Amplitude of the maximum motor response (Mmax) in human muscles typically decreases during the course of an experiment

 It was shown that the amplitude of the soleus M _max and H _max responses decreases in the course of long-lasting H-reflex studies. The peak-to-peak amplitudes of the M _max and H _max responses in the soleus muscle (and the M _max in the tibialis anterior muscle and small hand muscles) were measured repeatedly for 1–3 h in 20 subjects. 3–5 M _max responses and 5–10 H _max responses were elicited about every 3 min while the subject was at rest. Decreases in the soleus M _max response of up to 50.5% (mean 20.5% SEM 2.2) and of the soleus H _max of up to 49.7% (mean 19.1% SEM 3.7) in relation to the amplitudes measured at the beginning of the experiment were seen in 17 subjects. In 3 subjects no M _max amplitude decrease was seen. The maximum decrease was reached between 10 and 100 min (mean 44.2 min SEM 4.3). An M_max amplitude decrease was also seen in the tibialis anterior muscle and in two small hand muscles. In some subjects the decrease of the M _max response seemed to be initiated by the infrequent supramaximal stimulations. The possible causes for this amplitude reduction, as well as the methodological consequences of these findings for H-reflex studies and fatigue studies, are briefly discussed. Received: 1 July 1998 / Accepted: 9 October 1998     

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.     

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.     

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.     

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).     

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.     

Electrically evoked torque-velocity characteristics and isomyosin composition of the triceps surae in young and elderly men

The electrically evoked isokinetic torque-velocity relationship of the triceps surae of eight elderly and four non-trained young men was examined in relation to the isomyosin composition of the soleus and the gastrocnemius muscles, determined under non-denaturing conditions using pyrophosphate gel electrophoresis. The angle specific torque-velocity properties of the triceps surae were measured using maximal percutaneous electrical stimulation at 50 Hz and a release technique. The elderly subjects generated significantly (P < 0.05) less absolute torque at all angular velocities when compared with the young subjects. When the isokinetic data were normalized to the isometric torque, the lower normalized torques generated by the elderly subjects were not statistically different from the young. The total fast isomyosin (FM) content of the soleus and gastrocnemius in the elderly subjects was 22 ± 13 and 35 ± 18%, respectively. This compared with 29 ± 8 (n.s.) and 44 ± 8% (n.s.) in the young subjects. When the gastrocnemius and soleus muscles were given an equal weighting and considered together to represent the whole triceps surae, the normalized torque at the fixed angular velocity of 5 rad s^-1 was significantly associated with%FM (r = 0.90, P < 0.01), and the isomyosin bands%FM1 (r = 0.90, P < 0.01) and%FM2 (r = 0.93, P < 0.001) when only the elderly subjects were considered. No relationships were observed between contractile characteristics and contractile protein profile when only the young subjects were considered. This was despite the inclusion of a further two sprint and three endurance trained athletes to increase the range of contractile characteristics and differences in muscle composition.     

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.     

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.     

Quantification of T- and H-responses before and after a period of endurance training

Summary Tendon (T-) and Hoffmann (H-) responses in the soleus muscle were quantified either separately or in association to compare the mononeurons activated and to study their changes after a period of endurance training. In a first experiment T- and H-responses of the same amplitude were compared: the electrical stimulus (inducing the H-response) and the Achilles tendon tap (inducing the T-response) were associated so that the T-response firstly was concomitant with the H-response, and secondly shifted 10 ms forward or back compared to the H-response. From the study of these combined reflexes we would suggest that the same motoneurons are involved in T- or H-responses of the same amplitude. In a second experiment the maximal H-responses, the T-responses and maximal aerobic power (W _aer,max) were measured on 20 subjects before and after a period of endurance training. For 75% of the subjects the W _aer,max and the reflex parameters (T or H) varied in the same direction: most of them exhibited higher values of both W _aer,max and reflex amplitudes while the others had W _aer,max and reflex values hardly modified or decreased. The different effects of the training period could reflect the heterogeneity of the subject's status and involvement in sport. In most cases the T: Hmax ratios were also influenced, reflecting the fact that T- and H-responses were not identically affected by training. Thus it is suggested that an endurance training programme can influence not only the excitability of the motoneurons but also the response of the muscle receptors to stretch. An interpretation in terms of a change of spindle receptivity and/or a change in their recruitment due to a greater stiffness of the trained muscles is suggested.     

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.     

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.     

Comparison of metabolic adaptations between endurance‐ and sprint‐trained athletes after an exhaustive exercise in two different calf muscles using a multi‐slice 31P‐MR spectroscopic sequence

Measurements of exercise‐induced metabolic changes, such as oxygen consumption, carbon dioxide exhalation or lactate concentration, are important indicators for assessing the current performance level of athletes in training science. With exercise‐limiting metabolic processes occurring in loaded muscles, ³¹P‐MRS represents a particularly powerful modality to identify and analyze corresponding training‐induced alterations. Against this background, the current study aimed to analyze metabolic adaptations after an exhaustive exercise in two calf muscles (m. soleus – SOL – and m. gastrocnemius medialis – GM) of sprinters and endurance athletes by using localized dynamic ³¹P‐MRS. In addition, the respiratory parameters VO₂ and VCO₂, as well as blood lactate concentrations, were monitored simultaneously to assess the effects of local metabolic adjustments in the loaded muscles on global physiological parameters. Besides noting obvious differences between the SOL and the GM muscles, we were also able to identify distinct physiological strategies in dealing with the exhaustive exercise by recruiting two athlete groups with opposing metabolic profiles. Endurance athletes tended to use the aerobic pathway in the metabolism of glucose, whereas sprinters produced a significantly higher peak concentration of lactate. These global findings go along with locally measured differences, especially in the main performer GM, with sprinters revealing a higher degree of acidification at the end of exercise (pH 6.29 ± 0.20 vs. 6.57 ± 0.21). Endurance athletes were able to partially recover their PCr stores during the exhaustive exercise and seemed to distribute their metabolic activity more consistently over both investigated muscles. In contrast, sprinters mainly stressed Type II muscle fibers, which corresponds more to their training orientation preferring the glycolytic energy supply pathway. In conclusion, we were able to analyze the relation between specific local metabolic processes in loaded muscles and typical global adaptation parameters, conventionally used to monitor the training status of athletes, in two cohorts with different sports orientations.     

The Use of Pulsed Electromagnetic Fields with Complex Modulation in the Treatment of Patients with Diabetic Polyneuropathy

Clinical and electroneuromyographic studies were performed in 121 patients with diabetic polyneuropathy (DPN) before and after courses of treatment with pulsed electromagnetic fields with complex modulation (PEMF-CM) at different frequencies (100 and 10 Hz). Testing of patients using the TSS and NIS LL scales demonstrated a correlation between the severity and frequency of the main subjective and objective effects of disease and the stage of DPN. The severity of changes in the segmental-peripheral neuromotor apparatus – decreases in muscle bioelectrical activity, the impulse conduction rate along efferent fibers of peripheral nerves, and the amplitude of the maximum M response – depended on the stage of DPN and the duration of diabetes mellitus. The earliest and most significant electroneuromyographic signs of DPN were found to be decreases in the amplitude of the H reflex and the H_max/M_max ratio in the muscles of the lower leg. Application of PEMF-CM facilitated regression of the main clinical symptoms of DPN, improved the conductive function of peripheral nerves, improved the state of 1a afferents, and improved the reflex excitability of functionally diverse motoneurons in the spinal cord. PEMF-CM at 10 Hz was found to have therapeutic efficacy, especially in the initial stages of DPN and in patients with diabetes mellitus for up to 10 years.     

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.     

Identification of different size motoneurons labeled by the retrograde axonal transport of horseradish peroxidase

Summary Three main groups of motoneurons of different size have been labeled in adult cats by using the method for retrograde axonal transport following injection of horseradish peroxidase in the medial gastrocnemius and soleus muscles. In particular small, medium-size and large neurons which probably correspond respectively to gamma, small alpha and large alpha motoneurons innervating the calf muscles, have been identified and the corresponding area measured.     

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.     

H-reflexes are smaller in dancers from The Royal Danish Ballet than in well-trained athletes

Summary The size of the maximalH-reflex (H _max) was measured at rest and expressed as a percentage of the maximalM-response (M _max) in 17 untrained subjects, 27 moderately trained subjects, 19 well-trained subjects and 7 dancers from the Royal Danish Ballet. TheH _max/M _max was significantly larger in the moderately and well-trained subjects than in the untrained subjects but smaller in the ballet dancers. It is therefore suggested that both the amount and the type of habitual activity may influence the excitability of spinal reflexes.     

Comparative and reliability studies of neuromechanical leg muscle performances of volleyball athletes in different divisions

This study compared neural profiles of the leg muscles of volleyball athletes playing in different divisions of Taiwan’s national league to analyse the reliability and correlations between their profiles and biomechanical performances. Twenty-nine athletes including 12 and 17 from the first and second divisions of the league, respectively, were recruited. The outcome measures were compared between the divisions, including soleus H-reflex, first volitional (V) wave, normalised rate of electromyography (EMG) rise (RER) in the triceps surae muscles, and RER ratio for the tibialis anterior and soleus muscles, normalised root mean square (RMS) EMG in the triceps surae muscles, antagonist co-activation of the tibialis anterior muscle, rate of force development (RFD), and maximal plantar flexion torque and jump height. Compared to the results of the second division, the neural profiles of the first division showed greater normalised V waves, normalised RER in the lateral gastrocnemius, and normalised RMS EMG of the soleus and lateral gastrocnemius muscles with less antagonist co-activation of the tibialis anterior. First division volleyball athletes showed greater maximal torque, jump height, absolute RFD at 0–30, 0–100, and 0–200 ms, and less in the normalised RFD at 0–200 ms of plantar flexion when compared to the results of those in the second division. Neural profiles correlated to fast or maximal muscle strength or jump height. There are differences in the descending neural drive and activation strategies in leg muscles during contractions between volleyball athletes competing at different levels. These measures are reliable and correlate to biomechanical performances.     

Fatigue and caffeine effects in fast-twitch and slow-twitch muscles of the mouse

Summary In excised, curarized and massively stimulated fast-twitch mouse gastrocnemius muscles the early twitch tension enhancements (treppe) during 1/s activity between 10 and 36°C increase and affect more contractions as temperature increases. Tension output eventually declines at a temperature-independent rate. Half-relaxation time lengthens below 25°C and shortens above 25°C. During 1/0.63 s twitches half-relaxation time lengthens even at 25°C. In slow-twitch soleus muscles activity decreases twitch tension and half-relaxation time regardless of temperature. Activity shortens contraction times in both muscles. Oxygen lack induced by NaN₃ cannot account satisfactorily for these results. Activation is apparently more plastic in the gastrocnemius than in the soleus, and the relationship between the rates of their activation and relaxation processes and the temperature sensitivities of these rates also seem to differ.In both muscles caffeine can convert activity-induced shortening of half-relaxation times into prolongations. In the soleus this effect is more pronounced at 30 than at 25°C. At high temperature and twitch rates caffeine reduces treppe amplitude and duration without affecting the eventual twitch tension decline in the gastrocnemius while it greatly accelerates twitch tension decline in the soleus. In both muscles intrafiber Ca²⁺ movements are apparently major determinants of fatigue behavior.This work was partly supported by a grant of the Vocational Rehabilitation Administration, U.S. Department of Health, Education and Welfare, and by research funds of the Departments of Rehabilitation Medicine and Physiology, State University of New York, Downstate Medical Center