Neuromuscular factors contributing to in vivo eccentric moment generation

Webber, Sandra, and Dean Kriellaars. Neuromuscularfactors contributing to in vivo eccentric moment generation.J. Appl. Physiol. 83(1): 40-45, 1997.Muscle series elasticity and its contribution to eccentricmoment generation was examined in humans. While subjects [male,n = 30; age 26.3 ± 4.8 (SD) yr; body mass 78.8 ± 13.1 kg] performed an isometric contractionof the knee extensors at 60° of knee flexion, a quick stretch was imposed with a 12°-step displacement at 100°/s. The test wasperformed at 10 isometric activation levels ranging from 1.7 to 95.2%of maximal voluntary contraction (MVC). A strong linear relationship was observed between the peak imposed eccentric moment derived fromquick stretch and the isometric activation level(y = 1.44x + 7.08; r = 0.99). This increase in theeccentric moment is consistent with an actomyosin-dependent elasticitylocated in series with the contractile element of muscle. Byextrapolating the linear relationship to 100% MVC, the predictedmaximum eccentric moment was found to be 151% MVC, consistent with invitro data. A maximal voluntary, knee extensor strength test was alsoperformed (5-95°, 3 repetitions, ±50, 100, 150, 200, and250°/s). The predicted maximum eccentric moment was 206% of theangle- and velocity-matched, maximal voluntary eccentric moments. Thiswas attributed to a potent neural regulatory mechanism that limits therecruitment and/or discharge of motor units during maximalvoluntary eccentric contractions.

     

Effects of muscle kinematics on surface EMG amplitude and frequency during fatiguing dynamic contractions

Potvin, J. R. Effects of muscle kinematics on surfaceEMG amplitude and frequency during fatiguing dynamic contractions. J. Appl. Physiol. 82(1): 144-151, 1997.Fifteen male subjects performed a repetitive elbowflexion/extension task with a 7-kg mass until exhaustion. Average jointangle, angular velocity, and biceps brachii surface electromyographic(EMG) amplitude (aEMG) and mean powerfrequency (MPF) were calculated with each consecutive 250-ms segment ofdata during the entire trial. Data were separated into concentric oreccentric phases and into seven 20°-ranges from 0 to 140° ofelbow flexion. A regression analysis was used to estimate the restedand fatigued aEMG and MPF values. aEMG values were expressed as apercentage of amplitudes from maximum voluntary contractions (MVC).Under rested dynamic conditions, the average concentric aEMG amplitudewas 10% MVC higher than average eccentric values. Rested MPF valueswere similar for concentric and eccentric phases, although valuesincreased ~20 Hz from the most extended to flexed joint angles.Fatigue resulted in an average increase in concentric and eccentricaEMG of 35 and 10% MVC, respectively. The largest concentric aEMGincreases (up to 58% MVC) were observed at higher joint velocities,whereas eccentric increases appeared to be related to decreases invelocity. Fatigue had a similar effect on MPF during both concentricand eccentric phases. Larger MPF decreases were observed at shortermuscle lengths such that values within each angle range were verysimilar by the end of the trial. It was hypothesized that this findingmay reflect a biological minimum in conduction velocity beforepropagation failure occurs.

     

Impact of resistance exercise during bed rest on skeletal muscle sarcopenia and myosin isoform distribution

Bamman, Marcas M., Mark S. F. Clarke, Daniel L. Feeback,Robert J. Talmadge, Bruce R. Stevens, Steven A. Lieberman, and MichaelC. Greenisen. Impact of resistance exercise during bed rest onskeletal muscle sarcopenia and myosin isoform distribution. J. Appl. Physiol. 84(1): 157-163, 1998.Because resistance exercise (REx) and bed-rest unloading (BRU)are associated with opposing adaptations, our purpose was to test theefficacy of REx against the effects of 14 days of BRU on theknee-extensor muscle group. Sixteen healthy men were randomly assignedto no exercise (NoEx; n = 8) or REx(n = 8). REx performed five sets ofleg press exercise with 80-85% of one repetition maximum (1 RM)every other day during BRU. Muscle samples were removed from the vastuslateralis muscle by percutaneous needle biopsy. Myofiber distributionwas determined immunohistochemically with three monoclonal antibodiesagainst myosin heavy chain (MHC) isoforms (I, IIa, IIx). MHCdistribution was further assessed by quantitative gel electrophoresis.Dynamic 1-RM leg press and unilateral maximum voluntary isometriccontraction (MVC) were determined. Maximal neural activation (root meansquared electromyogram) and rate of torque development (RTD) weremeasured during MVC. Reductions(P < 0.05) in type I (15%) and typeII (17%) myofiber cross-sectional areas were found in NoEx but not inREx. Electrophoresis revealed no changes in MHC isoform distribution. The percentage of type IIx myofibers decreased(P < 0.05) in REx from 9 to 2% anddid not change in NoEx. 1 RM was reduced(P < 0.05) by 9% in NoEx but wasunchanged in REx. MVC fell by 15 and 13% in NoEx and REx,respectively. The agonist-to-antagonist root mean squaredelectromyogram ratio decreased (P < 0.05) 19% in REx. RTD slowed (P < 0.05) by 54% in NoEx only. Results indicate that REx preventedBRU-induced myofiber atrophy and also maintained training-specificstrength. Unlike spaceflight, BRU did not induce shifts in myosinphenotype. The reported benefits of REx may prove useful in prescribingexercise for astronauts in microgravity.

     

Tennis players show a lower coactivation of the elbow antagonist muscles during isokinetic exercises

PurposePrevious studies have suggested that muscle coactivation could be reduced by a recurrent activity (training, daily activities). If this was correct, skilled athletes should show a specific muscle activation pattern with a low level of coactivation of muscles which are typically involved in their discipline. In particular, the aim of this study was to verify the hypothesis that the amount of antagonist activation of biceps brachii (BB) and triceps brachii (TB) is different between tennis players and non-players individuals during maximal isokinetic contractions.MethodsTen young healthy men and eight male tennis players participated in the study. The surface electromyographic signals (sEMG) were recorded from the BB and TB muscles during three maximal voluntary isometric contractions (MVC) of elbow flexors and extensors and a set of three maximal elbow flexions and extensions at 15°, 30°, 60°, 120°, 180° and 240°/s. Normalized root mean square (RMS) of sEMG was calculated as an index of sEMG amplitude.ResultsAntagonist activation (%RMSmax) of TB was significantly lower in tennis players (from 14.0 ± 7.9% at MVC to 16.3 ± 8.9% at 240°/s) with respect to non-players (from 27.7 ± 19.7% at MVC to 38.7 ± 17.6% at 240°/s) at all angular velocities. Contrary to non-players, tennis players did not show any difference in antagonist activation between BB and TB muscles.ConclusionsTennis players, with a constant practice in controlling forces around the elbow joint, learn how to reduce coactivation of muscles involved in the control of this joint. This has been shown by the lower antagonist muscular activity of triceps brachii muscle during isokinetic elbow flexion found in tennis players with respect to non-players.     

Distribution of motor unit potential velocities in the biceps brachii muscle of sprinters and endurance athletes during prolonged dynamic exercises at low force levels

In surface electromyography (sEMG), the distribution of motor unit potential (MUP) velocities has been shown to reflect the proportion of faster and slower propagating MUPs. This study investigated whether the distribution of MUP velocities could distinguish between sprinters (n = 11) and endurance athletes (n = 12) in not-specifically trained muscle (biceps brachii) during prolonged dynamic exercises at low forces. sEMG was acquired during 4 min’ exercises: unloaded, 5%, 10% and 20% of maximal voluntary contraction (MVC). The features extracted from the sEMG were: the mean muscle conduction velocity – estimated using the inter-peak latency and cross-correlation methods, the within-subject skewness (expressing the proportions of faster and slower propagating MUPs) and the within-subject standard deviation of MUP velocities (SD-mup). Sprinters showed a greater proportion of faster propagating MUPs than endurance athletes. During fatigue, the SD-mup of sprinters broadened progressively, whereas that of endurance athletes did not. The findings suggest that sprinters conveyed a greater proportion of faster motor units than endurance athletes and that motor unit behavior during fatigue differed between groups. Thus, the distribution of MUP velocities enables distinction between a muscle of sprinters and endurance athletes during prolonged dynamic exercises at low forces.     

Normalized metabolic stress for 31P-MR spectroscopy studies of human skeletal muscle: MVC vs. muscle volume

Fowler, M. D., T. W. Ryschon, R. E. Wysong, C. A. Combs, andR. S. Balaban. Normalized metabolic stress for³¹P-MR spectroscopy studies ofhuman skeletal muscle: MVC vs. muscle volume. J. Appl.Physiol. 83(3): 875-883, 1997.A criticalrequirement of submaximal exercise tests is the comparability ofworkload and associated metabolic stress between subjects. In thisstudy, ³¹P-magnetic resonancespectroscopy was used to estimate metabolic strain in the soleus muscleduring dynamic, submaximal plantar flexion in which target torque was10 and 15% of a maximal voluntary contraction (MVC). In 10 healthy,normally active adults, (PCr + P_i)/PCr, where PCr isphosphocreatine, was highly correlated with power output normalized tothe volume of muscle in the plantar flexor compartment(r = 0.89, P < 0.001). The same variable was also correlated, although less strongly(r = 0.78, P < 0.001), with power normalized toplantar flexor cross-sectional area. These findings suggest thatcomparable levels of metabolic strain can be obtained in subjects ofdifferent size when the power output, or stress, for dynamic plantarflexion is selected as a function of plantar flexor muscle volume. Incontrast, selecting power output as a function of MVC resulted in apositive linear relationship between (PCr + P_i)/PCr and thetorque produced, indicating that metabolic strain was increasing ratherthan achieving constancy as a function of MVC. These findings providenew insight into the design of dynamic muscle contraction protocolsaimed at detecting metabolic differences between subjects of differentbody size but having similar blood flow capacity and mitochondrialvolume per unit of muscle.

     

Effects of fatigue duration and muscle type on voluntary and evoked contractile properties

Behm, D. G., and D. M. M. St-Pierre. Effects of fatigueduration and muscle type on voluntary and evoked contractile properties. J. Appl. Physiol. 82(5):1654-1661, 1997.The effects of fatigue duration and muscle typeon voluntary and evoked contractile properties were investigated withan isometric, intermittent, submaximal fatigue protocol. Four groupsperformed contractions of the plantar flexors and quadriceps at variousintensities to produce long (LDF; 19 min 30 s)- and short-durationfatigue (SDF; 4 min 17 s). The LDF group had a significantly greaterdecrease in muscle activation than did the SDF group (12 vs. 5.8%)during recovery, although there was no difference in the impairment of maximum voluntary contraction force beyond 30 s of recovery. The significant decrease in the compound muscle action potential of the LDFgroup (M-wave amplitude; 14.7%) contrasted with the M-wave potentiation of the SDF group (15.7%), suggesting changes in membrane excitation may affect LDF. The quadriceps group performing contractions at 50% MVC experienced a smaller decrease in agonist electromyograph activity than did other groups, indicating both muscle and fatigue duration specificity. Impairments in excitation-contraction coupling were indicated by changes in quadriceps peak twitch and time to peaktwitch while decreases in PF M-wave amplitudes suggested a disruptionof membrane potentials. Results suggest that fatigue mechanisms may beduration (activation, half relaxation time) or muscle specific(electromyograph, twitch torque) or a combination of both (M wave, timeto peak twitch torque).

     

Distribution of innervation zone and muscle fiber conduction velocity in the biceps brachii muscle

The spatial distributions of muscle innervation zone (IZ) and muscle fiber conduction velocity (CV) were examined in nine healthy young male participants. High-density surface electromyography (EMG) was collected from the biceps brachii muscle when subjects performed isometric elbow flexions at 20% to 80% of the maximal voluntary contraction (MVC). A total of 9498 samples of IZs were identified and CVs were calculated using the Radon transform. The center and width of IZ sample distribution were compared within four different force levels and six medial to lateral electrode column positions using repeated measures ANOVA and multiple comparison tests. Significant shifts of IZ center were observed in the medial columns (Columns 5, 6, and 7) compared with the lateral columns (Columns 3 and 4) (p < 0.05). Similarly, significant differences in the IZ width were found in Column 7 and 8 compared to Column 3 (p < 0.05). In contrast, muscle CV was unaffected by column position. Instead, muscle CV was faster at 40% and 80% MVC compared to 20% MVC (p < 0.05). The findings of this study add further insights into the physiological properties of the biceps brachii muscle.     

Neuromuscular fatigue following high versus low-intensity eccentric exercise of biceps brachii muscle

PurposeThis study investigated neuromuscular fatigue following high versus low-intensity eccentric exercise corresponding to the same amount of work.MethodsTen volunteers performed two eccentric exercises of the elbow flexors: a high-intensity versus a low-intensity exercise. Maximal voluntary contraction torque and surface electromyography of the biceps brachii muscle were recorded before, immediately and 48 h after exercises. Maximal voluntary activation level, neural (M-wave) and contractile (muscular twitch) properties of the biceps brachii muscle were analysed using electrical stimulation techniques.ResultsMaximal voluntary contraction torque was significantly (P < 0.01) reduced immediately and 48 h after exercise but the reduction was not different between the two conditions. Electromyography associated with maximal voluntary contraction significantly decreased (P < 0.05) immediately and 48 h after exercise for both conditions while maximal voluntary activation level was only significantly reduced immediately after the high-intensity exercise. Peak twitch alterations were observed immediately and 48 h after exercise for both conditions while M-wave did not change.ConclusionHigh and low-intensity eccentric exercises with the same amount of work induced the same reduction in maximal strength capacities of the biceps brachii muscles. The magnitude of peripheral and central fatigue was very similar in both conditions.     

Nonuniform shortening in the biceps brachii during elbow flexion.

This study tested the common assumption that skeletal muscle shortens uniformly in the direction of its fascicles during low-load contraction. Cine phase contrast magnetic resonance imaging was used to characterize shortening of the biceps brachii muscle in 12 subjects during repeated elbow flexion against 5 and 15% maximum voluntary contraction (MVC) loads. Mean shortening was relatively constant along the anterior boundary of the muscle and averaged 21% for both loading conditions. In contrast, mean shortening was nonuniform along the centerline of the muscle during active elbow flexion. Centerline shortening in the distal region of the biceps brachii (7.3% for 5% MVC and 3.7% for 15% MVC) was significantly less (P < 0.001) than shortening in the muscle midportion (26.3% for 5% MVC and 28.2% for 15% MVC). Nonuniform shortening along the centerline was likely due to the presence of an internal aponeurosis that spanned the distal third of the longitudinal axis of the biceps brachii. However, muscle shortening was also nonuniform proximal to the centerline aponeurosis. Because muscle fascicles follow the anterior contour and centerline of the biceps brachii, our results suggest that shortening is uniform along anterior muscle fascicles and nonuniform along centerline fascicles.     

Mechanical behavior of skeletal muscle during intermittent voluntary isometric contractions in humans

Vøllestad, N. K., I. Sejersted, and E. Saugen. Mechanical behavior of skeletal muscle duringintermittent voluntary isometric contractions in humans.J. Appl. Physiol. 83(5):1557-1565, 1997.Changes in contractile speed and force-fusionproperties were examined during repetitive isometric contractions withthe knee extensors at three different target force levels. Sevenhealthy subjects were studied at target force levels of 30, 45, and60% of their maximal voluntary contraction (MVC) force. Repeated 6-s contractions followed by 4-s rest were continued until exhaustion. Contractile speed was determined for contractions elicited by electrical stimulation at 1-50 Hz given during exercise and a subsequent 27-min recovery period. Contraction time remained unchanged during exercise and recovery, except for an initial rapid shift in thetwitch properties. Half relaxation time(RT_1/2) decreased gradually by 20-40% during exercise at 30 and 45% of MVC. In the recovery period, RT_1/2 values werenot fully restored to preexercise levels. During exercise at 60% MVC,the RT_1/2 decreased for twitches and increased for the 50-Hz stimulation. In the recovery period after60% MVC, RT_1/2 values declinedtoward those seen after the 30 and 45% MVC exercise. The forceoscillation amplitude in unfused tetani relative to the mean forceincreased during exercise at 30 and 45% MVC but remained unalteredduring the 60% MVC exercise. This altered force-fusion was closelyassociated with the changes inRT_1/2. The faster relaxation mayat least partly explain the increased energy cost of contractionreported previously for the same type of exercise.

     

Facilitation of triceps brachii muscle contraction by tendon vibration after chronic cervical spinal cord injury.

One way to improve the weak triceps brachii voluntary forces of people with chronic cervical spinal cord injury may be to excite the paralyzed or submaximally activated fraction of muscle. Here we examined whether elbow extensor force was enhanced by vibration (80 Hz) of the triceps or biceps brachii tendons at rest and during maximum isometric voluntary contractions (MVCs) of the elbow extensors performed by spinal cord-injured subjects. The mean +/- SE elbow extensor MVC force was 22 +/- 17.5 N (range: 0-23% control force, n = 11 muscles). Supramaximal radial nerve stimuli delivered during elbow extensor MVCs evoked force in six muscles that could be stimulated selectively, suggesting potential for force improvement. Biceps vibration at rest always evoked a tonic vibration reflex in biceps, but extension force did not improve with biceps vibration during triceps MVCs. Triceps vibration induced a tonic vibration reflex at rest in one-half of the triceps muscles tested. Elbow extensor MVC force (when >1% of control force) was enhanced by vibration of the triceps tendon in one-half of the muscles. Thus triceps, but not biceps, brachii tendon vibration increases the contraction strength of some partially paralyzed triceps brachii muscles.     

Effects of concentric and eccentric training on muscle strength, cross-sectional area, and neural activation

Higbie, Elizabeth J., Kirk J. Cureton, Gordon L. Warren III,and Barry M. Prior. Effects of concentric and eccentric trainingon muscle strength, cross-sectional area, and neural activation.J. Appl. Physiol. 81(5):2173-2181, 1996.We compared the effects of concentric (Con) andeccentric (Ecc) isokinetic training on quadriceps muscle strength,cross-sectional area, and neural activation. Women (age 20.0 ± 0.5 yr) randomly assigned to Con training (CTG;n = 16), Ecc training (ETG;n = 19), and control (CG;n = 19) groups were tested before andafter 10 wk of unilateral Con or Ecc knee-extension training. Averagetorque measured during Con and Ecc maximal voluntary knee extensions increased 18.4 and 12.8% for CTG, 6.8 and 36.2% for ETG, and 4.7 and1.7% for CG, respectively. Increases by CTG and ETG were greater than for CG (P < 0.05). ForCTG, the increase was greater when measured with Con than with Ecctesting. For ETG, the increase was greater when measured with Ecc thanwith Con testing. The increase by ETG with Ecc testing was greater thanthe increase by CTG with Con testing. Corresponding changes in theintegrated voltage from an electromyogram measured during strengthtesting were 21.7 and 20.0% for CTG, 7.1 and 16.7% for ETG, and8.0 and 9.1% for CG. Quadriceps cross-sectional areameasured by magnetic resonance imaging (sum of 7 slices) increased morein ETG (6.6%) than in CTG (5.0%) (P < 0.05). We conclude that Ecc is more effective than Con isokinetictraining for developing strength in Ecc isokinetic muscle actions andthat Con is more effective than Ecc isokinetic training for developingstrength in Con isokinetic muscle actions. Gains in strength consequentto Con and Ecc training are highly dependent on the muscle action usedfor training and testing. Muscle hypertrophy and neural adaptationscontribute to strength increases consequent to both Con and Ecctraining.

     

Spatial reorganisation of muscle activity correlates with change in tangential force variability during isometric contractions

The aim of this study was to quantify the effects of spatial reorganisation of muscle activity on task-related and tangential components of force variability during sustained contractions. Three-dimensional forces were measured from isometric elbow flexion during submaximal contractions (50 s, 5–50% of maximal voluntary contraction (MVC)) and total excursion of the centre of pressure was extracted. Spatial electromyographic (EMG) activity was recorded from the biceps brachii muscle. The centroids of the root mean square (RMS) EMG and normalised mutual information (NMI) maps were computed to assess spatial muscle activity and spatial relationship between EMG and task-related force variability, respectively. Result showed that difference between the position of the centroids at the beginning and at the end of the contraction of the RMS EMG and the NMI maps were different in the medial–lateral direction (P < 0.05), reflecting that muscle regions modulate their activity without necessarily modulating the contribution to the task-related force variability over time. Moreover, this difference between shifts of the centroids was positively correlated with the total excursion of the centre of pressure at the higher levels of contractions (>30% MVC, R² > 0.30, P < 0.05), suggesting that changes in spatial muscle activity could impact on the modulation of tangential forces. Therefore, within-muscle adaptations do not necessarily increase force variability, and this interaction can be quantified by analysing the RMS EMG and the NMI map centroids.     

Eccentric exercise increases EMG amplitude and force fluctuations during submaximal contractions of elbow flexor muscles.

The purpose of this study was to determine the effect of eccentric exercise on the ability to exert steady submaximal forces with muscles that cross the elbow joint. Eight subjects performed two tasks requiring isometric contraction of the right elbow flexors: a maximum voluntary contraction (MVC) and a constant-force task at four submaximal target forces (5, 20, 35, 50% MVC) while electromyography (EMG) was recorded from elbow flexor and extensor muscles. These tasks were performed before, after, and 24 h after a period of eccentric (fatigue and muscle damage) or concentric exercise (fatigue only). MVC force declined after eccentric exercise (45% decline) and remained depressed 24 h later (24%), whereas the reduced force after concentric exercise (22%) fully recovered the following day. EMG amplitude during the submaximal contractions increased in all elbow flexor muscles after eccentric exercise, with the greatest change in the biceps brachii at low forces (3-4 times larger at 5 and 20% MVC) and in the brachialis muscle at moderate forces (2 times larger at 35 and 50% MVC). Eccentric exercise resulted in a twofold increase in coactivation of the triceps brachii muscle during all submaximal contractions. Force fluctuations were larger after eccentric exercise, particularly at low forces (3-4 times larger at 5% MVC, 2 times larger at 50% MVC), with a twofold increase in physiological tremor at 8-12 Hz. These data indicate that eccentric exercise results in impaired motor control and altered neural drive to elbow flexor muscles, particularly at low forces, suggesting altered motor unit activation after eccentric exercise.     

Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training

McCall, G. E., W. C. Byrnes, A. Dickinson, P. M. Pattany,and S. J. Fleck. Muscle fiber hypertrophy, hyperplasia, and capillary density in college men after resistance training.J. Appl. Physiol. 81(5):2004-2012, 1996.Twelve male subjects with recreationalresistance training backgrounds completed 12 wk of intensifiedresistance training (3 sessions/wk; 8 exercises/session; 3 sets/exercise; 10 repetitions maximum/set). All major muscle groupswere trained, with four exercises emphasizing the forearm flexors.After training, strength (1-repetition maximum preacher curl) increasedby 25% (P < 0.05). Magneticresonance imaging scans revealed an increase in the biceps brachiimuscle cross-sectional area (CSA) (from 11.8 ± 2.7 to 13.3 ± 2.6 cm²;n = 8;P < 0.05). Muscle biopsies of thebiceps brachii revealed increases(P < 0.05) in fiber areas for type I(from 4,196 ± 859 to 4,617 ± 1,116 µm²;n = 11) and II fibers (from 6,378 ± 1,552 to 7,474 ± 2,017 µm²;n = 11). Fiber number estimated fromthe above measurements did not change after training (293.2 ± 61.5 × 10³ pretraining; 297.5 ± 69.5 × 10³ posttraining;n = 8). However, the magnitude ofmuscle fiber hypertrophy may influence this response because thosesubjects with less relative muscle fiber hypertrophy, but similarincreases in muscle CSA, showed evidence of an increase in fibernumber. Capillaries per fiber increased significantly(P < 0.05) for both type I(from 4.9 ± 0.6 to 5.5 ± 0.7;n = 10) and II fibers (from 5.1 ± 0.8 to 6.2 ± 0.7; n = 10). Nochanges occurred in capillaries per fiber area or muscle area. Inconclusion, resistance training resulted in hypertrophy of the totalmuscle CSA and fiber areas with no change in estimated fiber number,whereas capillary changes were proportional to muscle fiber growth.

     

Motor unit synchronization in FDI and biceps brachii muscles of strength-trained males

Motor unit (MU) synchronization is the simultaneous or near-simultaneous firing of two MUs which occurs more often than would be expected by chance. The present study sought to investigate the effects of exercise training, muscle group, and force level, by comparing the magnitude of synchronization in the biceps brachii (BB) and first dorsal interosseous (FDI) muscles of untrained and strength-trained college-aged males at two force levels, 30% of maximal voluntary contraction (MVC) and 80% MVC. MU action potentials were recorded directly via an intramuscular needle electrode. The magnitude of synchronization was assessed using previously-reported synchronization indices: k′, E, and CIS. Synchronization was significantly higher in the FDI than in the BB. Greater synchronization was observed in the strength-trained group with CIS, but not with E or k′. Also, synchronization was significantly greater at 80% MVC than at 30% MVC with E, but only moderately greater with CIS and there was no force difference with k′. Synchronization prevalence was found to be greater in the BB (80.1%) than in the FDI (71.5%). Thus, although the evidence is a bit equivocal, it appears that MU synchronization is greater at higher forces, and greater in strength-trained individuals than in untrained subjects.     

Duration of differential activations is functionally related to fatigue prevention during low-level contractions

The aim of this study was to investigate the importance of duration of differential activations between the heads of the biceps brachii on local fatigue during prolonged low-level contractions. Fifteen subjects carried out isometric elbow flexion at 5% of maximal voluntary contraction (MVC) for 30 min. MVCs were performed before and at the end of the prolonged contraction. Surface electromyographic (EMG) signals were recorded from both heads of the biceps brachii. Differential activation was analysed based on the difference in EMG amplitude (activation) between electrodes situated at the two heads. Differential activations were quantified by the power spectral median frequency of the difference in activation between the heads throughout the contraction. The inverse of the median frequency was used to describe the average duration of the differential activations. The relation between average duration of the differential activations and the fatigue-induced reduction in maximal force was explored by linear regression analysis. The main finding was that the average duration of differential activation was positively associated to relative maximal force at the end of the 30 min contraction (R² = 0.5, P < 0.01). The findings of this study highlight the importance of duration of differential activations for local fatigue, and support the hypothesis that long term differential activations prevent fatigue during prolonged low-level contractions.     

Muscle inactivation: assessment of interpolated twitch technique

Behm, D. G., D. M. M. St-Pierre, and D. Perez. Muscleinactivation: assessment of interpolated twitch technique.J. Appl. Physiol. 81(5):2267-2273, 1996.The validity, reliability, and protocol for theinterpolated twitch technique (ITT) were investigated with isometricplantar flexor and leg extension contractions. Estimates of muscleinactivation were attempted by comparing a variety of superimposed withpotentiated evoked torques with submaximal and maximal voluntarycontraction (MVC) torques or forces. The use of nerve and surfacestimulation to elicit ITT was reliable, except for problems inmaintaining maximal stimulation with nerve stimulation at 20°plantar flexion and during leg extension. The interpolated twitchratio-force relationship was best described by a shallow hyperboliccurve resulting in insignificant MVC prediction errors withsecond-order polynomials (1.1-6.9%). The prediction error under40% MVC was approximately double that over 60% MVC, contributing topoor estimations of MVC in non-weight-bearing postimmobilized anklefracture patients. There was no significant difference in the ITTsensitivity when twitches, doublets, or quintuplets were used.The ITT was valid and reliable when high-intensity contractions wereanalyzed with a second-order polynomial.

     

Effects of electromyostimulation versus voluntary isometric training on elbow flexor muscle strength

The purpose of this study was to determine whether 7 weeks of standardized (same number and duration of repetitions, sets and rest strictly identical) electromyostimulation training of the elbow flexor muscles would induce strength gains equivalent to those of voluntary isometric training in isometric, eccentric and concentric contractions. Twenty-five males were randomly assigned to an electromyostimulated group (EMS, n = 9), a voluntary isometric group (VOL, n = 8), or a control group (CON, n = 8). Maximal voluntary isometric, eccentric and concentric strength, electromyographic (EMG) activity of the biceps and triceps brachii muscles, elbow flexor muscle activation (twitch interpolation technique) and contractile properties were assessed before and after the training period. The main findings were that the isometric torque gains of EMS were greater than those of VOL after the training period (P < 0.01) and that the eccentric and concentric torque gains were equivalent. In both groups, we observed that the mechanical twitch (Pt) was increased (P < 0.05) and that torque improvements were not mediated by neural adaptations. Considering the respective intensities of the training programs (i.e., submaximal contractions for EMS versus maximal for VOL), it can be concluded that electromyostimulation training would be more efficient than voluntary isometric training to improve both isometric and dynamic strength.