Effect of mechanical stimulation of the support zones of soles on the muscle stiffness in 7-day dry immersion

Stiffness of m. soleus (Sol.) and m. tibialis anterior (TA) was evaluated in 16 volunteers during exposure to 7-days dry immersion alone and to the combination of immersion and mechanic stimulation of foot support zones. It was shown that Sol. stiffness decreased progressively starting from day-1 of immersion, whereas TA stiffness, on the contrary, made a sharp rise. Mechanic stimulation of foot support zones slowed down the rate and extent of changes in both muscles.     

Effect of vibratory stimulation of foot support areas in rats on the functional state of leg muscles and the content of N2A titin isoforms in gravity relief

In this work, we studied the effect of vibratory stimulation of the foot support zones on the functional state of the leg muscles and the content of N2A titin isoforms in rats under simulated microgravity (suspension model). The results of this study showed that vibratory stimulation of the support zones of the rat foot in a gravity discharge may reduce the drop in the amplitude of leg muscle motor response and undesirable reduction of the titin content.     

Mechanical stimulation of the support zones of soles: The method of noninvasive activation of the stepping movement generators in humans

The effects of mechanical stimulation of the soles’ support zones in the modes of slow and fast walking (75 and 120 steps per minute) were studied using the model of supportlessness (legs suspension). 20 healthy subjects participated in the study. EMG activity of hip and shin muscles was recorded. Kinematics of leg movements was assessed with the use of videoanalysis system. In 80% of cases support stimulation was followed by leg movements, in 69% of which they had characteristics of locomotions being accompanied by the burst-like electromyographic activities. The order of involvement of leg muscles and organization of antagonistic muscles activities were analogous to those of voluntary walking. The latencies of electromyographic activity in hip and shin muscles composed 5.17 ± 1.08 and 14.01 ± 2.82 s, respectively, the frequencies of bursts differed significantly depending on stimulation frequency. In 31% of cases the electromyographical activity following the stimulation of the soles’ support zones had not burst-like but uninterrupted pattern. Its amplitude rose smoothly reaching a certain level that was subsequently maintained. Results of the study showed that soles’ support zones stimulation in the mode of locomotion could activate a locomotor generator provoking the appearance of locomotion-like activity and that effect evoked by this stimulation includes not only rhythmical but also non-rhythmical (probably postural) components of walking.     

Effect of dry immersion in combination with stimulation of foot support zones upon muscle force-velocity characteristics

The complex of motor disturbances arising under conditions of real and simulated microgravity that include decrease of contractile characteristics of postural muscles is likely to be a result of withdrawal of support stimuli. Artificial stimulation of support zones of feet is shown to diminish partially or prevent completely the negative effects of microgravity on the motor system. The aim of the study was to evaluate and compare the changes of contractile properties of extensors and flexors of knee joint measured in isokinetic and isotonic regimes under conditions of simulated microgravity (7 days dry immersion).     

Electrical activity of the triceps surale muscle under static load in patients with chronic peripheral circulatory insufficiency

Combinations of various types of dynamics of integrated synchronously recorded bioelectrical activity of the gastrocnemius (lateral and medial) and soleus muscles were analyzed in healthy test subjects and patients with occlusive diseases of vessels of the lower limbs (obliterating endarteritis and atherosclerosis), who performed static work “to capacity”, maintaining a load of 50% of the maximum force of the muscles (plantar flexors of the sole of the foot). The frequency analysis of the elicited types of changes in the electromyogram integrals and their combinations testifies to general regularities in the functional organization of voluntary organization of an effort when fatigue develops in test subjects of two groups, and some features in the conjugate activity of the muscles ascribed to the group of the so-called “principal synergists,” recorded only in the patient group.     

Investigation of muscle tone in patients with Parkinson’s disease in unloading conditions

Parkinson’s disease (PD) is a progressive neurodegenerative disorder, the main symptoms of which are hypertonicity and difficulties emerging during performance of stepping movements due to increased muscle stiffness. Biomechanical (stiffness) and electrophysiological (shortening reaction, SR) characteristics of hip and shank muscles were examined in 25 patients with mild and moderate stages of PD (1 to 3 of Hoehn and Yahr Rating Scale, 61 ± 9 years) and 22 age-matched healthy controls in unloading leg conditions during passive flexion/extension of hip, knee, and ankle joints, as well as the changes in the tonic state of muscles under the influence of levodopa. The data obtained were compared with similar findings in healthy subjects. Essentially greater stiffness in all leg muscle groups (except foot extensors) was observed in patients with PD as compared to the healthy subjects. In patients with PD, SR values in hip and shank extensors as well as in foot flexors and extensors were essentially greater then in the healthy subjects. The medicine essentially reduced the stiffness of hip flexors and knee flexors and extensors. The SR persisted, although the frequency of its occurrence decreased in half of studied muscles, and a significant decrease in the SR value was observed in foot extensors. The medicine had no marked effect on the SR in the proximal muscles. Thus, the increased muscle stiffness in patients with PD manifests itself as distorted reactions to external disturbances and increased reflectory reactions of muscles.     

Nonlinear stiffness--force relationships in whole mammalian skeletal muscles

Small, sinusoidal length changes were superimposed on isometric contractions of fast- and slow-twitch mouse muscles, which were stimulated maximally via their nerves. Stiffness increased with increasing frequency of sinusoidal stimulation, but the relative time course of force and stiffness changes during twitch, tetanic, or partially fused contractions was quite invariant over a range of frequencies in both muscles. Typically, stiffness increases more rapidly than force during contraction and decreases less rapidly during relaxation. This pattern was observed at various temperatures and with various numbers of stimuli. It can be described by a nonlinear relation between stiffness and force with some hysteresis. The presence in the muscle of parallel and series elastic elements, whose stiffness varies with force, may account for the nonlinear relation. This nonlinearity can be used to relate the patterns for summation of force and stiffness observed with brief trains of stimuli under a variety of conditions.     

The role of skin afferent in regulation of locomotion evoked by electrical epidural stimulation of spinal cord in decerebrated cats

The role of hindpaw skin afferent input in the locomotor pattern formation induced by epidural spinal cord stimulation was investigated in decerebrated cats. Locomotor activity was evoked by continuous 3-5Hz stimulation of dorsal surface of L4-L5 spinal segments. Kinematic and electromyographic activity (EMG) of m. Quadriceps, m. Semitendinosus, m. Tibialis anterior an m. Gastrocnemius lateralis before and after blocking of skin receptors in one hind limb were recorded. In addition, reflex responses in the hind limb muscles to epidural stimulation with frequency 0.5 Hz were analysed. Blocking of skin receptors of the foot with chlorothane paw irrigation or 2 % lidocaine administrated into the hind paw was performed. After blocking of skin receptors of the foot the stepping pattern changed. Stepping with dorsal foot placement and dragging during swing phase was observed. Duration of stance phase significantly decreased. Inhibition of polysynaptic activity of proximal and distal extensor muscles and distal flexor muscles of hind paw during locomotion was found. Monosynaptic responses after blocking of skin receptors of the foot changed insignificantly.     

Contraction and stiffness changes in collagenous arm ligaments of the stalked crinoid Metacrinus rotundus (Echinodermata)

Shortening and stiffness were measured simultaneously in the aboral ligament of arms of sea lilies. Arm pieces were used from which oral tissues (including muscles) were removed, leaving only collagenous ligaments connecting arm ossicles. Chemical stimulation by means of artificial seawater with an elevated concentration of potassium caused both a bending movement and stiffness changes (either softening or stiffening). The movement lasted for 1.5-10 min, and bent posture was maintained. The observation that contraction was not necessarily associated with softening provided evidence against the hypothesis that the shortening of the aboral ligaments was driven by the elastic components that had been charged by the oral muscles and released their strain energy at the softening of the aboral ligaments. The speed of ligamental shortening was slower by at least one order of magnitude than that of muscles. Acetylcholine (10(-5)-10(-3) M) caused both contraction and softening. We conclude that the aboral ligament shows two mechanical activities based on different mechanisms: one is active contraction and the other is connective tissue catch in which passive mechanical properties show mutability. We suggest that there is neural coordination between the two mechanisms.     

Mechanical mutability in connective tissue of starfish body wall

Stiffness changes in response to mechanical and chemical stimulation were studied in muscle-free dermal samples from the body wall of the starfish Linckia laevigata. The ultrastructural study showed that the dermis was packed with collagen fibrils between which only a small number of cells were observed. Muscles were found only in the walls of coelomic extensions leading to papulae. Stress-strain tests were performed on isolated dermis containing no muscles. The tangent modulus was 27.5 MPa at 0.04% strain rate in the stress-strain tests. It was increased to 40.7 MPa by mechanical stimulation, which also increased the tensile strength and breaking-strain energy density. Dynamic mechanical tests showed that the increase in stiffness in response to mechanical stimulation was transient. Acetylcholine (10(-6)-10(-3) mol l(-1)) and artificial seawater with an elevated potassium concentration (KASW) stiffened the dermis. Mechanical stimulation caused a 12% mass loss. KASW also caused mass loss, which was inhibited by anesthesia. These results clearly showed that the stiffness changes in the starfish dermis were based on a non-muscular mechanism that was similar to that of other echinoderm connective tissues with mechanical mutability.     

Postnatal development of static vestibulo-ocular reactions in the rabbit--electromyographic studies

The changes in the electric activity of the extraocular muscles as a consequence of static tilt stimulation were investigated in rabbits of different postnatal ages by the registration of the electromyogram. The postnatal development of the tonic vestibulo-ocular reflex in the rabbit runs parallel with the transition from an irregular fluctuating eye muscle activity during the first postnatal days to a constantly tonic muscle activity depending on the tilt position.     

Morphology,mechanics, and locomotion: the relation between the notochord and swimming motions in sturgeon

Synopsis To examine the relation between morphology and performance, notochordal morphology was correlated with notochordal mechanics and with steady swimming motions in white sturgeon, Acipenser transmontanus. In a still-water tank, motions of four sturgeon varied with changes in swimming speed and axial position along the body. For a 1..34 m sturgeon, slow and fast swimming modes were distinguished, with speeds at the fast mode more than two times those at the slow mode without changes in tailbeat frequency. This increase in speed is correlated with an increase in the body's maximal midline curvature (m^–1), suggesting a role for curvature-related mechanical properties of the notochord. Maximal midline curvature also varied with axial position, and surprisingly was uncorrelated with axial changes in the notochord's cross-sectional shape - as measured by height, width, inner diameter, and lateral thickness of the sheaths. On the other hand, maximal midline curvature was negatively correlated with the axial changes in the notochord's angular stiffness (N m rad^–1) and change in internal pressure (% change from baseline of 58.6 kPa), both of which were measured during in vitro bending tests. In vivo curvature and in vitro angular stiffness were then used to estimate the bending moments (Nm) in the notochord during swimming. In the precaudal notochord, the axial pattern of maximal stiffness moments was congruent with the pattern of maximal notochordal curvature in the precaudal region, but in the caudal notochord maximal angular stiffness was located craniad to maximal curvature. One interpretation of this pattern is that the precaudal notochord resists bending moments generated by the muscles and that the caudal notochord resists bending moments generated by hydrodynamic forces acting on the tail.     

Muscle transverse stiffness and venous compliance under conditions of simulated supportlessness

Supportlessness arising as a result of 7 hrs dry immersion is accompanied by decreased transverse stiffness and increased calf venous compliance, as well as impaired orthostatic tolerance. Stimulation of support zones of foot is associated with smoothening of these effects. The latter may be considered as an indirect evidence in favor of the concept of support deafferentation triggering role in development of enumerated unfavorable effects of simulated microgravity.     

Simulation of the double limb support phase of human gait

Dynamic mechanical models of the double limb support phase of human gait were developed for both two-dimensional (sagittal plane) and three-dimensional motion. A "foot" model with a curved plantar surface was also developed such that the model foot motion was kinematically equivalent to that of a walking subject. This foot model was incorporated into the planar model for double limb support. The dynamic formulations were based on Kane's method and were implemented symbolically using MACSYMA. The development of the formulations for the constrained systems, application of these formulations to the study of normal gait, the sensitivity of the simulation to the frequency content of the input data, the sensitivity of limb displacements to changes in joint moments and the application of a nonlinear feedback controller to correct for perturbations in limb trajectories were investigated.     

Diaphragmatic plate electrode stimulation of the hamster diaphragm

We developed a new technique of diaphragmatic stimulation by apposing plate electrodes directly against the diaphragm (DPS) in adult Golden Syrian hamsters. The electrophysiological and the mechanical responses to DPS were compared with those with phrenic nerve stimulation. In four animals, evaluation of the electromyogram before and after curare demonstrated that plate electrode stimulation occurred via the phrenic nerve filaments. In four animals, similar transdiaphragmatic pressure was produced at maximal current with DPS and phrenic nerve stimulation. Using DPS increasing current beyond a certain level resulted in recruitment of muscles besides the diaphragm. In six animals, an external abdominal pressure of 15 cmH2O produced maximal transdiaphragmatic pressure, suggesting that the diaphragm was contracting near optimal position with this external abdominal pressure. In another four animals the twitch and pressure-frequency characteristics with the use of DPS were found to be reproducible over a 2-h period. We conclude that DPS is an effective method of diaphragmatic stimulation and should prove to be a valuable technique to study the diaphragm in long-term studies of small rodents.     

Intra-abdominal pressure increases stiffness of the lumbar spine

Intra-abdominal pressure (IAP) increases during many tasks and has been argued to increase stability and stiffness of the spine. Although several studies have shown a relationship between the IAP increase and spinal stability, it has been impossible to determine whether this augmentation of mechanical support for the spine is due to the increase in IAP or the abdominal muscle activity which contributes to it. The present study determined whether spinal stiffness increased when IAP increased without concurrent activity of the abdominal and back extensor muscles. A sustained increase in IAP was evoked by tetanic stimulation of the phrenic nerves either unilaterally or bilaterally at 20 Hz (for 5 s) via percutaneous electrodes in three subjects. Spinal stiffness was measured as the force required to displace an indentor over the L4 or L2 spinous process with the subjects lying prone. Stiffness was measured as the slope of the regression line fitted to the linear region of the force-displacement curve. Tetanic stimulation of the diaphragm increased IAP by 27-61% of a maximal voluntary pressure increase and increased the stiffness of the spine by 8-31% of resting levels. The increase in spinal stiffness was positively correlated with the size of the IAP increase. IAP increased stiffness at L2 and L4 level. The results of this study provide evidence that the stiffness of the lumbar spine is increased when IAP is elevated.     

Quantification of the forearm muscles mechanical properties using Myotonometer: Intra- and Inter-Examiner reliability and its relation with hand grip strength

The primary objective of this study was to investigate the reliability of the myotonometer in the mechanical properties of the forearm muscles [m. extensor carpi radialis brevis (ECRB), and m. flexor carpi ulnaris (FCU)] in healthy individuals. The secondary objective was to investigate the relationship between the handgrip strength and mechanical properties of these forearm muscles. The mechanical properties (muscle tone, stiffness, and elasticity) of the ECRB and FCU were measured using the MyotonPRO device. Examiner 1 performed two sets of measurements with a time interval of 30 min to determine intra-examiner reliability. Examiner 2 performed measurements during the interval between the two sets of examiner 1. The intra- and inter-examiner reliabilities were excellent (ICC˃0.82) for muscle tone, stiffness, and elasticity of the FCU. Both intra- and inter-examiner reliability in the evaluation of ECRB muscle tone, elasticity, and stiffness was moderate to excellent (ICCs = 0.56–0.98). The muscle tone and stiffness properties of the FCU were positively correlated with the handgrip strength (p <.05). The study findings indicate that the MyotonPRO device is a reliable tool to quantify ECRB, and FCU muscles mechanical properties in healthy individuals.     

Explanation of the bilateral deficit in human vertical squat jumping.

In the literature, it has been reported that the mechanical output per leg is less in two-leg jumps than in one-leg jumps. This so-called bilateral deficit has been attributed to a reduced neural drive to muscles in two-leg jumps. The purpose of the present study was to investigate the possible contribution of nonneural factors to the bilateral deficit in jumping. We collected kinematics, ground reaction forces, and electromyograms of eight human subjects performing two-leg and one-leg (right leg) squat jumps and calculated mechanical output per leg. We also used a model of the human musculoskeletal system to simulate two-leg and one-leg jumps, starting from the initial position observed in the subjects. The model had muscle stimulation as input, which was optimized using jump height as performance criterion. The model did not incorporate a reduced maximal neural drive in the two-leg jump. Both in the subjects and in the model, the work of the right leg was more than 20% less in the two-leg jump than in the one-leg jump. Peak electromyogram levels in the two-leg jump were reduced on average by 5%, but the reduction was only statistically significant in m. rectus femoris. In the model, approximately 75% of the bilateral deficit in work per leg was explained by higher shortening velocities in the two-leg jump, and the remainder was explained by lower active state of muscles. It was concluded that the bilateral deficit in jumping is primarily caused by the force-velocity relationship rather than by a reduction of neural drive.     

Energetics and mechanics of human running on surfaces of different stiffnesses.

Mammals use the elastic components in their legs (principally tendons, ligaments, and muscles) to run economically, while maintaining consistent support mechanics across various surfaces. To examine how leg stiffness and metabolic cost are affected by changes in substrate stiffness, we built experimental platforms with adjustable stiffness to fit on a force-plate-fitted treadmill. Eight male subjects [mean body mass: 74.4 +/- 7.1 (SD) kg; leg length: 0.96 +/- 0.05 m] ran at 3.7 m/s over five different surface stiffnesses (75.4, 97.5, 216.8, 454.2, and 945.7 kN/m). Metabolic, ground-reaction force, and kinematic data were collected. The 12.5-fold decrease in surface stiffness resulted in a 12% decrease in the runner's metabolic rate and a 29% increase in their leg stiffness. The runner's support mechanics remained essentially unchanged. These results indicate that surface stiffness affects running economy without affecting running support mechanics. We postulate that an increased energy rebound from the compliant surfaces studied contributes to the enhanced running economy.     

Influence of tongue muscle contraction and transmural pressure on nasopharyngeal geometry in the rat

The mammalian pharynx is a hollow muscular tube that participates in ingestion and respiration, and its size, shape, and stiffness can be altered by contraction of skeletal muscles that lie inside or outside of its walls. MRI was used to determine the interaction between pharyngeal pressure and selective stimulation of extrinsic tongue muscles on the shape of the rat nasopharynx. Pressure (-9, -6, -3, 3, 6, and 9 cmH?O) was applied randomly to the isolated pharyngeal airway of anesthetized rats that were positioned in a 4.7-T MRI scanner. The anterior-posterior (AP) and lateral diameters of the nasopharynx were measured in eight axial slices at each level of pressure, with and without bilateral hypoglossal nerve stimulation (0.1-ms pulse, 1/3 maximal force, 80 Hz). The rat nasopharynx is nearly circular, and positive pharyngeal pressure caused similar expansion of AP and lateral diameters; as a result, airway shape (ratio of lateral to AP diameter) remained constant. Negative pressure did not change AP or lateral diameter significantly, suggesting that a negative pressure reflex activated the tongue or other pharyngeal muscles. Stimulation of tongue protrudor muscles alone or coactivation of protrudor and retractor muscles caused greater AP than lateral expansion, making the nasopharynx slightly more elliptical, with the long axis in the AP direction. These effects tended to be more pronounced at negative pharyngeal pressures and greater in the caudal than rostral nasopharynx. These data show that stimulation of rodent tongue muscles can adjust pharyngeal shape, extending previous work showing that tongue muscle contraction alters pharyngeal compliance and volume, and provide physiological insight that can be applied to the treatment of obstructive sleep apnea.