EDL. It is suggested that changes in muscle architecture, which are often disregarded, could explain some variations in mechanical properties, especially when muscles undergo an increase in both mass and length.

Macroscopic-microscopic characterization of the passive mechanical properties in rat soleus muscle.

The purpose of the study was to investigate changes in passive mechanical properties of the soleus muscle of the rat during the first year of life. These mechanical changes were quantified at a macroscopic (whole muscle) and a microscopic level (fiber) and were correlated with biochemical and morphological properties. Three passive mechanical tests (a relaxation test, a ramp stretch test and a stretch release cycle test) with different amplitudes and velocities were performed on isolated soleus muscles and fibers in rats at ages 1 (R1), 4 (R4) and 12 (R12) months. Mechanical parameters (dynamic and static forces, stresses and normalized stiffness) were recorded and measured. The morphological properties (size of fibers and muscles) for the three groups of rats were assessed by light microscopy which allowed us to observe the evolution of the fiber type (I, IIc and IIa) in the belly region and along the longitudinal axis of the muscle. In addition, biochemical analyses were performed at the level of the whole muscle in order to determine the collagen content. The results of the passive mechanical properties between the macroscopic (muscle) and microscopic (fiber) levels showed a similar evolution. Thus, an increase of the dynamic and static forces appeared between 1 and 4 months while a decrease of the passive tension occurred between 4 and 12 months. These mechanical changes were correlated to the morphological properties. In addition, the size of the three fibers type which grew with age could explain the increase of forces between 1 and 4 months. Furthermore, the biochemical analysis showed an increase of the collagen content during the same period which could also be associated with the increase of the passive forces. After 4 months, the passive tension decreased while the size of the fiber continued to increase. The biochemical analysis showed a decrease of the collagen content after 4 months, which could explain the loss of passive tension in the whole muscle. Concerning the similar loss at the fiber level, other assumptions are required such as a myofibril loss process and an increase of intermyofibrillar spaces. The originality of this present study was to compare the passive mechanical properties between two different levels of anatomical organization within the soleus muscle of the rat and to explain these mechanical changes in terms of biochemical and morphological properties.

In vivo force-velocity relation of human muscle: a modelling from sinusoidal oscillation behaviour.

Isokinetic tests performed on human muscle in vivo during plantar flexion contractions lead to torque-angular velocity relationships usually fitted by Hill's equation expressed in angular terms. However, such tests can lead to discrepant results since they require maximal voluntary contractions performed in dynamic conditions. In the present study, another way to approach mechanical behaviour of a musculo-articular structure was used, i.e. sinusoidal oscillations during sub-maximal contractions. This led to the expression of (i). Bode diagrams allowing the determination of a damping coefficient (B(bode)); and (ii). a viscous parameter (B(sin)) using an adaptation of Hill's equation to sinusoidal oscillations. Then torque-angular velocity relationships were predicted from a model based on the interrelation between B(bode) and B(sin) and on the determination of optimal conditions of contraction. This offers the possibility of characterizing muscle dynamic properties by avoiding the use of isokinetic maximal contractions.

Passive stiffness changes in soleus muscles from desmin knockout mice are not due to titin modifications.

Passive stiffness was found to be increased in mouse soleus muscles lacking desmin. Because titin is considered to be the major source of muscle elasticity, the stiffening might be explainable by titin adaptation. To test this, passive mechanical properties of single skinned fibres of soleus muscles from desmin knockout and control mice were analysed by using various extension tests. Titin expression was studied by SDS-gel electrophoresis. Absence of desmin did not modify either electrophoretic mobility of the titin band (3700 kDa) or optical density-unit ratios between bands for titin and nebulin (congruent with 0.3) and bands for titin and myosin heavy chain (congruent with 0.08). Elastic properties of fibres were not altered in the absence of desmin since passive tensions were similar under quasi-static (56-66 kN m(-2)) and dynamic (100-118 kN m(-2)) conditions whatever the kind of fibre. Thus, titin is unlikely to be responsible for the large increase in passive stiffness observed in whole soleus muscles when desmin is lacking.

Models of skeletal muscle to explain the increase in passive stiffness in desmin knockout muscle.

Absence of desmin in skeletal muscle was found to induce an increase in passive stiffness. The present study aimed at developing rheological models of passive muscle to explain this stiffening. Models were elaborated by using experimental data depicting muscle viscoelastic behaviour. The experimental protocol included stepwise extension tests applied on control and desmin knockout soleus muscles from mice. Linear and non-linear models were composed of elastic and viscous elements. They were constructed with the aim at taking the presence or absence of desmin into account by simulating desmin as an elastic element. Furthermore, associated adaptation of connective tissues in absence of desmin was modelled as an additional elastic element. Differences in passive behaviour induced by absence of desmin were predicted by using a linear model and a non-linear one. The non-linear model was selected because: (1) it is able to predict experimental viscoelastic kinetics accounting for the increase in passive stiffness in muscles lacking desmin, (2) its design is consistent with morphological data, and (3) stiffness characteristics of its elements are in accordance with the literature. Finally, this modelling approach demonstrates that both absence of desmin and adaptation of connective tissue are required to explain the increase in passive stiffness in desmin knockout muscles.

Musculo-tendinous and joint elastic characteristics during elbow flexion in children.

OBJECTIVE: The aim of the present research was to determine joint stiffness and musculo-tendinous stiffness in children. BACKGROUND: No reliable data was available on differences in such parameters when comparing children and adults. METHODS: Sinusoidal perturbations and quick-released movements were applied to the right elbow during flexion to determine joint stiffness and musculo-tendinous stiffness, respectively. For normalization purposes, stiffness indexes were calculated as the slope of the relationship between stiffness and torque. RESULTS: Main results were that musculo-tendinous stiffness indexes obtained in children were similar to adult data deduced from the literature. On the other hand, joint stiffness indexes obtained in children were in the range 0.9-3.8 rad(-1) and were lower than those reported in the literature for adults (between 4 and 8.5 rad(-1)). CONCLUSION: Since normalized musculo-tendinous stiffness is similar in children and in adults, it can be hypothesized that, during growth, increase in ability to produce force parallels elastic tissue development. RELEVANCE: Stiffness parameters could be used, in the context of other functional tests, to characterize changes in muscle properties in neuromuscular disorders when elastic tissues are supposed to be altered.

Passive stiffness is increased in soleus muscle of desmin knockout mouse.

The effects of an absence of desmin on passive stiffness of skeletal muscle were analyzed using soleus muscles from desmin knockout mice. Stiffness was evaluated by analyzing the passive tension induced by a ramp-and-hold extension test. This test showed that passive resistance to stretch was largely increased for muscles lacking desmin. This study could facilitate interpretation of changes in muscle mechanics observed in humans affected by desmin-related myopathies.

Muscle and joint elastic properties during elbow flexion in Duchenne muscular dystrophy.

1. Maximal voluntary contraction (MVC), series elastic stiffness and total joint stiffness during elbow flexion were investigated in healthy boys and in boys with Duchenne muscular dystrophy (DMD) in order to assess changes in mechanical properties induced by the disease. 2. Two methods were used to perform stiffness measurements: (i) the application of sinusoidal perturbations to the joint during flexion efforts, allowing the calculation of total joint stiffness; (ii) the use of quick-release movements of the elbow, which had previously been maintained in isometric contraction, allowing the calculation of series elastic stiffness. In each case, stiffness was linearly related to torque, leading to the calculation of a normalized stiffness index as the slope of this stiffness-torque relationship. 3. As expected, mean MVC was found to be much higher for healthy boys (20.02 +/- 5.20 N m) than for DMD patients (3.09 +/- 2.44 N m). Furthermore, the results showed that it was possible to characterize healthy and DMD children by virtue of the mechanical properties measured. Mean series elastic stiffness index was higher for DMD children (142.55 +/- 136.58 rad(-1)) than for healthy subjects (4.39 +/- 2.53 rad(-1)). The same holds for mean total joint stiffness index: 43.68 +/- 67.58 rad(-1) for DMD children and 2.26 +/- 0.70 rad(-1) for healthy subjects. In addition, increases in stiffness were more marked in DMD patients exhibiting high levels of muscle weakness. 4. These changes are interpreted in terms of the adaptation of the properties of the muscles and joint involved, i.e. muscle fibres, tendons, peri- and intra-articular structures.

Effects of long-term spaceflight on mechanical properties of muscles in humans.

The effects of long-term spaceflight (90-180 days) on the contractile and elastic characteristics of the human plantarflexor muscles were studied in 14 cosmonauts before and 2-3 days after landing. Despite countermeasures practiced aboard, spaceflight was found to induce a decrease in maximal isometric torque (17%), whereas an index of maximal shortening velocity was found to increase (31%). In addition, maximal muscle activation evaluated during isokinetic tests decreased by 39%. Changes in musculotendinous stiffness and whole joint stiffness were characterized by means of quick-release movements and sinusoidal perturbations. Musculotendinous stiffness was found to be increased by 25%. Whole joint stiffness decreased under passive conditions (21%), whereas whole joint stiffness under active conditions remained unchanged after spaceflight (-1%). This invariance suggests an adaptive mechanism to counterbalance the decrease in stiffness of passive structures by an increased active stiffness. Changes in neural drive could participate in this equilibrium.

Changes in stiffness induced by hindlimb suspension in rat Achilles tendon.

The aim of this study was to measure the effects of hindlimb suspension on mechanical properties of the rat Achilles tendon. Adult male Wistar rats were randomly assigned to groups to be either suspended, or a control. After 21 days, Achilles tendons were removed for mechanical analysis. Classical tests of tensile performance were made, and mechanical parameters were derived from a stress-strain relationship. The tendons of animals that had been suspended presented values for maximal stress and tangent modulus which were 37.5% (P < 0.01) and 41% (P < 0.01), respectively, lower than the tendons of the control rats. In a similar way, the energy absorption capacity had largely decreased in animals that had been suspended. However, the maximal strain was similar in the two groups. These results showed that hindlimb suspension in rats has an important detrimental effect on mechanical properties of the Achilles tendon. Differences in tendon stiffness obtained here, along with those found by other investigators, encourage the hypothesis that homeostatic responses of soft tissues are due to changes in limb loadings. This study may be useful in providing a better understanding of the adaptation of human skeletal muscle when exposed to microgravity.

Viscous properties of human muscle during contraction.

The purpose of this study was to determine viscous properties of human muscle during plantarflexion efforts. Experiments were performed on 17 subjects with an ankle ergometer allowing sinusoidal oscillations during isometric contractions and isokinetic movements. Sinusoidal oscillations led to the expression of (i) Bode diagrams of the musculo-articular system allowing the determination of a damping coefficient (Bbode); and (ii) a viscous coefficient (Bsin) using an adaptation of Hill's equation to sinusoidal oscillations. Isokinetic movements led to torque-velocity relationships. They showed a fall in torque associated to an increase in angular velocity what was quantified by calculating a damping coefficient (Biso). Both experiments gave consistent results indicating that Bbode was the lowest viscous parameter. This difference is discussed in terms of (i) "analog" viscosity originating from muscle cross-bridges; and (ii) real mechanical damping of passive structures.

The rat suspension model is also a good tool for inducing muscle hyperactivity.

The aim of the present study was to investigate changes in the mechanical characteristics of the rat epitrochlearis muscle as a result of a period of hyperactivity. A tail suspension model was used to impose postural activity in the forelimb musculature with the intention of inducing a relative slowness in the fast epitrochlearis muscle. A method of dual-controlled releases was used to obtain force-velocity and tension-extension relationships characterizing muscle mechanics. Results showed that: (1) mechanical characteristics are that of a fast-twitch muscle and (2) suspension induces a decrease in the maximal shortening velocity and a decrease in the compliance of series elastic elements. Changes in fibre type proportions are consistent with these mechanical adaptations. This demonstrates that an usual model of muscle hypoactivity can also be used for imposing hyperactivity of a postural nature, inducing muscle transformations towards a slower twitch muscle.

Stiffness of knee extensors in Duchenne muscular dystrophy.

The series elastic component (SEC) of Hill's muscle model is sensitive to a modification in muscle functional demand. In this study, SEC stiffness was quantified from quick-released movements in knee extensors of Duchenne muscular dystrophy (DMD) boys to look for possible modifications with the stage of disease. A SEC stiffness index (SI) was defined and was found significantly lower for controls than for DMD. Moreover, a linear relationship was established between mean SI and knee extensors involvement. This suggests a role for this parameter, together with other functional tests, for following patients with DMD.

Effects of chronic low frequency stimulation on contractile and elastic properties of hindlimb suspended rat soleus muscle.

Chronic low frequency stimulation (10 Hz, 8 h x day[-1]) was used in this study to prevent the changes in the contractile and elastic properties of rat soleus muscles induced by 3 weeks of hindlimb suspension (HS). Results showed that electrostimulation was able to counteract in part the decrease in soleus muscle mass and tension output induced by unweighting. On the other hand, the increases in maximal shortening velocity and twitch speed following HS were not prevented by stimulation. Unweighting was responsible for an increase in series elastic compliance of soleus muscle. Chronic stimulation successfully counteracted this increase in series compliance probably by changing the properties of the tendon. The partial recovery of muscle mass and tension output as a result of stimulation enhanced the role of contractile activity in preventing muscle atrophy. Moreover, the inefficiency of the tonic activity imposed by stimulation in preventing the increase in twitch speed of soleus muscle during HS demonstrated the primacy of neuronal activity. Discrepant results concerning changes in contraction kinetics deduced from the twitch could have been due to the fact that such myograms also depend on the series compliance.

A mechanical device for studying mechanical properties of human muscles in vivo.

A mechanical device, primarily devoted to biomechanical studies of human calf muscles during microgravity experiments is presented. It allows investigation of both contractile and visco-elastic properties of musculo-articular systems using, respectively, isokinetic movements, quick-release tests and sinusoidal perturbations. This device is a specifically designed ergometer associated to an experimental protocol designed for pre- and post-flight tests. The protocol was evaluated on 22 healthy subjects and typical results are briefly presented. Preliminary results are discussed in terms of agreement with currently available data and a detailed evaluation of test-retest measurements is provided for quick-release experiments. Complementary investigations are suggested and potential fields of research are indicated.

Citrulline malate limits increase in muscle fatigue induced by bacterial endotoxins.

Citrulline malate is known to improve performance in weakened muscles. The present experiment was designed to test the hypothesis that citrulline malate can limit the effect of endotoxins on muscle fatigability. Endotoxemia was induced in rats by injection of lipopolysaccharides from Klebsiella pneumoniae. Resistance to fatigue was quantified by measuring tension production during repetitive electrical stimulation of the isolated epitrochlearis muscle. Oral treatment by citrulline malate was found to increase resistance to fatigue in infected rats, whereas twitch tension was not modified. This demonstrates the efficacy of citrulline malate for limiting an increase in muscle fatigue elicited with bacterial endotoxins.

Influence of plyometric training on the mechanical impedance of the human ankle joint.

The objective of this work was to study the effects of plyometric training on the mechanical properties of the ankle joint in humans. Changes in the mechanical parameters of this musculo-articular structure were quantified with the aid of a sinusoidal perturbation technique. This technique allowed the expression of the mechanical impedance of the musculo-articular system in terms of stiffness, viscosity and inertia. Measurements were performed under passive conditions and when the subject performed plantar flexion. A 7-week period of training induced a decrease in the slope of the relationship between stiffness and plantar flexion torque, whereas passive stiffness was increased. A slight decrease in viscosity and an invariability in inertia were also found. These results are interpreted in terms of the possible adaptations of the musculo-articular structure and ultrastructure involved in the performance of plantar flexion.

Neuromuscular adaptations in rats trained by muscle stretch-shortening.

The aim of this study was the analysis of neurophysiological, mechanical and histochemical parameters to demonstrate muscle adaptation with training. If the parameters studied were to show correlated changes, it would be possible to propose that the neural and the muscle components of motor units are both affected by the training programme used. The training consisted of repeated stretch-shortening cycles known to use extensively fast fibres. After the training period electromyographical reflex activities of the ankle plantar-flexors were recorded in awake rats and then mechanical and histochemical measurements were made on isolated soleus muscles of the control and trained rats. The reflexes studied were the H-response to electrical stimulation of the sciatic nerve and the T-response to an Achilles tendon tap. The H-response analysis indicated a decrease in reflex excitability of the trained muscles. The trained soleus muscle also presented a higher contractility as demonstrated by significantly smaller twitch contraction times and higher maximal velocities of shortening measured during tetanic contractions. The reflex and contractile muscle changes were accompanied by relative increases in the number of type II fibres. The T-response was not significantly modified by training despite the decrease in motoneuron excitability demonstrated by the decrease in H-response. This would suggest that the peripheral components of the reflex pathway such as tendon stiffness and/or spindle sensitivity might be modified by training. This would imply that both the motor and the sensory parts of a muscle are affected by training.

Reproducibility of kinetics of electromyogram spectrum parameters during dynamic exercise.

During incremental exercise on a cycle ergometer a study was made of the reproducibility of changes in electromyographic activity (EMG) of human quadriceps muscles. Seven subjects performed three periods of incremental exercise either every 2 weeks (G1: four subjects) or 6 weeks (G2: three subjects). Each test was normalized with respect to the maximal aerobic power (MAP) of the subject. It consisted of a quasilinear increase in mechanical power (from 20% to 100% MAP) during 8 min. For rectus femoris muscle activity, changes in total power spectrum (PEMG) and in mean power frequency (MPF) were fitted by a 3rd order polynomial function (named profile) and normalized with respect to the maximal value for PEMG and to the mean value for MPF. A curvilinear increase was found for PEMG. The MPF kinetics varied from one subject to another. These changes were either a continuous increase, or a continuous decrease, or an increase followed by a decrease. Only for G2, was MAP determined before each test in order to update the limits of the test. A good reproducibility of PEMG was shown by its mean magnitude [95.88%, (SD 3.92)] and by computing a mean correlation coefficient between profiles two by two [r2 = 0.948 (SD 0.028) n = 21]. Intraclass coefficient correlation (ICC) calculated for each subject indicated a high level of reproducibility for five of the seven subjects (ICC > 0.80). No clear effect of MAP updating on PEMG and MPF profile reproducibility was observed. Thus it is suggested that MPF kinetics may characterize a subject at a given moment while PEMG kinetics may illustrate a normal profile, and they may both characterize EMG changes for a population during incremental exercise.