Voluntary rate of torque development is impaired after a voluntary versus tetanic conditioning contraction

Introduction: Both voluntary and evoked conditioning contractions will potentiate muscle twitch contractile properties. The response of a voluntary contraction to each condition type is not well understood but it may be a more functional model than evoked twitch potentiation. Methods: Baseline measurements from tibialis anterior included: maximal isometric twitch torque and rate of torque development (RTD); maximal evoked 50‐Hz torque; and maximal voluntary ballistic RTD. Potentiation was induced by a 10‐s voluntary or tetanic contraction (∽78% MVC), followed by 2 twitches and 2 ballistic contractions. Results: Twitch properties (torque and RTD) were potentiated equally after each conditioning contraction. Ballistic RTD was greater post‐tetanus (390.2 ± 59.3 Nm/s) than post‐voluntary (356.4 ± 69.1 Nm/s), but both were reduced from baseline (422.0 ± 88.9 Nm/s). Conclusions: Twitch potentiation was similar between conditioning contraction types, but ballistic RTD was lower after post‐tetanus than post‐voluntary. The results indicate central inhibition or fatigue concurrent with peripheral potentiation. Muscle Nerve 49 : 218–224, 2014     

Assessment of quadriceps muscle inactivation with a new electrical stimulation paradigm

Introduction: In this study we evaluated the validity of garment‐based quadriceps stimulation (GQS) for assessment of muscle inactivation in comparison with femoral nerve stimulation (FNS). Methods: Inactivation estimates (superimposed doublet torque), self‐reported discomfort, and twitch and doublet contractile properties were compared between GQS and FNS in 15 healthy subjects. Results: Superimposed doublet torque was significantly lower for GQS than for FNS at 20% and 40% maximum voluntary contraction (MVC) (P < 0.01), but not at 60%, 80%, and 100% MVC. Discomfort scores were systematically lower for GQS than for FNS (P < 0.05). Resting twitch and doublet peak torque were lower for GQS, and time to peak torque was shorter for GQS than for FNS (P < 0.01). Conclusions: GQS can be used with confidence for straightforward evaluation of quadriceps muscle inactivation, whereas its validity for assessment of contractile properties remains to be determined. Muscle Nerve 51 : 117–124, 2015     

Activation of fast-twitch fibers assessed with twitch potentiation

Introduction: The augmentation of twitch response following brief muscle activation, called twitch potentiation, has been shown to be much more pronounced in fast‐twitch than in slow‐twitch fibers. We thus explored the possibility of twitch potentiation as a noninvasive measure of fast‐twitch fiber activation, by studying its dependence on the intensity of preceding contraction. Methods: Twitch contraction of plantar flexor muscles was evoked with supramaximal stimulation of the posterior tibial nerve, before and immediately after 6‐s voluntary contractions at intensities of 10–100% of maximal voluntary contraction (MVC). Results: Except for low‐intensity contractions (< 30%MVC), voluntary contraction induced twitch potentiation, the magnitude of which increased with increasing contraction intensity (P < 0.001). The shortened contractile process was associated with the potentiation. Conclusions: These results are consistent with the concept of “hierarchical order of fiber activation”, suggesting that the magnitude of twitch potentiation reflects the activation of fast‐twitch fibers during a brief contraction. Muscle Nerve 46: 218–227, 2012     

Post‐activation potentiation: The neural effects of post—activation depression

Introduction: Our knowledge of the neurophysiology of post‐activation potentiation (PAP) is limited. The purpose of this study was to examine the effect of PAP on twitch torque and H‐reflex amplitude after a 10‐s maximal voluntary contraction (MVC). Methods: PAP measurements were assessed with the plantarflexors in a relaxed state and during a tonic contraction at 10% MVC. Results: The H‐reflex/maximum M‐wave ratio (H/M) decreased significantly (P < 0.05) and returned to baseline levels after 1 min. The decrement in H/M was depressed when the plantarflexors were active at 10% MVC, and the depression was more obvious in the lateral gastrocnemius than in the soleus muscle. Conclusions: The inhibition induced immediately after contraction could be attributed to post‐activation depression. We conclude that PAP after a 10‐s MVC cannot be attributed to increased motor neuron excitability through the reflex pathway as assessed by the H‐reflex technique. Muscle Nerve 52 : 252–259, 2015     

Gene expression analysis in absence epilepsy using a monozygotic twin design

Purpose: To identify genes involved in idiopathic absence epilepsies by analyzing gene expression using a monozygotic (MZ) twin design. Methods: Genome‐wide gene expression in lymphoblastoid cell lines (LCLs) was determined using microarrays derived from five discordant and four concordant MZ twin pairs with idiopathic absence epilepsies and five unaffected MZ twin pairs. Gene expression was analyzed using three strategies: discordant MZ twins were compared as matched pairs, MZ twins concordant for epilepsy were compared to control MZ twins, and a singleton design of affected versus unaffected MZ twin individuals was used irrespective of twin pairing. An overlapping gene list was generated from these analyses. Dysregulation of genes recognized from the microarray experiment was validated using quantitative real time PCR (qRT‐PCR) in the twin sample and in an independent sample of 18 sporadic absence cases and 24 healthy controls. Results: Sixty‐five probe sets were identified from the three combined microarray analysis strategies. Sixteen genes were chosen for validation and nine of these genes confirmed by qRT‐PCR in the twin sample. Differential expression for EGR1 (an immediate early gene) and RCN2 (coding for the calcium‐binding protein Reticulocalbin 2) were reconfirmed by qRT‐PCR in the independent sample. Discussion: Using a unique sample of discordant MZ twins, our study identified genes with altered expression, which suggests novel mechanisms in idiopathic absence epilepsy. Dysregulation of EGR1 and RCN2 is implicated in idiopathic absence epilepsy.     

Neuromuscular fatigue of the knee extensors during repeated maximal intensity intermittent‐sprints on a cycle ergometer

Introduction: We studied the time course of neuromuscular fatigue during maximal intensity intermittent‐sprint cycling. Methods: Eight participants completed 10, 10‐s sprints interspersed with 180 s of recovery. The power outputs were recorded for each sprint. Knee extensor maximum voluntary contraction (MVC) force, voluntary activation, and evoked contractile properties were recorded presprint, postsprint 5, and postsprint 10. Results: Total work over the 10 sprints decreased significantly (P < 0.05) and could be described by 2 linear relationships from sprints 1–5 compared with sprints 6–10. Participants had significantly (P < 0.05) lower MVC and twitch forces postsprint 5 compared with presprint. MVC, voluntary activation, and twitch force were decreased (P < 0.05) postsprint 10 compared with postsprint 5. Conclusions: The maximal intermittent sprints induced neuromuscular fatigue. Neuromuscular fatigue in the first 5 sprints was mainly peripheral, whereas in the last 5 sprints it was both peripheral and central. Muscle Nerve 51: 569–579, 2015     

Effect of age on muscle activation and twitch properties during static and dynamic actions

In this investigation we examined age‐associated changes in peak torque, voluntary activation levels, and potentiated twitch properties of the knee extensors during isometric (ISO), shortening (SHO), and lengthening (LEN) actions in 18 young subjects (19–27 years) and 12 elderly subjects (64–77 years). Peak torque was lower for the elderly subjects under the ISO (?31%) and SHO (?28%) conditions (P < 0.05); however, the loss in LEN peak torque in the elderly was less marked (?17%) (P > 0.05). Voluntary activation levels within and between groups were not significantly different and ranged between 96.8% and 98.9% (P > 0.05). Peak twitch torque and some temporal twitch characteristics were altered with age (P < 0.05), but such changes were similar across all muscle actions (P > 0.05). These data suggest that the attenuated reduction in LEN muscle strength associated with age is probably not related to contraction‐specific changes in voluntary activation levels or potentiated twitch properties. Muscle Nerve, 2009     

Comparison between the effects of 4 different electrical stimulation current waveforms on isometric knee extension torque and perceived discomfort in healthy women

Introduction: We studied the effects of different neuromuscular electrical stimulation (NMES) currents, 2 kHz‐frequency alternating currents (KACs, Russian and Aussie) and 2 pulsed currents (PCs), on isometric knee extension torque and discomfort level, both in isolation and combined, with maximum voluntary contraction (MVC). Methods: Twenty‐one women (age 21.6 ± 2.5 years) were studied. We evaluated torque evoked by NMES or NMES combined with maximum voluntary contraction of the quadriceps muscle of healthy women. Discomfort level was measured using a visual analog pain scale. Results: Despite comparable levels of discomfort, evoked torque was lower for Russian current compared with the other modalities (Russian 50.8%, Aussie 71.7%, PC500 76.9%, and PC200 70.1%; P < 0.001). There was no advantage in combining NMES with MVC compared with isolated NMES. Conclusions: The Aussie and PC approaches proved superior to Russian current for inducing isometric knee extension torque. This information is important in guiding decision making with regard to NMES protocols for muscle strengthening. Muscle Nerve 51 : 76–82, 2015     

Creatine supplementation does not alter neuromuscular recovery after eccentric exercise

Introduction: The purpose of this study was to investigate the effects of creatine (CR) supplementation on recovery after eccentric exercise (ECC). Methods: Fourteen men were assigned randomly to ingest 0.3 g/kg of CR or placebo (PL) before and during recovery (48 hours) from 6 sets of 8 repetitions of ECC. Maximal voluntary contraction (MVC), voluntary activation (VA), muscle thickness (MT), electromyography (EMG), contractile properties, and soreness were assessed. Results: MVC, evoked twitch torque, and rate of torque development decreased for both groups immediately after ECC and recovered at 48 hours. MT increased and remained elevated at 48 hours for both groups. Soreness increased similarly for both groups. EMG activation was higher for CR versus PL only at 48 hours. There were no group differences for torque, total work, or fatigue index during ECC. Conclusions: CR supplementation before and during recovery from ECC had no effect on strength, voluntary activation, or indicators of muscle damage. Muscle Nerve 54 : 487–495, 2016     

Electrically induced torque decrease reflects more than muscle fatigue

The aim of the study was to compare the fatigue induced by different electrical stimulation (ES) protocols. The triceps surae muscle of 8 healthy subjects was fatigued with 4 protocols (30 Hz ?500 μs, 30 Hz ?1 ms, 100 Hz ?1 ms, and 100 Hz ?500 μs), composed of 60 trains (4 s on–6 s off), delivered at an intensity evoking 30% of maximal voluntary contraction (MVC). Fatigue was quantified by ES and MVC torque decreases. The amplitude of the twitch delivered at the intensity and pulse width used in each fatiguing protocol (twitch at I_stim) was analyzed. All parameters decreased significantly after all protocols. The ES torque decrease correlated positively with the twitch decrease elicited at I_stim only for the 30‐Hz protocols. Results show that, during the 100‐Hz protocols, phenomena not related to the fatigue of the solicited motor units may occur, including changes in the excitability threshold of the axonal terminal branches. Muscle Nerve 50: 604–606, 2014     

Influence of joint position on synergistic muscle activity after fatigue of a single muscle head

Introduction: We investigated synergistic muscle activity after fatigue of a single muscle in different joint positions. Methods: Two experimental groups (n = 12 each) performed maximal voluntary contractions (MVCs) before and after fatiguing the gastrocnemius lateralis (GL), using neuromuscular electrical stimulation (NMES). Neuromuscular tests, including muscle activity during MVC, H‐reflex, and twitch interpolation, were performed. One group completed the experiment in a knee‐extended position with the second group in a knee‐flexed position. Results: In the knee‐flexed position, the muscle activity increased in non‐stimulated synergistic muscles. In contrast, in the knee‐extended position, muscle activity of the synergistic muscles remained unaltered. The MVC force remained unaltered in the flexed position and decreased in the extended position. Conclusions: Synergistic muscles compensate for the fatigued muscle in the flexed position but not in the extended position. Compensation mechanisms seem to depend on joint position. Muscle Nerve 51 : 259–267, 2015     

The effects of lithium on muscle contractile function in humans

A side effect of lithium (Li⁺) treatment is fatigue. Li⁺ decreases inositol triphosphate (IP₃) accumulation and IP₃ may play a role in excitation-contraction (E-C) coupling in skeletal muscle. Li⁺ carbonate (600 mg b.i.d. × 6 days) was administered in a randomized, double-blind fashion to 12 males to measure the effect upon muscle contractile function: peak twitch torque (PTT), time to PTT, half-relaxation time, maximal voluntary contraction strength (MVC), percent motor unit activation, M-wave characteristics, and tetanic torque (3 min at 15 and 50 Hz). Li⁺ resulted in a significant decrease in 15- and 50-Hz tetanic torque (P < 0.001), MVC, and resting PTT (P < 0.05). There were no effects of Li⁺ upon any of the other measured variables. Li⁺ had a negative effect upon E-C coupling and did not affect central motor unit recruitment. Elucidation of the role of IP₃ in E-C coupling may help to understand fatigue in some neuromuscular disorders. © 1996 John Wiley & Sons, Inc.     

Functional implications of impaired control of submaximal hip flexion following stroke

Introduction: We quantified submaximal torque regulation during low to moderate intensity isometric hip flexion contractions in individuals with stroke and the associations with leg function. Methods: Ten participants with chronic stroke and 10 controls performed isometric hip flexion contractions at 5%, 10%, 15%, 20%, and 40% of maximal voluntary contraction (MVC) in paretic, nonparetic, and control legs. Results: Participants with stroke had larger torque fluctuations (coefficient of variation, CV), for both the paretic and nonparetic legs, than controls (P < 0.05) with the largest CV at 5% MVC in the paretic leg (P < 0.05). The paretic CV correlated with walking speed (r² = 0.54) and Berg Balance Score (r² = 0.40). At 5% MVC, there were larger torque fluctuations in the contralateral leg during paretic contractions compared with the control leg. Conclusions: Impaired low‐force regulation of paretic leg hip flexion can be functionally relevant and related to control versus strength deficits poststroke. Muscle Nerve 49 : 225–232, 2014     

Kinetics of neuromuscular changes during low‐frequency electrical stimulation

The purpose of the study was to examine the time course of neuromuscular fatigue components during a low‐frequency electrostimulation (ES) session. Three bouts of 17 trains of stimulation at 30 HZ (4 s on, 6 s off) were used to electrically induce fatigue in the plantar flexor muscles. Before and after every 17‐train bout, torque, electromyographic activity [expressed as root mean square (RMS) and median frequency (MF) values], evoked potentials (M‐wave and H‐reflex), and the level of voluntary activation (LOA, using twitch interpolation technique) were assessed. Torque during maximal voluntary contraction decreased significantly from the very first stimulation bout (?6.6 ± 1.11%, P < 0.001) and throughout the session (?10.32 ± 1.68% and ?11.53 ± 1.27%, for the second and third bouts, respectively). The LOA and RMS/Mmax values were significantly decreased during the ES session (?2.9 ± 1.07% and ?17.5 ± 6.14%, P < 0.01 and P< 0.001, respectively, at the end of the protocol), while MF showed no changes. The Hmax/Mmax ratio and Mmax were not significantly modified during the session. All twitch parameters were significantly potentiated after the first bout and throughout the session (P < 0.001). The maximal torque decrease was evident from the early phase of a low‐frequency ES protocol, with no concomitant inhibition of motoneuron excitability or depression of muscle contractile properties. These results are consistent with an early failure of the central drive to the muscle. Muscle Nerve, 2010     

Reliability of measures of quadriceps muscle function using magnetic stimulation

Introduction: Magnetic stimulation can be used to assess muscle function by calculating voluntary activation using an interpolated twitch during maximal voluntary contractions (MVCs) and control twitches to potentiated muscle. In this study we assessed the reliability of torque, electromyography (EMG), and voluntary activation variables. Methods: Fifteen men completed 5 testing sessions (2 familiarization and 3 reliability trials) to assess quadriceps femoris muscle function. Intra‐ and interday reliability levels of torque and EMG variables were estimated using typical error ± 90% confidence limits, expressed as percentage [coefficient of variation (CV)] and intraclass correlation coefficient. The smallest worthwhile change was calculated as 0.2 × between‐participant standard deviation. Results: Intra‐ and interday torque variables for MVC were reliable (CV < 4%, ICC 0.98, and CV < 5%, ICC 0.99, respectively). EMG variables were less reliable than torque variables, with CVs ranging from 7% to 18%. Conclusion: Magnetic stimulation of the femoral nerve is a reliable method for assessing muscle function. Muscle Nerve 53 : 770–778, 2016     

Twins discordant for schizophrenia: psychopathology of the non-schizophrenic co-twins

Objective: Patients with schizophrenia are more likely to suffer from mood and anxiety disorders compared with the general population. We explored the aetiology of this comorbidity using a twin study design. Method: We applied an additive genetic + unique environment (AE) random effects model in the analysis of 35 non‐schizophrenic co‐twins from pairs discordant for schizophrenia, and 131 control twins. Results: Non‐schizophrenic co‐twins had significantly increased rates of depression (P = 0.006) and anxiety disorders (P = 0.021) compared with the control twins. Conclusion: Our results provide evidence for a familial association between schizophrenia and anxiety and depression. This could reflect common aetiological factors contributing to each of the disorders. Future studies should attempt to investigate the relative genetic and environmental contribution to the shared risk factors for schizophrenia, mood and anxiety disorders.     

Neuromuscular fatigue after low‐ and medium‐frequency electrical stimulation in healthy adults

Introduction: In this study we investigated fatigue origins induced by low‐frequency pulsed current (PC) and medium‐frequency current (MF) neuromuscular electrical stimulation (NMES) after a clinical‐like session. Methods: Eleven healthy men randomly underwent 2 NMES sessions, PC and MF, on quadriceps muscle (15‐minute duration, 6 seconds on and 18 seconds off). Maximal voluntary contraction (MVC), central activation ratio (CAR), vastus lateralis electromyographic activity (EMG), and evoked contractile properties were determined before and after the sessions. Evoked torque and discomfort during the sessions were also measured. Results: Both currents produced decreases in MVC, EMG, and evoked contractile properties after the sessions. No difference was found between currents for all variables (P > 0.05). Evoked torque during sessions decreased (P < 0.05). No difference was observed in mean evoked torque and discomfort (P > 0.05). Discussion: Both currents induced similar neuromuscular fatigue. Clinicians can choose either PC or MF and expect similar treatment effects when the goal is to generate gains in muscle strength. Muscle Nerve 58 : 293–299, 2018     

Variability and distribution of muscle strength and its determinants in humans

Introduction: Inter‐individual variability in measurements of muscle strength and its determinants was identified to: (1) produce a normative data set describing the normal range and (2) determine whether some measurements are more informative than others when evaluating inter‐individual differences. Methods: Functional and morphological characteristics of the vastus lateralis were measured in 73 healthy, untrained adult men. Results: Inter‐individual variability (coefficient of variation) was greater for isometric maximal voluntary contraction (MVC) torque (18.9%) compared with fascicle force (14.6%; P = 0.025) and physiological cross‐sectional area (PCSA; 17.2%) compared with anatomical cross‐sectional area (ACSA, 13.0%; P < 0.0005). The relationship between ACSA and isometric MVC torque (r² = 0.56) was weaker than that between PCSA and fascicle force (r² = 0.68). Conclusions: These results provide a normative data set on inter‐individual variability in a variety of muscle strength‐related measurements and illustrate the benefit of using more stringent measures of muscle properties. Muscle Nerve 49 : 879–886, 2014     

Increased serum soluble interleukin-2 receptors in schizophrenic monozygotic twins

Summary There is a confusing history of immune findings associated with schizophrenia. At least some of these discrepant results may be artifacts caused by heterogeneity. In an attempt to decrease heterogeneity, we studied three groups of monozygotic twins who were either discordant for schizophrenia, concordant and ill, or concordant and well. This comparison minimizes environmental and genetic variance, and heightens differences that are actually due to the disorder. Overall, schizophrenic subjects had higher levels of serum soluble interleukin-2 receptors (SIL-2Rs) than unaffected individuals (480.8, SD 238.6 U/ml vs 380.9, SD 170.6 U/ml;F=5.256,df=1.61,P=0.02). When data from discordant and concordant twin groups were analysed separately, both the discordant ill twins (P=0.06) and concordant ill twin pairs (P=0.08) showed trends towards higher serum SIL-2R levels than their respective control groups. These data contribute to the growing body of evidence that immune activation is associated with some forms of schizophrenia.The Metabolism Branch of the National Cancer Institute