Spike-triggered average torque and muscle fiber conduction velocity of low-threshold motor units following submaximal endurance contractions.

The motor unit twitch torque is modified by sustained contraction, but the association to changes in muscle fiber electrophysiological properties is not fully known. Thus twitch torque, muscle fiber conduction velocity, and action potential properties of single motor units were assessed in 11 subjects following an isometric submaximal contraction of the tibialis anterior muscle until endurance. The volunteers activated a target motor unit at the minimum discharge rate in eight 3-min-long contractions, three before and five after an isometric contraction at 40% of the maximal torque, sustained until endurance. Multichannel surface electromyogram signals and joint torque were averaged with the target motor unit potential as trigger. Discharge rate (mean +/- SE, 6.6 +/- 0.2 pulses/s) and interpulse interval variability (33.3 +/- 7.0%) were not different in the eight contractions. Peak twitch torque and recruitment threshold increased significantly (93 +/- 29 and 12 +/- 5%, P <0.05) in the contraction immediately after the endurance task with respect to the preendurance values (0.94 +/- 0.26 mN.m and 3.7 +/- 0.5% of the maximal torque), whereas time to peak of the twitch torque did not change (74.4 +/- 10.1 ms). Muscle fiber conduction velocity decreased and action potential duration increased in the contraction after the endurance (6.3 +/- 1.8 and 9.8 +/- 1.8%, respectively, P <0.05; preendurance values, 3.9 +/- 0.2 m/s and 11.1 +/- 0.8 ms), whereas the surface potential peak-to-peak amplitude did not change (27.1 +/- 3.1 microV). There was no significant correlation between the relative changes in muscle fiber conduction velocity or surface potential duration and in peak twitch torque (R2= 0.04 and 0.10, respectively). In conclusion, modifications in peak twitch torque of low-threshold motor units with sustained contraction are mainly determined by mechanisms not related to changes in action potential shape and in its propagation velocity.     

Specificity of joint angle in isometric training

Six healthy women (21.8 +/- 0.4 y) did isometric strength training of the left plantarflexors at an ankle joint angle of 90 degrees. Training sessions, done 3 times per week for 6 weeks, consisted of 2 sets of ten 5 s maximal voluntary contractions. Prior to and following the training, and in random order, voluntary and evoked isometric contraction strength was measured at the training angle and at additional angles: 5 degrees, 10 degrees, 15 degrees, and 20 degrees intervals in the plantarflexion and dorsiflexion directions. Evoked contraction strength was measured as the peak torque of maximal twitch contractions of triceps surae. Training increased voluntary strength at the training angle and the two adjacent angles only (p less than 0.05). Time to peak twitch torque was not affected by training. Twitch half relaxation time increased after training (p = 0.013), but the increase was not specific to the training angle. There was a small (1.1%, p less than 0.05) increase in calf circumference after training. Evoked twitch torque did not increase significantly at any joint angle. It was therefore concluded that a neural mechanism is responsible for the specificity of joint angle observed in isometric training.     

Conduction velocity of low-threshold motor units during ischemic contractions performed with surface EMG feedback.

The aim of this study was to analyze the effect of ischemia on low-threshold motor unit conduction velocity. Nine subjects were trained to isolate the activity of a single motor unit (target motor unit) in the abductor pollicis brevis muscle with feedback on surface EMG signals recorded with a 16-electrode linear array. After training, the subjects activated the target motor unit at approximately 8 pulses per second (pps) for five 3-min-long contractions. During the third and fourth contractions, a cuff inflated at 180 mmHg around the forearm induced ischemia of the hand. The exerted force (mean +/- SE, 4.6 +/- 2.1% of the maximal voluntary contraction force), discharge rate (8.6 +/- 0.4 pps), interpulse interval variability (34.8 +/- 2.5%), and peak-to-peak amplitude of the target motor unit action potentials (176.6 +/- 18.2 microV) were not different among the five contractions. Conduction velocity, mean power spectral frequency, and action potential duration were the same in the beginning of the five contractions (2.8 +/- 0.2 m/s, 195.2 +/- 10.5 Hz, and 5.4 +/- 0.3 ms, respectively) and changed over the 3 min of sustained activation only during the fourth contraction. Conduction velocity and mean power spectral frequency decreased (10.05 +/- 1.8% and 8.50 +/- 2.18% during the 3 min, respectively) and action potential duration increased (8.2 +/- 4.6% in the 3 min) during the fourth contraction. In conclusion, 1) subjects were able to isolate the activity of a single motor unit with surface EMG visual feedback in ischemic conditions maintained for 16 min, and 2) the activation-induced decrease in single motor unit conduction velocity was significantly larger with ischemia than with normal circulation, probably due to the alteration of mechanisms of ion exchange across the fiber membrane.     

Length-dependent twitch contractile characteristics of skeletal muscle

The length dependence of force development of mammalian skeletal muscles was evaluated during twitch, double-pulse, and tetanic contractions, and the relation between muscle length and the time-dependent characteristics of twitch and double-pulse contractions were determined. In situ isometric contractions of the rat gastrocnemius muscle were analyzed at seven different lengths, based on a reference length at which the maximal response to double-pulse contractions occurred (Lopt-2P). Twitch and double-pulse contractions were analyzed for developed tension (DT), contraction time (tC), average rate of force development (DT-tC(-1)), half-relaxation time (t50%R), peak rate of relaxation (DT x dtmin(-1)), and 90%-relaxation time (t90%R). Considering the length at which maximal tetanic DT occurred to be the optimal length (Lo-TET), the peak DT for twitch contractions and double-pulse contractions was observed at Lo-TET + 0.75 mm (p < 0.05) and Lo-TET + 0.1 mm (p > 0.05), respectively. When measured at the length for which maximal twitch and double-pulse contractions were obtained, tetanic DT was 95.2 +/- 3 and 99.0 +/- 2% of the maximal value, respectively. These observations suggest that double-pulse contractions are more suitable for setting length for experimental studies than twitch contractions. Twitch and double-pulse contraction tC were 15.53 +/- 1.14 and 25.0 +/- 0.6 ms, respectively, at Lopt-2P, and increased above Lopt-2P and decreased below Lopt-2P. Twitch t50%R was 12.18 +/- 0.90 ms at Lopt-2P, and increased above Lopt-2P and below Lopt-2P. Corresponding changes for double-pulse contractions were greater. Stretching the muscle leads to slower twitch contractions and double-pulse contractions, but the mechanisms of this change in time course remain unclear.     

Motor unit firing rates and contractile properties in tibialis anterior of young and old men.

The effects of aging on motoneuron firing rates and muscle contractile properties were studied in tibialis anterior muscle by comparing results from six young (20.8 +/- 0.8 yr) and six old men (82.0 +/- 1.7 yr). For each subject, data were collected from repeated tests over a 2-wk period. Contractile tests included maximal voluntary contraction (MVC) with twitch interpolation and stimulated twitch contractions. The old men had 26% lower MVC torque (P < 0.01) than did the young men, but percent activation was not different (99.1 and 99.3%, respectively). Twitch contraction durations were 23% longer (P < 0.01) in the old compared with the young men. During a series of repeated brief steady-state contractions at 10, 25, 50, 75, and 100% MVC, motor unit firing rates were recorded. Results from approximately 950 motor unit trains in each subject group indicated that at all relative torque levels mean firing rates were 30-35% lower (P < 0.01) in the old subjects. Comparisons between young and old subjects' mean firing rates at each of 10%, 50%, and MVC torques and their corresponding mean twitch contraction duration yielded a range of moderate-to-high correlations (r = -0.67 to -0.84). That lower firing rates were matched to longer twitch contraction durations in the muscle of old men, and relatively higher firing rates were matched with shorter contraction times from the young men, indirectly supports the neuromuscular age-related remodeling principle.     

Effects of calcitonin gene-related peptide on neuromuscular transmission in the isolated rat diaphragm

The present study was undertaken to investigate a possible interaction between the cholinergic nerve neurotransmitter and CGRP on neuromuscular transmission in the isolated rat diaphragm. Electrical stimulation of the isolated phrenic nerve resulted in twitch contractions which were dose-dependently potentiated by CGRP in concentrations ranging from 1.2 x 10(-9) M to 3 x 10(-7) M. The potentiating action of CGRP (3 x 10(-7) M) disappeared in about 25 min. The same dose of CGRP 40 min later produced an augmentation of contraction amplitude similar to that observed prior to the administration of CGRP. The action of CGRP was dependent upon the stimulation pulse width ranging from 0.2 to 1.0 msec. Rat calcitonin (4.5 x 10(-7) M) caused a minimal change in the amplitude of twitch contractions. CGRP had no effect on the quiescent striated muscle. Twitch responses to direct electrical stimulation were also enhanced by CGRP (6 x 10(-8) M-6 x 10(-7) M) in the absence and presence of 10(-5) M d-tubocurarine. These results suggest that CGRP modulates the action of acetylcholine at the motor end plates of striated muscle.     

Force-frequency relationship and potentiation in mammalian skeletal muscle.

Repetitive activation of a skeletal muscle results in potentiation of the twitch contractile response. Incompletely fused tetanic contractions similar to those evoked by voluntary activation may also be potentiated by prior activity. We aimed to investigate the role of stimulation frequency on the enhancement of unfused isometric contractions in rat medial gastrocnemius muscles in situ. Muscles set at optimal length were stimulated via the sciatic nerve with 50-micros duration supramaximal pulses. Trials consisted of 8 s of repetitive trains [5 pulses (quintuplets) 2 times per second or 2 pulses (doublets) 5 times per second] at 20, 40, 50, 60, 70, and 80 Hz. These stimulation frequencies represent a range over which voluntary activation would be expected to occur. When the frequency of stimulation was 20, 50, or 70 Hz, the peak active force (highest tension during a contraction - rest tension) of doublet contractions increased from 2.2 +/- 0.2, 4.1 +/- 0.4, and 4.3 +/- 0.5 to 3.1 +/- 0.3, 5.6 +/- 0.4, and 6.1 +/- 0.7 N, respectively. Corresponding measurements for quintuplet contractions increased from 2.2 +/- 0.2, 6.1 +/- 0.5, and 8.7 +/- 0.7 to 3.2 +/- 0.3, 7.3 +/- 0.6, and 9.0 +/- 0.7 N, respectively. Initial peak active force values were 27 +/- 1 and 61.5 +/- 5% of the maximal (tetanic) force for doublet and quintuplet contractions, respectively, at 80 Hz. With doublets, peak active force increased at all stimulation frequencies. With quintuplets, peak active force increased significantly for frequencies up to 60 Hz. Twitch enhancement at the end of the 8 s of repetitive stimulation was the same regardless of the pattern of stimulation during the 8 s, and twitch peak active force returned to prestimulation values by 5 min. These experiments confirm that activity-dependent potentiation is evident during repeated, incompletely fused tetanic contractions over a broad range of frequencies. This observation suggests that, during voluntary motor unit recruitment, derecruitment or decreased firing frequency would be necessary to achieve a fixed (submaximal) target force during repeated isometric contractions over this time period.     

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.     

Inter-operator agreement in decomposition of motor unit firings from high-density surface EMG.

High-density surface EMG can be used to obtain a spatially selective representation of several motor unit action potentials. Recently, a decomposition of the signal into the underlying motor neuron firing patterns has been described. The reliability of the algorithm has not yet been tested. Eleven healthy subjects participated. High-density surface EMG was recorded from the vastus lateralis muscle during an isometric knee extension. Two independent operators analyzed the signals. After operator-supervised cluster analysis of spikes, motor unit action potential templates were constructed and an automatic template matching was performed. The decomposition was adjusted by hand. Agreement between operators was calculated for the number of coincident firings. Bland-Altman plots of peak-to-peak amplitude were constructed and limits of agreement were calculated. For completely decomposed motor unit action potential trains the between-operator agreement of firing events was very high. The peak-to-peak amplitude of monopolar motor unit action potentials was 115microV (SD 74microV). The agreement was within 3microV and independent of amplitude. With partial decomposition agreement within 26microV was achieved. For bipolarly derived motor unit action potentials the peak-to-peak amplitude was 54microV (SD 49microV), the agreement was within 3microV. Only for recordings obtained from a force level below 5% of the maximum voluntary contraction full decomposition was possible. It was concluded that when full decomposition is achieved, two independent operators are likely to arrive at nearly identical firing patterns.     

Age-related decline in rate of torque development is accompanied by lower maximal motor unit discharge frequency during fast contractions

The aim of this study was to investigate the association between the rate of torque development and maximal motor unit discharge frequency in young and elderly adults as they performed rapid submaximal contractions with the ankle dorsiflexors. Recordings were obtained of the torque exerted by the dorsiflexors during the isometric contractions and the surface and intramuscular electromyograms (EMGs) from the tibialis anterior. The maximal rate of torque development and integrated EMG (percentage of total EMG burst) at peak rate of torque development during fast contractions were lower in elderly than young adults by 48% (P < 0.05) and 16.5% (P < 0.05), respectively. The young adults, but not the elderly adults, exhibited a positive association (r2 = 0.33; P < 0.01) between the integrated EMG computed up to the peak rate of torque development and the maximal rate of torque development achieved during the fast contractions. These age-related changes during fast voluntary contractions were accompanied by a decline (P < 0.001) in motor unit discharge frequency (19, 28, and 34% for first 3 interspike intervals, respectively) and in the percentage of units (45%; P < 0.05) that exhibited double discharges (doublets) at brief intervals (<5 ms). Because aging decreased the maximal rate of torque development of fast voluntary contractions to a greater extent ( approximately 10%) than that of an electrically evoked twitch, collectively the results indicate that the age-related decline in maximal motor unit discharge frequency likely limit, in addition to the slowing of muscle contractile properties, the performance of fast voluntary contractions.     

Influence of gender on post-tetanic potentiation in human dorsiflexors

Twitch contractions of the ankle dorsiflexors were evoked before and after 7 s of tetanic stimulation at 100 Hz in young women and men. Torque decreased more in men (18%) than in women (12%) during the tetanus. There was no gender difference in twitch peak torque potentiation over the 5-min post-tetanus. Potentiation was 42% (women) and 45% (men) at 5 s post-tetanus, and still present at 5 min (women 24%, men 25%). The immediate (5 s) shortening of twitch rise time was similar in women (14%) and men (13%), but during the 5-min men's rise time came to exceed whereas women's only approached pretetanus values (e.g., +9% vs. -1% at 5 min). The immediate decrease in half-relaxation time was also similar in women (24%) and men (22%); however, women's but not men's values remained less than pretetanus values for most of the 5-min period. Twitch rate of torque development increased similarly (75%) in women and men at 5 s, with no gender difference over 5 min. In contrast, rate of torque relaxation increased significantly only in men. Rate of torque development normalized to peak torque was similar in women and men pretetanus and increased similarly 5 s post-tetanus, but women had greater values through most of the 5-min post-tetanus. Normalized rate of torque relaxation was similar in women and men and not affected by tetanus. In the dorsiflexor muscles, young women and men show a similar amount and pattern of twitch force potentiation, but there are gender differences in time-related twitch contractile properties in the first 5 min after tetanus.     

Supraspinal fatigue does not explain the sex difference in muscle fatigue of maximal contractions.

Young women are less fatigable than young men for maximal and submaximal contractions, but the contribution of supraspinal fatigue to the sex difference is not known. This study used cortical stimulation to compare the magnitude of supraspinal fatigue during sustained isometric maximal voluntary contractions (MVCs) performed with the elbow flexor muscles of young men and women. Eight women (25.6 +/- 3.6 yr, mean +/- SD) and 9 men (25.4 +/- 3.8 yr) performed six sustained MVCs (22-s duration each, separated by 10 s). Before the fatiguing contractions, the men were stronger than the women (75.9 +/- 9.2 vs. 42.7 +/- 8.0 N.m; P < 0.05) in control MVCs. Voluntary activation measured with cortical stimulation before fatigue was similar for the men and women during the final control MVC (95.7 +/- 3.0 vs. 93.3 +/- 3.6%; P > 0.05) and at the start of the fatiguing task (P > 0.05). By the end of the six sustained fatiguing MVCs, the men exhibited greater absolute and relative reductions in torque (65 +/- 3% of initial MVC) than the women (52 +/- 9%; P < 0.05). The increments in torque (superimposed twitch) generated by motor cortex stimulation during each 22-s maximal effort increased with fatigue (P < 0.05). Superimposed twitches were similar for men and women throughout the fatiguing task (5.5 +/- 4.1 vs. 7.3 +/- 4.7%; P > 0.05), as well as in the last sustained contraction (7.8 +/- 5.9 vs. 10.5 +/- 5.5%) and in brief recovery MVCs. Voluntary activation determined using an estimated control twitch was similar for the men and women at the start of the sustained maximal contractions (91.4 +/- 7.4 vs. 90.4 +/- 6.8%, n = 13) and end of the sixth contraction (77.2 +/- 13.3% vs. 73.1 +/- 19.6%, n = 10). The increase in the area of the motor-evoked potential and duration of the silent period did not differ for men and women during the fatiguing task. However, estimated resting twitch amplitude and the peak rates of muscle relaxation showed greater relative reductions at the end of the fatiguing task for the men than the women. These results indicate that the sex difference in fatigue of the elbow flexor muscles is not explained by a difference in supraspinal fatigue in men and women but is largely due to a sex difference of mechanisms located within the elbow flexor muscles.     

K+-induced twitch potentiation is not due to longer action potential

The objective of this study was to determine whether an increased duration of the action potential contributes to the K+-induced twitch potentiation at 37 degrees C. Twitch contractions were elicited by field stimulation, and action potentials were measured with conventional microelectrodes. For mouse extensor digitorum longus (EDL) muscle, twitch force was greater at 7-13 mM K+ than at 4.7 mM (control). For soleus muscle, twitch force potentiation was observed between 7 and 11 mM K+. Time to peak and half-relaxation time were not affected by the increase in extracellular K+ concentration in EDL muscle, whereas both parameters became significantly longer in soleus muscle. Decrease in overshoot and prolongation of the action potential duration observed at 9 and 11 mM K+ were mimicked when muscles were respectively exposed to 25 and 50 nM tetrodotoxin (TTX; used to partially block Na+ channels). Despite similar action potentials, twitch force was not potentiated by TTX. It is therefore suggested that the K+-induced potentiation of the twitch in EDL muscle is not due to a prolongation of the action potential and contraction time, whereas a longer contraction, especially the relaxation phase, may contribute to the potentiation in soleus muscle.     

Sprint training attenuates the deficits of force and dynamic stiffness in rat soleus muscle caused by eccentric contractions

We investigated whether sprint training attenuates the deficits in force and dynamic stiffness caused by eccentric contractions to the soleus muscles of Wistar rats. Two groups of male rats were analyzed: sedentary (C, n=8) and trained (T, n=8). T rats were sprint trained for 10 weeks. Subsequently, the right soleus muscles of rats were freed under anesthesia, leaving the bone insertion and blood supply intact. Eccentric contractions were induced by lengthening muscles during tetanic contractions. Force and dynamic stiffness were tested before and after 20 rounds of eccentric contractions. Tension decline was analyzed using a two-state model (first-order kinetics) in the context of Kramer's theory. Training improved the twitch tension (C, 6.44+/-0.6N/cm(2); T, 10.90+/-0.8N/cm(2)), tetanic force (C, 61.74+/-0.6N/cm(2); T, 85.62+/-0.8N/cm(2)), and increased the dynamic stiffness (C, 41.28+/-1.0N/cm(2); T, 49.56+/-3.2N/cm(2)). Twitch tension after eccentric contractions declined to 73% and 75% in C and T groups, respectively, while tetanic tension decreased to 60% and 36% in C and T groups, respectively. After eccentric contractions, dynamic stiffness decreases were smaller in T rats (from 49.56+/-3.2 to 36.09+/-2.1N/cm(2)) than in C rats (from 41.28+/-1.0 to 20.73+/-1.8N/cm(2)). Sprint training increased the dynamic stiffness and tetanic tension of the soleus muscle and protected against the attenuation induced by eccentric contractions. Finally, the two-state model provided evidence that the number of force-generating cross-bridges increases in trained muscle.     

Effects of an eccentric exercise session short-term recovery of muscle contractility

Short term effects of 5 sets of 10 maximal eccentric contractions of the elbow flexors, performed using an isokinetic ergometer, were studied. Maximal eccentric, isometric, concentric torque, myoelectrical activity of biceps and triceps brachii, voluntary activation, M-wave amplitude, as well as twitch and maximal contraction and relaxation velocities were measured before (Control), 2 minutes after (Post), 24 hours (Post24 h) and 48 hours (Post48 h) after the exercise session. Torque significantly decreased over the recovery period, whatever the contraction type, excepted concentric torque assessed at 240 degrees.s-1 which recovered its Control value at Post48 h. Activation level significantly decreased at Post (p < 0.05) and returned to its Control value at Post24 h. Twitch, as well as maximal contraction and relaxation velocities had significantly declined among the experimental procedure (p < 0.01). M-wave amplitude was not modified after the exercise. These results indicate that, over a 48 hour rest period, torque decrement following a maximal eccentric exercise session should mainly be due to a failure of the peripheral part of the neuromuscular system, and force recovery should closely be linked to the developed force value.     

The effects of isokinetic contraction velocity on concentric and eccentric strength of the biceps brachii

The purpose of this investigation was to determine the influence of contraction velocity on the eccentric (ECC) and concentric (CON) torque production of the biceps brachii. After performing warm-up procedures, each male subject (n = 11) completed 3 sets of 5 maximal bilateral CON and ECC isokinetic contractions of the biceps at speeds of 90, 180, and 300 degrees x s(-1) on a Biodex System 3 dynamometer. The men received a 3-minute rest between sets and the order of exercises was randomized. Peak torque (Nm) values were obtained for CON and ECC contractions at each speed. Peak torque scores (ECC vs. CON) were compared using a t-test at each speed. A repeated measures analysis of variance was used to determine differences between speeds. ECC peak torque scores were greater than CON peak torque scores at each given speed: 90 degrees x s(-1), p = 0.0001; 180 degrees x s(-1), p = 0.0001; and 300 degrees x s(-1), p = 0.0001. No differences were found between the ECC peak torque scores (p = 0.62) at any of the speeds. Differences were found among the CON scores (p = 0.004). Post hoc analysis revealed differences between 90 degrees x s(-1) (114.61 +/- 23) and 300 degrees x s(-1) (94.17 +/- 18). These data suggest that ECC contractions of the biceps brachii were somewhat resistant to a force decrement as the result of an increase in velocity, whereas CON muscular actions of the biceps brachii were unable to maintain force as velocity increased.     

Force-velocity relations and fiber composition in human knee extensor muscles.

Standardized measurements of dynamic strength of the kneee extensor muscles were performed in 25 healthy male subjects (17-37 yr) by means of isokinetic contractions, i.e., knee extensions with constant angular velocities. Overall variation between double determinations of maximal torque throughout the 90 degrees arc of motion (0 degrees = fully extended leg) averaged 10% for the different constant velocities chosen. At any given angle of the knee the torque produced was higher for isometric than for dynamic contractions. Dynamic torque decreased gradually with increased speed of shortening. Peak dynamic torque was reached at knee angles in the range: 55-66 degrees, with a displacement toward smaller knee angles with higher angular velocities. Correlations were demonstrated between peak torque produced at the highest speed of muscle shortening and percent as well as relative area of fast twitch fibers in the contracting muscle. In addition muscles with a high percentage of fast twitch fibers had the highest maximal contraction speeds. These observations on intact human skeletal muscle are consistent with earlier findings in animal skeletal muscle preparations.     

Experimental muscle pain reduces initial motor unit discharge rates during sustained submaximal contractions.

The aim of this human study was to investigate the effect of experimentally induced muscle pain on the modifications of motor unit discharge rate during sustained, constant-force contractions. Intramuscular and multichannel surface electromyographic (EMG) signals were collected from the right and left tibialis anterior muscle of 11 volunteers. The subjects performed two 4-min-long isometric contractions at 25% of the maximal dorsiflexion torque, separated by a 20-min rest. Before the beginning of the second contraction, hypertonic (painful; right leg) or isotonic (nonpainful; left leg) saline was injected into the tibialis anterior. Pain intensity scores did not change significantly in the first 150 s of the painful contraction. Exerted torque and its coefficient of variation were the same for the painful and nonpainful contractions. Motor unit discharge rate was higher in the beginning of the nonpainful contraction than the painful contraction on the right side [means +/- SE, 11.3 +/- 0.2 vs. 10.6 +/- 0.2 pulses/s (pps); P < 0.01] whereas it was the same for the two contractions on the left side (11.6 +/- 0.2 vs. 11.5 +/- 0.2 pps). The decrease in discharge rate in 4 min was smaller for the painful (0.4 +/- 0.1 pps) than for the control contractions (1.3 +/- 0.1 pps). Initial value and decrease in motor unit conduction velocity were not different in the four contractions (right leg, 4.0 +/- 0.1 m/s with decrease of 0.6 +/- 0.1 m/s in 4 min; left leg, 4.1 +/- 0.1 m/s with 0.7 +/- 0.1 m/s decrease). In conclusion, stimulation of nociceptive afferents by injection of hypertonic saline did not alter motor unit conduction velocity but reduced the initial motor unit discharge rates and the difference between initial and final discharge rates during sustained contraction.     

Motor unit activity and surface electromyogram power spectrum during increasing force of contraction

Twelve male subjects were tested to determine the relationship between motor unit (MU) activities and surface electromyogram (EMG) power spectral parameters with contractions increasing linearly from zero to 80% of maximal voluntary contraction (MVC). Intramuscular spike and surface EMG signals recorded simultaneously from biceps brachii were analyzed by means of a computer-aided intramuscular MU spike amplitude-frequency (ISAF) histogram and an EMG frequency power spectral analysis. All measurements were made in triplicate and averaged. Results indicate that there were highly significant increases in surface EMG amplitude (71 +/- 31.3 to 505 +/- 188 microV, p less than 0.01) and mean power frequency (89 +/- 13.3 to 123 +/- 23.5 Hz, p less than 0.01) with increasing force. These changes were accompanied by progressive increases in the firing frequency of MU's initially recruited, and of newly recruited MU's with relatively larger spike amplitudes. The group data in the ISAF histograms revealed significant increases in mean spike amplitude (412 +/- 79 to 972 +/- 117 microV, p less than 0.01) and mean firing frequency (17.8 +/- 5.4 to 24.7 +/- 4.1 Hz, p less than 0.01). These data suggest that surface EMG spectral analysis can provide a sensitive measure of the relative changes in MU activity during increasing force output.     

Hypothermia increases the gain of excitation-contraction coupling in guinea pig ventricular myocytes

Components of excitation-contraction (EC)-coupling were compared at 37 degrees C and 22 degrees C to determine whether hypothermia altered the gain of EC coupling in guinea pig ventricular myocytes. Ca(2+) concentration (fura-2) and cell shortening (edge detector) were measured simultaneously. Hypothermia increased fractional shortening (8.3 +/- 1.7 vs. 2.6 +/- 0.3% at 37 degrees C), Ca(2+) transients (157 +/- 33 vs. 35 +/- 5 nM at 37 degrees C), and diastolic Ca(2+) (100 +/- 9 vs. 60 +/- 6 nM at 37 degrees C) in field-stimulated myocytes (2 Hz). In experiments with high-resistance microelectrodes, the increase in contractions and Ca(2+) transients was accompanied by a twofold increase in action potential duration (APD). When voltage-clamp steps eliminated changes in APD, cooling still increased contractions and Ca(2+) transients. Hypothermia increased sarcoplasmic reticulum (SR) Ca(2+) stores (83 +/- 17 at 37 degrees C to 212 +/- 50 nM, assessed with caffeine) and increased fractional SR Ca(2+) release twofold. In contrast, peak Ca(2+) current was much smaller at 22 degrees C than at 37 degrees C (1.3 +/- 0.4 and 3.5 +/- 0.7 pA/pF, respectively). In cells dialyzed with sodium-free pipette solutions to inhibit Ca(2+) influx via reverse-mode Na(+)/Ca(2+) exchange, hypothermia still increased contractions, Ca(2+) transients, SR stores, and fractional release but decreased the amplitude of Ca(2+) current. The rate of SR Ca(2+) release per unit Ca(2+) current, a measure of EC-coupling gain, was increased sixfold by hypothermia. This increase in gain occurred regardless of whether cells were dialyzed with sodium-free solutions. Thus an increase in EC-coupling gain contributes importantly to positive inotropic effects of hypothermia in the heart.