Acute physiological responses and muscle recovery in females: a randomised controlled trial of muscle damaging exercise in hypoxia.

BACKGROUND: Studies have investigated the effects of training under hypoxia (HYP) after several weeks in a male population. However, there is still a lack of knowledge on the acute hypoxic effects on physiology and muscle recovery in a female population. METHODS: This randomized-controlled trial aimed to investigate the acute effects of muscle damaging exercise, performed in HYP and normoxia (CON), on physiological responses and recovery characteristics in healthy females. Key inclusion criteria were recreationally active female participants between the age of 18 to 35 years without any previous surgeries and injuries, whilst key exclusion criteria were acute pain situations, pregnancy, and medication intake. The females conducted a muscle-damaging protocol, comprising 5 × 20 drop-jumps, in either HYP (FiO2: 12%) or CON (FiO2: 21%). Physiological responses, including capillary oxygenation (SpO2), muscle oxygenation (SmO2), heart rate (HR), core- (Tcore) and skin- (Tskin) temperature were assessed at the end of each exercise set. Recovery characteristics were quantified by taking venous blood samples (serum creatine-kinase [CK], C-reactive protein [CRP] and blood sedimentation rate [BSR]), assessing muscle swelling of the quadriceps femoris muscle, maximum voluntary isometric contraction (MVIC) of the knee extensor muscles, countermovement jump (CMJ) performance and muscle soreness ratings (DOMS) at 24-, 48- and 72-hrs post-exercise. RESULTS: SpO2 (HYP: 76.7 ± 3.8%, CON: 95.5 ± 1.7%, p < 0.001) and SmO2 (HYP: 60.0 ± 9.3, CON: 73.4 ± 5.8%, p = 0.03) values were lower (p < 0.05) in HYP compared to CON at the end of the exercise-protocol. No physiological differences between HYP and CON were observed for HR, Tcore, and Tskin (all p > 0.05). There were also no differences detected for any recovery variable (CK, CRP, BSR, MVIC, CMJ, and DOMS) during the 72-hrs follow-up period between HYP and CON (all p > 0.05). CONCLUSION: In conclusion, our results showed that muscle damaging exercise under HYP leads to reduced capillary and muscle oxygenation levels compared to normoxia with no difference in inflammatory response and muscle recovery during 72 h post-exercise. TRIAL REGISTRATION: NCT04902924, May 26th 2021.

Short-term balance consolidation relies on the primary motor cortex: a rTMS study.

Structural and functional adaptations occur in the primary motor cortex (M1) after only a few balance learning sessions. Nevertheless, the role of M1 in consolidating balance tasks remains to be discussed, as direct evidence is missing due to the fact that it is unclear whether adaptations in M1 are indeed the driving force for balance improvements or merely the consequence of improved balance. The aim of the present study was to investigate whether the primary motor cortex is involved in the learning and consolidation of balance tasks. Thirty participants were randomly allocated into a repetitive transcranial magnetic stimulation (rTMS) or sham-rTMS group. The experimental design included a single balance acquisition phase, followed by either 15 min of low-frequency rTMS (1 Hz at 115% of resting motor threshold to disrupt the involvement of M1) or sham-rTMS, and finally a retention test 24 h later. During the acquisition phase, no differences in balance improvements were observed between the two groups. However, significant differences between the rTMS and the sham-rTMS group were found from the end of the acquisition phase to the retention test. While the rTMS group had a performance loss, the sham-rTMS group displayed significant off-line gains (p = 0.001). For the first time, this finding may propose a causal relationship between the involvement of M1 and the acquisition and consolidation of a balance task.

Modulation of intracortical inhibition during physically performed and mentally simulated balance tasks.

PURPOSE: Action observation (AO) during motor imagery (MI), so-called AO + MI, has been proposed as a new form of non-physical training, but the neural mechanisms involved remains largely unknown. Therefore, this study aimed to explore whether there were similarities in the modulation of short-interval intracortical inhibition (SICI) during execution and mental simulation of postural tasks, and if there was a difference in modulation of SICI between AO + MI and AO alone. METHOD: 21 young adults (mean ± SD = 24 ± 6.3 years) were asked to either passively observe (AO) or imagine while observing (AO + MI) or physically perform a stable and an unstable standing task, while motor evoked potentials and SICI were assessed in the soleus muscle. RESULT: SICI results showed a modulation by condition (F2,40 = 6.42, p = 0.009) with less SICI in the execution condition compared to the AO + MI (p = 0.009) and AO (p = 0.002) condition. Moreover, switching from the stable to the unstable stance condition reduced significantly SICI (F1,20 = 8.34, p = 0.009) during both, physically performed (- 38.5%; p = 0.03) and mentally simulated balance (- 10%, p < 0.001, AO + MI and AO taken together). CONCLUSION: The data demonstrate that SICI is reduced when switching from a stable to a more unstable standing task during both real task execution and mental simulation. Therefore, our results strengthen and further support the existence of similarities between executed and mentally simulated actions by showing that not only corticospinal excitability is similarly modulated but also SICI. This proposes that the activity of the inhibitory cortical network during mental simulation of balance tasks resembles the one during physical postural task execution.

Interference of balance tasks revisited: Consolidation of a novel balance task is impaired by subsequent learning of a similar postural task.

BACKGROUND: Interference effects have repeatedly been demonstrated for simple motor tasks but not for the complex whole-body task of balancing. It was therefore assumed that different balance tasks are so specific that they do not elicit interacting adaptations; neither in a positive (contextual interference) nor in a negative way (disruption of motor consolidation). RESEARCH QUESTION: Is a novel balancing task susceptible to interference if a similar balance task is learned shortly afterwards? METHODS: The common A1-B-A2 interference intervention design was applied. Participants were assigned to one of four intervention groups that differed with respect to task B. All four groups performed postural task A on a rocker board device (6 series of 8 trials of 8 s). Shortly after completion of task A, participants performed their respective task B (postural wobble board (P-WB), ballistic force, accuracy) or rested (control group). 24 h later, all groups performed a retention test of task A consisting of one series of 8 trials. To test for interference, we calculated repeated mixed design analysis of covariance (ANCOVA). RESULTS: For the retention test, the ANCOVA revealed a significant TIME*GROUP interaction (p = .010), which was followed up by separate Bonferroni-corrected post-hoc tests for each group. These tests showed a significant performance decrease for the P-WB group (p = .016) but no change in performance for the other three groups. SIGNIFICANCE: In contrast to previous findings, our results indicate that the complex whole-body task of balancing is susceptible to interference, but only, when task B consists of a similar balance task. This is of great functional relevance as for example fall prevention programs incorporate many different balance tasks to prepare participants for all sorts of situations. In such interventions, it seems therefore advisable to apply a random instead of a blocked practice design.

Intracortical Inhibition Increases during Postural Task Execution in Response to Balance Training.

Intracortical inhibitory modulation seems crucial for an intact motor control and motor learning. However, the influence of long(er) term training on short-interval intracortical inhibition (SICI) is scarcely investigated. With respect to balance, it was previously shown that with increasing postural task difficulty, SICI decreased but the effect of balance training (BT) is unknown. The present study tested whether improvements in postural control due to BT are accompanied by changes in SICI. SICI was measured in the tibialis anterior by applying paired-pulse magnetic stimuli to the motor cortex in a BT group (n = 13) training 2 weeks on an unstable platform and a control (CON) group (n = 13) while performing three progressively demanding postural tasks: stable stance ('Stable'), standing on a movable platform partly secured with elastic straps ('Straps') or freely moving ('Free'). The BT group improved postural control significantly more than the CON-group ('Free' condition: +80% vs. + 21%; p < 0.001). For SICI, there was a main effect of POSTURAL TASK (F2, 48 = 24.6; p < 0.001) with decreasing SICI when task difficulty increased and a TIME × GROUP interaction (F1, 24 = 5.9; p = 0.02) caused by significantly enhanced SICI in the BT group in all three postural tasks after the training. The increases in SICI were significantly correlated with improvements in balance performance (r = 0.56; p = 0.047). The present study confirms previous findings of task-specific modulation of SICI when balancing. More importantly, training was shown to increase SICI and this increase was correlated with changes in balance performance. Thus, changes in SICI seem to be involved not only for the control but also when adapting upright posture with training.

Age-Related Differences in Cortical and Subcortical Activities during Observation and Motor Imagery of Dynamic Postural Tasks: An fMRI Study.

Age-related changes in brain activation other than in the primary motor cortex are not well known with respect to dynamic balance control. Therefore, the current study aimed to explore age-related differences in the control of static and dynamic postural tasks using fMRI during mental simulation of balance tasks. For this purpose, 16 elderly (72 ± 5 years) and 16 young adults (27 ± 5 years) were asked to mentally simulate a static and a dynamic balance task by motor imagery (MI), action observation (AO), or the combination of AO and MI (AO + MI). Age-related differences were detected in the form of larger brain activations in elderly compared to young participants, especially in the challenging dynamic task when applying AO + MI. Interestingly, when MI (no visual input) was contrasted to AO (visual input), elderly participants revealed deactivation of subcortical areas. The finding that the elderly demonstrated overactivation in mostly cortical areas in challenging postural conditions with visual input (AO + MI and AO) but deactivation in subcortical areas during MI (no vision) may indicate that elderly individuals allocate more cortical resources to the internal representation of dynamic postural tasks. Furthermore, it might be assumed that they depend more strongly on visual input to activate subcortical internal representations.

Adopting an external focus of attention alters intracortical inhibition within the primary motor cortex.

AIM: Although it is well established that an external (EF) compared to an internal (IF) or neutral focus of attention enhances motor performance, little is known about the underlying neural mechanisms. This study aimed to clarify whether the focus of attention influences not only motor performance but also activity of the primary motor cortex (M1) when executing identical fatiguing tasks of the right index finger (first dorsal interosseous). Transcranial magnetic stimulation (TMS) at intensities below motor threshold was applied over M1 to assess and compare the excitability of intracortical inhibitory circuits. METHODS: In session 1, 14 subjects performed an isometric finger abduction at 30% of their maximal force to measure the time to task failure (TTF) with either an IF or EF. In session 2, the same task was performed with the other focus. In sessions 3 and 4, subthreshold TMS (subTMS) and paired-pulse TMS were applied to the contralateral M1 to compare the activity of cortical inhibitory circuits within M1 during EF and IF. RESULTS: With an EF, TTF was significantly prolonged (P = 0.01), subTMS-induced electromyographical suppression enhanced (P = 0.001) and short-interval intracortical inhibition (SICI) increased (P = 0.004). CONCLUSION: The level of intracortical inhibition was previously shown to influence motor performance. Our data shed new light on the ability to instantly modulate the activity of inhibitory circuits within M1 by changing the type of attentional focus. The increased inhibition with EF might contribute to the better movement efficiency, which is generally associated with focusing externally.

Task-dependent changes of corticospinal excitability during observation and motor imagery of balance tasks.

Non-physical balance training has demonstrated to be efficient to improve postural control in young people. However, little is known about the potential to increase corticospinal excitability by mental simulation in lower leg muscles. Mental simulation of isolated, voluntary contractions of limb muscles increase corticospinal excitability but more automated tasks like walking seem to have no or only minor effects on motor-evoked potentials (MEPs) evoked by transcranial magnetic stimulation (TMS). This may be related to the way of performing the mental simulation or the task itself. Therefore, the present study aimed to clarify how corticospinal excitability is modulated during AO+MI, MI and action observation (AO) of balance tasks. For this purpose, MEPs and H-reflexes were elicited during three different mental simulations (a) AO+MI, (b) MI and (c) passive AO. For each condition, two balance tasks were evaluated: (1) quiet upright stance (static) and (2) compensating a medio-lateral perturbation while standing on a free-swinging platform (dynamic). AO+MI resulted in the largest facilitation of MEPs followed by MI and passive AO. MEP facilitation was significantly larger in the dynamic perturbation than in the static standing task. Interestingly, passive observation resulted in hardly any facilitation independent of the task. H-reflex amplitudes were not modulated. The current results demonstrate that corticospinal excitability during mental simulation of balance tasks is influenced by both the type of mental simulation and the task difficulty. As H-reflexes and background EMG were not modulated, it may be argued that changes in excitability of the primary motor cortex were responsible for the MEP modulation. From a functional point of view, our findings suggest best training/rehabilitation effects when combining MI with AO during challenging postural tasks.

Cervical joint position sense in neck pain. Immediate effects of muscle vibration versus mental training interventions: a RCT.

BACKGROUND: Impaired cervical joint position sense is a feature of chronic neck pain and is commonly argued to rely on abnormal cervical input. If true, muscle vibration, altering afferent input, but not mental interventions, should have an effect on head repositioning acuity and neck pain perception. AIM: The aim of the present study was to determine the short-term effects of neck muscle vibration, motor imagery, and action observation on cervical joint position sense and pressure pain threshold in people with chronic neck pain. DESIGN: Forty-five blinded participants with neck pain received concealed allocation and were randomized in three treatment groups. A blinded assessor performed pre- and post-test measurement. SETTING: Patients were recruited from secondary outpatient clinics in the southwest of Germany. POPULATION: Chronic, non specific neck pain patients without arm pain were recruited for this study. METHODS: A single intervention session of 5 minutes was delivered to each blinded participant. Patients were either allocated to one of the following three interventions: (1) neck muscle vibration; (2) motor imagery; (3) action observation. Primary outcomes were cervical joint position sense acuity and pressure pain threshold. Repeated measures ANOVAs were used to evaluate differences between groups and subjects. RESULTS: Repositioning acuity displayed significant time effects for vibration, motor imagery, and action observation (all P<0.05), but revealed no time*group effect. Pressure pain threshold demonstrated a time*group effect (P=0.042) as only vibration significantly increased pressure pain threshold (P=0.01). CONCLUSION: Although motor imagery and action observation did not modulate proprioceptive, afferent input, they nevertheless improved cervical joint position sense acuity. This indicates that, against the common opinion, changes in proprioceptive input are not prerequisite to improve joint repositioning performance. However, the short-term applications of these cognitive treatments had no effect on pressure pain thresholds, whereas vibration reduced pressure pain thresholds. This implies different underlying mechanisms after vibration and mental training. CLINICAL REHABILITATION IMPACT: Mental interventions were effective in improving cervical joint position sense and are easy to integrate in rehabilitation regimes. Neck muscle vibration is effective in improving cervical joint position sense and pressure pain thresholds within 5 minutes of application.

Effect of surface stiffness on the neural control of stretch-shortening cycle movements.

AIM: It is accepted that leg stiffness (Kleg ) increases when surface stiffness decreases, and vice versa. However, little is known how the central nervous system fulfils this task. To understand the effect of surface stiffness on the neural control of stretch-shortening cycle movements, this study aimed to compare modulation of spinal and corticospinal excitability at distinct phases after ground contact during two-legged hopping when changing from solid to elastic ground. METHODS: Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) and H-reflexes were elicited at the time of the short (SLR)-, medium (MLR)- and long (LLR)-latency responses of the soleus muscle (SOL) during two-legged hopping on different stiffness surfaces, elastic and stiff. RESULTS: Soleus H-reflexes during two-legged hopping on the elastic surface were lower at SLR and larger at LLR than on the stiff surface (P < 0.05 for both comparisons). SOL MEP size was higher at the time of SLR during hopping on the elastic surface than on the stiff surface (P < 0.05) although the background EMG was similar. CONCLUSION: It is argued that this phase-specific adaptation in spinal reflex excitability is functionally relevant to adjust leg stiffness to optimally exploit the properties of the elastic surface. Thus, the increased corticospinal excitability on the elastic surface may reflect a more supraspinal control of the ankle muscles to compensate the decrease in reflexive stiffness at the beginning of touchdown and/or counteract the higher postural challenges associated with the elastic surface.

Ice skating promotes postural control in children.

High fall rates causing injury and enormous financial costs are reported for children. However, only few studies investigated the effects of balance training in children and these studies did not find enhanced balance performance in postural (transfer) tests. Consequently, it was previously speculated that classical balance training might not be stimulating enough for children to adequately perform these exercises. Therefore, the aim of this study is to evaluate the influence of ice skating as an alternative form of balance training. Volunteers of an intervention (n = 17; INT: 13.1 ± 0.4 years) and a control group (n = 13; CON: 13.2 ± 0.3 years) were tested before and after training in static and dynamic postural transfer tests. INT participated in eight sessions of ice skating during education lessons, whereas CON participated in normal physical education. Enhanced balance performance was observed in INT but not in CON when tested on an unstable free-swinging platform (P < 0.05) or when performing a functional reach test (P < 0.001). This is the first study showing significantly enhanced balance performance after ice skating in children. More importantly, participating children improved static and dynamic balance control in postural tasks that were not part of the training.

The effect of whole body vibration on the H-reflex, the stretch reflex, and the short-latency response during hopping.

The effect of whole body vibration (WBV) on reflex responses is controversially discussed in the literature. In this study, three different modalities of reflex activation with increased motor complexity have been selected to clarify the effects of acute WBV on reflex activation: (1) the electrically evoked H-reflex, (2) the mechanically elicited stretch reflex, and (3) the short-latency response (SLR) during hopping. WBV-induced changes of the H-reflex, the stretch reflex, and the SLR during hopping were recorded in the soleus and gastrocnemius muscles and were analyzed before, during (only the H-reflex), immediately after, 5 min and 10 min after WBV. The main findings were that (1) the H-reflexes were significantly reduced during and at least up to 5 min after WBV, (2) the stretch reflex amplitudes were also significantly reduced immediately after WBV but recovered to their initial amplitudes within 5 min, and (3) the SLR during hopping showed no vibration-induced modulation. With regard to the modalities with low motor complexities, the decreased H- and stretch reflex responses are assumed to point toward a reduced Ia afferent transmission during and after WBV. However, it is assumed that during hopping, the suppression of reflex sensitivity is compensated by facilitatory mechanisms in this complex motor task.

The drop height determines neuromuscular adaptations and changes in jump performance in stretch-shortening cycle training.

There is an ongoing discussion about how to improve jump performance most efficiently with plyometric training. It has been proposed that drop height influences the outcome, although longitudinal studies are missing. Based on cross-sectional drop jump studies showing height-dependent Hoffmann (H)-reflex activities, we hypothesized that the drop height should influence the neuromuscular activity and thus, the training result. Thirty-three subjects participated as a control or in one of two stretch-shortening cycle (SSC) interventions. Subjects either trained for 4 weeks doing drop jumps from 30, 50, and 75 cm drop heights (SSC1) or completed the same amount of jumps exclusively from 30 cm (SSC2). During training and testing (from 30, 50, and 75 cm), subjects were instructed to minimize the duration of ground contact and to maximize their rebound height. Rebound heights were significantly augmented after SSC1, but a trend was only observed after SSC2. In contrast, the duration of ground contact increased after SSC1 but decreased after SSC2. The performance index (rebound height/duration of ground contact) improved similarly after SSC1 (+14%) and SSC2 (+14%). Changes in performance were accompanied by neuromuscular adaptations: for SSC1, activity of the soleus increased toward take-off (between 120 and 170 ms after touchdown), whereas SSC2-trained subjects showed enhanced activity shortly after ground contact (20-70 ms after touch down). The present study demonstrates a strong link among drop height, neuromuscular adaptation, and performance in SSC training. As the improvement in the performance index was no different after SSC1 or SSC2, the decision whether to apply SSC1 or SSC2 should depend on the specific requirements of the sports discipline.

Improved postural control after slackline training is accompanied by reduced H-reflexes.

"Slacklining" represents a modern sports activity where people have to keep balance on a tightened ribbon. The first trials on the slackline result in uncontrollable lateral swing of the supporting leg. Training decreases those oscillations and therefore improves postural control. However, the underlying neural mechanisms are not known. Therefore, the present study aimed to highlight spinal adaptations going along with slackline training. Twenty-four subjects were either assigned to a training or a control group and postural control was assessed before and after the 10 training sessions. Additionally, soleus Hoffmann (H)-reflexes were elicited to evaluate changes in the excitability of the spinal reflex circuitry. Trained subjects were able to maintain balance on the slackline for at least 20 s (P<0.001) and reduced platform movements on the balance board (P<0.05). The H-reflexes were significantly diminished (P<0.05) while no changes occurred in the background electromyography (bEMG). The control group showed no significant changes. From a functional point of view the reflex reduction may serve to suppress uncontrollable reflex mediated joint oscillations. As the bEMG remained unchanged, presynaptic rather than post-synaptic mechanisms are speculated to be responsible for the changes in the Ia-afferent transmission.

Spinal reflex plasticity in response to alpine skiing in the elderly.

The present study was designed to assess the influence of 12 weeks (28.5 ± 2.6 skiing days) of alpine skiing on spinal reflex plasticity, strength and postural control in senior citizens. Therefore, soleus H-reflexes and postural stability were measured during bipedal quiet and unstable stance in 22 (12 male and 10 female) elderly subjects aged 66.6 ± 1 years. Furthermore, the maximal isometric force was determined in a leg press. The results showed an increased H-reflex excitability after the training (+39%; P<0.05) while no changes occurred in the background EMG. The postural sway decreased after training (-6.6 cm; P ≤ 0.05) and the maximal force increased (+16.1%; P<0.05). No adaptations in any parameter could be observed in the control group. The present study demonstrated that skiing training was effective to alter the spinal reflex activity in elderly individuals. The increased H-reflexes correspond to adaptations known from strength training in young subjects. It may be assumed that alpine skiing induced a functional adaptation in that subjects have learned to integrate Ia afferent feedback more efficiently to ensure adequate motoneuron output.

An enhanced level of motor cortical excitability during the control of human standing.

AIM: The study examined the role of the motor cortex in the control of human standing. METHODS: Subjects (n = 15) stood quietly with or without body support. The supported standing condition enabled subjects to stand with a reduced amount of postural sway. Peripheral electrical stimulation, transcranial magnetic stimulation (TMS) or transcranial electrical stimulation (TES) was applied to elicit a soleus (SOL) H-reflex, or motor-evoked potentials (MEPs) in the SOL and the tibialis anterior (TA). Trials were grouped based on the standing condition (i.e. supported vs. normal) as well as sway direction (i.e. forward and backward) while subjects were standing normally. RESULTS: During normal when compared to supported standing, the SOL H-reflex was depressed (-11 +/- 4%), while the TMS-evoked MEPs from the SOL and TA were facilitated (35 +/- 11% for the SOL, 51 +/- 15% for the TA). TES-evoked SOL and TA MEPs were, however, not different between the normal and supported standing conditions. The data based on sway direction indicated that the SOL H-reflex, as well as the SOL TMS- and TES-evoked MEPs were all greater during forward when compared to backward sway. In contrast, the TMS- and TES-evoked MEPs from the TA were smaller when swaying forward as compared to backward. CONCLUSIONS: The results indicated the presence of an enhanced cortical excitability because of the need to control for postural sway during normal standing. The increased cortical excitability was, however, unlikely to be involved in an on-going control of postural sway, suggesting that postural sway is controlled at the spinal and/or subcortical levels.

Balance training and ballistic strength training are associated with task-specific corticospinal adaptations.

The aim of this study was to investigate the role of presumably direct corticospinal pathways in long-term training of the lower limb in humans. It was hypothesized that corticospinal projections are affected in a training-specific manner. To assess specificity, balance training was compared to training of explosive strength of the shank muscles and to a nontraining group. Both trainings comprised 16 1-h sessions within 4 weeks. Before and after training, the maximum rate of force development was monitored to display changes in motor performance. Neurophysiological assessment was performed during rest and two active tasks, each of which was similar to one type of training. Hence, both training groups were tested in a trained and a nontrained task. H-reflexes in soleus (SOL) muscle were tested in order to detect changes at the spinal level. Corticospinal adaptations were assessed by colliding subthreshold transcranial magnetic stimulation to condition the SOL H-reflex. The short-latency facilitation of the conditioned H-reflex was diminished in the trained task and enhanced in the nontrained task. This was observable in the active state only. On a functional level, training increased the rate of force development suggesting that corticospinal projections play a role in adaptation of leg motor control. In conclusion, long-term training of shank muscles affected fast corticospinal projections. The significant interaction of task and training indicates context specificity of training effects. The findings suggest reduced motor cortical influence during the trained task but involvement of direct corticospinal control for new leg motor tasks in humans.

Spinal and supraspinal adaptations associated with balance training and their functional relevance.

Traditionally, balance training has been used to rehabilitate ankle injuries and postural deficits. Prospective studies have shown preventive effects with respect to ankle and knee joint injuries. Presently, balance training is not only applied for rehabilitation and prevention but also for improving motor performance, especially muscle power. The recent application of noninvasive electrophysiological and brain imaging techniques revealed insights into the central control of posture and the adaptations induced by balance training. This information is important for our understanding of the basic control and adaptation mechanisms and to conceptualize appropriate training programmes for athletes, elderly people and patients. The present review presents neurophysiological adaptations induced by balance training and their influence on motor behaviour. It emphasizes the plasticity of the sensorimotor system, particularly the spinal and supraspinal structures. The relevance of balance training is highlighted with respect to athletic performance, postural control within elderly people as well as injury prevention and rehabilitation.

Influence of falling height on the excitability of the soleus H-reflex during drop-jumps.

AIM: The stretch-shortening cycle (SSC) is characterized by stretching of the target muscle (eccentric phase) prior to a subsequent shortening in the concentric phase. Stretch reflexes in the eccentric phase were argued to influence the performance of short lasting SSCs. In drop-jumps, the short latency component of the stretch reflex (SLR) was shown to increase with falling height. However, in jumps from excessive heights, the SLR was diminished. So far, it is unclear whether the modulation of the SLR relies on spinal mechanisms or on an altered fusimotor drive. The present study aimed to assess the spinal excitability of the soleus Ia afferent pathway at SLR during jumps from low height (LH - 31 cm) and excessive height (EH - 76 cm). METHODS: In 20 healthy subjects (age 25 +/- 3 years), H-reflexes were timed to occur at the peak of the SLR during drop-jumps from LH and EH. RESULTS: H-reflexes were significantly smaller at EH than at LH (P < 0.05). Neither soleus and tibialis anterior background EMG nor the size of the maximum M-wave changed with falling height. CONCLUSION: Differences in the H-reflex between EH and LH indicate that spinal mechanisms are involved in the modulation of the SLR. A decreased excitability of the H-reflex pathway at EH compared with LH is argued to serve as a 'prevention strategy' to protect the tendomuscular system from potential injuries caused by the high load. It is argued that pre-synaptic inhibition of Ia afferents is most likely responsible for the change in H-reflex excitability between the two jump conditions.

Task-specific changes in motor evoked potentials of lower limb muscles after different training interventions.

This study aimed to identify sites and mechanisms of long-term plasticity following lower limb muscle training. Two groups performing either a postural stability maintenance training (SMT) or a ballistic ankle strength training (BST) were compared to a non-training group. The hypothesis was that practicing of a self-initiated voluntary movement would facilitate cortico-spinal projections, while practicing fast automatic adjustments during stabilization of stance would reduce excitatory influence from the primary motor cortex. Training effects were expected to be confined to the practiced task. To test for training specificity, motor evoked potentials (MEP) induced by transcranial magnetic stimulation (TMS) were recorded at rest and during motor tasks that were similar to each training. Intracortical, cortico-spinal, as well as spinal parameters were assessed at rest and during these tasks. The results show high task and training specificity. Training effects were only observable during performance of the trained task. While MEP size was decreased in the SMT group for the trained tasks, MEP recruitment was increased in the BST group in the trained task only. The control group did not show any changes. Background electromyogram levels, M. soleus H-reflex amplitudes and intracortical parameters were unaltered. In summary, it is suggested that the changes of MEP parameters in both training groups, but not in the control group, reflect cortical motor plasticity. While cortico-spinal activation was enhanced in the BST group, SMT may be associated with improved motor control through increased inhibitory trans-cortical effects. Since spinal excitability remained unaltered, changes most likely occur on the supraspinal level.