Post-activation potentiation after isometric contractions is strongly related to contraction intensity despite the similar torque-time integral

Post-activation potentiation (PAP) is defined as an enhanced contractile response of a muscle following its own contractile activity and is influenced by the intensity and duration of the conditioning contraction. The aim of this study was to determine if the combination of intensity and duration, that is, torque-time integral (TTI) is a determinant of PAP amplitude. We compared PAP amplitude following low-to-maximal voluntary conditioning contraction intensities with and without similar TTI in the knee extensors. Twelve healthy males completed two experimental sessions. Femoral nerve stimulation was applied to evoke single twitches on the relaxed quadriceps before and after isometric conditioning contractions of knee extensors. In one session, participants performed conditioning contractions without similar TTI (6 s at 100, 80, 60, 40 and 20% maximal voluntary contraction (MVC)), while they performed conditioning contractions with similar TTI in the other session (6 s at 100%, 7.5 s at 80%, 10 s at 60%, 15 s at 40%, and 30 s at 20% MVC). In both sessions, PAP amplitude was related to conditioning contraction intensity. The higher the conditioning contraction intensity with or without similar TTI, the higher PAP. Significant correlations were found (i) between PAP and conditioning contraction intensity with (r2 = 0.70; P < 0.001) or without similar TTI (r2 = 0.64; P < 0.001), and (ii) between PAP with and without similar TTI (r2 = 0.82; P < 0.001). The results provide evidence that TTI has a minor influence on PAP in the knee extensors. This suggests that to optimize the effect of PAP, it is more relevant to control the intensity of the contraction rather than the TTI.

Twitch Data in Health Promotion Research: Protocol for a Case Study Exploring COVID-19 Vaccination Views Among Young People

BACKGROUND

Social media platforms have emerged as a useful channel for health promotion communication, offering different channels to reach targeted populations. For example, social media has recently been used to disseminate information about COVID-19 vaccination across various demographics. Traditional modes of health communication such as television, health events, and newsletters may not reach all groups within a community. Health communications for younger generations are increasingly disseminated through social media to reflect key information sources. This paper explores a social media gaming platform as an alternative way to reach young people in health promotion research.

OBJECTIVE

This protocol study aimed to pilot-test the potential of Twitch, a live streaming platform initially designed for video gaming, to conduct health promotion research with young people. We used COVID-19 vaccination as a topical case study that was recommended by Australian health authorities at the time of the research.

METHODS

The research team worked with a Twitch Account Manager to design and test a case study within the guidelines and ethics protocols required by Twitch, identify suitable streamers to approach and establish a protocol for conducting research on the platform. This involved conducting a poll to initiate discussion about COVID-19 vaccination, monitoring the chat in 3 live Twitch sessions with 2 streamers to pilot the protocol, and briefly analyze Twitch chat logs to observe the range of response types that may be acquired from this methodology.

RESULTS

The Twitch streams provided logs and videos on demand that were derived from the live session. These included demographics of viewers, chat logs, and polling results. The results of the poll showed a range of engagement in health promotion for the case study topic: the majority of participants had received their vaccination by the time of the poll; however, there was still a proportion that had not received their vaccination yet or had decided to not be vaccinated. Analysis of the Twitch chat logs demonstrated a range of both positive and negative themes regarding health promotion for the case study topic. This included irrelevant comments, misinformation (compared to health authority information at the time of this study), comedic and conspiracy responses, as well as vaccine status, provaccine comments, and vaccine-hesitant comments.

CONCLUSIONS

This study developed and tested a protocol for using Twitch data for health promotion research with young people. With live polling, open text discussion between participants and immediate responses to questions, Twitch can be used to collect both quantitative and qualitative research data from demographics that use social media. The platform also presents some challenges when engaging with independent streamers and sensitive health topics. This study provides an initial protocol for future researchers to use and build on.

INTERNATIONAL REGISTERED REPORT IDENTIFIER (IRRID)

RR1-10.2196/48641.

Effect of 40 Hz Magnetic Field Application in Posttraumatic Muscular Atrophy Development on Muscle Mass and Contractions in Rats

Muscle atrophy refers to the deterioration of muscle tissue due to a long-term decrease in muscle function. In the present study, we simulated rectus femoris muscle atrophy experimentally and investigated the effect of pulsed electromagnetic field (PEMF) application on the atrophy development through muscle mass, maximal contraction force, and contraction-relaxation time. A quadriceps tendon rupture with a total tenotomy was created on the rats' hind limbs, inhibiting knee extension for 6 weeks, and this restriction of the movement led to the development of disuse atrophy, while the control group underwent no surgery. The operated and control groups were divided into subgroups according to PEMF application (1.5 mT for 45 days) or no PEMF. All groups were sacrificed after 6 weeks and had their entire rectus femoris removed. To measure the contraction force, the muscles were placed in an organ bath connected to a transducer. As a result of the atrophy, muscle mass and strength were reduced in the operated group, while no muscle mass loss was observed in the operated PEMF group. Furthermore, measurements of single, incomplete and full tetanic contraction force and contraction time (CT) did not change significantly in the operated group that received the PEMF application. The PEMF application prevented atrophy resulting from 6 weeks of immobility, according to the contraction parameters. The effects of PEMF on contraction force and CT provide a basis for further studies in which PEMF is investigated as a noninvasive therapy for disuse atrophy development. © 2022 Bioelectromagnetics Society.

Thick filament activation is different in fast- and slow-twitch skeletal muscle

The contractile properties of fast-twitch and slow-twitch skeletal muscles are primarily determined by the myosin isoform content and modulated by a variety of sarcomere proteins. X-ray diffraction studies of regulatory mechanisms in muscle contraction have focused predominately on fast- or mixed-fibre muscle with slow muscle being much less studied. Here, we used time-resolved X-ray diffraction to investigate the dynamic behaviour of the myofilament proteins in relatively pure slow-twitch-fibre rat soleus (SOL) and pure fast-twitch-fibre rat extensor digitorum longus (EDL) muscle during twitch and tetanic contractions at optimal length. During twitch contractions the diffraction signatures indicating a transition in the myosin heads from ordered OFF states, where heads are held close to the thick filament backbone, to disordered ON states, where heads are free to bind to thin filaments, were found in EDL and not in SOL muscle. During tetanic contraction, changes in the disposition of myosin heads as active tension develops is a quasi-stepwise process in EDL muscle whereas in SOL muscle this relationship appears to be linear. The observed reduced extensibility of the thick filaments in SOL muscle as compared to EDL muscles indicates a molecular basis for this behaviour. These data indicate that for the EDL, thick filament activation is a cooperative strain-induced mechano-sensing mechanism, whereas for the SOL, thick filament activation has a more graded response. These different approaches to thick filament regulation in fast- and slow-twitch muscles may be adaptations for short-duration, strong contractions versus sustained, finely controlled contractions, respectively. KEY POINTS: Fast-twitch muscle and slow-twitch muscle are optimized for strong, short-duration contractions and for tonic postural activity, respectively. Structural events (OFF to ON transitions) in the myosin-containing thick filaments in fast muscle help determine the timing and strength of contractions, but these have not been studied in slow-twitch muscle. The X-ray diffraction signatures of structural OFF to ON transitions are different in fast extensor digitorum longus (EDL) and slow soleus (SOL) muscle, being completely absent during twitches in soleus muscle and blunted during tetanic contractions SOL as compared to EDL Quasi-stepwise thick filament structural OFF to ON transitions in fast twitch muscle may be an adaptation for rapid, ballistic movements, whereas more graded OFF to ON structural transitions in slow-twitch muscle may be an adaptation for slower, finer motions.

Neuromuscular Adaptations after an Altitude Training Camp in Elite Judo Athletes

The aim of this study was to investigate neuromuscular adaptations in elite judo athletes after three weeks of power-oriented strength training at terrestrial altitude (2320 m). Nineteen men were assigned to altitude training (AL) (22.1 ± 2.3 years) and sea level training (SL) (22.6 ± 4.1 years). Neuromuscular assessment consisted of: (1) maximal isometric knee extensor (KE) torque, (2) KE rate of torque development (RTD), (3) quadriceps activity and voluntary activation, (4) soleus H-reflex, (5) quadriceps single (T_TW) and double twitch torque (T_DB100) and contraction time (CT_TW). There were no significant differences between groups at baseline for any of the observed parameters. Significant differences were found between groups in terms of change in RTD (p = 0.04). Cohen’s d showed a positive significant effect (0.43) in the SL group and a negative significant effect (−0.58) in the AL group. The difference between groups in changes in CT_TW as a function of altitude was on the edge of significance (p = 0.077). CT_TW increased by 8.1 ± 9.0% in the AL group (p = 0.036) and remained statistically unchanged in the SL group. Only the AL group showed a relationship between changes in T_TW and T_DB100 and changes in RTD at posttest (p = 0.022 and p = 0.016, respectively). Altitude induced differences in muscular adaptations likely due to greater peripheral fatigue.

Spatial Proteomic Approach to Characterize Skeletal Muscle Myofibers

Skeletal muscle myofibers have differential protein expression resulting in functionally distinct slow- and fast-twitch types. While certain protein classes are well-characterized, the depth of all proteins involved in this process is unknown. We utilized the Human Protein Atlas (HPA) and the HPASubC tool to classify mosaic expression patterns of staining across 49,600 unique tissue microarray (TMA) images using a visual proteomic approach. We identified 2164 proteins with potential mosaic expression, of which 1605 were categorized as "likely" or "real." This list included both well-known fiber-type-specific and novel proteins. A comparison of the 1605 mosaic proteins with a mass spectrometry (MS)-derived proteomic dataset of single human muscle fibers led to the assignment of 111 proteins to fiber types. We additionally used a multiplexed immunohistochemistry approach, a multiplexed RNA-ISH approach, and STRING v11 to further assign or suggest fiber types of newly characterized mosaic proteins. This visual proteomic analysis of mature skeletal muscle myofibers greatly expands the known repertoire of twitch-type-specific proteins.

Effect of paired-pulse stimulus parameters on the two phases of short interval intracortical inhibition in the quadriceps muscle group

BACKGROUND

Short interval intracortical inhibition (SICI) is commonly used to assess inhibition in the motor cortex and is known to be affected by the paired-pulse stimulus parameters (i.e., interstimulus interval [ISI], conditioning stimulus intensity [CSI] and test stimulus intensity [TSI]) used during testing. While the effects of stimulus parameters are well-studied in the upper-extremity, evidence in the lower-extremity is lacking.

OBJECTIVE

To comprehensively examine the effects of alterations in paired-pulse stimulus parameters on the two phases of SICI in the quadriceps muscle group.

METHODS

Seventeen adults (8 males, 9 females) volunteered to participate in this study. SICI was examined over a range of CSIs (70-90% active motor threshold [AMT]), TSIs (100-140% AMT), and ISIs (1.0-3.0 ms) using both EMG and torque responses elicited by transcranial magnetic stimulation (TMS).

RESULTS

The results indicated that SICI at 1.0 ms ISI was best revealed with a CSI of 70% and TSI ≥110% AMT, whereas SICI at 2.5 ms ISI was best revealed with a CSI of 80-90% and a TSI of ≥130% AMT. Unlike upper-extremity muscles, evaluating SICI with a CSI of 70% AMT and an ISI of 1.0 ms produced the greatest inhibition for all TSIs. In general, inhibitory effects were contaminated by facilitatory effects when using a TSI of 100% AMT.

CONCLUSIONS

The amount of inhibition was dependent on the stimulation parameters used during testing. A CSI of 70% AMT, ISI of 1.0 ms, and TSI of ≥110% AMT appear to be optimal for measuring SICI in the quadriceps muscle; however, other parameters can be used if careful consideration is given to the described interaction between the parameters.

Finger Twitches are More Frequent in REM Sleep Than in Non-REM Sleep

INTRODUCTION

Abnormal rapid eye movement (REM) sleep is often symptomatic of chronic disorders, however polysomnography, the gold standard method to measure REM sleep, is expensive and often impractical. Attempts to develop cost-effective ambulatory systems to measure REM sleep have had limited success. As elevated twitching is often observed during REM sleep in some distal muscles, the aim of this study was to assess the potential for a finger-mounted device to measure finger twitches, and thereby differentiate periods of REM and non-REM (NREM) sleep.

METHODS

One night of sleep data was collected by polysomnography from each of 18 (3f, 15m) healthy adults aged 23.2 ± 3.3 (mean ± SD) years. Finger movement was detected using a piezo-electric limb sensor taped to the index finger of each participant. Finger twitch densities were calculated for each stage of sleep.

RESULTS

Finger twitch density was greater in REM than in NREM sleep (p < 0.001). Each sleep stage had a unique finger twitch density, except for REM and stage N1 sleep which were similar. Finger twitch density was greater in late REM than in early REM sleep (p = 0.005), and there was a time-state interaction: the difference between finger twitch densities in REM and NREM sleep was greater in late sleep than in early sleep (p = 0.007).

CONCLUSION

Finger twitching is more frequent in REM sleep than in NREM sleep and becomes more distinguishable as sleep progresses. Finger twitches appear to be too infrequent to make definitive 30-second epoch determinations of sleep stage. However, an algorithm informed by measures of finger twitch density has the potential to detect periods of REM sleep and provide estimates of total REM sleep time and percentage.

Does postactivation potentiation (PAP) increase voluntary performance?

The transient increase in torque of an electrically evoked twitch following a voluntary contraction is called postactivation potentiation (PAP). Phosphorylation of myosin regulatory light chains is the most accepted mechanism explaining the enhanced electrically evoked twitch torque. While many authors attribute voluntary postactivation performance enhancement (PAPE) to the positive effects of PAP, few actually confirmed that contraction was indeed potentiated using electrical stimulation (twitch response) at the time that PAPE was measured. Thus, this review aims to investigate if increases in voluntary performance after a conditioning contraction (CC) are related to the PAP phenomenon. For this, studies that confirmed the presence of PAP through an evoked response after a voluntary CC and concurrently evaluated PAPE were reviewed. Some studies reported increases in PAPE when PAP reaches extremely high values. However, PAPE has also been reported when PAP was not present, and unchanged/diminished performance has been identified when PAP was present. This range of observations demonstrates that mechanisms of PAPE are different from mechanisms of PAP. These mechanisms of PAPE still need to be understood and those studying PAPE should not assume that regulatory light chain phosphorylation is the mechanism for such enhanced voluntary performance. Novelty The occurrence of PAP does not necessarily mean that the voluntary performance will be improved. Improvement in voluntary performance is sometimes observed when the PAP level reaches extremely high values. Other mechanisms may be more relevant than that for PAP in the manifestation of acute increases in performance following a conditioning contraction.

Neuromuscular Properties of the Human Wrist Flexors as a Function of the Wrist Joint Angle

The joint angle dependence of voluntary activation and twitch properties has been investigated for several human skeletal muscles. However, although they play a key role for hand function and possess a unique neural control compared to muscles surrounding other joint complexes, little is known about the wrist flexors innervated by the median nerve. Therefore, isometric voluntary and electrically evoked contractions of the wrist flexors were analyzed at three wrist joint angles (extension: -30°, neutral: 0°, flexion: 30°) to quantify the joint angle dependence of (i) voluntary activation (assessed via peripheral nerve stimulation and electromyography [EMG]), (ii) unpotentiated twitch torques, and (iii) potentiated twitch torques. Maximum voluntary torque was lower in extension compared to neutral and flexion. Although voluntary activation was generally high, data indicate that voluntary activation of the wrist flexors innervated by the median nerve was lower and the antagonist·agonist^-1 EMG ratio was higher with the wrist joint in flexion compared to extension. Peak twitch torque, rate of twitch torque development, and twitch half-relaxation time increased, whereas electromechanical delay decreased from flexion to extension for the unpotentiated twitch torques. Activity-induced potentiation partly abolished these differences and was higher in short than long wrist flexors. Different angle-dependent excitatory and inhibitory inputs to spinal and supraspinal centers might be responsible for the altered activation of the investigated wrist muscles. Potential mechanisms were discussed and might have operated conjointly to increase stiffness of the flexed wrist joint. Differences in twitch torque properties were probably related to angle-dependent alterations in series elastic properties, actin-myosin interaction, Ca²⁺ sensitivity, and phosphorylation of myosin regulatory light chains. The results of the present study provide valuable information about the contribution of neural and muscular properties to changes in strength capabilities of the wrist flexors at different wrist joint angles. These data could help to understand normal wrist function, which is a first step in determining mechanisms underlying musculoskeletal disorders and in giving recommendations for the restoration of musculoskeletal function after traumatic or overuse injuries.

Caffeine attenuates contraction-induced diminutions of the intracellular calcium transient in mouse lumbrical muscle ex vivo

The amount of calcium released from the sarcoplasmic reticulum in skeletal muscle rapidly declines during repeated twitch contractions. In this study, we test the hypothesis that caffeine can mitigate these contraction-induced declines in calcium release. Lumbrical muscles were isolated from male C57BL/6 mice and loaded with the calcium-sensitive indicator, AM-furaptra. Muscles were then stimulated at 8 Hz for 2.0 s in the presence or absence of 0.5 mM caffeine, at either 30 °C or 37 °C. The amplitude and area of the furaptra-based intracellular calcium transients and force produced during twitch contractions were calculated. For each of these measures, the values for twitch 16 relative to twitch 1 were higher in the presence of caffeine than in the absence of caffeine at both temperatures. We conclude that caffeine can attenuate contraction-induced diminutions of calcium release during repeated twitch contractions, thereby contributing to the inotropic effects of caffeine.

Twitches, Blinks, and Fidgets: Important Generators of Ongoing Neural Activity

Animals and humans continuously engage in small, spontaneous motor actions, such as blinking, whisking, and postural adjustments ("fidgeting"). These movements are accompanied by changes in neural activity in sensory and motor regions of the brain. The frequency of these motions varies in time, is affected by sensory stimuli, arousal levels, and pathology. These fidgeting behaviors can be entrained by sensory stimuli. Fidgeting behaviors will cause distributed, bilateral functional activation in the 0.01 to 0.1 Hz frequency range that will show up in functional magnetic resonance imaging and wide-field calcium neuroimaging studies, and will contribute to the observed functional connectivity among brain regions. However, despite the large potential of these behaviors to drive brain-wide activity, these fidget-like behaviors are rarely monitored. We argue that studies of spontaneous and evoked brain dynamics in awake animals and humans should closely monitor these fidgeting behaviors. Differences in these fidgeting behaviors due to arousal or pathology will "contaminate" ongoing neural activity, and lead to apparent differences in functional connectivity. Monitoring and accounting for the brain-wide activations by these behaviors is essential during experiments to differentiate fidget-driven activity from internally driven neural dynamics.

Post Activation Potentiation of the Plantarflexors: Implications of Knee Angle Variations

Flexing the knee to isolate the single joint soleus from the biarticular gastrocnemius is a strategy for investigating individual plantarflexor’s post activation potentiation (PAP). We investigated the implications of testing plantarflexor PAP at different knee angles and provided indirect quantification of the contribution of gastrocnemius potentiation to the overall plantarflexor enhancements post conditioning. Plantarflexor supramaximal twitches were measured in ten male power athletes before and after a maximal isometric plantarflexion (MVIC) at both flexed and extended knee angles. Mean torque and soleus (SOL_RMS) and medial gastrocnemius (MG_RMS) activity were measured during the MVIC. The mean torque and MG_RMS of the MVIC were lower (by 33.9 and 42.4%, respectively) in the flexed compared to the extended position, with no significant difference in SOL_RMS. After the MVIC, twitch peak torque (PT) and the rate of torque development (RTR) potentiated significantly more (by 17.4 and 14.7% respectively) in the extended as compared to the flexed knee position, but only immediately (5 s) after the MVIC. No significant differences were found in the twitch rate of torque development (RTD) potentiation between positions. It was concluded that knee joint configuration should be taken into consideration when comparing studies of plantarflexor PAP. Furthermore, results reflect a rather brief contribution of the gastrocnemius potentiation to the overall plantarflexor twitch enhancements.

Intersession reliability of the interpolated twitch technique applied during isometric, concentric, and eccentric actions of the human knee extensor muscles

INTRODUCTION

Although it has been shown that voluntary activation (%VA) of the knee extensors during isometric contractions can be reliably assessed with the interpolated twitch technique, little is known about the reliability of %VA during concentric and eccentric muscle actions. Therefore, relative and absolute intersession reliability of quadriceps muscle's %VA during different contraction modes was determined.

METHODS

After a familiarization session, 21 participants (17 males, 25 ± 2 yrs) completed two testing sessions. Paired supramaximal electrical stimuli were administered to the femoral nerve during isometric, concentric, eccentric MVCs, and at rest to assess %VA (stimuli were applied at 70° knee flexion).

RESULTS AND DISCUSSION

Data indicate that %VA of the knee extensors can be reliably measured during isometric [intraclass correlation coefficient (ICC) = 0.89, coefficient of variation (CV) = 4.1%], concentric (ICC = 0.87, CV = 6.6%), and eccentric muscle actions (ICC = 0.86, CV = 7.0%). Muscle Nerve 56: 324-327, 2017.

Neuromuscular responses of the plantar flexors to whole-body vibration

Enhanced physical performance following whole-body vibration (WBV) has been attributed to increased muscle activity; however, few studies have measured the mechanisms underlying these changes. The objective of this study was to measure the responsiveness of the Ia pathway as well as contractile properties in 16 young adults (24±2 years, eight men, eight women) following repeated bouts of acute WBV (45 Hz, 2 mm). Hoffman reflexes (H-reflex), compound muscle action potentials (M-wave), and twitch contractile properties were measured prior to and immediately following five 1-minute WBV exposures, and at 3, 5, 10, and 20 minute post-WBV. M-wave and H-reflex amplitudes decreased by 8% (P<.001) and by 46% (P<.05), respectively, whereas peak twitch torque decreased by 9% (P<.01) and rate of twitch torque development slowed 8% (P<.05). Percent voluntary activation and maximal plantar flexor torque were unchanged as a consequence of WBV (P>.05). In response to acute WBV, the root mean square of the soleus electromyography signal (EMG_RMS ) increased by 8%, while the EMG_RMS of the lateral gastrocnemius increased by 3% (P<.05). These data indicate that the responsiveness of the Ia pathway is diminished and contractile function is impaired immediately following WBV, and that the neural mechanisms underlying improved performance following WBV lie in alternative hypotheses possibly involving spindle disfacilitation or Golgi afferent modulation.

Myofilament Calcium Sensitivity: Role in Regulation of In vivo Cardiac Contraction and Relaxation

Myofilament calcium sensitivity is an often-used indicator of cardiac muscle function, often assessed in disease states such as hypertrophic cardiomyopathy (HCM) and dilated cardiomyopathy (DCM). While assessment of calcium sensitivity provides important insights into the mechanical force-generating capability of a muscle at steady-state, the dynamic behavior of the muscle cannot be sufficiently assessed with a force-pCa curve alone. The equilibrium dissociation constant (K_d) of the force-pCa curve depends on the ratio of the apparent calcium association rate constant (k_on) and apparent calcium dissociation rate constant (k_off) of calcium on TnC and as a stand-alone parameter cannot provide an accurate description of the dynamic contraction and relaxation behavior without the additional quantification of k_on or k_off, or actually measuring dynamic twitch kinetic parameters in an intact muscle. In this review, we examine the effect of length, frequency, and beta-adrenergic stimulation on myofilament calcium sensitivity and dynamic contraction in the myocardium, the effect of membrane permeabilization/mechanical- or chemical skinning on calcium sensitivity, and the dynamic consequences of various myofilament protein mutations with potential implications in contractile and relaxation behavior.

Modulation of spinal motor output by initial arm postures in anesthetized monkeys

Proper execution of voluntary movement requires a sensorimotor transformation based on the initial limb state. For example, successfully reaching to a stable target requires the recruitment of different muscle groups depending on limb position at movement initiation. To test whether this transformation could occur at the spinal level, we stimulated the cervical spinal cord of anesthetized monkeys while systematically changing initial posture and examined the modulation of the twitch response induced in the upper limb muscles. In three monkeys, a multichannel microelectrode array was implanted into the C6 segment of the spinal cord and electromyographic electrodes were implanted in 12 limb muscles (five hand, four elbow, and three shoulder muscles). The magnitude and onset latency of the evoked response in each electrode-muscle pair were examined by systematically changing the hand position through nine positions in a horizontal plane with the monkey prone. Among 330 electrode-muscle pairs examined, 61% of pairs exhibited significant modulation of either magnitude or latency of twitch responses across different hand/arm configurations (posture dependency). We found that posture dependency occurred preferentially in the distal rather than proximal muscles and was not affected by the location of the electrode within the stimulated spinal segment. Importantly, this posture dependency was not affected by spinalization at the C2 level. These results suggest that excitability in the cervical spinal cord is affected by initial arm posture through spinal reflex pathways. This posture dependency of spinal motor output could affect voluntary arm movement by adjusting descending motor commands relative to the initial arm posture.

Potentiation and electrical stimulus frequency during self-paced exercise and recovery

The aim of this study was to investigate the effect of potentiation on stimulation-induced muscle function during and after an intense bout of self-paced dynamic exercise. Ten active subjects performed a time trial involving repetitive concentric extension-flexion of the right knee using a Biodex dynamometer. Electrical stimulation before and after a 5 s maximal isometric voluntary contraction was performed before the start of the time trial and immediately (< 5 s) after each 20% of the time trial as well as 1, 2, 4 and 8 min after time trial termination. Potentiation was observed before the time trial and as early as 1–2 min after the time trial, but no potentiation was detected during or immediately after the time trial for neither single or paired stimuli. At termination of the time trial, “potentiated” peak torque was significantly more reduced than “unpotentiated” peak torque for single stimulus (−65 ± 10% and −42 ± 18%, respectively) and paired stimuli at 100 Hz (−51 ± 10% and −33 ± 15%, respectively). Faster recovery for “potentiated” compared to “unpotentiated” peak torque indicate that potentiate peak torque measurements or delay the post-exercise measurements more than a few seconds, will underestimate peripheral fatigue. In conclusion, the potentiation after maximal contraction disappears during intense exercise. Whether the muscle is already potentiated during intense contraction or fatiguing mechanisms inhibits potentiation remains to be clarified.

Dynamic changes of twitchlike responses to successive stimuli studied by decomposition of motor unit tetanic contractions in rat medial gastrocnemius

Unfused tetanic contractions evoked by trains of stimuli at variable interpulse intervals (IPIs) were recorded for 10 fast fatigable (FF), 10 fast resistant (FR), and 10 slow (S) motor units (MUs) and subsequently decomposed with a mathematical algorithm into trains of twitch-shape responses to successive stimuli. The mean stimulation frequencies were matched for each MU to evoke tetani of similar fusion degrees, whereas the variability range of IPIs was in each case 50-150% of the mean IPI. Force and time parameters of decomposed twitches were analyzed and related to the first response. Considerable variability of the analyzed twitch parameters was observed in each MU, although the largest range of variability occurred in slow MUs. In general, the decomposed twitch responses had longer duration and higher force than single-twitch contractions, although for nine FF and six FR MUs some of the decomposed responses were slightly weaker (but not faster) than the first twitches of these MUs. Comparison of the strongest decomposed twitch to the first decomposed twitch revealed ratios of forces up to 2.35, 3.33, and 6.89 for FF, FR, and S MUs and ratios of force-time areas up to 3.54, 4.67, and 14.26 for FF, FR, and S MUs, whereas for the contraction times the ratios of the longest decomposed twitch to the first twitch amounted to 2.46, 2.07, and 3.52 for FF, FR, and S MUs, respectively. The results indicate that contractile responses to successive action potentials are considerably variable, especially for slow MUs.

Myo-mechanical analysis of isolated skeletal muscle

To assess the in vivo effects of therapeutic interventions for the treatment of muscle disease, quantitative methods are needed that measure force generation and fatigability in treated muscle. We describe a detailed approach to evaluating myo-mechanical properties in freshly explanted hindlimb muscle from the mouse. We describe the atraumatic harvest of mouse extensor digitorum longus muscle, mounting the muscle in a muscle strip myograph (Model 820MS; Danish Myo Technology), and the measurement of maximal twitch and tetanic tension, contraction time, and half-relaxation time, using a square pulse stimulator (Model S48; Grass Technologies). Using these measurements, we demonstrate the calculation of specific twitch and tetanic tension normalized to muscle cross-sectional area, the twitch-to-tetanic tension ratio, the force-frequency relationship curve and the low frequency fatigue curve. This analysis provides a method for quantitative comparison between therapeutic interventions in mouse models of muscle disease, as well as comparison of the effects of genetic modification on muscle function.

Brainstem-mediated locomotion and myoclonic jerks. I. Neural substrates

Eleven of 40 decerebrated cats were found to exhibit periods of spontaneous or sensory myoclonus and locomotion beginning 24 h after decerebration. Histological analysis showed that the cats generating myoclonus hemorrhagic lesions in the retrorubral nucleus (RRN) and ventral mesopontine junction (vMPJ). However, with intact RRN and vMPJ never showed myoclonus. To verify that the lesions were responsible for myoclonus, 6 additional cats received N-methyl-D-aspartate (NMDA, 0.5 M/0.5 microliter) injections in the areas of RRN and vMPJ to produce bilateral lesions. Coordinated rhythmic leg movement (locomotion) or myoclonic twitches developed in all of these cats beginning 3 hours after NMDA injection. These NMDA lesion-induced movements appeared either spontaneously (5 out of 6 cats) or after sensory stimulation (1 cat). Four cats received saline control injections in the RRN and vMPJ and did not have spontaneous, or sensory stimulation-induced, myoclonic twitches during the 48 h observation period. These results indicate that the RRN and vMPJ have a suppressive effect on myoclonic twitches and rhythmic leg movement. Dysfunction of these regions could release motor activity into sleep and waking states.