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     

Quantifying the effects of voluntary contraction and inter-stimulus interval on the human soleus <Emphasis Type="Italic">H</Emphasis>-reflex

The human soleus H-reflex is commonly tested as an indicator of the reflex excitability of the calf muscles with infrequent stimuli to a subject seated and at rest. However, the reflex varies widely with the level of voluntary contraction and with the time history of stimulation. We studied two aspects of this variation. Antagonist (tibialis anterior) activation decreases the response, while increasing agonist (soleus) activation increases the H-reflex to a peak after which it declines. In subjects with large H-reflexes at rest, the reflex peaked at low levels of contraction. In contrast, in subjects with small H-reflexes at rest, the reflex peaked at higher levels of contraction for reasons that were elucidated using a realistic computer model. A parabolic curve fitted the maximum amplitude of the H-reflex in the model and over the entire range of contractile levels studied. The second aspect studied was post-activation depression or homosynaptic depression (HD), which has been described previously as a reduction of a second H-reflex elicited shortly after an initial reflex. We confirmed the presence of HD in resting, seated subjects for intervals up to 4 s. However, by voluntarily activating the soleus muscle, HD was drastically reduced when seated and abolished when standing. This suggests that HD may be absent in normal, functional movements and perhaps in clinical conditions that alter H-reflexes. Meaningful, quantitative measurements of reflex excitability can only be made under voluntary activity that mimics the condition of interest.     

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.     

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.     

Phase- and task-specific modulation of soleus <Emphasis Type="Italic">H</Emphasis>-reflexes during drop-jumps and landings

H-reflexes in the soleus muscle were previously shown to be decreased in drop-jumps at the instant of the short latency response (SLR) of the stretch reflex when falling height was increased. The aim of the present study was to elucidate task-specific modulation of H-reflexes during drop-jumps in more detail. Therefore, soleus H-reflex excitability was compared in drop-jumps from three different falling heights (30, 50, 75 cm) and between drop-jumps and landings. In landings, there is no need for high tendomuscular stiffness like in drop-jumps. Therefore, we assumed reduced spinal excitability of soleus Ia afferents which should be reflected as reduced H-reflexes. H-reflex recruitment curves were recorded in 23 subjects (24 ± 2 years) at the instant of the SLR during drop-jumps (30 cm-LH, 50 cm-MH, 75 cm-EH) and landings (30 cm falling height). Furthermore, recruitment curves were recorded at ground contact (GND) as a reference to SLR. At SLR, H/M ratios were higher during drop-jumps (LH: 0.49 ± 0.18) than during landings (0.33 ± 0.18, P < 0.001). H/M ratios did not differ at GND (LH: 0.46 ± 0.15; landings: 0.46 ± 0.14). H/M ratios were progressively decreased at the SLR from LH, to MH, to EH (MH: 0.44 ± 0.17; EH: 0.40 ± 0.16, P < 0.001). Again, there were no differences at GND (MH: 0.46 ± 0.15; EH: 0.44 ± 0.16). The present study provides further evidence of phase-specific (GND vs. SLR) and task-specific modulation of SOL Ia afferent input. These modulations were thought to be initiated prior to touch down.     

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.     

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.     

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.     

Modulation of soleus <Emphasis Type="Italic">H</Emphasis>-reflexes during gait in healthy children

During locomotion spinal short latency reflexes are rhythmically modulated and depressed compared to rest. In adults this modulation is severely disturbed after bilateral spinal lesions indicating a role for supra-spinal control. Soleus reflex amplitudes are large in the stance phase and suppressed in the swing phase contributing to the reciprocal muscle activation pattern required for walking. In early childhood the EMG pattern during gait underlies an age-dependent process changing from co-contraction of agonists and antagonists to a reciprocal pattern at the age of 5–7 years. It is unknown whether at this stage apart from the EMG also reflexes are modulated, and if so, whether the reflex modulation is fully mature or still underlies an age-dependent development. This may give important information about the maturation of CNS structures involved in gait control. Soleus Hoffmann H-reflexes were investigated in 36 healthy children aged 7–16 years during treadmill walking at 1.2 km/h and 3.0 km/h. At 7 years old a rhythmic modulation similar to adults was observed. The H-reflex size during the stance phase decreased significantly with age while the maximum H-reflex (H _max) at rest remained unchanged. At 3.0 km/h H-reflexes were significantly larger during the stance phase and smaller during the swing phase as compared to 1.2 km/h but the age-dependent suppression was observed at both walking velocities. In conclusion H-reflex modulation during gait is already present in young children but still underlies an age-dependent process independent of the walking velocity. The finding that the rhythmic part of the modulation is already present at the age of 7 years may indicate that the supra-spinal structures involved mature earlier than those involved in the tonic reflex depression. This may reflect an increasing supra-spinal control of spinal reflexes under functional conditions with maturation.     

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.     

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.     

Functional maximal strength training induces neural transfer to single-joint tasks

The purpose of this study was to investigate whether neural adaptations following functional multiple-joint leg press training can induce neural adaptations to the plantar flexor muscles in a single-joint contraction task. Subjects were randomised to a maximal strength training (MST) (n = 10) or a control group (n = 9). MST consisted of 24 sessions (8 weeks) of 4 × 4 repetitions of horizontal leg press using maximal intended velocity in the concentric phase with the movement ending in a plantar flexion. Neural adaptations in the soleus and gastrocnemius medialis (GM) were assessed by surface electromyographic activity and V-waves during maximum voluntary isometric contraction (MVIC), and also by H-reflexes in the soleus during rest and 20% MVIC. One repetition maximum leg press increased by 44 ± 14% (mean ± SD; P < 0.01). Plantar flexion MVIC increased by 20 ± 14% (P < 0.01), accompanied by 13 ± 19% (P < 0.05) increase in soleus, but not GM surface electromyography. Soleus V/M_SUP increased by 53 ± 66% and in GM by 59 ± 64% (P < 0.05). Normalised soleus H-reflexes remained unchanged by training. No changes occurred in the control group. These results suggest that leg press MST can induce neural adaptations in a single-joint plantar flexion MVIC task.     

A sigmoid function is the best fit for the ascending limb of the Hoffmann reflex recruitment curve

The Hoffmann (H)-reflex has been studied extensively as a measure of spinal excitability. Often, researchers compare the H-reflex between experimental conditions with values determined from a recruitment curve (RC). An RC is obtained experimentally by varying the stimulus intensity to a nerve and recording the peak-to-peak amplitudes of the evoked H-reflex and direct motor (M)-wave. The values taken from an RC may provide different information with respect to a change in reflex excitability. Therefore, it is important to obtain a number of RC parameters for comparison. RCs can be obtained with a measure of current (HCRC) or without current (HMRC). The ascending limb of the RC is then fit with a mathematical analysis technique in order to determine parameters of interest such as the threshold of activation and the slope of the function. The purpose of this study was to determine an unbiased estimate of the specific parameters of interest in an RC through mathematical analysis. We hypothesized that a standardized analysis technique could be used to ascertain important points on an RC, regardless of data presentation methodology (HCRC or HMRC). For both HCRC and HMRC produced using 40 randomly delivered stimuli, six different methods of mathematical analysis [linear regression, polynomial, smoothing spline, general least squares model with custom logistic (sigmoid) equation, power, and logarithmic] were compared using goodness of fit statistics (r-square, RMSE). Behaviour and robustness of selected curve fits were examined in various applications including RCs generated during movement and somatosensory conditioning from published data. Results show that a sigmoid function is the most reliable estimate of the ascending limb of an H-reflex recruitment curve for both HCRC and HMRC. Further, the parameters of interest change differentially with respect to the presentation methodology and the analysis technique. In conclusion, the sigmoid function is a reliable analysis technique which mimics the physiologically based prediction of the input/output relation of the ascending limb of the recruitment curve. Therefore, the sigmoid function should be considered an acceptable and preferable analytical tool for H-reflex recruitment curves obtained with reference to stimulation current or M-wave amplitude.     

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).     

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.     

Men and women exhibit a similar time to task failure for a sustained,submaximal elbow extensor contraction

Sex differences in muscle fatigue-resistance have been observed in a variety of muscles and under several conditions. This study compared the time to task failure (TTF) of a sustained isometric elbow extensor (intensity 15% of maximal strength) contraction in young men (n = 12) and women (n = 11), and examined if their neurophysiologic adjustments to fatigue differed. Motor-evoked potential amplitude (MEP), silent period duration, interference electromyogram (EMG) amplitude, maximal muscle action potential (M _max), heart rate, and mean arterial pressure were measured at baseline, during the task, and during a 2-min ischemia period. Men and women did not differ in TTF (478.2 ± 31.9 vs. 500.4 ± 41.3 s; P = 0.67). We also performed an exploratory post hoc cluster analysis, and classified subjects as low (n = 15) or high endurance (n = 8) based on TTF (415.3 ± 16.0 vs. 626.7 ± 25.8 s, respectively). The high-endurance group exhibited a lower MEP and EMG at baseline (MEP 16.3 ± 4.1 vs. 37.2 ± 3.0% M _max, P < 0.01; EMG 0.98 ± 0.18 vs. 1.85 ± 0.26% M _max, P = 0.03). These findings suggest no sex differences in elbow extensor fatigability, in contrast to observations from other muscle groups. The cluster analyses results indicated that high- and low-endurance groups displayed neurophysiologic differences at baseline (before performing the fatigue task), but that they did not differ in fatigue-induced changes in their neurophysiologic adjustments to the task.     

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.     

Neural adaptations underlying cross-education after unilateral strength training

The purpose of this study was to investigate the effects of 4-week (16 sessions) unilateral, maximal isometric strength training on contralateral neural adaptations. Subjects were randomised to a strength training group (TG, n = 15) or to a control group (CG, n = 11). Both legs of both groups were tested for plantar flexion maximum voluntary isometric contractions (MVCs), surface electromyogram (EMG), H-reflexes and V-waves in the soleus (SOL) and gastrocnemius medialis (GM) superimposed during MVC and normalised by the M-wave (EMG/M_SUP, H_SUP/M_SUP, V/M_SUP, respectively), before and after the training period. For the untrained leg, the TG increased compared to the CG for MVC torque (33%, P < 0.01), SOL EMG/M_SUP (32%, P < 0.05) and SOL V/M_SUP (24%, P < 0.05). For the trained leg, the TG increased compared to the CG for MVC torque (40%, P < 0.01), EMG/M_SUP (SOL: 38%, P < 0.05; GM: 60%, P < 0.05) and SOL V/M_SUP (72%, P < 0.01). H_SUP/M_SUP remained unchanged for both limbs. No changes occurred in the CG. These results reinforce the concept that enhanced neural drive to the contralateral agonist muscles contributes to cross-education of strength.     

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.     

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.