Improvement of dy/dy dystrophic diaphragm by 3,4-diaminopyridine

Concerns have been raised that inotropic agents may worsen function of dystrophic muscle due to structural fragility. Studies tested the hypothesis that force increments elicited by potassium (K(+)) channel blockade can be maintained during the course of repetitive stimulation. In vitro twitch force of dy/dy dystrophic mouse diaphragm was significantly lower than normal (796 versus 1271 g/cm(2)). 3,4-Diaminopyridine (DAP) increased twitch force of dystrophic diaphragm by 111 +/- 12% (P <.0001) and increased force at stimulation frequencies of 5-50 Hz by 41-77%. During fatigue-inducing stimulation, force augmentation by DAP was well maintained in dystrophic muscle throughout 25 Hz (P =.0047) and 50 Hz (P =.0059) stimulation. These findings indicate that the K(+) channel blocker DAP augments the force of dystrophic muscle to values close to that of normal muscle over a range of stimulation frequencies. Furthermore, these functional increments can be achieved without causing force to eventually deteriorate below that of untreated dystrophic muscle during fatiguing stimulation. It is possible that DAP may be useful for the clinical management of a variety of disorders causing muscle weakness.     

Distributed low‐frequency functional electrical stimulation delays muscle fatigue compared to conventional stimulation

We present a low‐frequency stimulation method via multi‐pad electrodes for delaying muscle fatigue. We compared two protocols for muscle activation of the quadriceps in paraplegics. One protocol involved a large cathode at 30 HZ (HPR, high pulse‐rate), and the other involved four smaller cathodes at 16 HZ (LPR, low pulse‐rate). The treatment included 30‐min daily sessions for 20 days. One leg was treated with the HPR protocol and the other with the LPR protocol. Knee‐joint torque was measured before and after therapy to assess the time interval before the knee‐joint torque decreased to 70% of the initial value. The HPR therapy provided greater increases in muscle endurance and force in prolonged training. Yet the LPR stimulation produced less muscle fatigue compared to the HPR stimulation. The results suggest that HPR is the favored protocol for training, and LPR is better suited for prolonged stimulation. Muscle Nerve, 2010     

Contractile properties of slow and fast skeletal muscles from protease activated receptor‐1 null mice

Introduction: Protease‐activated receptors (PARs) may play a role in skeletal muscle development. We compared the contractile properties of slow‐twitch soleus muscles and fast‐twitch extensor digitorum longus (EDL) muscles from PAR‐1 null and littermate control mice. Methods: Contractile function was measured using a force transducer system. Fiber type proportions were determined using immunohistochemistry. Results: Soleus muscles from PAR‐1 null mice exhibited longer contraction times, a leftward shift in the force–stimulation frequency relationship, and decreased fatiguability compared with controls. PAR‐1 null soleus muscles also had increased type 1 and decreased type IIb/x fiber numbers compared with controls. In PAR‐1 null EDL muscles, no differences were found, except for a slower rate of fatigue compared with controls. Conclusions: The absence of PAR‐1 results in a slower skeletal muscle contractile phenotype, likely due to an increase in type I and a decrease in type IIb/x fiber numbers. Muscle Nerve 50: 991–998, 2014     

Surface‐distributed low‐frequency asynchronous stimulation delays fatigue of stimulated muscles

Introduction: One important reason why functional electrical stimulation (FES) has not gained widespread clinical use is the limitation imposed by rapid muscle fatigue due to non‐physiological activation of the stimulated muscles. We aimed to show that asynchronous low‐pulse‐rate (LPR) electrical stimulation applied by multipad surface electrodes greatly postpones the occurrence of muscle fatigue compared with conventional stimulation (high pulse rate, HPR). Methods: We compared the produced force vs. time of the forearm muscles responsible for finger flexion in 2 stimulation protocols, LPR (f_L = 10 Hz ) and HPR (f_H = 40 Hz ). Results: Surface‐distributed low‐frequency asynchronous stimulation (sDLFAS) doubles the time interval before the onset of fatigue (104 ± 80%) compared with conventional synchronous stimulation. Conclusions: Combining the performance of multipad electrodes (increased selectivity and facilitated positioning) with sDLFAS (decreased fatigue) can improve many FES applications in both the lower and upper extremities. Muscle Nerve 48 : 930–937, 2013     

Adjustments in the force–frequency relationship during passive and exercise‐induced hyperthermia

Introduction: We examined the extent to which fatiguing cycling exercise in the heat influences contractile function in modulating the force–frequency relationship. Methods: Before (∽37.0°C) and after (∽38.5°C) exercise (ExH) and passive (PaH) hyperthermia, an 8‐s train of stimulation at 10, 20, 50, and 100 Hz (2 s per frequency) and a potentiated twitch were evoked on the relaxed knee extensors using percutaneous stimulation. Results: ExH and PaH produced a decrease in the 20:50 Hz force ratio, indicative of low‐frequency fatigue (P < 0.01). This adjustment was more pronounced after ExH than PaH (P < 0.01). A rightward displacement in the force–frequency relationship occurred after ExH and PaH (P < 0.05) and was exacerbated by ExH (P < 0.05). Peak twitch force also decreased after ExH (P < 0.05). Conclusions: ExH reduces force summation due to development of skeletal muscle fatigue, exacerbating the shift in force–frequency to the right relative to PaH. Muscle Nerve 50 : 822–829, 2014     

Upregulation of MHC class I in transgenic mice results in reduced force‐generating capacity in slow‐twitch muscle

Expression of major histocompatibility complex (MHC) class I in skeletal muscle fibers is an early and consistent finding in inflammatory myopathies. To test if MHC class I has a primary role in muscle impairment, we used transgenic mice with inducible overexpression of MHC class I in their skeletal muscle cells. Contractile function was studied in isolated extensor digitorum longus (EDL, fast‐twitch) and soleus (slow‐twitch) muscles. We found that EDL was smaller, whereas soleus muscle was slightly larger. Both muscles generated less absolute force in myopathic compared with control mice; however, when force was expressed per cross‐sectional area, only soleus muscle generated less force. Inflammation was markedly increased, but no changes were found in the activities of key mitochondrial and glycogenolytic enzymes in myopathic mice. The induction of MHC class I results in muscle atrophy and an intrinsic decrease in force‐generation capacity. These observations may have important implications for our understanding of the pathophysiological processes of muscle weakness seen in inflammatory myopathies. Muscle Nerve, 2008     

Changes in joint angle,muscle‐tendon complex length,muscle contractile tissue displacement,and modulation of EMG activity during acute whole‐body vibration

It has been suggested that vibration causes small changes in muscle length, but to the best of our knowledge, these have yet to be demonstrated during whole‐body vibration (WBV). This was an observational study to determine whether acute WBV would result in muscle lengthening. We hypothesized that acute WBV would increase electromyography (EMG) activity concurrently with measurable changes in muscle contractile length. Nine healthy males performed two conditions on a Galileo vibration machine for 15 s at 0 HZ (resting) and 6 HZ at a set knee angle of 18°. Muscle tendon complex length, contractile tissue displacement of the medial gastrocnemius muscle, and EMG of soleus, tibialis anterior, and vastus lateralis muscles were measured. At 6 HZ the medial gastrocnemius (MG) muscle tendon complex (MTC) amplitude (375 μm) was significantly greater (P < 0.05) compared to 0 HZ (35 μm). The MG contractile length (CD) amplitude at 6 HZ (176 μm) was significantly greater (P < 0.01) compared to 0 HZ (4 μm). Significant increases (P < 0.05) in EMG modulation were found for all muscles during the 6 HZ compared to the 0 HZ condition. The major finding was that ≈50% of the elongation occurred within the muscle itself and was associated with preceding changes in EMG. This indicates muscle lengthening may be a prerequisite for eliciting stretch reflexes. In conclusion, there is a temporal association between EMG activity and muscle contractile tissue displacement where low‐frequency WBV results in small muscle length changes and increases muscle activation. Muscle Nerve, 2009     

Maximal versus submaximal intensity stimulation with variable patterns

It is essential to determine optimal parameters of stimulation to maintain muscle force during neuromuscular electrical stimulation (NMES). Protocols that increase in frequency and include doublets can prolong force output over time. However, stimulation intensity level could differentially affect muscle force output during variable-frequency NMES. We compared three intermittent stimulation patterns at maximal and submaximal intensities of stimulation of the median nerve: (1) a constant 20-HZ pattern; (2) 90 s at 20 HZ followed by a 90-s increase from 20 to 40 HZ; and (3) 90 s at 20 HZ followed by 90 s of doublets at 20 HZ. At submaximal intensities, the doublet pattern produced the highest overall force-time integral (FTI). At maximal intensities, the doublet pattern produced the lowest FTI and the increasing frequency pattern produced the least amount of fatigue. Thus, double-pulse stimulation was more effective during submaximal than maximal intensity NMES. These data demonstrate that intensity level must be taken into consideration when programming frequency patterns for NMES devices.     

Failure of neuromuscular transmission and contractility during muscle fatigue

In previous studies of muscle fatigue, tension was monitored from whole muscle, while action potentials were recorded from a few muscle fibers. To compare more accurately changes in these responses, an in vitro fluid electrode technique was employed to record the action potential of whole muscle simultaneously with tension during fatigue induced by nerve stimulation in the rat extensor digitorum longus (EDL), soleus, and diaphragm muscles. In each muscle, tension declined from the start of stimulation, while action potential amplitude initially increased slightly and then declined most rapidly in EDL, more slowly in diaphragm, and most slowly in soleus. Direct stimulation of the fatigued muscle produced the greatest increase in tension in EDL, next in diaphragm, and least in soleus. These results indicate that while failure of excitation-contraction coupling or of the contractile mechanism is the initial cause of fatigue in all the muscles studied, and remains the predominant cause throughout in the soleus muscle, failure of neuromuscular transmission plays an important role in fatigue after the first 15 seconds in EDL, and to a lesser extent, after the first 90 seconds in diaphragm.     

Functional Electrical Stimulation‐Supported Interval Training Following Sensorimotor‐Complete Spinal Cord Injury: A Case Series

Objective. To investigate the effect of interval training supported by Functional Electrical Stimulation (FES) on ambulation ability in complete spinal cord injury (SCI). Methods. We trained four men with sensorimotor‐complete (ASIA A) SCI, who achieved gait through FES of the quadriceps femoris, gluteus maximus, and common peroneal nerve on each side on a motorized treadmill. Training involved progressive interval walking exercise, consisting of periods of activity followed by equal periods of rest, repeated until muscle fatigue. We used time to muscle fatigue during continuous treadmill ambulation as the primary outcome measure. We also recorded the patterns of incremental stimulation for all training and testing sessions. Results. All subjects increased their ambulation capacity; however, the responses varied from subject to subject. Some subjects increased the total distance walked by as much as 300% with progressive improvement over the entire training period; however, others made more modest gains and appeared to reach a performance plateau within a few training sessions. Conclusions. FES‐supported interval training offers a useful and effective strategy for strength‐endurance improvement in the large muscle groups of the lower limb in motor‐complete SCI. We believe that this training protocol offers a viable alternative to that of continuous walking training in people with SCI using FES to aid ambulation.     

3,4‐Diaminopyridine is more effective than placebo in a randomized,double‐blind,cross‐over drug study in LEMS

The purpose of this study was to investigate the clinical and electrophysiological efficacy of 3,4‐diaminopyridine (DAP) in patients with Lambert–Eaton myasthenic syndrome (LEMS) in a randomized, double‐blind, cross‐over drug trial. The diagnosis of LEMS was made based on the combination of fluctuating muscle weakness, diminished or absent reflexes, and more than 60% increment of the compound muscle action potential (CMAP) amplitude after brief exercise or 50‐HZ stimulation on a repetitive nerve stimulation (RNS) test. Evaluations were done at baseline, with placebo, and with 3,4‐DAP (up to 75–80 mg/day). Assignment of placebo or 3,4‐DAP was done in a double‐blinded manner. Measurements included subjective symptoms score, objective clinical measurements [LEMS classification, muscle strength score, quantitative myasthenia gravis (QMG) score] and RNS test and single‐fiber electromyography (SFEMG). The differences between placebo and baseline values (placebo change) were compared with the differences between 3,4‐DAP and baseline or placebo values (DAP change). Seven patients with LEMS (QMG score >9) participated in the study. One patient had major side‐effects with 3,4‐DAP and withdrew from the study. Statistically significant efficacy was noted with DAP change (N = 13) compared with placebo change (N = 7) according to the subjective symptoms score (P = 0.01), LEMS classification (P < 0.001), muscle strength score (P < 0.006), QMG score (P = 0.02), and CMAP (P = 0.03). For long‐term treatment, 2 patients preferred 3,4‐DAP, 1 chose guanidine hydrochloride, 1 preferred pyridostigmine, and 2 chose no treatment. A randomized, double‐blind, cross‐over drug trial of 3,4‐DAP showed significant efficacy over placebo in patients with LEMS. As a long‐term treatment, however, not all patients preferred this drug. Muscle Nerve, 2009     

Predicting fatigue during electrically stimulated non‐isometric contractions

Mathematical prediction of power loss during electrically stimulated contractions is of value to those trying to minimize fatigue and to those trying to decipher the relative contributions of force and velocity. Our objectives were to: (1) develop a model of non‐isometric fatigue for electrical stimulation–induced, open‐chain, repeated extensions of the leg at the knee; and (2) experimentally validate the model. A computer‐controlled stimulator sent electrical pulses to surface electrodes on the thighs of 17 able‐bodied subjects. Isometric and non‐isometric non‐fatiguing and fatiguing leg extension torque and/or angle at the knee were measured. Two existing mathematical models, one of non‐isometric force and the other of isometric fatigue, were combined to develop the non‐isometric force–fatigue model. Angular velocity and 3 new parameters were added to the isometric fatigue model. The new parameters are functions of parameters within the force model, and therefore additional measurements from the subject are not needed. More than 60% of the variability in the measurements was explained by the new force–fatigue model. This model can help scientists investigate the etiology of non‐isometric fatigue and help engineers to improve the task performance of functional electrical stimulation systems. Muscle Nerve, 2010     

Fiber-type dependence of stretch-induced force enhancement in rat skeletal muscle

When an active muscle is stretched, the force increases due to strain of contractile and noncontractile proteins. We examined this force enhancement in rat extensor digitorum longus (EDL) and soleus muscles, which differ in their composition of these proteins, and their susceptibility to damage. Small stretches were applied at different velocities during isometric contractions from which we quantified the velocity-dependent contractile and velocity-independent noncontractile contributions to force enhancement. Whereas the contractile contribution was significantly greater in soleus than EDL, the noncontractile force enhancement was significantly greater in EDL than soleus, and increased ≈6-fold after damaging eccentric contractions. The increased contractile stiffness may be functionally beneficial in slow muscle, as resistance to lengthening is fundamental to maintaining posture. Following stretch-induced muscle damage this capacity is compromised, leading to increased strain of noncontractile proteins that may facilitate the activation of signaling pathways involved in muscle adaptation to injury.     

Effects of botulinum toxin type A on non‐injected bi‐articular muscle include a narrower length range of force exertion and increased passive force

Introduction: The goal of this study was to test the hypothesis that botulinum toxin type A (BTX‐A) injection in rat tibialis anterior (TA) muscle affects the mechanics of its bi‐articular synergist, both actively and passively. Methods: Two groups of Wistar rats were tested: control (no BTX‐A) and BTX (0.1 U of BTX‐A) animals were injected exclusively to the mid‐belly of TA. Extensor digitorum longus (EDL) muscle isometric forces were measured after proximal and distal lengthening. Results: Five days after injection, BTX‐A administration changed EDL mechanics: (1) active forces decreased (proximal muscle length dependently); (2) length range of active force exertion decreased both proximally and distally; and (3) passive muscle forces increased. Conclusions: Effects of BTX‐A appear to not be limited to decreased active muscle tone, but may cause also a narrower active range of movement and increased passive resistance. Through spread of BTX‐A to a bi‐articular muscle, such effects are plausible for both joints spanned. Muscle Nerve 49 : 866–878, 2014     

Isotonic fatigue in laminin alpha2-deficient dy/dy dystrophic mouse diaphragm

Laminin alpha2 deficiency causes approximately 50% of human congenital muscular dystrophies. Muscle in the corresponding dy/dy mouse model has reduced force but increased fatigue resistance during isometric contractions. To determine whether a similar pattern of alterations is present during isotonic contractions, dy/dy diaphragm was studied in vitro. During 20% load, dystrophic diaphragm had significantly reduced shortening, shortening velocity, work and power deficits, which persisted during the fatigue-inducing stimulation. In contrast, during 40% load, isotonic contractile performance of diseased muscle was impaired only mildly and only for some contractile parameters. At both loads, rate of isotonic fatigue when expressed relative to initial contractile values was similar for dystrophic and normal muscle, or in some instances slightly higher for dystrophic muscle. Therefore, fatigue resistance is considerably impaired during isotonic contractions relative to that reported previously for isometric contractions. This has important implications for increased susceptibility to respiratory failure in laminin alpha2-deficient muscular dystrophy.     

Variable stimulation patterns for poststroke hemiplegia

Neuromuscular electrical stimulation can improve motor function in those affected by paralysis, but its use is limited by a high rate of muscular fatigue. Variable stimulation patterns have been examined in young adults with and without spinal cord injury, but much less investigation has been devoted to studying the effects of variable stimulation patterns administered to older adults or those paralyzed by stroke. Significant changes occur in the neuromuscular system with age that may affect the response to variable stimulation patterns. We administered three, 3‐min intermittent stimulation patterns to the median nerves of 10 individuals with hemiplegia from stroke and 10 age‐matched able‐bodied adults: (1) constant 20 HZ , (2) a pattern that began at 20 HZ and progressively increased to 40 HZ in the latter half of the task, and (3) a 20‐HZ pattern that switched to a 20‐HZ doublet pattern after 90 s. In the able‐bodied group the doublet pattern produced significantly higher force time integrals (FTI) (1409.72 ± 3.15 N · s) than the 20–40‐HZ pattern (1067.46 ± 1.15 N · s) or the 20‐HZ pattern (831 ± 1.87 N · s). In the poststroke individuals the doublet pattern also produced the highest FTI (724.04 ± 2.02 N · s), and there was no significant difference between the 20–40‐HZ (636.42 ± 1.65 N · s) and 20‐HZ (583.64 ± 3.02 N · s) patterns. These results indicate that protocols that incorporate doublets in the later stages of fatigue are effective in older adults and in older adults with paralysis from stroke. © 2009 Wiley Periodicals, Inc. Muscle Nerve 39: 54–62, 2009     

Effects of training on contractile properties of paralyzed quadriceps muscle

Effects of two different training regimens on the contractile properties of the quadriceps muscle were studied in six individuals with spinal cord injury. Each subject had both limbs trained with the two regimens, consisting of stimulation with low frequencies (LF) at 10 HZ or high frequencies (HF) at 50 HZ; one limb of each subject was stimulated with the LF protocol and the other with the HF regimen. Twelve weeks of daily training increased tetanic tension by approximately 20%, which was not significantly different between training regimens. Interestingly, after HF but not LF training, the unusual high forces at the low frequency range of the force-frequency relationship decreased, possibly due to a reduced activation per impulse. After LF but not HF training, force oscillation amplitudes declined (by 33%) as relaxation tended to slow, which may have opposed possible effects of reduced activation as seen after HF training. Finally, fatigue resistance also increased rapidly after LF training (by 43%) but not after HF training. These results indicate that different types of training may selectively change different aspects of function in disused muscles.     

Twitch potentiation after voluntary contraction and neuromuscular electrical stimulation at various frequencies in human quadriceps femoris

Introduction: In this study we aimed to compare the extent of twitch potentiation (TP) after voluntary contraction and percutaneous electrical stimulation of muscles (neuromuscular electrical stimulation: NMES) with various stimulation frequencies at equivalent target levels. Methods: Isometric knee extensions of 10 s were performed at a 40% maximal voluntary contraction level by voluntary or NMES conditioning contractions at 20, 40, and 80 HZ of the quadriceps femoris muscle. Twitch responses were elicited by stimulating the femoral nerve transcutaneously at supramaximal intensity. Results: NMES at 80 HZ induced significantly less TP (128.7 ± 17.1%) than voluntary contraction (156.2 ± 23.1%), whereas no statistical difference was found in TP among voluntary and 20‐ and 40‐HZ NMES conditioning contractions (170.8 ± 21.1% and 162.7 ± 16.9% for 20‐ and 40‐HZ NMES, respectively). Conclusion: Stimulation frequency of NMES determines whether NMES can induce TP comparable to that after voluntary contraction. Muscle Nerve 45: 110–115, 2012     

Inflammatory response during slow‐ and fast‐twitch muscle regeneration

Introduction: Skeletal muscles are characterized by their unique ability to regenerate. Injury of a so‐called fast‐twitch muscle, extensor digitorum longus (EDL), results in efficient regeneration and reconstruction of the functional tissue. In contrast, slow‐twitch muscle (soleus) fails to properly reconstruct and develops fibrosis. This study focuses on soleus and EDL muscle regeneration and associated inflammation. Methods: We determined differences in the activity of neutrophils and M1 and M2 macrophages using flow cytometry and differences in the levels of proinflammatory cytokines using Western blotting and immunolocalization at different times after muscle injury. Results: Soleus muscle repair is accompanied by increased and prolonged inflammation, as compared to EDL. The proinflammatory cytokine profile is different in the soleus and ED muscles. Conclusions: Muscle repair efficiency differs by muscle fiber type. The inflammatory response affects the repair efficiency of slow‐ and fast‐twitch muscles. Muscle Nerve 55 : 400–409, 2017