Voluntary activation level and muscle fiber recruitment of human quadriceps during lengthening contractions.

Voluntary activation levels during lengthening, isometric, and shortening contractions (angular velocity 60 degrees/s) were investigated by using electrical stimulation of the femoral nerve (triplet, 300 Hz) superimposed on maximal efforts. Recruitment of fiber populations was investigated by using the phosphocreatine-to-creatine ratio (PCr/Cr) of single characterized muscle fibers obtained from needle biopsies at rest and immediately after a series of 10 lengthening, isometric, and shortening contractions (1 s on/1 s off). Maximal voluntary torque was significantly higher during lengthening (270 +/- 55 N.m) compared with shortening contractions (199 +/- 47 N.m, P < 0.05) but was not different from isometric contractions (252 +/- 47 N.m). Isometric torque was higher than torque during shortening (P < 0.05). Voluntary activation level during maximal attempted lengthening contractions (79 +/- 8%) was significantly lower compared with isometric (93 +/- 5%) and shortening contractions (92 +/- 3%, P < 0.05). Mean PCr/Cr values of all fibers from all subjects at rest were 2.5 +/- 0.6, 2.0 +/- 0.7, and 2.0 +/- 0.7, respectively, for type I, IIa, and IIax fibers. After 10 contractions, the mean PCr/Cr values for grouped fiber populations (regardless of fiber type) were all significantly different from rest (1.3 +/- 0.2, 0.7 +/- 0.3, and 0.8 +/- 0.6 for lengthening, isometric, and shortening contractions, respectively; P < 0.05). The cumulative distributions of individual fiber populations after either contraction mode were significantly different from rest (P < 0.05). Curves after lengthening contractions were less shifted compared with curves from isometric and shortening contractions (P < 0.05), with a smaller shift for the type IIax compared with type I fibers in the lengthening contractions. The results indicate a reduced voluntary drive during lengthening contractions. PCr/Cr values of single fibers indicated a hierarchical order of recruitment of all fiber populations during maximal attempted lengthening contractions.     

Electromyographic measures of muscle activation and changes in muscle architecture of human elbow flexors during fatiguing contractions.

The study compared changes in intramuscular and surface recordings of EMG amplitude with ultrasound measures of muscle architecture of the elbow flexors during a submaximal isometric contraction. Ten subjects performed a fatiguing contraction to task failure at 20% of maximal voluntary contraction force. EMG activity was recorded in biceps brachii, brachialis, and brachioradialis muscles using intramuscular and surface electrodes. The rates of increase in the amplitude of the surface EMG for the long and short heads of biceps brachii and brachioradialis were greater than those for the intramuscular recordings measured at different depths. The amplitude of the intramuscular recordings from three muscles increased at a similar rate (P = 0.13), as did the amplitude of the three surface recordings from two muscles (P = 0.83). The increases in brachialis thickness (27.7 +/- 5.7 to 30.9 +/- 3.5 mm; P < 0.05) and pennation angle (10.9 +/- 3.5 to 16.5 +/- 4.8 degrees ; P = 0.003) were not associated with the increase in intramuscular EMG amplitude (P > 0.58). The increase in brachioradialis thickness (22.8 +/- 4.8 to 25.5 +/- 3.4 mm; P = 0.0075) was associated with the increase in the amplitude for one of two intramuscular EMG signals (P = 0.007, r = 0.79). The time to failure was more strongly associated with the rate of increase in the amplitude of the surface EMG than that for the intramuscular EMG, which suggests that the surface measurement provides a more appropriate measure of the change in muscle activation during a fatiguing contraction.     

Variable-frequency train stimulation of canine latissimus dorsi muscle during shortening contractions

George, David T., Stuart A. Binder-Macleod, Thomas N. Delosso, and William P. Santamore. Variable-frequency trainstimulation of canine latissimus dorsi muscle during shorteningcontractions. J. Appl. Physiol. 83(3):994-1001, 1997.In cardiomyoplasty, the latissimus dorsi muscle(LDM) is wrapped around the heart ventricles and electrically activatedwith a constant-frequency train (CFT). This study tested the hypothesesthat increased mechanical performance from the LDM could be achieved byactivating the muscle with variable-frequency trains (VFTs) of shorterduration or containing fewer stimulus pulses than the CFT now used. Themechanical performance of the canine LDM (n = 7) during shortening contractionswas measured while the muscle was stimulated with 5- and 6-pulse CFTs(of duration 132 and 165 ms, respectively) and 5- and 6-pulse VFTs (ofduration 104 and 143 ms, respectively) that were designed to takeadvantage of the catchlike property of skeletal muscle. Measurementswere made from fresh and fatigued muscles. For the fresh muscles, the VFTs elicited significantly greater peak power than did the 6-pulse CFT. When the muscles were fatigued, VFT stimulation significantly improved both the peak and mean power produced compared withstimulation by CFTs. These results show that stimulation of the LDMwith shorter duration VFTs is potentially useful for application incardiomyoplasty.

     

Accessory muscle activation during the superimposed burst technique

Quadriceps muscle activation is assessed using the superimposed burst technique. This technique involves percutaneous muscle stimulation superimposed during maximal isometric volitional knee extension. It is unknown whether accessory muscle activation during maximal knee extension influences estimates of quadriceps muscle activation. Our aim was to compare accessory muscle activation while performing the superimposed burst technique using investigator delivered verbal instruction to constrain the system (CS) and a participant preferred (PP) technique. Twenty five healthy, active individuals (13M/12F, age=23.8 ± 3.35, height=72.73 ± 14.51 cm, and weight=175.29 ± 9.59 kg) were recruited for this study. All participants performed superimposed burst testing with (CS) and without (PP) verbal instruction to encourage isolated quadriceps activation during maximal isometric knee extension. The main outcome variables measured were knee extension torque, quadriceps central activation ratio and mean EMG of vastus lateralis, biceps femoris, and lumbar paraspinal muscles. There were significant differences in knee extension torque (CS=2.87 ± 0.93 Nm/kg, PP=3.40 ± 1.12 Nm/kg, p<0.001), superimposed burst torque (CS=3.40 ±0.98 Nm/kg, PP=3.75 ± 1.11 Nm/kg, p=0.002) and quadriceps CAR (CS=84.1 ± 12.0%, PP=90.2 ± 9.9%, p<0.001) between the techniques. There was also a significant difference in lumbar paraspinal EMG (CS=6.40 ± 8.52%, PP=11.86 ± 14.89%, p=0.043) between the techniques however vastus lateralis EMG was not significantly different. Patient instruction via verbal instruction to constrain proximal structures may help patient minimize confounders to knee extension torque generation while maximizing quadriceps activation.     

Fatigue of mouse diaphragm muscle in isometric and isotonic contractions

Fatiguabilities of mouse diaphragm muscle in vitro in isometric and isotonic contractions were compared in this study. Isolated mouse diaphragm muscle was stimulated repetitively to induce fatigue during both isometric and isotonic contractions. The supramaximal electrical stimulation used was a train of 100-Hz, 0.5-ms pulses delivered to the muscle every 2 s for 0.5 s. The percentage decrease in isometric tension from beginning to end of the fatiguing process was used as the index of fatigue. The experiments were carried out at different PO2 levels in both normal and zero-glucose Ringer solutions. It was found that fatigue developed more rapidly in isotonic contractions than in isometric ones. Also, the extracellular glucose level demonstrated little effect on the muscle's short-term fatiguability, whereas reductions in the extracellular PO2 exerted a profound effect, especially in the case of isotonic fatigue.     

Rate of fatigue during repeated submaximal contractions of human quadriceps muscle.

Our purpose was to determine the effect of eight different combinations of contraction intensity, duration, and rest on the rate of fatigue in vastus lateralis muscle. A single combination consisted of contractions at 30 or 70% maximal voluntary contraction (MVC), held for 3 or 7 s with 3- or 7-s rest intervals. Contractions were repeated until the subject could not hold the force for the requisite duration. At regular intervals during each experiment, a brief MVC, a single twitch, and the response to eight stimulation pulses at 50 Hz were elicited. The rate of fatigue was the rate of decline of MVC calculated from regression analysis. Mean rate of fatigue (n = 8) ranged from 0.3 to 25% MVC/min and was closely related (r = 0.98) to the product of the relative force and the duty cycle. Force from 50 Hz stimulation fell linearly and in parallel with MVC. Twitch force was first potentiated and then fell twice as fast as 50 Hz stimulation and MVC (p less than 0.05). Differentiated twitch contraction and relaxation rates were higher at potentiation and lower at the limit of endurance, compared with control values (p less than 0.05). The maximal electromyogram decreased 25% and the submaximal EMG increased to maximal by the end of the protocol, indicating that the entire motor unit pool had been recruited. The close relation between rate of fatigue and the force x time product probably reflects the off-setting interaction of contraction amplitude, duration, and rest interval. This occurs despite the changes in twitch characteristics and the apparent recruitment of fast fatiguing motor units.     

Effect of quadriceps femoris muscle length on neural activation during isometric and concentric contractions.

The effect of muscle length on neural drive (here termed "neural activation") was investigated from electromyographic activities and activation levels (twitch interpolation). The neural activation was measured in nine men during isometric and concentric (30 and 120 degrees /s) knee extensions for three muscle lengths (35, 55, and 75 degrees knee flexion, i.e., shortened, intermediate, and lengthened muscles, respectively). Long (76 degrees ), medium (56 degrees ), and short (36 degrees ) ranges of motion were used to investigate the effect of the duration of concentric contraction. Neural activation was found to depend on muscle length. Reducing the duration of contraction had no effect. Neural activation was higher with short muscle length during isometric contractions and was weaker for shortened than for intermediate and lengthened muscles performing 120 degrees /s concentric contractions. Muscle length had no effect on 30 degrees /s concentric neural activation. Peripheral mechanisms and discharge properties of the motoneurons could partly explain the observed differences in the muscle length effect. We thus conclude that muscle length has a predominant effect on neural activation that would modulate the angular velocity dependency.     

MRI measures of perfusion-related changes in human skeletal muscle during progressive contractions.

Although skeletal muscle perfusion is fundamental to proper muscle function, in vivo measurements are typically limited to those of limb or arterial blood flow, rather than flow within the muscle bed itself. We present a noninvasive functional MRI (fMRI) technique for measuring perfusion-related signal intensity (SI) changes in human skeletal muscle during and after contractions and demonstrate its application to the question of occlusion during a range of contraction intensities. Eight healthy men (aged 20-31 yr) performed a series of isometric ankle dorsiflexor contractions from 10 to 100% maximal voluntary contraction. Axial gradient-echo echo-planar images (repetition time = 500 ms, echo time = 18.6 ms) were acquired continuously before, during, and following each 10-s contraction, with 4.5-min rest between contractions. Average SI in the dorsiflexor muscles was calculated for all 240 images in each contraction series. Postcontraction hyperemia for each force level was determined as peak change in SI after contraction, which was then scaled to that obtained following a 5-min cuff occlusion of the thigh (i.e., maximal hyperemia). A subset of subjects (n = 4) performed parallel studies using venous occlusion plethysmography to measure limb blood flow. Hyperemia measured by fMRI and plethysmography demonstrated good agreement. Postcontraction hyperemia measured by fMRI scaled with contraction intensity up to approximately 60% maximal voluntary contraction. fMRI provides a noninvasive means of quantifying perfusion-related changes during and following skeletal muscle contractions in humans. Temporal changes in perfusion can be observed, as can the heterogeneity of perfusion across the muscle bed.     

Behavior of human muscle fascicles during shortening and lengthening contractions in vivo.

The aim of the present study was to investigate the behavior of human muscle fascicles during dynamic contractions. Eight subjects performed maximal isometric dorsiflexion contractions at six ankle joint angles and maximal isokinetic concentric and eccentric contractions at five angular velocities. Tibialis anterior muscle architecture was measured in vivo by use of B-mode ultrasonography. During maximal isometric contraction, fascicle length was shorter and pennation angle larger compared with values at rest (P < 0.01). During isokinetic concentric contractions from 0 to 4.36 rad/s, fascicle length measured at a constant ankle joint angle increased curvilinearly from 49.5 to 69.7 mm (41%; P < 0.01), whereas pennation angle decreased curvilinearly from 14.8 to 9.8 degrees (34%; P < 0.01). During eccentric muscle actions, fascicles contracted quasi-isometrically, independent of angular velocity. The behavior of muscle fascicles during shortening contractions was believed to reflect the degree of stretch applied to the series elastic component, which decreases with increasing contraction velocity. The quasi-isometric behavior of fascicles during eccentric muscle actions suggests that the series elastic component acts as a mechanical buffer during active lengthening.     

Activation of human quadriceps femoris muscle during dynamic contractions: effects of load on fatigue.

Muscle fatigue is both multifactorial and task dependent. Electrical stimulation may assist individuals with paralysis to perform functional activities [functional electrical stimulation (FES), e.g., standing or walking], but muscle fatigue is a limiting factor. One method of optimizing force is to use stimulation patterns that exploit the catchlike property of skeletal muscle [catchlike-inducing trains (CITs)]. Although nonisometric (dynamic) contractions are important parts of both normal physiological activation of skeletal muscles and FES, no previous studies have attempted to identify the effect that the load being lifted by a muscle has on the fatigue produced. This study examined the effects of load on fatigue during dynamic contractions and the augmentation produced by CITs as a function of load. Knee extension in healthy subjects was electrically elicited against three different loads. The highest load produced the least excursion, work, and average power, but it produced the greatest fatigue. CIT augmentation was greatest at the highest load and increased with fatigue. Because CITs were effective during shortening contractions for a variety of loads, they may be of benefit during FES applications.     

Recording and evaluation of isotonic and isometric contractions of skeletal muscle in situ

A procedure is described for obtaining records and identifying curves of isotonic and isometric contractions (both tetanic and twitch) of the gastrocnemius muscle of the rabbit in situ. The experimental conditions have made it possible to follow the above mentioned phenomena and to measure further parameters in series at sequentially altered muscle lengths and loads acting in the course of isotonic contractions.     

Nonisometric behavior of fascicles during isometric contractions of a human muscle

Fascicle length, pennation angle, and tendonelongation of the human tibialis anterior were measured in vivo byultrasonography. Subjects (n = 9) wererequested to develop isometric dorsiflexion torque gradually up tomaximal at the ankle joint angle of 20° plantarflexion from theanatomic position. Fascicle length shortened from 90 ± 7 to 76 ± 7 (SE) mm, pennation angle increased from 10 ± 1 to 12 ± 1°, and tendon elongation increased up to 15 ± 2 mm with gradedforce development up to maximum. The tendon stiffness increased withincreasing tendon force from 10 N/mm at 0-20 N to 32 N/mm at240-260 N. Young's modulus increased from 157 MPa at 0-20 Nto 530 MPa at 240-260 N. It can be concluded that, in isometriccontractions of a human muscle, mechanical work, some of which isabsorbed by the tendinous tissue, is generated by the shortening ofmuscle fibers and that ultrasonography can be used to determine thestiffness and Young's modulus for human tendons.

     

Endothelial modulation of skeletal muscle blood flow and VO(2) during low- and high-intensity contractions.

In the present study, we determined whether endothelin (ET)-1 contributed to the observed reduction in muscle blood flow (Q) during contractions with nitric oxide synthase (NOS) inhibition and whether muscle O(2) uptake (VO(2)) would be affected by the decrease in muscle Q with NOS inhibition at different contraction intensities. Muscle Q, VO(2), O(2) extraction ratio (OER), and tension development (TD) were studied in the in situ gastrocnemius muscle preparation in anesthetized dogs. A decrease in the VO(2)-to-TD ratio (VO(2)/TD) was used as an indicator of O(2) limitation. Three contraction protocols were used: 1) isometric twitch contractions at 2 twitches (tw)/s, 2) the same contractions at 4 tw/s, and 3) pretreatment with an ET(A)-receptor antagonist (BQ-123) before 2 tw/s contractions. The muscle was stimulated to contract, and measures were obtained at steady state (approximately 5-8 min). NOS inhibition (N(omega)-nitro-L-arginine methyl ester) was then induced, and measures were repeated at 2, 5, 10, and 15 min. During 2 tw/s contractions, NOS inhibition reduced Q with and without ET(A)-receptor blockade. In both groups, OER increased in response to the fall in Q, with the result being no change in VO(2)/TD. NOS inhibition also decreased Q during 4 tw/s contractions, but OER did not increase, resulting in a reduction in VO(2)/TD 5 and 15 min after N(omega)-nitro-L-arginine methyl ester. These data indicated that 1) a reciprocal increase in ET-1 during NOS inhibition does not influence active hyperemia in skeletal muscle, and 2) during 4 tw/s contractions, the ischemia with NOS inhibition was associated with either an O(2) limitation or an alteration in the efficiency of muscle contractions.     

Optimal hematocrit for canine skeletal muscle during rhythmic isotonic exercise.

Contractile power, blood flow, O2-uptake, and O2-extraction during isotonic, rhythmic exercise were determined in the isolated canine gastrocnemius muscle during perfusion with blood with hematocrits between 0.21 and 0.81. The results obtained in 36 measurements on nine muscles showed that maximal O2-delivery to the muscle if found at hematocrits between 0.5 and 0.6. Both in the range of hemodilution, and in the range of extreme hemoconcentration, O2-delivery decreases significantly. O2-consumption and contractile power of the muscles are almost unaffected in the hematocrit range between 0.4 and 0.7; beyond and below this hematocrit range both parameters decrease. O2-extraction is virtually constant in the hematocrit range between 0.3 and 0.6, but increase both below and above these hematocrit levels. It is concluded that due to reduced vasodilatory reserve in working skeletal muscle compared to resting muscle the optimal hematocrit is shifted to higher values.     

Cyclic nucleotide levels during carbachol-induced smooth muscle contractions.

Cyclic nucleotide levels and tension were measured at various times during carbachol-induced smooth muscle contractions. Cyclic GMP levels were markedly increased during contractions of rat vas deferens, guinea pig myometrium and guinea pig taenia coli, but were unchanged during contractions of rat uterus or guinea pig ileum. No significant changes in cyclic GMP levels could be detected in estrogen-primed rat uteri at any of the times or drug concentrations studied. Even in tissues in which large increases in cyclic GMP levels could be detected during carbachol-induced contractions (i.e. guinea pig myometrium and taenia coli) the contractions appeared to precede the cyclic GMP increases by several seconds. No significant changes in cyclic AMP levels were observed during carbachol-induced contractions in any of the smooth muscles studied. Thus, changes in tissue levels of the cyclic nucleotides do not appear to be responsible for the initiation of carbachol-induced smooth muscle contractions.     

Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions.

We investigated the firing rate of motor units in the vastus lateralis muscle in five healthy young men (mean = 21.4 yr, SD = 0.9) during a sequence of isometric constant-torque contractions repeated to exhaustion. The contractions were sustained at 20% of the maximal voluntary level, measured at the beginning of the test sequence. Electromyographic (EMG) signals were recorded via quadrifilar fine-wire electrodes and subsequently decomposed into their constituent motor unit action potentials to obtain the motor unit firing times. In addition, we measured the whole muscle mechanical properties during the fatigue task using electrical stimulation. The firing rate of motor units first decreased within the first 10-20% of the endurance time of the contractions and then increased. The firing rate increase was accompanied by recruitment of additional motor units as the force output remained constant. The elicited twitch and tetanic torque responses first increased and then decreased. The two processes modulated in a complementary fashion at the same time. Our data suggest that, when the vastus lateralis muscle is activated to maintain a constant torque output, its motoneuron pool receives a net excitatory drive that first decreases to compensate for the short-lived potentiation of the muscle force twitch and then increases to compensate for the diminution of the force twitch. The underlying inverse relationship between the firing rate and the recruitment threshold that has been reported for nonfatigued contractions is maintained. We, therefore, conclude that the central nervous system control of vastus lateralis motor units remains invariant during fatigue in submaximal isometric isotonic contractions.