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Background Current evidence suggests that Brugada syndrome (BrS), far from being a purely electrical condition, is associated with subtle mechanical abnormalities primarily affecting the right ventricle (RV). We aimed to characterize RV function in BrS and investigate the echocardiographic profile of patients with arrhythmic events, with a special focus on parameters of RV dyssynchrony using speckle-tracking echocardiography (STE). Methods An echocardiogram was performed in 71 BrS patients and 25 healthy controls. STE was performed to assess regional and global RV mechanics, including RV outflow tract shortening (RVOTS). RVOT contraction time was considered to calculate the modified RV mechanical dispersion (RVMDm). Arrhythmic events were prospectively evaluated in the BrS cohort. Results Compared to controls, BrS patients showed subtle contractile abnormalities, including impaired RV longitudinal strain, higher RV index of myocardial performance (RIMP) and lower RVOTS. BrS patients also exhibited a greater contraction delay between the lateral and the septal aspect of the RV. After a median follow-up of 7.3 year (IQR 5.2–10.8), 6 patients presented malignant arrhythmic events. RIMP >0.50, RVOTS 42 ms showed high sensitivity for the identification of BrS patients with arrhythmic events during follow-up. Conclusions Subtle RV mechanical abnormalities were present in BrS patients. RIMP and RVOTS, a novel STE-derived parameter, were found to be sensitive markers of arrhythmic events. Adding the RVOT contraction time to the analysis of RVMD may help identify patients at higher risk, reflecting the importance of the RVOT mechanical substrate in the assessment of the arrhythmic risk in BrS.

Tensiomyography detects the contraction time (Tc) and amplitude (Dm) of muscle belly thickening during maximal isometric twitch contraction. The assessment of both parameters is highly reliable; however, it seems that their calculation depends on the measurement point. The aim of the study was to determine spatial relative error distribution of Tc and/or Dm within a two-dimensional array of 27 (3 × 9) measurement points in comparison to the reference point (RP) in 12 male participants (22.5 ± 3.1 years). The RPs were determined as follows: in the biceps brachii (BB) at 50% of the humerus length; in the erector spinae (ES) at the height of the iliac crest; in the vastus lateralis (VL), vastus medalis (VM), and rectus femoris (RF) at 30%, 20%, and 50% of femur length above the patella, respectively. The surface area under the 3% relative error in Dm (BB: 4.0; VL: 3.8; VM; 8.2; RF: 6.2; ES: 2.4 cm2) was lower than in Tc (BB: 6.9; VL: 3.8; VM; 4.6; RF: 9.5; ES: 3.7 cm2), yielding merged values (BB: 3.9; VL: 3.7; VM; 4.8; RF: 5.1; ES: 2.4 cm2). Dm show twice as steep relative error rate when moving away from the RP in comparison to Tc, which seems to be less sensitive to spatial sensor positioning.

Summary Objectives The aim of this study was to investigate the effect of changing concentric-eccentric contraction time ratio in a single session of resistance exercise on hormonal response in trained men. Methods In a double-blind cross section study design, ten young men (mean age: 26.3 ± 1.2 years, mean weight: 81.2 ± 4.25 kg, and mean height: 178 ± 6 cm) were selected and completed 4 resistance exercise sessions with 3 days’ between sessions interval. Protocols included, 1 second concentric and 1 second eccentric contraction; 3 second concentric and 3 second eccentric contraction; 4 second concentric and 2 seconds eccentric contraction; 2 seconds concentric and 4 seconds eccentric contraction. In each session subjects performed one of the four protocols. The exercise protocol was consisted of leg extension, bench press, hamstring curls and lat pull-down with three sets of eight repetitions and intensity of 0.65 of one repetition maximum. Rest times for sets and exercises were 1 and 2 minutes respectively. Blood samples were collected before and immediately after exercise. Results Increases in lactate were observed after all protocols. Serum levels of testosterone and growth hormone increased significantly in all, but 1 second concentric and 1 second eccentric contraction protocol, while level of serum cortisol did not change significantly in none of the protocols. Conclusions The result of this study showed that in a single session of RE, changes in concentric-eccentric contraction time ratio had no effect on hormonal response, however slower contractions produced greater hormonal response compare to normal protocol.

Background Latent trigger point (LTrP) can cause motor dysfunction and disturb normal patterns of motor recruitment. Objective To analyze the effects of DN in the upper trapezius (UT) LTrP on pain and the mechanical and contractile properties of the muscle. Design A randomized, double-blinded, parallel-group-trial. Methods Fifty healthy volunteers with LTrPs in the UT were randomly divided into a DN-group (n = 26) and a Sham-DN-group (n = 24) and received one session of DN or placebo treatment. Mechanical and contractile properties of the muscle and pressure pain perception (PPP) were evaluated before treatment and in a 30min, 24 h and 72 h follow-up after treatment. Results In the mechanical properties, the DN-group showed lower values than the Sham-DN-group for dynamic stiffness at 72 h (p = 0.04). The DN-group showed lower values for dynamic stiffness at 72 h from baseline (278.74 ± 38.40 to 261.54 ± 33.64 N/m; p = 0.01) and for tone at 72 h from 30min (16.62 ± 1.27 to 15.88 ± 1.31 Hz; p = 0.01). In the contractile properties, the DN-group showed higher values for maximal radial displacement (Dm) of the muscle belly at 72 h from baseline (5.38 ± 1.67 to 6.13 ± 1.70 mm; p = 0.04), higher values for contraction time at 30min (28.53 ± 8.80 s; p = 0.03) and lower ones at 72 h (24.74 ± 4.36 s; p = 0.04) from baseline (26.97 ± 6.63 s). The DN-group showed a decrease of PPP from baseline to 72 h after treatment (5.16 ± 1.33 to 4.02 ± 0.97 mm; p Conclusion The application of DN in healthy volunteers over LTrPs in the UT decreased dynamic stiffness, tone and contraction time and increased Dm at 72 h after treatment. Additionally, the PPP showed a decrease at 72 h after needling. CLINICALTRIALS.GOV: NCT04466813.

We propose a force estimation method in fatigue condition using a muscle-twitch model and surface electromyography (sEMG). The twitch model, which is an estimate of force by a single spike, was obtained from sEMG features and measured forces. Nine healthy subjects performed isometric index finger abduction until exhaustion for a series of dynamic contractions (0–20% MVC) to characterize the twitch model and static contractions (50% MVC) to induce muscle fatigue. Muscle fatigue was identified based on the changes of twitch model; the twitch peak decreased and the contraction time increased as muscle fatigue developed. Force estimation performance in non-fatigue and fatigue conditions was evaluated and its results were compared with that of a conventional method using the mean absolute value (MAV). In non-fatigue conditions, the performance of the proposed method (0.90 ± 0.05) and the MAV method (0.88 ± 0.06) were comparable. In fatigue conditions, the performance was significantly improved for the proposed method (0.87 ± 0.05) compared with the MAV (0.78 ± 0.09).

To assess the validity and reliability of the mechanomyogram (MMG) as a tool to investigate the fatigue-induced changes in the muscle during relaxation, the torque and MMG signals from the gastrocnemius medialis muscle of 23 participants were recorded during tetanic electrically-elicited contractions before and immediately after fatigue, as well as at min 2 and 7 of recovery. The peak torque (pT), contraction time (CT) and relaxation time (RT), and the acceleration of force development (d2RFD) and relaxation (d2RFR) were calculated. The slope and τ of force relaxation were also determined. MMG peak-to-peak was assessed during contraction (MMG p-p) and relaxation (R-MMG p-p). After fatigue, pT, d2RFD, d2RFR, slope, MMG p-p and R-MMG p-p decreased significantly, while CT, RT and τ increased (P0.05 for all comparisons), remaining altered throughout the entire recovery period. R-MMG p-p correlated with pT, MMG p-p, slope, τ and d2RFR both before and after fatigue. Reliability measurements always ranged from high to very high. In conclusion, MMG may represent a valid and reliable index to monitor the fatigue-induced changes in muscle mechanical behavior, and could be therefore considered an effective alternative to the force signal, also during relaxation.

A recovery model is proposed to represent the relationship between the muscle fatigue and the rest time. Three experiments were conducted at 50% MVC with the contraction time of 10 s, 30 s and 50 s. Every experiment consists of 5 handgrip tasks with different rest interval. The maximal isometric forces during the pre-fatigue and post-fatigue were recorded to compute the muscle fatigue developed from each handgrip tasks. An exponential function is used to model the relationship between the muscle recovery and rest interval. With this model, the amount of muscle fatigue that is recovered given the rest duration can be estimated. The results suggest that the rate of muscle recovery is correlated to the degree of muscle fatigue at any particular moment, regardless the contraction level or contraction time of the tasks. It is demonstrated that the performance is not significant improved by increasing the number of experiments during calibration process. After that, the recovery model is integrated with the existing fatigue index proposed previously. This is then evaluated on cyclic handgrip tasks. The results show that during the high repetitive motion tasks, the effect of muscle recovery is essential to be taken into consideration for quantifying the degree of muscle fatigue. In average, the estimation error is between 5% MVC, comparing between the estimated value and the force loss measured using dynamometer. This concludes the effectiveness of utilizing force loss as the index to quantify the muscle fatigue and recovery.

Skeletal muscles consist of muscle fibers that are responsible for contracting and generating force. Skeletal muscle fibers are categorized into distinct subtypes based on several characteristics such as contraction time, force production and resistance to fatigue. The composition of distinct muscle fibers in terms of their number and cross-sectional areas is characterized by a histological examination. However, manual delineation of individual muscle fibers from digitized muscle histology tissue sections is extremely time-consuming. In this study, we propose an automated image analysis system for quantitative characterization of muscle fiber type composition. The proposed system operates on digitized histological muscle tissue slides and consists of the following steps: segmentation of muscle fibers, registration of successive slides with distinct stains, and classification of muscle fibers into distinct subtypes. The performance of the proposed approach was tested on a dataset consisting of 25 image pairs of successive muscle histological cross-sections with different ATPase stain. Experimental results demonstrate a promising overall segmentation and classification accuracy of 89.1% in identifying muscle fibers of distinct subtypes.

Unfused tetani of motor units (MUs) evoked by stimulation at variable interpulse intervals at mean frequencies of 20, 25, 33, 40 and 50 Hz were studied using ten functionally isolated fast-type MUs from the medial gastrocnemius muscle of adult Wistar rats. A previously proposed algorithm and computer program for mathematical decomposition of unfused tetani into a series of twitches, representing responses to individual pulses, were used. Analysis of the parameters of the decomposed twitches showed considerable variability in force of successive contractions. These twitches were extremely variable with up to 2-fold higher forces and longer contraction times than a single twitch evoked by one stimulus. However, when the stimulation frequency was decreased, the decomposed twitches became similar to the single twitch with respect to amplitude and contraction time. It was found that the basic contractile parameters of decomposed twitches could be predicted with high accuracy on the basis of the tetanus force level at which the next contraction begins. This analysis of the parameters of decomposed twitches demonstrated that the contractile responses of the muscle fibers to successive action potentials generated by motoneurons are highly variable and depend on the previous MU state.

In the present study a previously proposed model of a twitch based on an analytical function with four-parameters (lead, contraction and half-relaxation times and maximum force of the twitch) was validated on 115 motor units (MUs), divided into slow (S), fast-fatigue resistant (FR) and fast fatigable (FF) types. The original records were collected from electrophysiological experiments performed on MUs from the medial gastrocnemius muscle of five rats. Besides the easy calculation of the twitch parameters and their variability, the usefulness of the model was confirmed by eliminating artifacts and noise in the original twitch records, as well as by calculations of the velocity of force increase and decrease, the area under force records, and by normalization of all twitches with respect to the maximal force and contraction time. It was concluded that: (1) the four-parameter twitch model describes precisely the individual contractions of various MUs; (2) all physiological twitch parameters are distributed continuously and located within overlapping intervals for different MU types; this distribution is not linear, but exponential; (3) S MUs can be distinguished from fast ones on the basis of some twitch parameters (contraction and half-relaxation times, velocity of contraction), but the same cannot be applied for FF and FR MUs; (4) the analysis of the normalized twitches reveals the differences in shapes for different types of MUs, which shows that twitches of different MUs cannot be obtained from one standard pattern scaled in time and force. These results may have functional implications for studying effectiveness of twitch summation during tetanic contractions and the work performed by various types of MUs.

The avian cardiovascular system is of special interest because avian hearts are relatively larger than mammalian hearts, and activation of ventricular myocardium in birds has a "flash" pattern. Systolic time intervals and indices of myocardial contractility were examined in anaesthetized open-chest chickens by polycardiography, including synchronous recordings of electrocardiogram, phonocardiogram, and apex cardiogram. The asynchronous contraction time, isometric contraction time, pre-ejection period and ejection time were 26 +/- 3 (Mean +/- SD), 21 +/- 9, 47 +/- 12, and 83 +/- 23 ms, respectively, for heart rates of 260 +/- 57 bpm. The myocardial tension index, isometric contraction index and the pre-ejection period/ejection time ratio were 0.39 +/- 0.11, 0.42 +/- 0.10, and 0.54 +/- 0.14, respectively. A "flash" pattern of ventricular myocardial depolarization causes more rapid excitation and as a consequence shorter asynchronous contraction time of relatively larger chicken hearts compared with rabbit hearts. Inverse relation (P0.05) of the asynchronous contraction time to the heart rate in chickens is probably associated with the specific activation pattern of avian ventricles. Establishment of the values of systolic time intervals will facilitate a better understanding of cardiac function in birds. The obtained results are discussed in comparison with the rabbit. The indices calculated from the systolic time intervals show disadvantageous contractile function of chicken heart compared to rabbit heart.

The variability of fiber type distribution in nine limb muscles was examined with histochemical and tensiomyographical (TMG) methods in two groups of 15 men aged between 17 and 40 years. The aim of this study was to determine the extent to which the relative occurrence of different fiber types and subtypes varies within human limb muscles in function to depth and to predict fiber type proportions with a non-invasive TMG method. The distribution of different fiber types varied within the muscles, as a function of depth, with a predominance of type 2b fibers at the surface and type 1 fibers in deeper regions of the muscle. For all the analyzed muscles the contraction times measured at stimulus intensity 10% of supramaximal stimulus (10% MS) were significantly (p

Cohen, L., Contraction of Pulses, Digital Signal Processing12 (2002) 405–415 We give an explicit criterion for when a pulse will first contract before eventually spreading. The contraction time is explicitly given and it is shown that the important quantity is the initial correlation between position and group velocity. A simple physical model is presented that explains why sometimes pulses contract before eventually expanding.

Objectives : The aim of the study was to investigate the effects of different loads on tremor around 10 Hz during fatiguing contractions. Methods : Eighteen healthy volunteers performed sustained isometric knee extensions at 30%, 50% and 70% maximum voluntary contraction (MVC). During the fatiguing contractions, mechanical recordings were made with a high-resolution force sensor. Tremor-power was calculated for the 6–20 Hz frequency window as a function of time normalized to endurance time. Results : Initial tremor power was different between the high and low load tasks. Changes of tremor with contraction time differed between the three tasks, in that tremor of the 30% MVC contraction showed the least decrease throughout the sustained contraction, whilst that of the 50% and 70% MVC showed progressively higher decreases. At failure, all 3 contractions merged to the same tremor level. Conclusion : Load-dependent, fatigue-related 6–20 Hz tremor changes during sustained submaximum voluntary contractions seem mainly the consequence of recruitment of new units and fatigue-related properties of the high threshold motor units of muscles.

We recorded twitches of single motor units (MUs) in the human first dorsal interosseus muscle using either spike-triggered averaging (STA; 236 MUs in 12 normal subjects) or low-rate intramuscular microstimulation of motor axons (IMS; 200 MUs in 20 normal subjects). We analysed twitch force (TF), maximal rate of rise of force (MRRF), contraction time (CT) and half-relaxation time (HRT). MRRF, CT and HRT were significantly smaller with STA than with IMS whereas TFs were fairly similar. Higher stimulus rates (up to 14 Hz) in IMS resembling the voluntary MU firing rates in STA were associated with a decrease of all twitch parameters because of partial fusion of the twitches (20 MUs). Concerning MRRF, CT and HRT, the reduced values matched those obtained by STA, suggesting that the underestimation of these parameters in STA can be mainly attributed to partial fusion. The reduction of TF with high rate IMS but not with STA reveals that other factors such as MU synchronization and non-linear force summation of MU contractions must counteract the effects of partial fusion in STA. We conclude that both STA and IMS are appropriate for assessing TFs in man while the time-dependent parameters MRRF, CT and HRT will be underestimated with STA.

Is there a static load limit which is low enough to allow unlimited duration of contraction? This question was addressed by measuring changes of surface EMG of the biceps brachii muscle during sustained static contractions of 1 hr duration at load levels of 4%, 8% and 15% maximum voluntary contraction force (MVC). Experimental findings show that (a) integrated EMG (iEMG) increased whereas muscular action potential conduction velocity (CV) and mean power frequency of the EMG signal (MPF) decreased, (b) the changes were significant even at the lowest contraction force, (c) at all force levels the effects increased nearly linearly over the contraction time, and (d) the time course of CV changes differed from that of iEMG and MPF. The results suggest that homeostasis in the working muscle was not preserved even at very low levels of load. Since homeostasis is a prerequisite for continuous activity, it may be concluded that a load level which allows an unlimited duration of contraction cannot be defined.

Intramuscular microstimulation of motor axons was used to study twitch responses of 209 motor units (MUs) in the first dorsal interosseus muscle (FDI) of 20 normal subjects. Twitch peak force (TF), maximum rate of rise of force (MRRF), contraction time (CT) and one-half relaxation time (HRT) were determined. The distributions of TF (mean 16.0 mN, median 10.3 mN) and MRRF (mean 0.88 N s-1, median 0.66 N s-1) were skewed to the right with the majority of the values lying in the lower ranges, whereas CT (mean 63 ms, median 62 ms) and HRT (mean 61 ms, median 58 ms) were approximately normally distributed. TF was significantly correlated with MRRF, but not with CT in contrast with studies of cat gastrocnemius muscle. TF values were similar to those obtained by spike-triggered averaging in the same muscle. The method proved to be reliable and appropriate for use in patients. Examples of MU twitch parameters from three patients with chronic partial denervation of the FDI are described.

It has been suggested that protrusion of the mandible results in an alteration of the functional activity of the lateral pterygoid muscle. If this is true, however, it is unclear whether this altered muscle function is a transient phenomenon with no long-term effect or whether it results in structural and functional adaptation of the involved musculature. The purpose of this study was to determine whether or not physiologic and metabolic changes take place within two jaw-protruder muscles--the lateral pterygoid muscle and the superficial masseter muscle--in rats after treatment with a protrusive appliance. Thirty 45-day-old male Sprague-Dawley rats were divided equally into experimental and control groups. The experimental animals wore bonded protrusive-type appliances for 2 weeks. Histochemical analysis of muscle fiber types and in vivo whole-muscle contractile-property analysis were used to evaluate structural and functional muscle adaptations. Mandibular length was slightly but significantly greater in the experimental group, indicating that the protrusive appliance had the expected positive effect on mandibular growth. Histochemically, the lateral pterygoid muscle in the experimental group exhibited a significantly greater area occupied by type I fibers at the expense of type IIb fibers. The superficial masseter muscle exhibited a significantly greater percentage of areas for both type IIa and type IIb fibers in the experimental group. Contraction time (TPT) increased in both muscles; that is, the muscles became slower. The histochemical and contractile-properties data indicate that the protrusive appliance caused the lateral pterygoid muscle to become more active with respect to tonic (postural) activity, whereas the superficial masseter muscle became more active phasically.

collecting EG conducted PNF based training in the dominant lower limb to 15 sessions in 5 weeks. Before the training sessions, new MVC and RMS data were collected of all samples. Results:RegardingMVCmuscular strength (Smax), there was an increase in both groups, significant in CG (p< 0.01) and EG (p< 0.05) for RMS and MVC time (MVC t), there wasn’t significant increase in EG, but the interaction Vxt was significantly increased for this group. The results corroborate the literature by showing that muscles with a predominance of resistant fibers (fiber I/IIA) have greater contraction time with more electrical activation and that the PNF is able to type IIB fibers II A. Conclusion(s): The training sample was efficient to increase muscle strength and EMG data presented show strong evidence of the conversion of muscle fibers trained. Implications:This research shows that new low-cost tools for implementation and little risk to health are available for the physiotherapist increase their options for functional evaluation.

The effects of temperature and repetitive stimulation on isometric twitch contractions of normal, self-innervated and cross-innervated fast extensor digitorum longus and slow soleus muscles of the rat were studied in situ. Normal and self-innervated extensor digitorum longus showed a 2-fold increase in twitch tension following either a train of 200 stimuli at 35 C (post-tetanic potentiation) or lowering of temperature from 35 C to 20 C (cooling potentiation), while under these conditions the twitch tensions of normal and self-innervated slow soleus fell by about 15%. Post-tetanic and cooling potentiation were virtually abolished in cross-innervated extensor digitorum longus but appeared in cross-innervated soleus. The degree of potentiation in these and control muscles was inversely proportional to contraction time. These experiments suggest that mammalian motor nerves exert a controlling influence on the degree of activation of skeletal muscle during a twitch. An hypothesis of the molecular mechanism of post-titanic and cooling potentiation is proposed. According to this hypothesis, neural regulation of muscle activation results from the transformation of myosin after nerve cross-union.

Summary A hypothesis about the origin of the physiological tremor on the basis of other hypotheses is presented. In order to find the muscle fiber candidates that produce the tremor. the muscle fibers and their motoneurons are classified into three groups: The silent fibers (1). the fibers that fire with their initial firing rate (2) and those firing with a high frequency (3). This classification is dynamical. A given muscle fiber can belong to anyone of the three groups. It depends on the threshold of the fiber and the actual excitation level of the motoneuron pool. to which group a fiber belongs. The group two fibers are responsible for, the genesis of the tremor. The reasons for this assumption are: 1. The twitches of the muscle fibers of group two are the least fused. 2. The motoneurons of this group have an augmented tendency to synchronisation. 3. The motor units of group two are the strongest (active) units. Therefore they are overrepresented in the total muscle force.

To determine the effect of a fixed chronic muscle length on maintaining muscle mass and selected physiologic responses in the characterization of the immobilization model, rat hind limbs were bilaterally fixed at various joint angles for 4 weeks. The slow-twitch soleus (Sol) and fast-twitch medial gastrocnemius (MG) ankle extensors and fast-twitch flexor tibialis anterior (TA) were studied after being immobilized under extreme lengthening, shortening, or at an intermediate (neutral) length. A fourth group of animals was sham treated. Contraction time (CT) and half-relaxation time ( 1 2 RT ) were shorter than normal for the Sol fixed in the neutral position. No other position induced a change in the CT or 1 2 RT for the Sol, nor was any change at any position seen in these parameters for the MG or TA muscles. The maximal intrinsic velocity of shortening (normalized to sarcomere length) was not affected in any muscle. Twitch (P t ) and tetanic (P o ) tensions were reduced more in the MG and Sol muscles fixed in shortened positions than in the lengthened or neutral muscles. The P o of lengthened and neutrally fixed Sol and MG muscles were reduced to similar extents compared with control muscles. Both P t and P o of the TA for all positions of immobilization were similar. Subtetanic tension expressed as percent of P o was enhanced relative to control in the MG and TA, but not in the Sol. These data demonstrate that regardless of the position of joint fixation, the capacity of slow- or fast-twitch muscle to produce maximal tetanic tension was significantly impaired with chronic immobilization. Force expressed per whole muscle cross-sectional area of the Sol and MG was not changed in either the lengthened or neutral condition. In a chronically shortened position, the tension-producing capacities of these muscles was significantly impaired. The changes in isometric speed-related properties observed in the slow Sol immobilized in a neutral position did not occur when this muscle was immobilized at an extreme length. These data demonstrate that the length at which muscles are immobilized can affect the specific physiologic adaptation both quantitatively and qualitatively.

Single muscle units of the retractor bulbi and lateral rectus muscles were activated with brief (0.5-ms) current pulses delivered through an intracellular micropipet penetrating single motoneuron cell bodies or axons in the principal abducens nucleus of the cat. Muscle unit mechanical characteristics were measured. A total of 40 retractor bulbi muscle units was studied. Thirty muscle units were contained in 2, 3, or all 4 retractor bulbi muscle slips. Three muscle units involved one retractor bulbi muscle slip and seven units were contained in the lateral rectus muscle plus one or both of the lateral retractor bulbi muscle slips. The largest percentage of muscle units (32.5%) was contained in the two inferior retractor bulbi muscle slips. The 33 retractor bulbi muscle units, having no muscle fibers in the lateral rectus muscle, caused the contraction of 76 retractor bulbi muscle slips. Twitch tension was measured in 58 slips and maximum tetanic tension in 27 slips. The average twitch tension per measured slip was 68.0 mg and the average maximum tetanic tension was 459.8 mg. The average retractor bulbi muscle unit contraction time was 10 ms and the lateral rectus average contraction time was 7.9 ms. We propose that peripheral branching of abducens nerve axons can account for the division of muscle unit components among the retractor bulbi muscle slips and between the lateral rectus and retractor bulbi muscles.

1. 1. Myocardial contractility and electrical activities were studied in strips of ventricular muscle of the marine toad, Bufo marinus . 2. 2. The responses of the heart exhibited significant seasonal variations. 3. 3. Contractile tension and the velocity of contraction and of relaxation were lowest in December, increased gradually starting in February, and reached a maximum value in the period July through September. 4. 4. The contraction-relaxation cycle also was altered in the cold season. However, the parameters of relaxation (time and velocity) appeared to be more affected than the parameters of contraction. 5. 5. Resting potential, duration of the action potential and the amplitude of the overshoot were different in summer and winter. 6. 6. During December, an increase in the calcium concentration in the tissue bath to twice the normal concentration was without any effect on the membrane responses or tension development although the velocity of contraction increased. As spring approached, addition of calcium became increasingly effective in augmenting the tension and velocity of contraction. 7. 7. High calcium concentrations invariably shortened the contraction time while relaxation time was prolonged. The velocity of relaxation was unaffected by high calcium concentration. 8. 8. High calcium concentrations reduced the duration of the action potential in summer but were without effect in winter. 9. 9. Epinephrine, in contrast, consistently increased the contractile tension and the velocities of contraction and of relaxation without significantly changing the contraction-relaxation cycle time. 10. 10. It is suggested that these changes reflect seasonal alterations in a calcium modulated component of the myocardium.

Fetal systolic time intervals (pre-ejection period, iso-volumetric contraction time and ventricular ejection time) can be measured using Doppler ultrasound to detect valve movements and the fetal ECG to indicate the onset of ventricular depolarisation. Continuous recordings of FSTI, fetal heart rate and uterine activity depict changes reflecting progressive cord compression and have the potential for providing improved differentiation of late and variable decelerations. However, the continual adjustment of the ultrasound transducer required to maintain a good signal limits the technique. Recent improvements in phono indicate that this may provide a useful alternative, provided that suitable signal-processing is developed.

This paper describes a project conducted at Queen Alexandra Hospital, Cosham, which investigated the use of a microcomputer to provide visual feedback during an occupational therapy session. The results achieved were very encouraging. Significant evidence (at the 2 1 2 % level) was found in support of Hypothesis I which suggested that change in reaction time due to treatment of the affected lower limb of a patient in the mid stroke category is different to that of a patient in the late stroke category. Hypothesis II was also supported (at the 2 1 2 % level) while Hypothesis III was refuted (at the 5% level), and suggested that: there would be a significant improvement in the contraction time (II) or relaxation time (III) of patients after treatment. In general terms, computer assisted visual feedback does appear to offer a number of advantages; in particular, the study carried out suggests a clear role for this approach as a prognostic tool in adult hemiplegia.

Compensatory hypertrophy of the rat soleus was compared between normal rats (N) and rats bearing a non-metastasizing mammary tumor (TB). Two weeks after the tumor inoculation, introduced subcutaneously on the midline of the upper back, the two groups of rats underwent unilateral tenotomy of the plantaris and gastrocnemius muscles to induce functional overload or compensatory hypertrophy of the soleus. One week later, the body weight (BW), wet muscle weights (MW), percent of Type I (slow-twitch) and Type II (fast-twitch) muscle fibers and contractile parameters of the isometric twitch (Pt) and tetanic (Po) tensions were evaluated. The TB animals did not show any signs of cancer cachexia. The sham-operated control soleus muscles of the two groups were similar in wet MW, maximum isometric Pt and Po. The latent period (LP), the contraction time (CT), the half relaxation time (HRT) and the number of Type I (slow-twitch) fibers were increased significantly. The hypertrophied muscles in the N and TB rats showed significant declines in the maximum isometric Pt, compared to their respective controls. These data suggest that the compensatory hypertrophy of muscle is expressed similarly in both the normal and tumor-bearing rats but that it interferes with the generation of isometric twitch tension in the muscle.

In order to compare the regenerative ability of skeletal muscle between young (5 month) and old (26 month) rats, sliced or intact extensor digitorum longus muscles were freely autografted into young and old rats and also reciprocally grafted from young to old inbred animals and vice versa. Sixty days after grafting, the transplants were analyzed for contractile and histochemical properties. There was a relative similarity between the contraction times of both normal control muscles and of all groups of transplants, although the contraction time tended to be prolonged and histochemical fiber pattern was more often found to be uniform in grafts of senescent animals. All groups of transplants possessed histochemically heterogeneous fiber types at 60 days. The experiments demonstrate that skeletal muscle in old rats possesses a substantial degree of regenerative ability and that the free tranpllantation of entire muscles in old animals is feasible.

Soleus motor units of 5 and 34 week-old rats, weighing 150 and 520 g respectively, were examined for contraction time at 36°C, innervation ratio as well as cross-sectional area, ATPase and SDH activity of fibres. The muscle was continuously adjusted to the growth of the animal. Between 5 and 34 weeks the proportion of Type II fibre units with 15–26 msec contraction time decreased from 33 to 10% and Type I fibre units with 27–40 msec contraction time thus increased from 67 to 90%. This was parallelled by approximately the same relative changes in number of Type II and Type I fibres. The average innervation ratio of Type I fibre units appeared unchanged between 5 and 34 weeks, 83 ± 16, resp. 83 ± 17 per unit (N = 18) and about 99% of the fibres of the units were histochemically uniform. Moreover, at 5 weeks there occurred continuous transitions between Type II and Type I fibre units as regards all tested markers including contraction time, intensity of alkali-, formaldehydeand acid-stable ATPase, SDH activity and fibre area. It is concluded that the increase in Type I fibre units with growth results from transformation in toto of Type II fibre units to Type I owing to changing properties of their motoneurones from phasic to tonic. Thus one and the same motoneurone determines not only the histochemical and related functional characteristics of its muscle fibres at a specific level but may also alter them qualitatively. With increasing contraction time of units transformation of fibres was manifested first by an increase in intensity of acid-stable ATPase, followed by a decrease in formaldehyde-stable and later by a decrease in alkali-stable ATPase. This led to the following sequence of relative intensities of ATPase at pH 9.4 and 4.35, respectively: dark-light (Type II A) → dark intermediate (Type II C) → (dark-dark) → intermediate-dark → light-dark (Type I). Fibres under transformation (ATPase intermediate fibres) were retarded in growth and showed within the unit variations in intensity of ATPase reactions which to some extent were topographically orientated. Transformation from Type II to Type I was complemented at a slower rate by a decrease in SDH activity, which is opposite to that in larger mammals indicating that myofibrillar ATPase and mitochondria-bound oxidative enzymes are independently controlled by the motoneurone.

SUMMARY Twitch tension, train-of-four and tetanus—clinical parameters of neuromuscular transmission— were studied in 37 infants and children anaesthetized with halothane. The contraction time was slower in infants less than 1 month old when compared with their elder counterparts. This may indicate a possible transformation from slow to faster muscle types. The train-of-four reached 100% in most of the infants older than 2 months. The post-tetanic facilitation and the tetanus twitch ratios were significantly greater in infants older than 2 months when compared with infants less than 1 month. Post-tetamc exhaustion was seen in all the infants and children after stimulation at 100 Hz. Tetanus : twitch ratios were greater in infants more than 2 months old and in children, and the tetanic force was more marked at 50 Hz than at 20 Hz. The data indicate that maturation of neuromuscular transmission occurs in infants in and around the first 2 months of age.

The contractile properties of the rat masseter muscle were studied by direct stimulation of the muscle while in situ in the anaesthetized rat. The muscle was found to be a fast muscle with a contraction time of approximately 50 msec. Variation of the position of the lower jaw showed that the twitch tension, produced by discrete single stimuli, was greatest when the mouth was opened by about 8 mm from occlusion. This finding differs from that for voluntary biting power in the human subject.

Tibialis anterior muscles were transplanted between 12-week-old normal and dystrophic mice with intact or polydimethyl silicone-capped peroneal nerve. After 150 days the transplants were removed and their isometric twitch contraction properties were studied in vitro at 20 C. Intact normal and dystrophic muscles of equivalent age were used as controls. Dystrophic muscles developed lower twitch and tetanus tension than normal muscles and showed prolonged half relaxation time. The contraction time and twitch/tetanus ratio of both types of muscle were similar. Of all transplantations performed, only those in normal mice with intact nerve responded upon stimulation. Both normal and dystrophic transplants in normal hosts showed similar isometric properties. Although intact dystrophic muscles and viable dystrophic transplants in normal hosts were similar in weight, the transplants developed about three to four times more tension. In addition, dystrophic transplants showed relaxation times similar to normal muscles. It is suggested that the dystrophic lesion in mice may have a neural origin.

It is clear that there are events which occur in response to neuromuscular activity that are essential to maintain normal muscle properties. Two factors thought to influence muscle contractile properties are tension and electrical activity. To study these variables the hind limbs of 10 postpubertal female Sprague-Dawley rats were prevented from supporting the weight of the body for 4 weeks, then in situ isometric properties were determined and compared with 10 age-matched control rats. The soleus (slow plantarflexor), the medial gastrocnemius (fast plantarflexor), and the tibialis anterior (fast dorsiflexor) were studied. The suspended soleus wet weight was 42% lower and the maximum isometric twitch (Pt) and tetanic (P0) tensions were 62 and 69% lower respectively, than in the control rats. The suspended medial gastrocnemius was 18% lower in wet weight and 14 and 9% lower in P0 and Pt than the controls. No differences in wet weight, P0 or Pt were observed in tibialis anterior. Mean contraction time was 22% shorter in the soleus, 9% in the medial gastrocnemius, and unchanged in the tibialis anterior following suspension. Further, the percent of P0 attained during a 330-ms tetanus at 20 Hz was 15% lower in the suspended soleus and unchanged in the other two muscles. The fatigue index (ratio of tension after 2 min of stimulation at 40 Hz for 330 ms once per second to the maximum tension developed during the test) was unaffected by suspension in the soleus and tibialis anterior but was reduced from 49% to 36% in the medial gastrocnemius. The maximum rate of shortening of all three muscles was unaffected by suspension. These results indicate that suspension of the hind limbs selectively affects the mass and force-generating capabilities of the plantarflexors, particularly the predominantly slow soleus. In contrast, the fatigability of only the fast plantarflexor was increased whereas the slow plantarflexor was unaffected. These results, considered in light of collaborative studies, suggest that the chronic force-time levels in a muscle have an important influence on tension- and speed-related properties but not necessarily the fatigability of that muscle. Further, a marked difference in the sensitivity of the contractile elements of slow and fast muscle to this influence was evident.

In 16 rats, the extensor digitorum longus (EDL) — a fast muscle — was grafted, complete with its innervation, into the bed of the (removed) soleus — a slow muscle. Following the transposition, performed on one side only, EDL recovered and showed the contraction time of the contralateral EDL that served as control. However, as demonstrated histochemically, the transposed EDL exhibited a different succinic dehydrogenase activity: it acquired the muscle fiber pattern of the slow red soleus muscle. This method of transposition of muscle plus nerve allows the study of nerve-muscle relationships with respect to a multiple regulation of muscle properties, including the decisive role of neuronal specificity and other mechanisms such as substrate (oxygen) supply.

Contraction time of fast and slow muscles is rather constant in rat up to 2 years after birth but increases in both muscles between the second and third year of life. This corresponds to a decrease of myofibrillar and myosin ATPase activity observed in muscles of 3-year-old rats.

The functional properties of 30 soleus and 46 anterior tibial motor units in the cat were investigated by isolating and stimulating filaments of ventral rootlets containing one functional axon. The soleus data support previous studies which indicated slow conducting axons and slow contracting motor units for that muscle. Force output was greater than reported by others but consistent with the weight-beraing role of soleus in posture and locomotion. Anterior tibial units were supplied by fast conducting axons. Their contraction times were short, but average force output was similar to that of the soleus units. At low rates of stimulation soleus units developed substantially more tension than anterior tibial units. This finding provides additional evidence that tone can be developed and maintained in postural muscles by the asynchronous activation of slowly firing motor units. Within the populations of soleus or anterior tibial motor units there was no clear relation between axonal conduction velocity, contraction time or peak tetanic tension. A relationship among these parameters has been generally assumed for large as well as small muscles of the cat hind limb. The physiological properties of motor units reflect the individuality of muscle structure and function. The concept of motor unit types has been a useful one, yet the contractile properties of any one type may vary from muscle to muscle.

the speed of contraction of mammalian skeletal muscle is determined, at least in part, by the peripheral nerve. At birth all muscles contract slowly, but shortly thereafter the contraction time of fast muscle shortens. Cross-union of nerves to fast and slow muscle tends to prolong contraction time of the former and shorten that of the latter. The present experiment was performed to determine whether the innervation is responsible for chemical as well as biophysical differences between red and white muscle. Supernatants from aqueous homogenates of rat muscle were subjected to acrylamide gel disc electrophoresis and the protein patterns visualized with amido black dye. Consistent differences were observed between the soleus (a slow, red muscle) and the plantaris (a fast, white one). Denervation of the plantaris caused its protein pattern to gradually resemble that of the soleus; reinnervation restored the normal pattern. Denervation or reinnervation of the soleus (by crushing the soleus nerve) was without effect on its protein patterns. But when the denervated soleus was reinnervated by nerve fibers that predominantly supply white muscles (by transecting and resuturing the sciatic nerve) the soleus pattern came to resemble that of the plantaris. These results are consistent with observations on speed of muscle contraction and indicate that the nerve is responsible for some of the differences between protein constituents of slow and fast muscle.

To test functional properties of regenerating skeletal muscle, the triceps surae muscles of 1-month-old rats were minced into 1-mm3 pieces and replaced. Myogenic cells originating from the degenerating implanted muscle fragments form a model of the original muscle and contractile activity is first detected after 7–8 days. Early regenerates exhibited contractile properties of very slow muscles. Contraction time speeded up until by 30–40 days it approached those of the normal fast muscles. The development of contractile function closely resembles that of normal muscles during the first postnatal weeks and it thus appears that the regenerating muscle undergoes a functional “developmental recapitulation.” It is concluded that, despite a quantitative deficiency in the mass of regenerating muscle, those fibers which are present rapidly approach the normal condition in functional as well as morphological characteristics.