The auditory startle response in post-traumatic stress disorder

Changes in the reflex amplitude throughout the day have been observed in non-human mammals. The present experiment tested whether diurnal fluctuations also occur in humans. Hoffmann reflex (H-reflex) amplitude was measured in soleus and flexor carpi radialis (FCR) muscles from the data collected over a 12-h period between 7:00–9:00 a.m. and 7:00–9:00 p.m. At 4-h intervals, M/H recruitment curves were obtained, and two measures of H-reflex excitability were calculated. The maximal H-reflex (H _max) was calculated as the average of the three largest H-reflexes. H-reflexes were also sampled from the ascending limb of the M/H recruitment curve (H _A, n=10), with a corresponding M-wave of 5% M _max. All values were normalized to the maximal M-wave (M _max). Soleus H-reflex amplitude and plantar flexion maximal voluntary isometric contraction force (MVIC) were significantly smaller (p<0.05) in the morning (H _max=57.2% M _max, H _A=42.3%, M _max, MVIC=162.1 Nm) than in the evening (H _max=69.1% M _max, a 20.1% increase, H _A=54.1% M _max, a 27.4% increase and MVIC=195.8 Nm, a 20.8% increase). In contrast, FCR H-reflex amplitude and FCR MVIC were unchanged across all testing sessions. The data show that diurnal fluctuations are present in the amplitude of the human soleus but not in the FCR H-reflex. Diurnal fluctuation in the human soleus H-reflex amplitude must be considered when interpreting H-reflex data, especially when a repeated measures design spanning several days is utilized.     

Acute passive stretching alters the mechanical properties of human plantar flexors and the optimal angle for maximal voluntary contraction

The purpose of this study was to investigate whether acute passive stretching (APS) reduced maximal isometric voluntary contraction (MVC) of the plantar flexors (PF) and if so, by what mechanisms. The PF in 15 female volunteers were stretched for 10 min (5×120 s) by a torque motor to within 2° of maximum dorsiflexion (D) range of motion (ROM). MVC with twitch interpolation, maximal Hoffmann reflex (H_max) and compound action potentials (M_max) were recorded at 20° D. Stretch reflexes (SR) were mechanically induced at 200° s^–1 between 0° and 10° D and SR torque and EMG amplitude were determined. All tests were assessed pre- (pre) and post-APS (post-test₁). MVC, SR, and M_max were again assessed after additional stretch was applied [mean 26 (1)° D; post-test₂] to test if the optimal angle had been altered. EMG was recorded from soleus (SOL), medial gastrocnemius (MG) and tibialis anterior (TA) using bipolar surface electrodes. APS resulted in a 27% decrease in mean peak passive torque (P<0.05). MVC and SR torque were 7% (P<0.05) and 13% lower at post-test₁ (P<0.05), respectively. SR EMG amplitude of SOL and MG was reduced by 27% (P<0.05) and 22% (P<0.05), respectively. The H_max/M_max EMG and H_max/M_max torque ratios were unchanged at post-test₁. At post-test₂, MVC and SR EMG recovered to pre-APS values, while the SR and M_max torque increased by 19% and 13%, respectively (P<0.05). The decrease in MVC during post-test₁ was attributed to changes in the mechanical properties of PF and not to reduced muscle activation.     

The stability of M<Subscript>max</Subscript> and H<Subscript>max</Subscript> amplitude over time

The stability of the maximal muscle response (M_max) is critical to H reflex methodology. It has previously been reported that the amplitude of M_max declines over time. If reproducible, this finding would have implications for all experimental studies that normalise the output of the motoneurone pool against the M wave. We investigated the effect of time on changes in M_max and the maximal H reflex (H_max) evoked at 4-s intervals over 60 min. To identify an influence of homosynaptic depression, we extended the interstimulus interval to 10 s and the time to 100 min. Two recording montages over soleus were used to ensure that interelectrode distance was not a critical factor. The soleus M_max and H reflex were evoked by stimulation of the tibial nerve in the popliteal fossa in 7 subjects who sat with the knee flexed to 30° and the ankle plantar flexed by ~30°. We found no change in the pooled data for M_max, H_max, a reflex 50% of maximal, or the current required to produce it. However, one subject had a statistically significant increase in M_max and a concurrent decrease in H_max regardless of the interstimulus interval. On average, there was no change in the H_max/M_max ratio over time. While both M_max and H_max may change in response to many factors, these results suggest that, typically, time is not one of them.     

Soleus H-reflex modulation during body weight support treadmill walking in spinal cord intact and injured subjects

The soleus H-reflex modulation pattern was investigated in ten spinal cord intact subjects during treadmill walking at varying levels of body weight support (BWS), and nine spinal cord injured (SCI) subjects at a BWS level that promoted the best stepping pattern. The soleus H-reflex was elicited by tibial nerve stimulation with a single 1-ms pulse at an intensity that the M-waves ranged from 4 to 8% of the maximal M-wave (M_max). During treadmill walking, the H-reflex was elicited every four steps, and stimuli were randomly dispersed across the gait cycle which was divided into 16 equal bins. EMGs were recorded with surface electrodes from major left and right hip, knee, and ankle muscles. M-waves and H-reflexes at each bin were normalized to the M_max elicited at 60–100 ms after the test reflex stimulus. For every subject, the integrated EMG area of each muscle was established and plotted as a function of the step cycle phase. The H-reflex gain was determined as the slope of the relationship between H-reflex and soleus EMG amplitudes at 60 ms before H-reflex elicitation for each bin. In spinal cord intact subjects, the phase-dependent H-reflex modulation, reflex gain, and EMG modulation pattern were constant across all BWS (0, 25, and 50) levels, while tibialis anterior muscle activity increased with less body loading. In three out of nine SCI subjects, a phase-dependent H-reflex modulation pattern was evident during treadmill walking at BWS that ranged from 35 to 60%. In the remaining SCI subjects, the most striking difference was an absent H-reflex depression during the swing phase. The reflex gain was similar for both subject groups, but the y-intercept was increased in SCI subjects. We conclude that the mechanisms underlying cyclic H-reflex modulation during walking are preserved in some individuals after SCI.     

Morning to evening changes in the electrical and mechanical properties of human soleus motor units activated by H reflex and M wave

The aim of the present study was to compare the relative contribution of the soleus motor units (MUs) activated by H and M waves to the plantar-flexion torque in the morning and in the evening. Twelve healthy male subjects (physical education students) took part in this investigation. The electromechanical properties of the plantar flexor muscles were recorded at two different times of day: between 06:00 and 08:00 h and between 17:00 and 19:00 h. Plantar-flexion torque and concomitant electromyographic activity of soleus muscle were assessed under voluntary and evoked conditions. The results indicated a significant decrease in maximal voluntary muscle torque of triceps surae and associated soleus EMG in the evening as compared with the morning. The mean values of MVC ranged from 131.6±9.6 N m in the morning to 125.1±9.0 N m in the evening. Peak-to-peak values of soleus H _max and M _max potentials were comparable in the morning and in the evening (2.97 vs 3.18 mV and 7.95 vs 7.44 mV for H _max and M _max, respectively). The H _max/M _max ratio was not modified between the two experimental test sessions (34.8 vs 41.3%). The peak amplitude of the twitch produced by the H _max wave decreased significantly. When estimating the mechanical contribution to of the slowest and fastest-twitch MUs reflexively and directly activated, we observed that the contribution of the slowest MUs did not change while those of the fastest decreased significantly in the evening. To conclude, a weaker reflex twitch torque caused by higher fatigue state of the MUs directly activated by the M wave which accompanied H _max in the evening may be regarded as a possible explanation of the weaker plantar-flexion torque production in the evening.     

Alpha-motoneuron excitability at high altitude

Summary It has been hypothesized that chronic hypobaric hypoxia could lead to inhibition of the-motoneuron pool, thus limiting the maximal activation of working skeletal muscles. To test this hypothesis six subjects [32 (SEM 2) years] were evaluated in resting conditions, at sea level and after acclimatization at 5,050 m. The recruitment curves of the Hofmann-reflex (H-) and the direct muscle-response (M-) of the right soleus muscle were obtained by stimulating the posterior tibeal nerve with different intensities while recording the electromyogram of the soleus muscle. From the recorded data the net-motoneuron excitability (ratio of maximal H-reflex to M-response H_max : M_max ratio), the threshold and gain for both responses, obtained from linear regressions through the rising phase of the recruitment curves of both responses, as well as the latency times of both responses were determined. The latency times and the H_max :M_max ratio were unchanged at altitude. The thresholds of both responses and the gain of the M-response were unaltered. The gain of the H-response was significantly higher at altitude when compared to sea level. It is concluded that in the acclimatized subjects at rest the signal conduction velocity through the different parts of both pathways was unaltered and therefore nerve and muscle conduction velocity as well as synaptic and muscle end-plate transmission were unchanged, that the recruitment of the H-reflex was slightly facilitated after acclimatization to high altitude suggesting increased excitability of the-motoneurons, through either postsynaptic facilitatory changes in the soma or a different descending drive, and that the unchanged H_max:M_max ratio indicated no change in the net excitatory and inhibitory influences on the-motoneuron pool. The above hypothesis is thus not strengthened by the results that were, however, obtained in resting conditions.     

Coactivation of the ankle musculature during maximal isokinetic dorsiflexion at different angular velocities

The objectives of this study were to determine the differences in the level of coactivation among the ankle agonist and antagonist muscles during maximal isokinetic dorsiflexion and to determine whether velocity alters the activation levels. Raw surface electromyograms (EMG) were recorded from six muscle sites – two over the agonist tibialis anterior (upper and lower sites) and four over the antagonist (lateral and medial gastrocnemius, plus lateral and medial soleus) muscles – in ten healthy subjects (two males and eight females) as they performed maximal dorsiflexor efforts against an isokinetic dynamometer at each of three angular velocity settings (30, 90 and 150°/s). The root-mean-square amplitude (RMS) was calculated over a 35° angular displacement for each EMG recording, then was normalized (RMS_{N ji} ) to the amplitude measured during maximal voluntary isometric contractions (MVIC) of the dorsifexors and plantarflexors (i.e. RMS_{MVIC j} ). A two-factor repeated-measures analysis of variance (ANOVA) tested the muscle site by velocity interaction and the main effects of muscle site and velocity separately. The raw EMG signals were then full-wave rectified and low-pass filtered (6 Hz) and ensemble average curves of the sample were calculated, for each muscle site, at each velocity. The ANOVA revealed a statistically significant muscle-by-velocity interaction (P < 0.05). The RMS_{N ji} values for the two tibialis anterior sites were not significantly different from each other for any of the three velocity settings, nor for the two gastrocnemius muscles at 30°/s velocity. All other between-muscle comparisons were statistically significant (P < 0.05). The RMS_{N ji} means were 28, 23, 59, 52, 98 and 98% MVIC for the lateral and medial gastrocnemius, the lateral and medial sites on the soleus, and for the upper and lower sites on the tibialis anterior, respectively. The RMS_{N ji} from the lower site on the tibialis anterior yielded the only statistically significant difference (P < 0.05) among velocities; at this site, the RMS_{N ji} at 150°/s was higher than at the other two velocities. The ensemble-average curves revealed that not all muscle sites are activated to a consistent level throughout the entire movement. The interaction between agonist and antagonist activation during isokinetic dorsiflexion has implications for interpreting the results of isokinetic dynamometry for strength assessment and for understanding the neuromuscular control strategies used in this exercise modality. Accepted: 3 March 2000     

Hypercapnic impairment of neuromuscular function is related to afferent depression

Acetazolamide (ACZ), a carbonic anhydrase inhibitor, results in altered neuromuscular function secondary to depressed afferent transmission in intact humans. One effect of ACZ is hypercapnia. Thus, to test if the neuromuscular depression observed following ACZ treatment is related to elevated CO₂, human subjects (n=10) were exposed to 15 min of room air (0% CO₂) or hypercapnia (7% inspired CO₂), and neuromuscular function was evaluated. Isometric force (36.8 to 31.1 N) and peak-to-peak electromyographic amplitude (EMG, 1.5 to 1.0 mV) associated with an Achilles tendon tap, and soleus H_max:M_max ratio (69.0 to 62.2%) were depressed, while EMG latency (34.8 to 39.8 ms) was increased by hypercapnia. Reflex recovery profiles (following a conditioning tap to the contralateral Achilles tendon), motor nerve conduction velocity, amplitude of the maximum M-wave, and peak twitch tension at M_max were unaltered by hypercapnia. We conclude that elevated CO₂ impairs neuromuscular function through effects on afferent transmission or synaptic integrity between type Ia fibers of the muscle spindle and the alpha motor neuron, without affecting the muscle spindle, efferent conduction or skeletal muscle force-generating capacity.     

Differences in H-reflex between athletes trained for explosive contractions and non-trained subjects

Summary The efficacy of type la synapse on alpha-motoneurons of soleus and lateral gastrocnemius muscles has been investigated, using the H-reflex technique, in athletes engaged in sports requiring very rapid and intense contractions (sprinters and volley-ball players) as well as in non-trained subjects. It has been observed, in both muscles, that the ratio between the mean value of the maximal reflex response (H_max) and the mean value of the maximal direct response (M_max) elicited upon electrical stimulation of the tibial nerve is significantly smaller in athletes trained for explosive-type movements than in non-trained subjects. This difference in the H_max:M_max ratio was dependent on a smaller amplitude of H_max and not on a greater amplitude of M_max. No significant differences were observed between sprinters and volley-ball players. In both trained and non-trained subjects, soleus and lateral gastrocnemius muscles displayed significant differences in H_max: M_max ratio and M_max amplitude but not in H_max amplitude. Since the H-response is considered to be due mainly to activation of the smallest motoneurons in the motoneuronal pools, the difference in H_max amplitude and H_max:M_max ratio between athletes and non-trained subjects could have been dependent on a lower incidence of these motoneurons in the athletes. This is in accord with the mechanical needs of muscles during explosive-type power training. Although this difference ,ay have been wholly determined genetically, the possibility is discussed as to whether the lower incidence in sprinters and volley-ball players of small motoneurons could have been related to a training-induced transformation of small and slow motoneurons into large and fast ones.     

Cortical and spinal adaptations induced by balance training: correlation between stance stability and corticospinal activation

Aim: To determine the sites of adaptation responsible for improved stance stability after balance (=sensorimotor) training, changes in corticospinal and spinal excitability were investigated in 23 healthy subjects. Methods: Neural adaptations were assessed by means of H‐reflex stimulation, transcranial magnetic stimulation (TMS) and conditioning of the H‐reflex by TMS (H_cond) before and after 4 weeks of balance training. All measurements were performed during stance perturbation on a treadmill. Fast posterior translations induced short‐ (SLR), medium‐ and long‐latency responses (LLR) in the soleus muscle. Motor‐evoked potential‐ (MEP) and H_cond‐amplitudes as well as H_max/M_max ratios were determined at SLR and LLR. Postural stability was measured during perturbation on the treadmill. Results: Balance training improved postural stability. H_max/M_max ratios were significantly decreased at LLR. MEPs and H_cond revealed significantly reduced facilitation at LLR following training. A negative correlation between adaptations of H_cond and changes in stance stability was observed (r = ?0.87; P < 0.01) while no correlation was found between stance stability and changes in H_max/M_max ratio. No changes in any parameter occurred at the spinally organized SLR and in the control group. Conclusion: The decrease in MEP‐ and H_cond‐facilitation implies reduced corticospinal and cortical excitability at the transcortically mediated LLR. Changes in cortical excitability were directly related to improvements in stance stability as shown by correlation of these parameters. The absence of such a correlation between H_max/M_max ratios and stance stability suggests that mainly supraspinal adaptations contributed to improved balance performance following training.     

Motor-neuron pool excitability of the lower leg muscles after acute lateral ankle sprain

Context:

Neuromuscular deficits in leg muscles that are associated with arthrogenic muscle inhibition have been reported in people with chronic ankle instability, yet whether these neuromuscular alterations are present in individuals with acute sprains is unknown.

Objective:

To compare the effect of acute lateral ankle sprain on the motor-neuron pool excitability (MNPE) of injured leg muscles with that of uninjured contralateral leg muscles and the leg muscles of healthy controls.

Design:

Case-control study.

Setting:

Laboratory.

Patients or Other Participants:

Ten individuals with acute ankle sprains (6 females, 4 males; age = 19.2 ± 3.8 years, height = 169.4 ± 8.5 cm, mass = 66.3 ±11.6 kg) and 10 healthy individuals (6 females, 4 males; age = 20.6 ± 4.0 years, height = 169.9 ± 10.6 cm, mass = 66.3 ± 10.2 kg) participated.

Intervention(s):

The independent variables were group (acute ankle sprain, healthy) and limb (injured, uninjured). Separate dependent t tests were used to determine differences in MNPE between legs.

Main Outcome Measure(s):

The MNPE of the soleus, fibularis longus, and tibialis anterior was measured by the maximal Hoffmann reflex (H_max) and maximal muscle response (M_max) and was then normalized using the H_max:M_max ratio.

Results:

The soleus MNPE in the ankle-sprain group was higher in the injured limb (H_max:M_max = 0.63; 95% confidence interval [CI], 0.46, 0.80) than in the uninjured limb (H_max:M_max = 0.47; 95% CI, 0.08, 0.93) (t₆ = 3.62, P = .01). In the acute ankle-sprain group, tibialis anterior MNPE tended to be lower in the injured ankle (H_max:M_max = 0.06; 95% CI, 0.01, 0.10) than in the uninjured ankle (H_max:M_max = 0.22; 95% CI, 0.09, 0.35), but this finding was not different (t₉ = −2.01, P = .07). No differences were detected between injured (0.22; 95% CI, 0.14, 0.29) and uninjured (0.25; 95% CI, 0.12, 0.38) ankles for the fibularis longus in the ankle-sprain group (t₉ = −0.739, P = .48). We found no side-to-side differences in any muscle among the healthy group.

Conclusions:

Facilitated MNPE was present in the involved soleus muscle of patients with acute ankle sprains, but no differences were found in the fibularis longus or tibialis anterior muscles.     

Effect of electromyostimulation training on soleus and gastrocnemii H- and T-reflex properties

When muscle is artificially activated, as with electromyostimulation (EMS), action potentials are evoked in both intramuscular nerve branches and cutaneous receptors, therefore activating spinal motoneurons reflexively. Maximal soleus and gastrocnemii H- and T-reflex and the respective mechanical output were thus quantified to examine possible neural adaptations induced at the spinal level by EMS resistance training. Eight subjects completed 16 sessions of isometric EMS (75 Hz) over a 4-week period. Maximal soleus and gastrocnemii M wave (M_max), H reflex (H_max) and T reflex (T_max) were compared between before and after training, together with the corresponding plantar flexor peak twitch torque. No significant changes were observed for electromechanical properties of H_max reflex following EMS. On the other hand, peak twitch torque produced by T_max, but not by equal-amplitude H reflex, significantly increased as a result of training (+21%, P<0.05). These changes were associated with a trend towards a significant increase for normalized gastrocnemii (+21%, P=0.07) but not soleus T_max reflex. It is concluded that, contrary to results previously obtained after voluntary physical training, EMS training of the plantar flexor muscles did not affect alpha motoneuron excitability and/or presynaptic inhibition, as indicated by H-reflex results. On the other hand, in the absence of change in a control group, T_max electromechanical findings indicated that: (1) equal-amplitude H- and T-reflex adapted differently to EMS resistance training; and (2) EMS had an effect on gastrocnemii but not on soleus muscle, perhaps because of the differences in respective motor unit characteristics (e.g., axon diameter).     

Oxygen uptake kinetics during horizontal and uphill treadmill running in humans

The aim of this study was to examine the effect of increasing the ratio of concentric to eccentric muscle activation on oxygen uptake (V˙O₂) kinetics during treadmill running. Nine subjects [2 women; mean (SD) age 29 (7) years, height 1.77 (0.07) m, body mass 73.0 (7.5) kg] completed incremental treadmill tests to exhaustion at 0% and 10% gradients to establish the gradient-specific ventilatory threshold (VT) and maximal oxygen uptake (V˙O_2max). Subsequently, the subjects performed repeated moderate intensity (80% of gradient-specific VT) and heavy intensity (50% of the difference between the gradient specific VT and V˙O_2max) square-wave runs with the treadmill gradient set at 0% and 10%. For moderate intensity exercise, there were no significant differences between treadmill gradients for V˙O₂ kinetics. For heavy intensity exercise, the amplitude of the primary component of V˙O₂ was not significantly different between 0% and 10% treadmill gradients [mean (SEM) 2,940 (196) compared to 2,869 (156) ml·min^–1, respectively], but the amplitude of the V˙O₂ slow component was significantly greater at the 10% gradient [283 (43) compared to 397 (37) ml·min^–1; P<0.05]. These results indicate that the muscle contraction regimen (i.e. the relative contribution of concentric and eccentric muscle action) significantly influences the amplitude of the V˙O₂ slow component. Electronic Publication     

Temperature dependence of soleus H-reflex and M wave in young and older women

The purpose of this study was to investigate the effect of altered local temperature on soleus H-reflex and compound muscle action potential (M wave) in young and older women. H-reflex and M wave responses were elicited in 10 young (22.3±3.3 years) and 10 older (72.5±3.2 years) women at three muscle temperatures: control (34.2±0.3°C), cold (31.3±0.5°C) and warm (37.1±0.2°C). H-reflex output, expressed as the ratio between maximal H-reflex and maximal M wave (H_max/M_max), was lower in the older, compared with the younger, group, regardless of temperature. In control temperature conditions, for example, the H_max/M_max ratio was 36.8±24% in the young and 25.4±20% in the older (P<0.05). Warming had no effect on the H-reflex output in either group, whilst cooling increased H-reflex output only in the younger group (+28%). In both groups, cooling increased (+5.3%), and warming decreased (–5.5%) the H-reflex latency. This study confirms that older individuals experience a reduced ability to modulate the reflex output in response to a perturbation. In a cold environment, for example, the lack of facilitation in the reflex output, along with a delayed reflex response could be critical to an older individual in responding to postural perturbations thus potentially compromising both static and dynamic balance.     

Threshold of the soleus muscle H-reflex is less sensitive to the change in excitability of the motoneuron pool during plantarflexion or dorsiflexion in humans

In the present study, we investigated whether weak (10% of maximal voluntary contraction) tonic dorsiflexion (DF) and plantarflexion (PF) affects the two conventional parameters used for evaluating the excitability of the soleus motoneuron (MN) pool, i.e. the ratio of the threshold of H-reflex to that of M-response (H_th:M_th) and the ratio of the maximal amplitude of H-reflex to that of M-response (H_max:M_max) in human subjects. The results showed that the H_max:M_max decreased during DF and increased during PF compared with that during rest, whereas no clear alteration was observed in H_th:M_th. These results are consistent with the scheme proposed by earlier workers, who have argued that neither inhibitory nor facilitatory effects of the conditioning stimulus apply to specific spinal reflex circuits occurring around the threshold of the test H-reflex. It is suggested, therefore, that the conventional use of the H_th:M_th ratio as a parameter reflecting the excitability of the MN pool should be reconsidered.     

A diazo-2 study of relaxation mechanisms in frog and barnacle muscle fibres: effects of pH, MgADP, and inorganic phosphate

 The aim of this study was to compare the effects of increased concentrations of MgADP, inorganic phosphate (P_i) and H⁺ ([MgADP], [Pi] and [H⁺], respectively) on the rate of relaxation in two different muscle types: skinned muscle fibres from the frog Rana temporaria and myofibrillar bundles from the giant Pacific acorn barnacle Balanus nubilus. Relaxation transients are produced by the photolysis of diazo-2 and are well fitted with a double exponential curve, giving two rate constants: k₁ [5.6±0.1 s^–1 for barnacle, n=30; 26.3±0.7 s^–1 for frog, n=14 (mean±SEM)] and k₂ [0.6±0.1 s^–1 in barnacle, n=30; 10.4±1.0 s^–1 in frog, n=14 (mean±SEM)], at 10°C. Decreasing the pH by 0.5 pH units did not significantly affect k₁ for barnacle relaxation [5.6±0.1 s^–1 (mean±SEM), n=15] compared to the decrease in k₁ of 40% seen in frog. Use of the Ca²⁺-sensitive fluorescent label acrylodan on barnacle wild-type troponin C demonstrated that decreasing the pH from 7.0 to 6.6 only alters the pCa₅₀ value by 0.23 in the cuvette, while stopped-flow experiments with acrylodan revealed no significant change in k_off from the labelled protein [322±32 s^–1 at pH 7.0 and 381±24 s^–1 (mean±SEM) at pH 6.6]. Increasing [MgADP] by 20 μM (50 μM added ADP) from control values of 50 μM in frog decreased k₁ to 12.3±0.4 s^–1 (mean±SEM, n=8), and at 400 μM MgADP, k₁=9.6±0.1 s^–1 (mean±SEM, n=12). In barnacle, 500 μM MgADP had a much smaller effect on k₁ (4.0±0.9 s^–1, mean±SEM, n=8). Increasing the free [P_i] from the contaminant level of 0.36 mM to 1.9 mM slowed k₁ by ≈15% in barnacle [4.8±0.8 s^–1, mean±SEM, n=7], compared to a ≈30% reduction seen in frog. We conclude that the differences between barnacle and frog seen here are most probably due to different isomers of the contractile proteins, and that events underlying the crossbridge cycle are the same or similar. We interpret our results according to a model of crossbridge transitions during relaxation. Received: 18 May 1998 / Received after revision and accepted: / 1 September 1998     

Eccentric exercise inhibits the H reflex in the middle part of the trapezius muscle

The objectives of this study were to (1) investigate the modulation of the H reflex immediately after and 24 h after eccentric exercise in the presence of delayed-onset muscle soreness (DOMS) and (2) test the reproducibility of the H reflex in trapezius across days. H reflexes were recorded from the dominant middle trapezius muscle by electrical stimulation of the C3/4 cervical nerve in ten healthy subjects. DOMS was induced by eccentric exercise of the dominant shoulder. H reflexes were obtained in four sessions: “24 h before”, “Pre”, “Post”, and “24 h after” eccentric exercise. Ratios of maximal H reflex and M wave responses (H _max/M _max) were compared between sessions. In addition, a between session comparison was done for the ratios of H reflex amplitudes (H _{i_75}/M _max, and H _{i_50}/M _max) obtained from the stimulus intensity needed to obtain 75 and 50 % of H _max at “24 h before”. No ratio changes were found when comparing “24 h before” and “Pre” recordings. A decrease in H _{i_50}/M _max was found at “Post” (P < 0.05) and decreases in both H _{i_75}/M _max and H _{i_50}/M _max were observed at “24 h after” (P < 0.05). This study presented evidence that an acceptable day-to-day reproducibility of the H reflex could be obtained with the applied experimental setup. Furthermore, immediately after and 24 h after exercise a stronger stimulus intensity was needed to reach the same magnitude of the H reflex reflecting that the recruitment curve was shifted to the right. This modulation of the stimulus–response relationship could be caused by presynaptic inhibition of Ia afferent fibres’ input to the motoneuron by group III and IV afferents.     

The force–velocity relationship of the human soleus muscle during submaximal voluntary lengthening actions

In experiments on isolated animal muscle, the force produced during active lengthening contractions can be up to twice the isometric force, whereas in human experiments lengthening force shows only modest, if any, increase in force. The presence of synergist and antagonist muscle activation associated with human experiments in situ may partly account for the difference between animal and human studies. Therefore, this study aimed to quantify the force–velocity relationship of the human soleus muscle and assess the likelihood that co-activation of antagonist muscles was responsible for the inhibition of torque during submaximal voluntary plantar flexor efforts. Seven subjects performed submaximal voluntary lengthening, shortening(at angular, velocities of +5, –5, +15, –15 and +30, and –30° s^–1) and isometric plantar flexor efforts against an ankle torque motor. Angle-specific (90°) measures of plantar flexor torque plus surface and intramuscular electromyography from soleus, medial gastrocnemius and tibialis anterior were made. The level of activation (30% of maximal voluntary isometric effort) was maintained by providing direct visual feedback of the soleus electromyogram to the subject. In an attempt to isolate the contribution of soleus to the resultant plantar flexion torque, activation of the synergist and antagonist muscles were minimised by: (1) flexing the knee of the test limb, thereby minimising the activation of gastrocnemius, and (2) applying an anaesthetic block to the common peroneal nerve to eliminate activation of the primary antagonist muscle, tibialis anterior and the synergist muscles, peroneus longus and peroneus brevis. Plantar flexion torque decreased significantly (P<0.05) after blocking the common peroneal nerve which was likely due to abolishing activation of the peroneal muscles which are synergists for plantar flexion. When normalised to the corresponding isometric value, the force–velocity relationship between pre- and post-block conditions was not different. In both conditions, plantar flexion torques during shortening actions were significantly less than the isometric torque and decreased at faster velocities. During lengthening actions, however, plantar flexion torques were not significantly different from isometric regardless of angular velocity. It was concluded that the apparent inhibition of lengthening torques during voluntary activation is not due to co-activation of antagonist muscles. Results are presented as mean (SEM).     

Central fatigue during a long-lasting submaximal contraction of the triceps surae

Our purpose was to study central fatigue and its dependence on peripheral reflex inhibition during a sustained submaximal contraction of the triceps surae. In 11 healthy subjects, superimposed twitches, surface electromyograms (EMG) from the medial head of the gastrocnemius (MG) and soleus (SOL) muscles, maximal compound motor action potentials (M_max), tracking error and tremor were recorded during sustained fatiguing contractions at a torque level corresponding to 30% of maximal voluntary contraction (MVC). When the endurance limit (401±91 s) of the voluntary contraction (VC-I) was reached, the triceps surae could be electrically stimulated to the same torque level for an additional 1 min in 10 of the 11 subjects. These subjects were then able to continue the contraction voluntarily (voluntary contraction II, VC-II) for another 85±48 s. At the endurance limit of VC-I, the superimposed twitch was larger than during the unfatigued MVC, while there was no significant difference between the twitch at the endurance limit of VC-II and MVC. The EMG amplitude of both MG and SOL at the endurance limit of VC-I was significantly less than that during the MVC. While the EMG amplitude of MG increased further during VC-II, SOL EMG remained unchanged, neither muscle reaching their unfatigued MVC values. This difference was diminished for SOL by taking into account its decrease in M_max found during VC-II, and relative EMG levels approached their MVC values. These results clearly indicate that a higher voluntary muscle activation was achievable after 1 min of electrical muscle stimulation, which continued metabolic stress and contractile fatigue processes but allowed for supraspinal, muscle spindle and/or motoneuronal recovery. It is concluded that peripheral reflex inhibition of -motoneurons via small-diameter muscle afferents is of minor significance for the development of the central fatigue that was found to occur during the first voluntary contraction.     

Calf and shin muscle oxygenation patterns and femoral artery blood flow during dynamic plantar flexion exercise in humans

The effects of dynamic plantar flexion exercise [40, 60, and 80 contractions·min^–1 (cpm)] on calf and shin muscle oxygenation patterns and common femoral artery blood flow ( ) were examined in six female subjects [mean age 21 (SD 1) years] who exercised for 1 min at 33% of their maximal voluntary contraction at ankle angles between 90° and 100°. Spatially resolved near-infrared spectroscopy was used to measure medial gastrocnemius, lateral soleus (synergist) and anterior tibialis (antagonist) muscle oxygen saturation (SO₂, %). was measured by ultrasound Doppler. The SO₂ changed significantly only in the medial gastrocnemius and its decrease (up to about 30%) was independent of the contraction frequencies examined. The increase in , at the end of exercise, was highest at 80 cpm. When the exercise at 60 cpm was prolonged until exhaustion [mean 2.7 (SD 1.1) min], medial gastrocnemius SO₂ decreased, reaching its minimal value [mean 30 (SD 10)%] within the 1st min, and had partially recovered before the end of the exercise with concomitant increases in total haemoglobin content and . These results suggest that the medial gastrocnemius is the muscle mostly involved in dynamic plantar flexion exercise and its oxygen demand with increases in contraction frequency and duration is associated with an up-stream increase in . Electronic Publication