Basic characteristics between mechanomyogram and muscle force during twitch and tetanic contractions in rat skeletal muscles

The mechanomyogram (MMG) is a signal measured by various vibration sensors for slight vibrations induced by muscle contraction, and it reflects the muscle force during electrically induced-contraction or until 60%–70% maximum voluntary contraction, so the MMG is considered an alternative and novel measurement tool for muscle strength. We simultaneously measured the MMG and muscle force in the gastrocnemius (GC), vastus intermedius (VI), and soleus (SOL) muscles of rats. The muscle force was measured by attaching a hook to the tendon using a load cell, and the MMG was measured using a charged-coupled device-type displacement sensor at the middle of the target muscle. The MMG-twitch waveform was very similar to that of the muscle force; however, the half relaxation time and relaxation time (10%), which are relaxation parameters, were prolonged compared to those of the muscle force. The MMG amplitude correlated with the muscle force. Since stimulation frequencies that are necessary to evoke tetanic progression have a significant correlation with the twitch parameter, there is a close relationship between twitch and tetanus in the MMG signal. Therefore, we suggest that the MMG, which is electrically induced and detected by a laser displacement sensor, may be an alternative tool for measuring muscle strength.     

Sarcomere length-dependence of activity-dependent twitch potentiation in mouse skeletal muscle

Background  

It has been reported that potentiation of a skeletal muscle twitch response is proportional to muscle length with a negative slope during staircase, and a positive slope during posttetanic potentiation. This study was done to directly compare staircase and posttetanic responses with measurement of sarcomere length to compare their length-dependence.     

Myosin phosphorylation, twitch potentiation, and fatigue in human skeletal muscle

Twitch tension and phosphate incorporation into the phosphorylatable light chains (P-light chains) of myosin were studied during a 10-min recovery period following a 10- or 60-s maximal voluntary isometric contraction (MVC) in 18 subjects. Analysis of muscle biopsy samples obtained before, immediately after, 1 min, and 10 min following the 10-s MVC revealed that the 10-s MVC produced a modest but transient metabolic displacement from rest, a 35% decrease in phosphocreatine, and a threefold elevation in lactate concentration. Immediately after the 60-s MVC, ATP was decreased by 20%, phosphocreatine decreased by 84%, and lactate was elevated by 15-fold. Lactate remained elevated over the 10-min recovery period. Twitch force was maximally potentiated following the 10-s MVC and declined to rest by 10 min of recovery. Twitch force was 0.66 of rest value immediately after the 60-s MVC, then increased over the next 4 min to reach a potentiated value 21% greater than rest, before declining. Significant phosphate incorporation into P-light chains was observed immediately after both contractions, but dephosphorylation to rest values at the end of recovery was only noted for the 60-s condition. These results demonstrate an inconsistent relationship between twitch tension enhancement and P-light chain phosphorylation in the in vivo human model.     

Collagen of slow twitch and fast twitch muscle fibres in different types of rat skeletal muscle

The appearance of collagen around individual fast twitch (FT) and slow twitch (ST) muscle fibres was investigated in skeletal muscles with different contractile properties using endurance trained and untrained rats as experimental animals. The collagenous connective tissue was analyzed by measuring hydroxyproline biochemically and by staining collagenous material histochemically in M. soleus (MS), M. rectus femoris (MRF), and M. gastrocnemius (MG). The concentration of hydroxyproline in the ST fibres dissected from MS (2.72 +/- 0.35 micrograms X mg-1 d.w.) was significantly higher than that of the FT fibres dissected from MRF (1.52 +/- 0.33 micrograms X mg-1 d.w.). Similarly, the concentration of hydroxyproline was higher in ST (2.54 +/- 0.51 micrograms X mg-1 d.w.) than in FT fibres (1.60 +/- 0.43 micrograms X mg-1 d.w.), when the fibres were dissected from the same muscle, MG. Histochemical staining of collagenous material agreed with the biochemical evidence that MS and the slow twitch area of MG are more collagenous than MRF and the fast twitch area of MG both at the level of perimysium and endomysium. The variables were not affected by endurance training. When discussing the role of collagen in the function of skeletal muscle it is suggested that the different functional demands of different skeletal muscles are also reflected in the structure of intramuscular connective tissue, even at the level of endomysial collagen. It is supposed that the known differences in the elastic properties of fast tetanic muscle compared to slow tonic muscle as, e.g., the higher compliance of fast muscle could at least partly be explained in terms of the amount, type, and structure of intramuscular collagen.     

Length-dependence of isometric twitch tension potentiation and myosin phosphorylation in mouse skeletal muscle

The effect of changes in muscle length on post-tetanic isometric twitch tension potentiation and myosin P-light chain phosphorylation-was studied at 23°C in the mouse extensor digitorum longus muscle. The length-tension relationship was determined for the same muscles after a 30 min period of quiescence and between 30 s and 3 min after a 1.5 s tetanus at L₀. Isometric twitch tension is increased at all muscle lengths after the tetanus; however, the fractional increase in twitch tension rises from 0.2 at L₀ to a maximum of 0.3 at 1.2 L_0. The fractional increase in twitch tension measured at any fixed muscle length is constant between 30 s and 3 min post-tetanus. P-light chain phosphorylation remains constant between 30 s and 3 min post-tetanus followed by a slow decline to basal values. Under fixed length conditions, there is linear relationship between the relative magnitude of the twitch tension and the extent of P-light chain phosphor-ylation. Net myosin phosphorylalion measured after a 1.5 s tetanus at 1.23 L₀ is 35% less than that obtained under the same conditions at L_0. Thus, contraction-induced phosphorylation of P-light chain decreases with increased muscle length and post-tetanic potentiation at a constant level of P-light chain phosphorylation increases with increasing muscle length. These observations may be consistent with alterations in the sarcoplasmic Ca²⁺ ion transient as the muscle is lengthened.     

Effects of pH on the length-dependent twitch potentiation in skeletal muscle.

When muscle is elongated, there is a length dependence of twitch potentiation and an increased Ca(2+) sensitivity of the myofilaments. Changes in the charge potential of myofilaments, induced by a decrease in pH, are known to abolish the length dependence of Ca(2+) sensitivity. This study was aimed at testing the hypothesis that a decrease in pH, and the concomitant loss of length dependence of Ca(2+) sensitivity, depresses the length dependence of staircase potentiation. In vitro, isometric twitch contractions of fiber bundles dissected from the mouse extensor digitorum longus, performed before and after 10 s of 10-Hz stimulation (i.e., the staircase potentiation protocol) were analyzed at five different lengths, ranging from optimal length for maximal force production (L(o); = 12 +/- 0.7 mm) to L(o) + 1.2 mm (L(o) + 10%). These measurements were made at an extracellular pH of 6.6, 7.4, and 7.8 (pH changes induced by altering the CO(2) concentration of the bath solution). At pH 7.4 and 7.8, the degree of potentiation after 10-Hz stimulation showed a linear decrease with increased fiber bundle length (r(2) = 0.95 and r(2) = 0.99, respectively). At pH 6.6, the length dependence of potentiation was abolished, and the slope of the length-potentiation relationship was not different from zero (r(2) = 0.05). The results of this study indicate that length dependence of potentiation in intact skeletal muscle is abolished by lowering the pH. Because decreasing the pH decreases Ca(2+) sensitivity and changes the charge potential of the filaments, the mechanism of length-dependent potentiation may be closely related to the length dependence of Ca(2+) sensitivity, and changes in the charge potential of the myofilaments may be important in regulating this relationship.     

Transverse tubular system depolarization reduces tetanic force in rat skeletal muscle fibers by impairing action potential repriming

When muscle fibers are repeatedly stimulated, they may become depolarized and force output decline. Excitation of the transverse tubular system (T-system) is critical for activation, but its role in muscle fatigue is poorly understood. Here, mechanically skinned fibers from rat fast-twitch muscle were used, because the sarcolemma is absent but the T-system retains normal excitability and its properties can be studied in isolation. The T-system membrane was fully polarized by bathing the skinned fiber in an internal solution with 126 mM K⁺ (control solution) or set at partially depolarized levels (approximately –63 and –58 mV) in solutions with 66 or 55 mM K⁺, respectively, and action potentials (APs) were triggered in the sealed T-system by field stimulation. Prolonged depolarization of the T-system reduced tetanic force proportionately more than twitch force, with greater effect at higher stimulation frequency (responses at 20 and 100 Hz reduced to 71 and 62% in 66 mM K⁺ and to 54 and 35% in 55 mM K⁺, respectively). Double-pulse stimulation showed that depolarization increased the repriming period (estimated minimum time before a second AP can be produced) from 4 ms to 7.5 and 15 ms in the 66 and 55 mM K⁺ solutions, respectively. These results demonstrate that T-system depolarization reduces tetanic force by impairing AP repriming, rather than by preventing AP generation per se or by inactivating the T-system voltage sensors. The findings also explain why it is advantageous to reduce the rate of motoneuron stimulation to muscles during repeated or prolonged periods of activity. T-system; muscle fatigue; excitation-contraction coupling     

Coexistence of potentiation and low-frequency fatigue during voluntary exercise in human skeletal muscle

The role of muscle potentiation in overcoming low-frequency fatigue (LFF) as it developed during submaximal voluntary exercise was investigated in eight males (age 26.4 +/- 0.7 years, mean +/- SE) performing isometric leg extension at approximately 30% of maximal voluntary contraction for 60 min using a 0.5-duty cycle (1 s contraction, 1 s rest). At 5, 20, 40, and 60 min, exercise was interrupted for 3 min, and the maximum positive rate of force development (+dF/dtmax) and maximal twitch force (Pt) were measured in maximal twitch contractions at 0, 1, 2, and 3 min of rest (R0, R1, R2, R3); they were also measured at 15 min of recovery following the entire 60-min exercise period. These measures were compared with pre-exercise (PRE) as an indicator of potentiation. Force at low frequency (10 Hz) was also measured at R0, R1, R2, and R3, and at 15 min of recovery, while force at high frequency (100 Hz) was measured only at R0 and R3 and in recovery. Voluntary exercise increased twitch +dF/dtmax at R0 following 5, 20, 40, and 60 min of exercise, from 2553 +/- 150 N/s at PRE to 39%, 41%, 42%, and 36% above PRE, respectively (P<0.005). Twitch +dF/dtmax decayed at brief rest (R3) following 20, 40, and 60 min of exercise (P<0.05). Pt at R0 following 5 and 20 min of exercise was above that at PRE (P<0.05), indicating that during the early phase of moderate-intensity repetitive exercise, potentiation occurs in the relative absence of LFF. At 40 and 60 min of exercise, Pt at R0 was unchanged from PRE. The LFF (10 Hz) induced by the protocol was evident at 40 and 60 min (R0-R3; P<0.05) and at 15 min following exercise (P<0.05). High-frequency force was not significantly compromised by the protocol. Since twitch force was maintained, these results suggest that as exercise progresses, LFF develops, which can be compensated for by potentiation.     

Prediction of summation in incompletely fused tetanic contractions of rat muscle

Summation is the accumulating contractile force resulting from sequential activations applied to a muscle without sufficient interval to permit complete relaxation. The purpose of this study was to evaluate summation in the rat medial gastrocnemius muscle, and to determine if the contractile responses during summation could be predicted from the relationship between force and activation pattern. In the first part of this study, the consistency of summation in the rat gastrocnemius muscle was assessed and prediction equations were derived. The second part compared predicted summation with actual contractions obtained in a new set experiments. Summation was assessed by calculation of the contractile response, per stimulation, for up to five stimulating pulses at these frequencies: 20, 40, 60 and 80Hz. This was done by subtraction of the force transient for j-1 pulses of stimulation (where j=1-5 pulses) from the force response with j pulses of stimulation. Each of these force differences was evaluated for peak rate of force development, contraction time and half-relaxation time. Contraction and half-relaxation times changed by only a small magnitude from values obtained for the twitch. Peak rate of force development was proportional to the active force for all force transients obtained by subtraction. The force per activation increased from the first to the fifth stimulus, and was dependent on interpulse delay. In the second series of experiments, the predicted force was related to the actual force for brief tetanic contractions at 40, 50 and 60Hz (r(2)=0.875). These experiments demonstrate that the force response to sequential activations is consistent and predictable. Summation can be predicted, knowing only the amplitude of the twitch contraction and the relationship between delay and force for each activating stimulus.     

Stimulation frequency and force potentiation in the human adductor pollicis muscle

The effect of stimulation frequency on twitch force potentiation was examined in the adductor pollicis muscle of ten normal subjects. The ulnar nerve was supramaximally stimulated at the wrist and isometric twitch force was measured from a 3-Hz train lasting 1 s. Test stimulation frequencies of 5, 10, 20, 25, 30, 40, 50 and 100 Hz were applied for 5 s each in random order (5 min apart) and the twitches (3 Hz) were applied immediately before and after (1 s) the test frequency and at intervals up to 5 min afterwards (10 s, and 1, 2 and 5 min). Poststimulation twitches were expressed as a percentage of the prestimulation twitch. Low frequency fatigue was not induced by the protocol since the 20:50 Hz ratio did not alter within each session. The degree of twitch potentiation was frequency dependent, with potentiation increasing up to 50 Hz [mean 173 (SD 16)%] but the effect was markedly less at 100 Hz [mean 133 (SD 25)%, P less than 0.01] for all subjects. The reduced potentiation at 100 Hz may have occurred due to high frequency fatigue produced by the 100-Hz test stimulation train. The optimal frequency of those examined in the experimental group was 50 Hz but this only produced maximal potentiation in six of the ten subjects and 100 Hz always produced less potentiation. These findings have implications for electrical stimulation of muscle in the clinical setting.     

The variation of characteristics of twitch and tetanic contractions with sarcomere length in isolated muscle fibres of the frog.

The relation between sarcomere length, tension and time course of tension development in twitch and tetanic contractions at 20 degrees C was determined for isolated fibres from the semitendinosus muscle of the frog (Rana esculenta). In twenty fibres at about 2.15 micron sarcomere length, the peak twitch tension, the maximum tetanic tension and the twitch/tetanus ratio ranged, respectively, from 0.22 to 1.6 kg/cm2, from 2.13 o 3.96 kg/cm2 an from 0.07 to 0.53. The peak twitch tension was found to be: i) directly correlated with the twitch/tetanus ratio and the time to the peak of the first derivative of the twitch tension, ii) inversely correlated with the time to the peak of the first derivative of tetanic tension. No significant correlation was found between the maximal tetanic tension and the peak twitch tension or the twitch/tetanus ratio. Peak twitch tension and twitch/tetanus ratio were not correlated with the fibre cross-sectional area which ranged from 1.052 to 6,283 micron2. Sarcomere length-tension curves for twitch and tetanic isometric contractions at 20 degrees C were determined in twelve fibres. Increases in sarcomere length from about 2.15 to 2.85 micron produced, depending on the peak twitch tension or the twitch/tetanus ratio at about 2.15 micron, either decrease and no change or increase in peak twitch tension, but constantly enhanced the twitch/tetanus ratio and the degree of this potentiation was inversely correlated with the twitch/tetanus ratio at 2.15 micron. Increase in sarcomere length above 2.15 micron did not alter the course of the early development of twitch and tetanic tensions, reduced considerably the variation in peak twitch tension and twitch/tetanus ratio, without altering that of tetanic tension and swamped the correlation between the peak twitch tension and the time to peak of the differentiated twitch tension. However, the peak twitch tension at about 2.85 micron resulted to be directly correlated with the peak twitch tension at about 2.15 micron and in addition the relative length-dependent change in the time of the peak of the first derivative of the twitch tension resulted to be directly correlated with the relative length-dependent change in the peak twitch tension. It is concluded that both the duration of the active state and the rate factors of activation contribute to the determining of the large variation in peak twitch tension at about 2.15 micron, whereas the length-dependent increase in twitch/tetanus ratio appears to be mainly determined by prolongation of the active state duration.     

Development of muscle fiber specialization in the rat hindlimb

The appearance of fast and slow fiber types in the distal hindlimb of the rat was investigated using affinity-purified antibodies specific to adult fast and slow myosins, two-dimensional electrophoresis of myosin light chains, and electron microscope examination of developing muscle cells. As others have noted, muscle histogenesis is not synchronous; rather, a series of muscle fiber generations occurs, each generation forming along the walls of the previous generation. At the onset of myotube formation on the 15th d of gestation, the antimyosin antibodies do not distinguish among fibers. All fibers react strongly with antibody to fast myosin but not with antibody to slow myosin. The initiation of fiber type differentiation can be detected in the 17-d fetus by a gradual increase in the binding of antibody to slow myosin in the primary, but not the secondary, generation myotubes. Moreover, neuromuscular contacts at this crucial time are infrequent, primitive, and restricted predominantly, but not exclusively, to the primary generation cells, the same cells which begin to bind large amounts of antislow myosin at this time. With maturation, the primary generation cells decrease their binding of antifast myosin and become type I fibers. Secondary generation cells are initially all primitive type II fibers. In future fast muscles the secondary generation cells remain type II, while in future slow muscles most of the secondary generation cells eventually change to type I over a prolonged postnatal period. We conclude that the temporal sequence of muscle development is fundamentally important in determining the genetic expression of individual muscle cells.     

Different isometric force - [Ca2+] relationships in slow and fast twitch skinned muscle fibres of the rat

Skinned muscle fibres prepared from fast and slow twitch muscles of rat have been activated in Ca²⁺-buffered solutions using a new activation procedure (Moisescu, D.G. and Thieleczek, R. (1978) J. Physiol. 275, 241–262). The results indicate that (i) the Ca²⁺ activation curve is less steep for slow fibres, (ii) physiologically relevant force levels are attained considerably faster at constant [Ca²⁺] in fast fibres, and (iii) active force becomes noticeable at lower [Ca²⁺], but reaches saturation at higher [Ca²⁺] for slow fibres.     

Charge movement in a fast twitch skeletal muscle from rat

Voltage-dependent charge movement in the rat omohyoid muscle was investigated using the three microelectrode voltage clamp technique. The charge that moved during a depolarization from the holding potential (-90 mV) to the test potential, V, increased with increasing V, saturating around 0 mV. The charge vs. voltage relationship was well fitted by Q = Q_max/{1 + exp[-(V - V)/k]}, with Q_max = 28.5 nC/μF, V = -34.2 mV, and k = 8.7 mV. Repolarization of the fiber from the test potential back to the holding potential caused an equal but opposite amount of charge to move. The kinetics of ON charge movement could be well described by a model developed for frog muscle by Horowicz and Schneider (1981b), which suggests that rat and frog charge movements are similar. This model failed to describe the kinetics of OFF charge movement for steps in potential from 0 mV to test potentials of -10 to -90 mV. OFF-charge movement rose to a peak more slowly and decayed more slowly than predicted by the theory.     

Endogenous triacylglycerol utilization by rat skeletal muscle during tetanic stimulation

An isolated perfused rat hindquarter preparation was used to examine the utilization of endogenous triacylglycerol (TG) during 20 min of electrical stimulation. The sciatic nerve was stimulated with maximal tetanic trains at 0.5 Hz. The isometric tension generated by the gastrocnemius-plantaris-soleus muscle group was recorded, and muscle samples were taken pre- and poststimulation. Twenty minutes of stimulation significantly reduced endogenous TG from 6.78 +/- 0.84 to 4.64 +/- 0.64 mumol X g dry wt-1 (32%) in the red gastrocnemius muscle and from 7.70 +/- 0.61 to 6.66 +/- 0.80 mumol X g dry wt-1 (13.5%) in the plantaris muscle. Although TG content decreased by 16% in the soleus (28.2 +/- 5.0 to 23.8 +/- 4.4 mumol X g-1), the change was not significant. Stimulation had no effect on white gastrocnemius TG concentration (6.84 +/- 1.22 to 6.25 +/- 1.41 mumol X g-1). Thus oxidation of TG occurred primarily in muscles with a large proportion of fast-twitch oxidative-glycolytic fibers. Calculations from measurements of muscle energy stores and fuel uptake indicated that up to 62% of the aerobic energy was provided by endogenous TG. Carbohydrate oxidation contributed up to 28% and the remaining 10% may be accounted for by the oxidation of exogenous free fatty acids originating in the perfusate or from hindquarter adipose tissue. The magnitude of the fall in TG concentration in a given muscle was inversely related to the fall in glycogen concentration.     

Development of muscle fiber types in the prenatal rat hindlimb

Immunohistochemistry was used to examine the expression of embryonic, slow, and neonatal isoforms of myosin heavy chain in muscle fibers of the embryonic rat hindlimb. While the embryonic isoform is present in every fiber throughout prenatal development, by the time of birth the expression of the slow and neonatal isoforms occurs, for the most part, in separate, complementary populations of fibers. The pattern of slow and neonatal expression is highly stereotyped in individual muscles and mirrors the distribution of slow and fast fibers found in the adult. This pattern is not present at the early stages of myogenesis but unfolds gradually as different generations of fibers are added. As has been noted by previous investigators (e.g., Narusawa et al., 1987, J. Cell Biol. 104, 447-459), all of the earliest generation (primary) muscle fibers initially express the slow isoform but some of these primary fibers later lose this expression. In this study we show that loss of slow myosin in these fibers is accompanied by the expression of neonatal myosin. This switch in isoform expression occurs in all primary fibers located in specific regions of particular muscles. However, in other muscles primary fibers which retain their slow expression are extensively intermixed with those that switch to neonatal expression. Later generated (secondary) muscle fibers, which are interspersed among the primary fibers, express neonatal myosin, although a few of them in stereotyped locations later switch from neonatal to slow myosin expression. Many of the observed changes in myosin expression occur coincidentally with the arrival of axons in the limb or the invasion of axons into individual muscles. Thus, although both fiber birth date and intramuscular position are grossly predictive of fiber fate, neither factor is sufficient to account for the final pattern of fiber types seen in the rat hindlimb. The possibility that fiber diversification is dependent upon innervation is tested in the accompanying paper (K. Condon, L. Silberstein, H.M. Blau, and W.J. Thompson, 1990, Dev. Biol. 138, 275-295).