Muscle excitation in elderly adults: the effects of training.

Muscle membrane excitability is thought to decline with aging; the extent of this decline may be noninvasively assessed by measurement of the electrically evoked compound muscle action potential (M-wave). The intent of this study was two-fold: (1) to compare the M-wave in the brachioradialis (BR), tibialis anterior (TA), and thenar (TH) muscles of elderly (mean age = 66.3 +/- 3.7 years) and young (mean age = 31.2 +/- 4.9 years) adults, and (2) to determine the effects of 12 weeks of resistance training on M-wave characteristics in elderly adults. Prior to training, the elderly subjects had significantly smaller (P less than 0.05) resting M-waves than the young adults in the BR (4.8 mV vs. 8.7 mV), TA (8.8 mV vs. 11.0 mV), and TH (5.2 mV vs. 10.2 mV) muscles. During a 2-minute voluntary fatigue paradigm (3 seconds MVC per 2 seconds rest for 2 minutes), there was no evidence of excitability failure in either group. Following training, there was a significant increase (P less than 0.05) in the size of the M-wave of the TH (pretraining: 5.2 mV; posttraining: 8.96 mV) and BR (pretraining: 4.8 mV; posttraining: 6.1 mV), and a nonsignificant increase in the M-wave of the TA, but there was no change in the relative behavior of the M-wave during the 2-minute voluntary fatigue paradigm. It is suggested that the decline in muscle membrane excitation with aging may be due, at least in part, to the effects of a decreased membrane potential on the muscle fiber action potential.(ABSTRACT TRUNCATED AT 250 WORDS)     

Within-train neuromuscular propagation varies with torque in paralyzed human muscle

Electromyographic (EMG) recordings may serve an important role in predicting torque during repetitive activation of paralyzed muscle. We compared the initial M-wave to the subsequent M-waves of the same train under fatigued and recovered conditions in the paralyzed human soleus muscle. Sixteen individuals with chronic (n = 13) or acute paralysis (n = 3) had the tibial nerve activated before and after a repetitive supramaximal stimulation protocol. The mean within-train M-wave amplitude and median frequency increased approximately 20%, whereas the duration decreased approximately 15% compared with the initial M-wave of each train. During fatigue, there was a linear decrease in the difference between the initial M-wave amplitude and subsequent train ( approximately 20% to 8%). Following fatigue, this difference recovered to approximately 12%. The difference between the M-wave train average and the initial M-wave for amplitude, duration, and median frequency closely followed torque (Pearson correlations = 0.99, 0.94, and 0.98, respectively) during fatigue. We conclude that the difference between the later-occurring M-waves (average of the train) and initial M-wave is large when muscle torque is high and less when torque is low and, therefore, predicts torque during activation of paralyzed muscle. This difference in the within-train M-wave amplitude, duration, and median frequency may reflect a mechanical change, such as muscle shortening and increased muscle cross-sectional area during isometric contractions. Electromyographic feedback may assist in the optimization of neuromuscular electrical stimulation of paralyzed muscle.     

Neuromuscular propagation after fatiguing contractions of the paralyzed soleus muscle in humans

We analyzed the M wave and torque after repetitive activation and recovery of the human soleus muscle in individuals with spinal cord injury. Fifteen individuals with complete paralysis had the tibial nerve activated for 330 ms every second with a 20-Hz train. The M wave and torque were analyzed before fatigue, immediately after fatigue, and during recovery. The torque and three M-wave measurements (amplitude, duration, median frequency) changed significantly after fatigue in the chronic group, but the M-wave area was not changed. The M wave was completely recovered after 5 min of rest, even though the torque remained depressed during recovery. The M-wave changes appeared to contribute minimally to the reduced torque in individuals with chronic paralysis. The disassociation in the M-wave–torque relationship during fatigue and recovery suggests, that electrical stimulation under electromyography control is not an ideal method to optimize torque in paralyzed muscle. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21:776–787, 1998.     

Quantitative evaluation of electromyogram activity in rat extensor and flexor muscles immobilized at different lengths

Because immobilization of muscles in the "long" position mitigates the effects of inactivity and rapid wasting occurs when muscles are immobilized in the "short" position, a study was made of the EMG activity in the soleus (SOL)--an extensor muscle--and the tibialis anterior (TA)--a flexor muscle--in order to clarify the possible role of muscle function in modifying the course of disuse atrophy. EMG activity was recorded in the SOL and TA muscles in adult rats in which the ankle had been immobilized in a plaster cast either in plantar flexion or dorsiflexion. The number of action potentials per minute in samples of the EMG activity from control and immobilized muscles was assessed before, for 10 days during immobilization, and up to 9 days after removal of the cast. Immobilization in the short position (plantar flexion) led to a dramatic reduction in the EMG activity of the SOL (to 10% of the control). On the other hand, fixation of the SOL in the long position was without effect upon resting EMG activity. In the TA, EMG activity was exclusively phasic in character and corresponded to about 3% of that of the SOL. Neither the fixation of the ankle in plantar flexion nor dorsiflexion had any appreciable effect upon EMG activity in the TA. We conclude that, because immobilization in the lengthened position does not increase EMG activity in either extensor or flexor muscles, passive stretch appears to be the factor mainly responsible for mitigating the effects of disuse in this situation. On the other hand, when a typical extensor muscle (SOL) is immobilized in the shortened position and undergoes rapid wasting, an accessory role of decreased activity cannot be excluded.     

Depression of human electromyographic activity by fatigue of a synergistic muscle

In human volunteers, lateral gastrocnemius muscles were stimulated electrically under ischemic conditions so as to produce fatigue. Recordings of electromyographic (EMG) activity were then made from those muscles and simultaneously from untreated medial gastrocnemius muscles during maximal voluntary efforts. In the lateral gastrocnemius the mean amount of EMG activity declined by 52% and was associated with a 35% reduction in the mean amplitude of the M wave (muscle compound action potential) and an insignificant change in M-wave area. In the medial gastrocnemius the EMG was also diminished, by 29%, but there were no significant changes in M-wave amplitude or area. The findings in the medial gastrocnemius are consistent with the existence of an inhibitory reflex effect which originates in the fatigued lateral gastrocnemius muscle and serves to depress excitation in motoneurons supplying that muscle and also in those innervating synergists. The inhibitory effect appears to be long-lasting, in that a significant reduction of the EMG could still be demonstrated 10 min after release of the arterial cuff. © 1997 John Wiley & Sons, Inc. Muscle Nerve 20, 710–717, 1997.     

Co-contractions in antagonistic hindlimb muscles during simulated step cycle rates

To determine whether the step cycle rate during locomotion may be limited by mechanical interactions of antagonistic muscles about the ankle, muscle activation patterns were simulated through electrical stimulation and muscle forces monitored in the rat lateral gastrocnemius-soleus (LGS) and tibialis anterior (TA) muscles. Simulations were made in acute experiments on Nembutal-anesthetized rats. Based on EMG data recorded from free-moving rats at 3 treadmill speeds, SOL mean cycle durations, cycles per second (eps), burst durations, and TA onset times following SOL EMG onset were determined. With muscle nerve stimulation based on these temporal patterns, muscle force was monitored from the detached tendons or directly at the approximate insertion sites. To investigate mechanical interactions attributed to the slow-twitch SOL muscle, forces were measured with the LGS intact, and after the SOL was denervated or the tendon was cut. With tendons detached, TA contraction force temporally overlapped with LGS or LG contraction force over periods ranging between 20 and 50 ms at 3.4 cps, 0-10 ms at 4.1 cps, and 0-10 ms at 4.6 cps. LGS force-time traces were asymmetrical, i.e. return time toward baseline force was more prolonged than the time to reach peak force. Without contributions to force from SOL activation, LG rate of relaxation was consistently increased and rate of rise and fall in force-time patterns were more symmetrical. Co-contractions were not observed when LGS contractions followed TA activation. At all cycle rates, forces remained above baseline (range 0.5-4.0 N) between contractions in LGS.(ABSTRACT TRUNCATED AT 250 WORDS)     

Putative effects of nerve extract on carbonic anhydrase III expression in rat muscles

Carbonic anhydrase III (CA III), the predominant CA isoform in skeletal muscle is very sensitive to neuronal influences. We aimed to determine whether CA III expression could be influenced by neurotrophic factor(s) present in sciatic nerve extract (SNE). Intact muscles were thus compared with denervated soleus (SOL), extensor digitorum longus (EDL), and tibialis anterior (TA) muscles injected daily for 7 days with saline solution (SS) or with SNE. CA III activity was significantly increased in SS-treated EDL and TA muscles compared to control (CTR), while SNE injections partially prevented this increase. There was no significant difference for CA III activity in the SOL between CTR, SS, and SNE groups. The CA III mRNA increase observed in response to denervation was reduced by 40% in SNE-treated EDL and TA muscles. While SOL CA III mRNA level was not affected by denervation, a 52% decrease was observed with SNE. We concluded that neuronal modulation of CA III expression in type II fibers may involve a neurotrophic component. © 1994 John Wiley & Sons, Inc.     

Examination of pain ratings associated with elicitation of the maximal H-wave and maximal M-wave in the soleus and flexor carpi radialis muscles

The present study involved an examination of pain ratings associated with elicitation of the maximal H-wave and maximal M-wave in the soleus and flexor carpi radialis (FCR) muscles. Sixteen male students provided pain intensity ratings associated specifically with elicitation of the maximal H-wave and maximal M-wave in the soleus and FCR muscles before and after a 1 h period of seated rest on two trials separated by 1 wk. There were no significant interactions or main effects of Muscle Group, Time, or Trials for the pain ratings associated with the maximal H-wave and maximal M-wave in the soleus and FCR muscles. Pain ratings and current were higher for the maximal M-wave than the maximal H-wave in both the soleus and FCR muscles. We concluded that pain ratings associated with the maximal H-wave and maximal M-wave do not differ between the soleus and the FCR muscles, and that pain ratings are higher for the maximal M-wave than the maximal H-wave.     

Fatigue associated EMG behavior of the first dorsal interosseous and adductor pollicis muscles in different groups of subjects

We have studied the fatigue-associated behavior of surface EMG in two histochemically different muscles of the hand: first dorsal interosseous (FDI) and adductor pollicis (AP; relatively more type I fibers in AP than in FDI). During a fatigue test evoked by electrical stimulation of the ulnar nerve, the mean amplitudes of compound muscle action potentials (M-waves) exhibited the same overall pattern for both muscles: a rapid phase of potentiation followed by a gradual decline. However, if the group of subjects was subdivided on the basis of hand length, significant differences emerged in the reactions of AP: in large hands, no fatigue-associated M-wave decline was seen, whereas in small hands a distinct decline was observed. A possible explanation for this phenomenon might be the presence of a greater amount of EMG contamination from other muscles in smaller hands. In the supposedly “cleaner” recordings from larger hands, significant differences between FDI and AP were observed with regard to their fatigue-associated EMG reactions (M-wave depression in FDI but not in AP). The direction of these differences was in accordance with expectations on the basis of known differences in histochemical fiber type composition. © 1994 John Wiley & Sons, Inc.     

Effect of precocious locomotor activity on the development of motoneurones and motor units of slow and fast muscles in rat

We have investigated the effect of precociously increasing locomotor activity during early postnatal development by daily treatment with the monoaminergic precursor L-DOPA on the survival of motoneurones supplying the slow soleus (SOL) muscle and the fast, tibialis anterior (TA) and extensor digitorum longus (EDL) muscles as well as the contractile and histochemical properties of these muscles. L-DOPA treatment resulted in a significant loss of motoneurones to the slow SOL muscle, but not to the fast TA and EDL muscles. Moreover, motoneurones to fast muscles also die as when exposed to increased activity in early life, if their axons are repeatedly injured. The loss of normal soleus motoneurones was accompanied by an increase in force of the remaining motor units and sprouting of the surviving axons suggesting a remodelling of motor unit organisation. The time to peak contraction of both SOL and EDL muscles from L-DOPA treated rats was prolonged at 8 weeks of age. At 4 weeks the soleus muscles of the L-DOPA treated animal developed more tension than the saline treated one. This difference between the two groups did not persist and by 8 weeks of age the muscle weight and tetanic tension from either group were not significantly different from control animals. The present study shows that early transient, precocious locomotor activity induced by L-DOPA is damaging to normal soleus but not to normal EDL/TA motoneurones.     

EXAMINATION OF PAIN RATINGS ASSOCIATED WITH ELICITATION OF THE MAXIMAL H-WAVE AND MAXIMAL M-WAVE IN THE SOLEUS AND FLEXOR CARPI RADIALIS MUSCLES

The present study involved an examination of pain ratings associated with elicitation of the maximal H-wave and maximal M-wave in the soleus and flexor carpi radialis (FCR) muscles. Sixteen male students provided pain intensity ratings associated specifically with elicitation of the maximal H-wave and maximal M-wave in the soleus and FCR muscles before and after a 1 h period of seated rest on two trials separated by 1 wk. There were no significant interactions or main effects of Muscle Group, Time, or Trials for the pain ratings associated with the maximal H-wave and maximal M-wave in the soleus and FCR muscles. Pain ratings and current were higher for the maximal M-wave than the maximal H-wave in both the soleus and FCR muscles. We concluded that pain ratings associated with the maximal H-wave and maximal M-wave do not differ between the soleus and the FCR muscles, and that pain ratings are higher for the maximal M-wave than the maximal H-wave     

Long-term electrical stimulation of muscles in children with Duchenne and Becker muscular dystrophy.

Nine children suffering from progressive muscular dystrophy (7 Duchenne and 2 Becker) were included in a program of low-frequency electrical stimulation (LFES) of the right tibialis anterior (TA) muscle. Muscle strength and muscle fatigue were estimated by measuring torques in the ankle during attempts of maximal voluntary contraction (MVC) in the direction of dorsal flexion of the foot and during electrically evoked contractions (EEC). No important increase in the strength of the stimulated muscles was noticed in 4 boys whose muscles were stimulated for 3 months. The muscles of 5 boys who were subjected to electrical stimulation for 9 months showed an improvement; 6 measurements made during the stimulation program revealed that changes of torques in the ankle of the right stimulated extremity were significantly different (P less than 0.001) from the changes of torques in the ankle of the left nonstimulated extremity.     

Repeatability of electrically evoked EMG signals in the human vastus medialis muscle

The repeatability of spectral and amplitude parameters and conduction velocity estimates of M-waves was tested on the vastus medialis muscle of 9 normal subjects. Isometric contractions sustained for 30 s were elicited by stimulation of the main muscle motor point and repeated on 5 different days. The initial value and two indicators of myoelectric manifestations of muscle fatigue were computed for each variable. The test—retest Pearson correlation coefficient, the paired Wilcoxon test, and the intraclass correlation coefficient (ICC) showed that parameters of spectral variables are more repeatable than those of amplitude variables and conduction velocity. The ICC ranged from 86.4% for the initial value of median frequency to 21.6% for the initial value of the average rectified value. Fatigue indices showed ICCs in the range of 20–64%. It is concluded that: (a) initial values and fatigue indices based on spectral variables are more repeatable than those based on amplitude variables; (b) the repeatability of conduction velocity estimates is not yet sufficient for clinical application; (c) M-wave shape, rather than amplitude or width, is a characteristic of individual muscles; and (d) electrode location is critical. © 1998 John Wiley & Sons, Inc. Muscle Nerve 21: 184–193, 1998.     

Fatigability of normal and dystrophic chicken muscle in vivo

Changes in the indirectly elicited isometric twitch tension, tetanic tension, twitch-to-tetanus ratio, and post-tetanic potentiation in response to fatigue were examined in the posterior latissimus dorsi muscles of normal and genetically dystrophic New Hampshire chickens. Contractile parameters were studied during 3-h fatigue stimulations and during continuous infusion of potassium chloride (0.24 m equiv/min) initiated at the conclusion of the fatigue period. Both twitch and tetanic tension of dystrophic muscles showed a relative resistance to fatigue; no significant changes in either twitch-to-retanus ratio or post-tetanic potentiation occurred during the fatigue period. In contrast, twitch and tetanic tension of normal muscles decreased more rapidly and to a greater extent in response to fatigue. The twitch-to-tetanus ratio decreased and post-tetanic potentiation increased such that after 30 min they were not significantly different from values seen in dystrophic muscles. Potassium chloride infusion produced a significant recovery (two- to ninefold improvement) of the fatigued twitch response to dystrophic muscle but did not have a significant effect on fatigued normal muscles. A comparison of directly and indirectly elicited twitch contractions indicated that part of the decrement of contractile response in dystrophic muscle was due to synaptic failure at the neuromuscular junction and that potassium chloride infusion resulted in restoration of neuromuscular transmission. It is suggested that the difference in fatigue pattern observed between normal and dystrophic muscle was a function of an altered distribution in the physiological types of motor units present in the diseased muscle.     

Effect of hind-limb suspension on young and adult skeletal muscle. II. Dystrophic mice

Disuse atrophy induced by limb immobilization reportedly protects dystrophic mouse muscle from histopathological changes. This study was conducted to determine whether disuse atrophy induced by hind-limb suspension (HS) limits the histopathology and contractile abnormalities typically observed in the dystrophic mouse. Two weeks of hind-limb suspension were applied to dystrophic mice (line 129B6F1) at two ages, 4 weeks (6 mice) and 12 weeks (8 mice). Thirty-one untreated dystrophics served as controls. In general, HS exaggerated the dystrophic signs, especially in the younger mice; it reduced animal weight, muscle weight, maximum tetanic and twitch tensions, and rates of tetanic and twitch tension development. HS further slowed the contractile properties of soleus (SOL) and extensor digitorum longus (EDL) muscles, and increased their fatigue resistance. HS reduced the size of type I and IIA fibers in the 6-week SOL and EDL, but not in the 14-week muscles. HS produced a preferential atrophy of SOL type I fibers, with a parallel increase in type IIA fibers. However, it did not alleviate the fiber size variability, degree of necrosis, central nucleation, inflammation, or muscle fibrosis in dystrophic muscles. These data demonstrate that disuse by hind-limb suspension does not prevent the histopathological deterioration or loss of muscle function in 6- and 14-week dystrophic mice.     

EFFECT OF NEOSTIGMINE, d-TUBOCURARINE AND CLINICAL CONDITION ON EVOKED ELECTROMYOGRAM FROM ORBICULARIS OCULI IN MYASTHENIA GRAVIS

In our previous report, evoked electromyograms of the orbicularis oculi muscle with repetitive stimulation given at frequencies of 5, 10, 20, 50 and 100 c/s were examined in myasthenia gravis. The amplitude of M-waves showed several specific changes against the stimulation frequencies employed. The patterns of the amplitude of M-waves appearing with each frequency could be classified into 3 types, Type I, Type II and Type III according to relation between the frequency of stimulation and decrease and/ or increase of M-wave amplitude. In the present study, the effect of neostigmine, d-tubocurarine and clinical condition on the various pattern of M-wave amplitude curve classified into above mentioned three types was examined.     

Fatigue‐related changes in fascicle–tendon geometry over repeated contractions: Difference between synergist muscles

Fatigue‐induced changes in force production of synergist muscles were evaluated through observation of fascicle‐tendon geometry and electromyography (EMG). Seven subjects performed 60 maximal isometric plantar flexions intermittently. No statistically significant intermuscle difference was observed in the decrease of mean EMG amplitudes or mean power frequency for the medial gastrocnemius (MG) and soleus (SOL) muscles. The tendon elongation of MG significantly decreased after the 19th contraction, and MG fascicle length increased after the 29th contraction, while SOL fascicle and tendon length did not change except for the last contraction. The declines in torques were highly correlated with the increase of MG fascicle length and decrease in tendon elongation in each subject, while no consistent relationship was found for SOL. These results suggest that changes in force‐production of MG and SOL over repeated contractions differ, which is reflected in fascicle–tendon geometry. Muscle Nerve 40: 395–401, 2009     

Changes in the cholinergic system of rat sciatic nerve and skeletal muscle following suspension-induced disuse

Muscle disuse-induced changes in the cholinergic system of sciatic nerve, slow-twitch soleus (SOL), and fast-twitch extensor digitorum longus (EDL) muscles were studied in rats. Rats with hind limbs suspended for 2 to 3 weeks showed marked elevation in the activity of choline acetyltransferase in sciatic nerve (38%), in the SOL (108%), and in the EDL (67%). Acetylcholinesterase (AChE) activity in the SOL increased 163% without changing the molecular forms pattern of 4S, 10S, 12S, and 16S. No significant (P greater than 0.05) changes in the activity and molecular forms pattern of AChE were seen in the EDL or in AChE activity of sciatic nerve. Nicotinic receptor binding of [3H]acetylcholine was increased in both muscles. When measured after 3 weeks of hind limb suspension the normal distribution of type I fibers in the SOL (87%) was reduced (to 58%) and a corresponding increase in types IIa and IIb fibers occurred. In the EDL no significant change in fiber proportion was observed. Muscle activity, such as loadbearing, appeared to have a greater controlling influence on the characteristics of the slow-twitch SOL muscle than on the fast-twitch EDL muscle.     

Contractile and fatigue properties of thyrotoxic rat skeletal muscle

The effects of thyrotoxicosis on the contractile properties and development of muscle fatigue in the slow soleus (SOL) and fast extensor digitorum longus (EDL) muscles were examined in rats given 3 mg of L-thyroxine and 1 mg of L-triiodothyronine per kilogram of diet for 6 weeks. The hormone treatment produced significant decreases in the contraction time, one-half relaxation time, and twitch tension in the SOL, while the peak rate of tension development (+ dP/dt) and decline (? dP/dt) in this muscle were elevated. Additionally, the forcefrequency curve was shifted to the right and, thus, resembled the curve of a normal fast-twitch muscle. In contrast, the contractile properties of the fast EDL were relatively unaltered by the hormone administration. Thyrotoxicosis also changed the SOL response to contractile activity as twitch tension, + dP/dt, and ? dP/dt remained high, and a faster decline in muscle glycogen and an increase in lactate occurred compared to control muscles. These results clearly demonstrate a preferential effect of thyroid hormone on slow compared to fast skeletal muscle.     

Effects of denervation and immobilisation during development upon [3H]ouabain binding by slow- and fast-twitch muscle of the rat

The effect of innervation and of muscle inactivity upon the normal production of Na+-K+-ATPase sites, assayed by [3H]ouabain binding, in muscle surface membranes has been determined for the rat. In both slow-twitch soleus (SOL) and fast-twitch extensor digitorum longus (EDL) muscles a large increase was found to occur in [3H]ouabain binding per unit weight of muscle over the first 3 weeks of life. Interruptions of development, brought about by fixation of muscles at different lengths at 5 days of age, had no significant effect upon [3H]ouabain binding by EDL. In contrast, fixation led to a decrease in binding in SOL. When fixed in a shortened position profound morphological changes occurred, although these were not apparent when SOL was fixed in a stretched position. Denervation of SOL at 5 days of age significantly reduced the age related increase in the density of [3H]ouabain binding, whilst denervation of EDL had little effect. It was concluded that normal development of SOL is dependent upon innervation and possibly the resulting muscle activity, whereas development of EDL was relatively independent of innervation.