The application of Hilbert�CHuang transform in the analysis of muscle fatigue during cyclic dynamic contractions

Surface electromyography (sEMG) is a common technique used in the assessment of local muscle fatigue. As opposed to static contraction situations, sEMG recordings during dynamic contractions are particularly characterised by non-stationary (and non-linear) features. Standard signal processing methods using Fourier and wavelet based procedures demonstrate well known restrictions on time-frequency resolution and the ability to process non-stationary and/or non-linear time-series, thus aggravating the spectral parameters estimation. The Hilbert-Huang transform (HHT), comprising of the empirical mode decomposition (EMD) and Hilbert spectral analysis (HSA), provides a new approach to overcome these issues. The time-dependent median frequency estimate is used as muscle fatigue indicator, and linear regression parameters are derived as fatigue quantifiers. The HHT method is utilised for the analysis of the sEMG signals recorded over quadriceps muscles during cyclic dynamic contractions. The results are compared with those obtained by the Fourier and wavelet based methods. It is shown that HHT procedure provides the most consistent and reliable assessment of spectral and derived linear regression parameters, given the time epoch width and sampling interval in the time domain. The suggested procedure successfully deals with non-stationary and non-linear properties of biomedical signals.     

Delayed onset muscle soreness at tendon–bone junction and muscle tissue is associated with facilitated referred pain

Delayed onset muscle soreness (DOMS) involves central and peripheral pain mechanisms. Referred pain patterns following stimulation of DOMS affected tissue have not been fully described. Referred pain may provide information on how central mechanisms are involved in DOMS, as referred pain is a central mechanism. Further, tendon tissue involvement in DOMS is not clear. This study assessed pressure pain threshold (PPT) sensitivity at the tendon, tendon–bone junction (TBJ) and muscle belly sites of tibialis anterior pre- and during DOMS in 45 subjects (34 males, 11 females). Furthermore, pain and referred pain areas at these three sites in response to hypertonic saline injection (n = 15 per injection site) were investigated pre- and during DOMS. DOMS was induced using controlled plantarflexion from a platform (bodyweight as resistance) causing eccentric contraction of the tibialis anterior muscle. DOMS induced PPT decrease was found at the TBJ and muscle belly sites only (P < 0.001). No mechanical effect was found in the unexercised limb. Maximal pain intensity induced by hypertonic saline given pre-DOMS was significantly higher for the tendon and TBJ injections compared to intramuscular injections (P < 0.05). Significantly higher referred pain frequency and enlarged pain areas were found at the muscle belly and TBJ sites following injection during DOMS compared to pre-DOMS. The results indicate that muscle belly and TBJ sites are sensitised while tendon tissue per se is unaffected by DOMS. Central sensitivity changes caused by DOMS may explain the increase in referred pain frequency and enlarged pain areas.     

Viscoelastic creep in the human skeletal muscle–tendon unit

The purposes of the present study were to (1) characterize viscoelastic creep in vivo in the human skeletal muscle–tendon unit and (2) to examine the consistency of these responses during a single 30-s stretch. Twelve volunteers (mean ± SD = 22 ± 3 years; height = 169 ± 11 cm; mass = 70 ± 17 kg) participated in two separate experimental trials. Each trial consisted of a 30-s constant-torque stretch of the plantar flexor muscles. Position (°) values were quantified at every 5-s period (0, 5, 10, 15, 20, 25, and 30 s) and the percent change in position was quantified for each 5-s epoch (0–5, 5–10, 10–15, 15–20, 20–25, and 25–30 s) relative to the total increase in the range of motion. In addition, the intraclass correlation coefficient (ICC) and standard errors of the measurement (SEM) were calculated for test–retest reliability. These results indicated that position increased over the entire 30-s stretch (P < 0.05), while the majority of the increases in position (73–85%) occurred during the first 15–20 s. ICC values were >0.994 and SEM values (expressed as percentage of the mean) were <1.54%. In conclusion, these results demonstrate viscoelastic creep in vivo in the human skeletal muscle–tendon unit and suggest that these responses may be reliable for future studies.     

Age-related degeneration in leg-extensor muscle–tendon units decreases recovery performance after a forward fall: compensation with running experience

The goals of this study were to investigate whether the lower muscle-tendon units (MTUs) capacities in older affect their ability to recover balance with a single-step after a fall, and to examine whether running experience enhances and protects this motor skill in young and old adults. The investigation was conducted on 30 older and 19 younger divided into two subgroups: runners versus non-active. In previous studies we documented that the older had lower leg extensor muscle strength and tendon stiffness while running had no effect on MTUs capacities. The current study examined recovery mechanics of the same individuals after an induced forward fall. Younger were better able to recover balance with a single-step compared to older (P < 0.001); this ability was associated with a more effective body configuration at touchdown (more posterior COM position relative to the recovery foot, P <0.001). MTUs capacities classified 88.6% of the subjects into single- or multiple-steppers. Runners showed a superior ability to recover balance with a single-step (P < 0.001) compared to non-active subjects due to a more effective mechanical response during the stance phase (greater knee joint flexion, P <0.05). We concluded that the age-related degeneration of the MTUs significantly diminished the older adults' ability to restore balance with a single-step. Running seems to enhance and protect this motor skill. We suggested that runners, due to their running experience, could update the internal representation of mechanisms responsible for the control of dynamic stability during a forward fall and, thus, were able to restore balance more often with a single-step compared to the non-active subjects.     

Na+–K+ pump location and translocation during muscle contraction in rat skeletal muscle

Muscle contraction may up-regulate the number of Na(+)-K(+) pumps in the plasma membrane by translocation of subunits. Since there is still controversy about where this translocation takes place from and if it takes place at all, the present study used different techniques to characterize the translocation. Electrical stimulation and biotin labeling of rat muscle revealed a 40% and 18% increase in the amounts of the Na(+)-K(+) pump alpha(2) subunit and caveolin-3 (Cav-3), respectively, in the sarcolemma. Exercise induced a 36% and 19% increase in the relative amounts of the alpha(2) subunit and Cav-3, respectively, in an outer-membrane-enriched fraction and a 41% and 17% increase, respectively, in sarcolemma giant vesicles. The Na(+)-K(+) pump activity measured with the 3-O-MFPase assay was increased by 37% in giant vesicles from exercised rats. Immunoprecipitation with Cav-3 antibody showed that 17%, 11% and 14% of the alpha(1) subunits were associated with Cav-3 in soleus, extensor digitorum longus, and mixed muscles, respectively. For the alpha(2), the corresponding values were 17%, 5% and 16%. In conclusion; muscle contraction induces translocation of the alpha subunits, which is suggested to be caused partly by structural changes in caveolae and partly by translocation from an intracellular pool.     

Nerve–muscle activation by rotating permanent magnet configurations

Key points

• The standard method of magnetic nerve activation using pulses of high current in coils has drawbacks of high cost, high electrical power (of order 1 kW), and limited repetition rate without liquid cooling.
• Here we report a new technique for nerve activation using high speed rotation of permanent magnet configurations, generating a sustained sinusoidal electric field using very low power (of order 10 W).
• A high ratio of the electric field gradient divided by frequency is shown to be the key indicator for nerve activation at high frequencies.
• Activation of the cane toad sciatic nerve and attached gastrocnemius muscle was observed at frequencies as low as 180 Hz for activation of the muscle directly and 230 Hz for curved nerves, but probably not in straight sections of nerve.
• These results, employing the first prototype device, suggest the opportunity for a new class of small low‐cost magnetic nerve and/or muscle stimulators.

Abstract

Conventional pulsed current systems for magnetic neurostimulation are large and expensive and have limited repetition rate because of overheating. Here we report a new technique for nerve activation, namely high‐speed rotation of a configuration of permanent magnets. Analytical solutions of the cable equation are derived for the oscillating electric field generated, which has amplitude proportional to the rotation speed. The prototype device built comprised a configuration of two cylindrical magnets with antiparallel magnetisations, made to rotate by interaction between the magnets’ own magnetic field and three‐phase currents in coils mounted on one side of the device. The electric field in a rectangular bath placed on top of the device was both numerically evaluated and measured. The ratio of the electric field gradient on frequency was approximately 1 V m⁻² Hz⁻¹ near the device. An exploratory series of physiological tests was conducted on the sciatic nerve and attached gastrocnemius muscle of the cane toad (Bufo marinus). Activation was readily observed of the muscle directly, at frequencies as low as 180 Hz, and of nerves bent around insulators, at frequencies as low as 230 Hz. Nerve–muscles, with the muscle elevated to avoid its direct activation, were occasionally activated, possibly in the straight section of the nerve, but more likely in the nerve where it curved up to the muscle, at radius of curvature 10 mm or more, or at the nerve end. These positive first results suggest the opportunity for a new class of small, low‐cost devices for magnetic stimulation of nerves and/or muscles.

Abbreviations

MQS

magnetoquasistatics

NdFeB

neodymium iron boron

TENS

transcutaneous electrical nerve stimulation

     

Muscle–tendon interaction and EMG profiles of world class endurance runners during hopping

The present study examined the muscle–tendon interaction of ten international level Kenyan runners. Ultrasonography and kinematics were applied together with EMG recordings of lower limb muscles during repetitive hopping performed at maximal level. The ten Kenyans had longer gastro Achilles tendon at rest (p < 0.01) as compared with ten control subjects matched in height. Conversely, the stretching and shortening amplitudes of the tendinous tissues of the medial gastrocnemius (MG) muscle were significantly smaller in the Kenyans than in controls during the contact phase of hopping. This applied also to the fascicle length changes, which were smaller and more homogeneous among Kenyans. These limited musculo-tendinous changes resulted in higher maximal hopping height and in larger power despite their reduced body weight. The associated finding of a greater shortening to stretching ratio of the MG tendinous tissues during contact could imply that the Kenyan MG muscle–tendon unit is optimized to favor efficient storage and recoil of elastic energy, while operating at optimal muscle fascicle working range (plateau region).     

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

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

In vivo determination of triceps surae muscle–tendon complex viscoelastic properties

Viscoelastic properties of muscles and tendons have an important influence on human motion performance. Proper determination of these properties is essential in the analysis and modelling of human motion dynamics. The purpose of our study was to develop a method for in vivo determination of the viscoelastic properties of the entire triceps surae muscle–tendon complex (MTC) including the gastrocnemius. Ten trained male subjects participated in this study. The measurement procedure consisted of two parts: soleus and Achilles tendon stiffness and viscosity were determined in the first part while the gastrocnemius stiffness and viscosity were determined in the second part. The measurement device and the procedure have been designed in such a manner that as few human body segments move as possible during the measurement. Thus, the measurement uncertainty due to the approximation of the properties of the human body segments was minimized. Triceps surae MTC viscoelastic properties of both legs were measured for each subject. There were no significant differences in viscoelastic coefficients for left and right lower extremities; however, there were noticeable differences between subjects. The soleus stiffness coefficient was greater than the gastrocnemius stiffness coefficient by 87.6 m^–1 in average. For all subjects, soleus viscosity was equal or greater than gastrocnemius viscosity. Values of viscoelastic parameters obtained by our method can be used in the analysis and modelling of human movement in situations where the knee joint is not necessarily flexed and there is coactivation of the soleus and the gastrocnemius.     

Cross-bridge movement and stiffness during the rise of tension in skeletal muscle – a theoretical analysis

Predictions for the time courses of cross-bridge attachment, N(t), stiffness, S(t), and force, T(t), during the tetanus rise were analysed for a special class of cross-bridge models where cross-bridges initially attach in a non-stereospecific weak-binding state, A _w. This state is in rapid equilibrium (equilibrium constant K) with detached states and the force generating transition (rate constant F ₊) is delayed. One model (model IA) which assumed step-function rise of activation at onset of tetanus, gave a poor fit to the experimental data (judged by root mean square error, RMSe 0.038) but the experimentally observed lead of N(t) over T(t) was reproduced qualitatively. An activation mechanism where K increased towards its maximum value according to an exponential function (Model IB) improved the fit considerably (RMSe 0.013). However, the activation time constant ( = 30 ms) derived in the fit was too high to reflect Ca²⁺ binding to troponin. In a further developed model (model II) both Ca²⁺-binding to troponin and cross-bridge attachment were assumed to be required for full activation. This more complex model gave a good fit to the experimental data (RMSe 0.013) with a realistic time constant for Ca²⁺ binding to troponin (9 ms). In both model IB and model II the best fit was obtained with F ₊ 40 s^–1 . An extended version of model IB, with distributed cross-bridge attachment and a series elastic element, gave a fit of similar quality (RMSe 0.009) as obtained with model IB and model II and with a similar value of F₊. The results support the view that weakly bound cross-bridges (state A _w) may account for the lead of cross-bridge movement over force during tension rise. It is also shown that, if the stiffness of the myofilaments is non-linear (stiffness increasing with tension) the experimentally observed lead of S(t) over T(t) may, to a significant degree, be attributed to cross-bridges in the state A _w.     

Voltage-clamp analysis of membrane currents and excitation–contraction coupling in a crustacean muscle

A single fibre preparation from the extensor muscle of a marine isopod crustacean is described which allows the analysis of membrane currents and simultaneously recorded contractions under two-electrode voltage-clamp conditions. We show that there are three main depolarisation-gated currents, two are outward and carried by K⁺, the third is an inward Ca²⁺ current, I _Ca. Normally, the K⁺ currents which can be isolated by using K⁺ channel blockers, mask I _Ca. I _Ca activates at potentials more positive than –40 mV, is maximal around 0 mV, and shows strong inactivation at higher depolarisation. Inactivation depends on current rather than voltage. Ba²⁺, Sr²⁺ and Mg²⁺ can substitute for Ca²⁺. Ba²⁺ currents are about 80% larger than Ca²⁺ currents and inactivate little. The properties of I _Ca characterise it as a high threshold L-type current. The outward current consists primarily of a fast, transient A current, I _K(A), and a maintained, delayed rectifier current, I _K(V). In some fibres, a small Ca²⁺-dependent K⁺ current is also present. I _K(A) activates fast at depolarisation above –45 mV, shows pronounced inactivation and is almost completely inactivated at holding potentials more positive than –40 mV. I _K(A) is half-maximally blocked by 70 M 4-aminopyridine (4-AP), and 70 mM tetraethylammonium (TEA). I _K(V) activates more slowly, at about –30 mV, and shows no inactivation. It is half-maximally blocked by 2 mM TEA but rather insensitive to 4-AP. Physiologically, the two K⁺ currents prevent all-or-nothing action potentials and determine the graded amplitude of active electrical responses and associated contractions. Tension development depends on and is correlated with depolarisation-induced Ca²⁺ influx mediated by I _Ca. The voltage dependence of peak tension corresponds directly to the voltage dependence of the integrated I _Ca. The threshold potential for contraction is at about –38 mV. Peak tension increases with increasing voltage steps, reaches maximum at around 0 mV, and declines with further depolarisation.This revised version was published online in September 2005 with corrections to the Cover Date.     

Modeling of skeletal muscle: the influence of tendon and aponeuroses compliance on the force–length relationship

The aim of this study was to investigate the influence of changing elastic properties of tendon and aponeuroses on force production and muscle geometry. A three-dimensional, structural, continuum mechanics model of the cat medial gastrocnemius was used for this purpose. Increasing compliance in tendon and aponeuroses caused a decrease in the peak isometric force and a shift of the force–length relationship to the right of the length axis (i.e. toward greater muscle lengths). This result can be explained with the stability condition of the force–length relationship which produced a history dependence of force production that is conceptually in agreement with experimental observations.     

Inhibition of return in cue–target and target–target tasks

Inhibition of return (IOR), the term given for the slowing of a response to a target that appeared at the same location as a previously presented stimulus, has been studied with both target–target (TT; participants respond to each successive event) and cue–target (CT; participants only respond to the second of two events) tasks. Although both tasks have been used to examine the processes and characteristics of IOR, few studies have been conducted to understand if there are any differences in the processes that underlie the IOR that results from ignoring (CT paradigm) or responding to (TT paradigm) the first stimulus. The purpose of the present study was to examine the notion that IOR found in TT tasks represents “true” IOR whereas IOR found in CT tasks consist of both “true” IOR and response inhibition (Coward et al. in Exp Brain Res 155:124–128, 2004). Consistent with the pattern of effects found by Coward et al. (Exp Brain Res 155:124–128, 2004), IOR was larger in the CT task than in the TT task when a single detection response was required (Experiment 1). However, when participants completed one of two spatially-directed responses (rapid aiming movement to the location of the target stimulus), IOR effects from the CT and TT tasks were equal in magnitude (Experiment 2). Rather than CT tasks having an additional response inhibition component, these results suggest that TT tasks may show less of an inhibitory effect because of a facilitatory response repetition effect.     

Fascicle–tendon behavior of the gastrocnemius and soleus muscles during ankle bending exercise at different movement frequencies

The present study investigated the effect of movement frequencies on the behavior of fascicles and tendons of synergistic muscles. Seven male subjects performed ankle bending (calf-raise) exercises at four movement frequencies (1.33, 1.67, 1.84, and 2.00 Hz), performed with an identical range of ankle joint motion. The fascicle and tendon behavior of medial gastrocnemius (MG) and soleus (SOL) was measured by ultrasonography while kinematic and kinetic parameters of the ankle were recorded. The torque of ankle joint was larger at higher exercise frequencies. The length change of muscle decreased and that of tendon increased at higher frequencies both for MG and for SOL, with no significant inter-muscle differences in the relative changes of muscle or tendon lengths to that of MTU. Changes of pennation angles and electromyographic activities as a function of movement frequency were also comparable for MG and SOL. These results suggest that under a stretch–shortening cycle action, the muscle–tendon interaction is altered by the movement frequency toward greater use of tendon elastic energy to provide greater MTU power at a higher frequency. Results also suggest that the movement frequency dependence of fascicle and tendon behavior is comparable between MG and SOL.     

Force–velocity and unloaded shortening velocity during graded potassium contractures in frog skeletal muscle fibres

Steady-state conditions of contraction, at maximal and submaximal forces, were produced in intact single muscle fibres, from Rana esculenta, using full tetani and graded K⁺-contractures. The uniformity in radial direction, of spreading of activation produced in K⁺-contractures, was checked in relation to the fibre diameters. The absolute isometric force was similar in tetani and maximal contractures, for fibres with diameters between 40 and 60 m, but not for fibres with diameters greater than about 70 m in which contracture force never reached tetanic force. The force–[K⁺]_o relation was similar for fibres with diameters between 40 and 60 m, but it was right shifted and it had a minor slope for fibres with diameters greater than 65–70 m. This suggests that only in the small diameter fibres (40–60 m) the activation does not fail to penetrate uniformly from the surface towards the fibre core. For fibres selected in the diameter range between 40 and 60 m, force–velocity relations and unloaded shortening velocities were determined in tetani and maximal and submaximal contractures. Data were obtained across a force range of 0.3 to 1 P ₀ (tetanic plateau force). Controlled velocity method was used to obtain force–velocity relations, and slack test to determine the unloaded shortening velocity (V _U). The values of the parameters characterising the force–velocity relation (V ₀ and a/P ₀) and V _U as determined by the slack test did not differ significantly in tetani and contractures, independent of the activation level or absolute force developed by the fibre. These results show that, at least within the range of forces tested, crossbridge kinetics is independent of the number of cycling crossbridges, in agreement with the prediction of the recruitment model of myofilament activation.     

The effects of agonist and antagonist muscle activation on the knee extension moment–angle relationship in adults and children

The present study examined the effect of agonist activation and antagonist co-activation on the shape of the knee extension moment–angle relationship in adults and children. Isometric knee extension maximum voluntary contractions (MVCs) were performed at every 5° of knee flexion between 55° and 90° (full extension = 0°) by ten men, ten women, ten boys and ten girls. For each trial, the knee extensors’ voluntary activation level was quantified using magnetic stimulation and the level of antagonist co-activation was quantified from their electromyographical activity. Peak MVC moment was greater for men (264 ± 63 N m) than women (177 ± 60 N m), and greater for adults than children (boys 78 ± 17 N m, girls 91 ± 28 N m) (p < 0.01). The agonistic activation level was greater for adults (~85%) than children (~70%). Similarly, antagonist co-activation was greater for adults than children, but relative to the agonist moment there were no differences between groups (all groups 7–8%). Correcting the peak moment for agonist and antagonist activation levels resulted in moments produced by fully activated agonist muscles of 334 ± 83, 229 ± 70, 114.2 ± 32 and 147 ± 46 N m, for men, women, boys and girls, respectively. Although correcting for shifts in joint angle during contraction altered the angle of peak moment by ~10° (p < 0.01), the peak moment occurred at ~60° for all groups. Changes in tendon stiffness, muscle size and architecture, and the pattern of the moment arm–angle relationship may in combination occur so that as children develop and mature into adults the shape of the moment–angle relationship is not altered.     

Dynamic force responses of skeletal muscle during stretch–shortening cycles

Muscle damage due to stretch–shortening cycles (i.e., cyclic eccentric/concentric muscle actions) is one of the major concerns in sports and occupational related activities. Mechanical responses of whole muscle have been associated with damage in neural motor units, in connective tissues, and the force generation mechanism. The objective of this study was to introduce a new method to quantify the real-time changes in skeletal muscle forces of rats during injurious stretch–shortening cycles. Male Sprague Dawley rats (n=24) were selected for use in this study. The dorsi flexor muscle group was exposed to either 150 stretch–shortening cycles (n=12) or 15 isometric contractions (n=12) in vivo using a dynamometer and electrical stimulation. Muscle damage after exposure to stretch–shortening cycles was verified by the non-recoverable force deficit at 48 h and the presence of myofiber necrosis. Variations of the dynamic forces during stretch–shortening cycles were analyzed by decomposing the dynamic force signature into peak force (F_peak), minimum force (F_min), average force (F_mean), and cyclic force (F_a). After the 15th set of stretch–shortening cycles, the decrease in the stretch–shortening parameters, F_peak, F_min, F_mean, and F_a, was 50% (P<0.0001), 26% (P=0.0055), 68% (P<0.0001), and 50% (P<0.0001), respectively. Our results showed that both isometric contractions and stretch–shortening cycles induce a reduction in the isometric force. However, the force reduction induced by isometric contractions fully recovered after a break of 48 h while that induced by stretch–shortening cycles did not. Histopathologic assessment of the tibialis anterior exposed to stretch–shortening cycles showed significant myofiber degeneration and necrosis with associated inflammation, while muscles exposed to isometric contractions showed no myofiber degeneration and necrosis, and limited inflammation. Our results suggest that muscle damage can be identified by the non-recoverable isometric force decrement and also by the variations in the dynamic force signature during stretch–shortening cycles.     

Inflammasome activation and IL-1β and IL-18 processing during infection

Interleukin-1β (IL-1β) and IL-18 contribute to host defense against infection by augmenting antimicrobial properties of phagocytes and initiating Th1 and Th17 adaptive immune responses. Protein complexes called inflammasomes activate intracellular caspase-1 autocatalytically, which cleaves the inactive precursors of IL-1β and IL-18 into bioactive cytokines. In this review, we discuss the controversies regarding inflammasome activation and the role of the inflammasome during infection. We highlight alternative mechanisms for processing IL-1β and IL-18 during infection, which involve extracellular cleavage of the inactive cytokines by neutrophil-derived serine proteases or proteases released from cytotoxic T cells.