Changes in passive tension after stretch of unexercised and eccentrically exercised human plantarflexor muscles

The study measured the effect of stretch on passive mechanical properties in unexercised and eccentrically exercised plantarflexor muscles, to obtain insight into how stretch might serve athletes as a warm-up strategy. Passive torque, voluntary contraction strength and muscle soreness were measured before and after a large amplitude stretch given before and after a period of eccentric exercise and at 0, 1, 2 and 24 h later. Stretch of the unexercised muscle led to a 20% fall in passive torque which recovered within an hour. About 40% of the fall could be recovered immediately with a voluntary contraction. After eccentric exercise there was a rise in passive torque by 20% at 2 h post-exercise. This rise was postulated to result from an injury contracture in muscle fibres damaged by the exercise. It was accompanied by a fall in maximum voluntary torque and the development of muscle soreness at 24 h. Stretch of the exercised muscle led to a fall in passive torque and rise in pain threshold. It is proposed that in response to a stretch there is a fall in passive tension in the muscle due to stable cross-bridges in sarcomeres which could be recovered with a voluntary contraction and an additional component attributable to the elastic filament, titin. The size of the fall was not significantly different between exercised and unexercised muscle. These observations provide a physiological basis for the effects of passive stretches on skeletal muscle and help to explain why they are used as a popular warm-up strategy.     

Changes in stiffness induced by hindlimb suspension in rat soleus muscle

Soleus muscle atrophy was induced by hindlimb suspension of rats for 3 weeks with the intention of inducing a relative increase in the percentage of fast twitch fibres and assessing modifications in muscle stiffness. A method of dual controlled releases was used to obtain tension/extension curves and force/velocity relationships characterizing the mechanical behaviour of the soleus. Fibre typing was achieved by myofibrillar adenosine 5-triphosphatase staining. Results showed that hindlimb suspension decreased the percentage of slowtwitch fibres (–31%) to the profit of fast-twitch fibres (+370%) and intermediate fibres (+255%). This led to an increase in maximal shortening velocity. Tension/extension curves indicated a decrease in soleus stiffness after 3 weeks of unloading. Changes in elastic properties are interpreted in terms of modifications occurring in the active part and the passive part of the so-called series elastic component. These changes also suggest that the parameters derived from a twitch are inappropriate to account for modifications in speed-related properties of muscle.     

In vivo specific tension of the human quadriceps femoris muscle

It is not known to what extent the inter-individual variation in human muscle strength is explicable by differences in specific tension. To investigate this, a comprehensive approach was used to determine in vivo specific tension of the quadriceps femoris (QF) muscle (Method 1). Since this is a protracted technique, a simpler procedure was also developed to accurately estimate QF specific tension (Method 2). Method 1 comprised calculating patellar tendon force (F _t) in 27 young, untrained males, by correcting maximum voluntary contraction (MVC) for antagonist co-activation, voluntary activation and moment arm length. For each component muscle, the physiological cross-sectional area (PCSA) was calculated as volume divided by fascicle length during MVC. Dividing F _t by the sum of the four PCSAs (each multiplied by the cosine of its pennation angle during MVC) provided QF specific tension. Method 2 was a simplification of Method 1, where QF specific tension was estimated from a single anatomical CSA and vastus lateralis muscle geometry. Using Method 1, the variability in MVC (18%) and specific tension (16%) was similar. Specific tension from Method 1 (30 ± 5 N cm⁻²) was similar to and correlated with that of Method 2 (29 ± 5 N cm⁻²; R ² = 0.67; P < 0.05). In conclusion, most of the inter-individual variability in MVC torque remains largely unexplained. Furthermore, a simple method of estimating QF specific tension provided similar values to the comprehensive approach, thereby enabling accurate estimations of QF specific tension where time and resources are limited.     

Electromechanical delay in human skeletal muscle under concentric and eccentric contractions

Summary In contraction of skeletal muscle a delay exists between the onset of electrical activity and measurable tension. This delay in electromechanical coupling has been stated to be between 30 and 100 ms. Thus, in rapid movements it may be possible for electromyographic (EMG) activity to have terminated before force can be detected. This study was designed to determine the dependence of the EMG-tension delay upon selected initial conditions at the time of muscle activation. The rigth forearms of 14 subjects were passively oscillated by a motor-driven dynamometer through flexion-extension cycles of 135 deg at an angular velocity of 0.5 rad/s. Upon presentation of a visual stimulus the subjects maximally contracted the relaxed elbow flexors during flexion, extension, and under isometric conditions. The muscle length at the time of the stimulus was the same in all three conditions. An on-line computer monitoring surface EMG (Biceps and Brachioradialis) and force calculated the electromechanical delay. The mean value for the delay under eccentric condition, 49.5 ms, was significantly different (p<0.05) from the delays during isometric (53.9 ms) and concentric activity (55.5 ms). It is suggested that the time required to stretch the series elastic component (SEC) represents the major portion of the measured delay and that during eccentric muscle activity the SEC is in a more favorable condition for rapid force development.     

Role of myofibrillar creatine kinase in the relaxation of rigor tension in skinned cardiac muscle

In the absence of creatine phosphate, MgATP produced relaxation of rigor tension in chemically-skinned right papillary muscles of the rat, the half maximal effect being obtained at 1.8 mM MgATP. In the presence of 12 mM creatine phosphate and 250 M ADP, a decrease in MgATP concentration even to 10^–9 M never induced rigor tension. At a very low MgATP concentration (10^–6 M), the half maximal relaxing effect was obtained with 2 mM creatine phosphate, a value close to theK _m of isolated MM-creatine kinase for this substrate, or with 14 M MgADP, a value 5 times lower than the reportedK _m. An exogenous MgATP regenerating system (phosphoenol pyruvate + pyruvate kinase) was not able to fully relax the fibres. When MM-creatine kinase was inhibited by fluorodinitrobenzene, the dependency of rigor tension on MgATP became the same as it was without creatine phosphate. After washing out the fluorodinitrobenzene the addition of exogenous MM-creatine kinase for half an hour fully relaxed rigor tension; moreover, this effect persisted even after prolonged washout. These results show that endogenous MM-creatine kinase is able to ensure maximal efficiency of myosin ATPase by producing a localized high MgATP/MgADP ratio; they also suggest the existence of rapidly exchangeable binding sites for MM-creatine kinase in cardiac myofibrils.     

Learned maintenance of pain: muscle tension reduces central nervous system processing of painful stimulation in chronic and subchronic pain patients.

The effect of level of muscle tension on the perception of painful stimuli was assessed in 13 chronic back pain patients, 14 subjects at high risk for chronic back pain, and 14 matched healthy controls. Subjects received painful intracutaneous electric stimuli to the forearm or the lower back while they produced either high or low muscle tension levels. Visual analog scale (VAS) ratings of acute pain were obtained after each trial. Electroencephalograms, electromyograms, skin conductance levels, and blood pressure were measured during the trials. Although subjective pain ratings were not significantly affected by muscle tension levels, the chronic pain patients displayed elevated N150 and N150/P260 amplitudes of the somatosensory-evoked potentials in the low as compared to the high muscle tension condition. The high risk group showed a trend toward higher N150 amplitudes in the low as compared to the high tension condition. The results of this study partially support the hypothesis that increases in muscle tension might serve as a pain-reducing mechanism in chronic pain patients and those at risk for chronicity, thus leading to a vicious pain-tension cycle.     

硝普钠诱导被动致敏的人气道平滑肌细胞凋亡

目的： 研究一氧化氮(NO)供体硝普钠（SNP）是否诱导被动致敏的人气道平滑肌细胞(HASMCs)凋亡。方法：将人气道平滑肌细胞用哮喘患者血清被动致敏后，用SNP（NO的供体）干预，通过四甲基偶氮唑盐（MTT）法测定细胞生长率的变化，用原位末端标记法和流式细胞术检测SNP对被动致敏的人气道平滑肌细胞凋亡的影响。结果：（1）MTT法测定：SNP+哮喘血清致敏组的细胞存活率明显低于哮喘血清致敏组细胞存活率（n=5，P<0.05）。（2）TUNEL(原位末端标记)法检测：SNP干预的被动致敏的HASMC组的凋亡指数明显高于哮喘血清致敏组（n=4，P<0.01）。（3）流式细胞术测定SNP+哮喘血清致敏组的HASMCs凋亡率明显高于哮喘血清致敏组（n=4，P<0.05）。结论： SNP 能抑制被动致敏的HASMCs的增殖，并且能诱导被动致敏的HASMCs凋亡，这种作用可能与治疗哮喘患者的气道重建有关。     

Muscle tension in generalized anxiety disorder: Elevated muscle tonus or agitated movement?

The purpose of this study was to investigate the amplitude characteristics of frontalis and gastrocnemius electro-myographic (EMG) activity in clinically anxious and nonanxious populations. Eighteen women with generalized anxiety disorder (GAD) and 19 nonanxious women were compared during baseline, laboratory stressor, and recovery conditions. EMG mean levels were greater for the GAD group, but there were no group differences in EMG skewness. During the stressor the GAD group had a significant reduction in frontalis EMG variability. Gastrocnemius muscle activity for both groups during the stressor condition increased in mean levels and variability while decreasing in skewness. These results indicate that clinically anxious individuals have elevated muscular tonus and have reduced variability in frontalis activity during stressful tasks. Also, the gastrocnemius muscle exhibited a stressor reactivity, whereas the frontalis did not. This study presents an approach to EMG analysis that could be useful in distinguishing unique features of anxiety as well as other emotional disorders.     

Comparison of blood pressure and heart rate responses to isometric exercise and passive muscle stretch in humans

The responses of mean arterial blood pressure (BP_a) and heart rate (f _c) to isometric contraction and passive stretch were compared in seven healthy male subjects at identical external forces. They were investigated in the sitting position with the hip and knee joint flexed to 90°. Each subject performed two tests, separated by a day, in which the stimuli were applied in random order. After 5 min of rest they performed either 10-min static plantar flexion of one calf (200 N) or 10 min of passive calf muscle stretch at the same load. After 5-min rest, the second stimulus was applied for a further 10 min followed by 5-min rest. The second test was identical except for the sequence of the stimuli. The BP_a was measured by a noninvasive and continuous method. Contraction of the vastus lateralis, gastrocnemius lateralis, and soleus muscles were determined by the myo-electric activity (electromyogram, EMG) by means of surface electrodes. The EMG activity of the vastus lateralis muscle remained at resting values throughout the experiments. Increases in EMG activity could only be detected for the triceps surae muscles during isometric contraction. During the initial 2 min of stimulation the BP_a and (f _c), responses to active contraction and passive stretch were comparable. Thereafter, both parameters showed significantly higher values during contraction. It was concluded that mechanical stress may have contributed to the early response of BP_a during both passive stretch and voluntary contraction but that chemical stimuli were needed to maintain the peripheral cardiovascular drive.     

Training-induced changes in muscle architecture and specific tension

Five men underwent unilateral resistance training of elbow extensor (triceps brachii) muscles for 16 weeks. Before and after training, muscle layer thickness and fascicle angles of the long head of the triceps muscle were measured in vivo using B-mode ultrasound, and fascicle lengths were estimated. Series anatomical cross-sectional areas (ACSA) of the triceps brachii muscle were measured by magnetic resonance imaging, from which muscle volume (Vm) was determined and physiological cross-sectional area (PCSA) was calculated. Elbow extension strength (isometric; concentric and eccentric at 30, 90 and 180°·s^–1) was measured using an isokinetic dynamometer to determine specific tension. Muscle volumes, ACSA, PCSA, muscle layer thickness and fascicle angles increased after training and their relative changes were similar, while muscle and fascicle length did not change. Muscle strength increased at all velocities; however, specific tension decreased after training. Increase in fascicle angles, which would be the result of increasedV _m and PCSA, would seem to imply the occurrence of changes in muscle architecture. This might have given a negative effect on the force-generating properties of the muscles.     

Transmission phenomena and early tension recovery in skinned muscle fibres of the frog

Tension responses due to small rapid length changes completed in 50 s were obtained from segments with different length of single fibres of the ileofibularis muscle of the frog. The very early parts of the responses varied with segment length. A simulation of the early parts of the response was carried out by means of a linear model in which the fibre is regarded as a rod of infinitesimally small segments containing undamped elasticity, damped elasticity and mass in series. In the simulation corrections were included for the effects caused by the viscosity and density of the surrounding fluid and for the force transducer characteristics. The results indicate the presence of a very rapid component in the fast recovery with a time constant of 5–15 s. The undamped elasticity of the activated fibres corrected for their passive properties was such that a sudden shortening corresponding to 2.6 nm/half sarcomere would reduce active tension to zero.     

Aging related changes in determinants of muscle force generating capacity: A comparison of muscle aging in men and male rodents

Human aging is associated with a progressive decline in skeletal muscle mass and force generating capacity, however the exact mechanisms underlying these changes are not fully understood. Rodents models have often been used to enhance our understanding of mechanisms of age-related changes in human skeletal muscle. However, to what extent age-related alterations in determinants of muscle force generating capacity observed in rodents resemble those in humans has not been considered thoroughly. This review compares the effect of aging on muscle force generating determinants (muscle mass, fiber size, fiber number, fiber type distribution and muscle specific tension), in men and male rodents at similar relative age. It appears that muscle aging in male F344*BN rat resembles that in men most; 32–35-month-old rats exhibit similar signs of muscle weakness to those of 70–80-yr-old men, and the decline in 36–38-month-old rats is similar to that in men aged over 80 yrs. For male C57BL/6 mice, age-related decline in muscle force generating capacity seems to occur only at higher relative age than in men. We conclude that the effects on determinants of muscle force differ between species as well as within species, but qualitatively show the same pattern as that observed in men.     

Active contraction of cardiac muscle: in vivo characterization of mechanical activation sequences in the beating heart

Progressive alterations in cardiac wall strains are a classic hallmark of chronic heart failure. Accordingly, the objectives of this study are to establish a baseline characterization of cardiac strains throughout the cardiac cycle, to quantify temporal, regional, and transmural variations of active fiber contraction, and to identify pathways of mechanical activation in the healthy beating heart. To this end, we insert two sets of twelve radiopaque beads into the heart muscle of nine sheep; one in the anterior-basal and one in the lateral-equatorial left ventricular wall. During three consecutive heartbeats, we record the bead coordinates via biplane videofluoroscopy. From the resulting four-dimensional data sets, we calculate the temporally and transmurally varying Green–Lagrange strains in the anterior and lateral wall. To quantify active contraction, we project the strains onto the local muscle fiber directions. We observe that mechanical activation is initiated at the endocardium slightly after end diastole and progresses transmurally outward, reaching the epicardium slightly before end systole. Accordingly, fibers near the outer wall are in contraction for approximately half of the cardiac cycle while fibers near the inner wall are in contraction almost throughout the entire cardiac cycle. In summary, cardiac wall strains display significant temporal, regional, and transmural variations. Quantifying wall strain profiles might be of particular clinical significance when characterizing stages of left ventricular remodeling, but also of engineering relevance when designing new biomaterials of similar structure and function.     

Kv在正常与被动致敏人气道平滑肌张力调控中的作用

目的：探讨电压依赖性延迟整流钾通道（Kv）对正常与体外致敏人气道平滑肌（HASM）的张力调控作用及其机制。 方法： 采用肌张力试验、逆转录聚合酶链反应（RT-PCR）和免疫细胞化学等技术，观察钾通道阻断剂对正常与哮喘患者血清致敏HASM张力、细胞Kv的mRNA和蛋白质表达。 结果： Kv 阻断剂4-氨基吡啶（4-AP）可引起正常HASM肌环产生浓度依赖性收缩反应，达到最大效应一半所需浓度的负对数值（pD2）为2.09±0.09，人哮喘血清致敏组为2.44±0.16，差异显著（P＜0.01）；但最大反应强度（Emax）分别为正常组（24.0±6.4）g/g，人哮喘血清致敏组（31.8±7.3）g/g，差异无显著（P＞0.05）。在培养的HASM细胞上，Kv1.2、Kv1.3和 Kv1.5基因mRNA均有表达；人哮喘血清致敏的HASM细胞Kv1.5基因mRNA（P＜0.01）和蛋白质（P＜0.01）的表达均下降。 结论： 体外被动致敏HASM细胞Kv功能较正常下调，致平滑肌兴奋性增高，该变化可能主要与Kv1.5基因相关。     

Study of human muscle contraction using electrically evoked twitch responses during passive shortening and lengthening movements

Summary Electrically evoked twitch properties of the human plantarflexor muscles were measured with the muscles at a constant length (static) and during passive shortening and lengthening. A Kin-Com dynamometer system was used to passively move the ankle joint at 0.52 rad s⁻¹ (30‡ s⁻¹), as well as to record the twitch responses which were elicited by supramaximal electric shocks applied over the tibial nerve in the popliteal fossa. In the lengthening and shortening conditions, twitches were evoked by triggering the shocks so that the twitch response occurred at a similar angular position for all three conditions. The lengthening twitch peak torque was about twice as large as that recorded for the shortening condition. There was, however, no statistical difference in the twitch time course in these three testing conditions. This twofold increase in the peak twitch torque during lengthening, compared to shortening, is much greater than the torque increase reported during eccentric, as compared to concentric maximal voluntary contractions. These findings suggest that a deactivation process of the contractile system occurs during muscle shortening, while in contrast, during passive lengthening a potentiation mechanism is acting, and that both these mechanisms are independent of volitional muscle activation. The present study is the first to demonstrate the possibility of electrically evoked contractions of human muscles during passive lengthening and shortening. We believe that the use of such evoked contractions may be promising for the study of contractile behaviour of human skeletal muscles during eccentric and concentric conditions. This work was supported by a grant from l'Institut de Recherche en Santé et en Sécurite du Travail du Québec (IRSST)     

Activation by ADP and the correlation between tension and ATPase activity in insect fibrillar muscle

Summary The effect of ADP on the tension and ATPase activity of glycerinated fibres of the flight muscles of the giant water bugLethocerus cordofanus has been investigated. Both increased, by a variable amount, when the ADP concentration was raised to 1 mM, but the increase was reversed when the concentration was raised further. In another series of experiments, the effect of temperature on the ATPase activity and tension was measured. There was a positive correlation between ATPase activity and tension in both types of experiment, the slopes being similar to those reported by other workers when calcium ion concentration or mean muscle length were altered. The positive feedback resulting from the increase of ATPase activity caused by ADP explains at least in part the variability of the results, and provides a possible explanation of the high tension state in glycerinated fibrillar muscle.     

In vivo human triceps surae and quadriceps femoris muscle function in a squat jump and counter movement jump

An optic fibre method was used to measure in humans in vivo Achilles (ATF) and patellar tendon forces (PTF) during submaximal squat jumps (SJ) and counter movement jumps (CMJ). Normal two-legged jumps on a force plate and one-legged jumps on a sledge apparatus were made by four volunteers. Kinetics, kinematics, and muscle activity from seven muscles were recorded. The loading patterns of the tendomuscular system differed among the jumping conditions, but were similar when the jumping height was varied. Peak PTF were greater than ATF in each condition. In contrast to earlier simulation studies it was observed that tendomuscular force could continue to increase during the shortening of muscle-tendon unit in CMJ. The concentric tendomuscular output was related to the force at the end of the stretching phase while the enhancement of the output in CMJ compared to SJ could not be explained by increases in muscle activity. The stretching phase in CMJ was characterised by little or no electromyogram activity. Therefore, the role of active stretch in creating beneficial conditions for the utilisation of elastic energy in muscle was only minor in these submaximal performances. The modelling, as used in the present study, showed, however, that tendon underwent a stretch-shortening cycle, thus having potential for elastic energy storage and utilisation. In general, the interaction between muscle and tendon components may be organised in a manner that takes advantage of the basic properties of muscle at given submaximal and variable activity levels of normal human locomotion. Accepted: 28 June 2000     

Nonexercise muscle tension and behavioral fidgeting are positively correlated with food availability/palatability and body weight in rats

While certain measures of energy expenditure such as respiratory quotient and thermogenesis are readily quantifiable using existing animal models, the mechanism for and measurement of energy expenditure via nonexercise activity have not been thoroughly characterized. This low intensity form of physical exertion, associated with involuntary fidgeting and postural changes in man, was quantified in the present studies using passive measurement of muscle tension in rats. In particular, long-term weight loss and gain were induced using diet yoking and feeding of preferred foods in order to assess corresponding changes in locomotor activity and radiotelemetered measures of muscle tension, temperature and global activity. Hind limb muscle tension, but not body temperature, was increased 30-60% by enhancing the availability or palatability of food relative to the decreased muscle tension resulting from limited food availability. Enhancing food availability or palatability also produced a relative 5-15% increase in the amount of telemetered global activity. Importantly, neither diet yoking nor provision of a highly preferred diet altered a precise measure of behavioral locomotor activity. These results suggest that muscle tension and activity-in-place are positively correlated with weight change in the present studies and that these mechanisms of energy expenditure are mobilized by environmental changes in diet composition and meal pattern.     

Viscoelastic properties of short calf muscle-tendon units of older women: effects of slow and fast passive dorsiflexion stretches in vivo

Changes in connective tissues of the skeletal muscle-tendon unit (MTU) of aging animal muscles have been associated with increased passive viscoelastic properties. This study examined whether similar changes in the viscoelastic properties were present in short calf MTUs of older women in vivo. Fifteen women 68–87 years of age with short calf MTUs, as represented by limited active dorsiflexion (DF) range of motion (ROM) of 5°, and 15 women 20–26 years of age without decreased DF ROM participated. A Kin-Com dynamometer stretched the MTU from plantarflexion to maximal DF at the slow velocity of 5° s^–1 (0.087 rad s^–1) and the fast velocity of 120° s^–1 (2.094 rad s^–1) with minimal surface electromyogram activity in the soleus, gastrocnemius, and tibialis anterior muscles. Two-way analysis of variance (ANOVA) tests for repeated measures (Velocity × Group) indicated that all women showed greater passive torque, average passive elastic stiffness, and total absorbed passive elastic energy for the fast stretch than for the slow stretch (P<0.001). The older women had greater percent increases for the average passive torque (30%) and total absorbed passive elastic energy (26%) for the fast stretch than the younger women (P<0.05), who had 17.5 and 13% increases, respectively. The older women had less maximal and average passive torque (Nm) and total absorbed passive elastic energy (°Nm), but greater average passive elastic stiffness (Nm°^–1) at both stretch velocities (P<0.001). The results indicated that short calf MTUs of older women have increased passive viscoelastic properties that could have implications for balance and ambulatory function.     

Skeletal muscle mechanics in osteoporotic and nonosteoporotic postmenopausal women

The purpose of this study was to evaluate single-joint, dynamic muscle function of osteoporotic (OST) and nonosteoporotic (N-OST) women. Knee flexor and extensor function in postmenopausal women (6th decade OST,n = 15; 7th decade OST,n = 10; 6th decade N-OST,n = 6; 7th decade N-OST,n = 5) were evaluated at five angular velocities from 60° · s^–1 to 300° · s^–1. All subject groups had similar anthropometric measurements, but the 6th decade N-OST group were more physically active than the age-matched OST group. The OST and N-OST women produced peak torque at similar knee angles. The 6th decade N-OST women produced significantly greater knee extensor mean peak torque and angle specific torque, and mean work than any of the other three groups (P<0.05). However, knee flexor function was equivalent throughout the groups for most comparisons, except those between the 6th decade N-OST and 7th decade OST. While previous research has shown an early loss of flexor muscle function in ageing women, our data indicated that women with osteoporosis also experience a deterioration in quadriceps muscle function not encountered within the N-OST subjects. It is possible that such a change is precipitated by reduced physical activity, and may mirror deterioration in bone mineral content.