Christian Leukel

Differential Modulation of Spinal and Corticospinal Excitability During Drop Jumps

Previously it was shown that spinal excitability during hopping and drop jumping is high in the initial phase of ground contact when the muscle is stretched but decreases toward takeoff. To further understand motor control of stretch-shortening cycle, this study aimed to compare modulation of spinal and corticospinal excitability at distinct phases following ground contact in drop jump. 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, LLR(2))-latency responses of the soleus muscle (SOL) after jumps from 31 cm height. MEPs and H-reflexes were expressed relative to the background electromyographic (EMG) activity. H-reflexes were highly facilitated at SLR (172%) and then progressively decreased (MLR = 133%; LLR = 123%; LLR(2) = 110%). TMS showed no effect at SLR, MLR, and LLR, whereas MEPs were significantly facilitated at the LLR(2) (122%; P = 0.003). Background EMG was highest at LLR and lowest at LLR(2). Strong H-reflex facilitation at the beginning of the stance phase indicated significant contribution of Iotaa-afferent input to the alpha-motoneurons during this phase that then progressively declined toward takeoff. Conversely, corticospinal excitability was exclusively increased at the phase of push off (LLR(2), approximately 120 ms). It is argued that corticomotoneurons increased their excitability at LLR(2). At LLR ( approximately 90 ms), Iotaa-afferent transmission as well as corticospinal excitability was low, whereas background EMG was high. Therefore it is speculated that other sources, presumably subcortical in origin, contributed to the EMG activity at LLR in drop jumps.

Contribution of afferent feedback and descending drive to human hopping

During hopping an early burst can be observed in the EMG from the soleus muscle starting about 45 ms after touch‐down. It may be speculated that this early EMG burst is a stretch reflex response superimposed on activity from a supra‐spinal origin. We hypothesised that if a stretch reflex indeed contributes to the early EMG burst, then advancing or delaying the touch‐down without the subjects knowledge should similarly advance or delay the burst. This was indeed the case when touch‐down was advanced or delayed by shifting the height of a programmable platform up or down between two hops and this resulted in a correspondent shift of the early EMG burst. Our second hypothesis was that the motor cortex contributes to the first EMG burst during hopping. If so, inhibition of the motor cortex would reduce the magnitude of the burst. By applying a low‐intensity magnetic stimulus it was possible to inhibit the motor cortex and this resulted in a suppression of the early EMG burst. These results suggest that sensory feedback and descending drive from the motor cortex are integrated to drive the motor neuron pool during the early EMG burst in hopping. Thus, simple reflexes work in concert with higher order structures to produce this repetitive movement.

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.

How neurons make us jump: the neural control of stretch-Shortening cycle movements

How can the human central nervous system (CNS) control complex jumping movements task- and context-specifically? This review highlights the complex interaction of multiple hierarchical levels of the CNS, which work together to enable stretch-shortening cycle contractions composed of activity resulting from feedforward (preprogrammed) and feedback (reflex) loops.

Pathway‐specific plasticity in the human spinal cord

The aim of the present study was to artificially induce plasticity in the human spinal cord and evaluate whether this plasticity is pathway specific. For this purpose, a technique called paired associative stimulation (PAS) was applied. Volleys evoked by transcranial magnetic stimulation over the primary motor cortex and peripheral nerve stimulation of the nervus tibialis in the popliteal fossa were timed to coincide at the spinal level. The transmission of different corticospinal projections was assessed before and after PAS using conditioned H‐reflexes. Different groups of healthy volunteers (28 ± 5 years) were tested; intervention groups 1 (n = 9) and 2 (n = 8) received spinal PAS (360 paired stimuli) and the induced effects were evaluated using cortical (group 1) or cervicomedullary (group 2) conditioning of musculus soleus H‐reflexes. After spinal PAS, the conditioned H‐reflexes were significantly facilitated when tested with cortical and cervicomedullary stimulation. The effect of the latter technique is independent of changes in the excitability of cortical neurons. Therefore, the finding that conditioned H‐reflexes were increased after spinal PAS when tested with both cortical and cervicomedullary stimulation suggests that neural plasticity was induced within the spinal cord. The facilitation could only be observed for specific inter‐stimulus intervals between volleys induced by peripheral nerve stimulation and transcranial magnetic stimulation. As the specific inter‐stimulus intervals were assumed to relate to transmission within specific motor pathways, it is argued that changes in the corticospinal transmission were pathway‐specific. These findings may be helpful in inducing and assessing neural plasticity in pathological conditions like spinal cord injuries.

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).

Evidence that the cortical motor command for the initiation of dynamic plantarflexion consists of excitation followed by inhibition.

At the onset of dynamic movements excitation of the motor cortex (M1) is spatially restricted to areas representing the involved muscles whereas adjacent areas are inhibited. The current study elucidates whether the cortical motor command for dynamic contractions is also restricted to a certain population of cortical neurons responsible for the fast corticospinal projections. Therefore, corticospinal transmission was assessed with high temporal resolution during dynamic contractions after both, magnetic stimulation over M1 and the brainstem. The high temporal resolution could be obtained by conditioning the soleus H-reflex with different interstimulus intervals by cervicomedullary stimulation (CMS-conditioning) and transcranial magnetic stimulation (TMS) of M1 (M1-conditioning). This technique provides a precise time course of facilitation and inhibition. CMS- and M1-conditioning produced an ‘early facilitation’ of the H-reflex, which occurred around 3 ms earlier with CMS-conditioning. The early facilitation is believed to be caused by activation of direct monosynaptic projections to the spinal motoneurons. CMS-conditioning resulted in a subsequent ‘late facilitation’, which is considered to reflect activity of slow-conducting and/or indirect corticospinal pathways. In contrast, M1-conditioning produced a ‘late dis-facilitation’ or even ‘late inhibition’. As the late dis-facilitation was only seen following M1- but not CMS-conditioning, it is argued that cortical activation during dynamic tasks is restricted to fast, direct corticospinal projections whereas corticomotoneurons responsible for slow and/or indirectly projecting corticospinal pathways are inhibited. The functional significance of restricting the descending cortical drive to fast corticospinal pathways may be to ensure a temporally focused motor command during the execution of dynamic movements.

Repetitive Activation of the Corticospinal Pathway by Means of rTMS may Reduce the Efficiency of Corticomotoneuronal Synapses

Low-frequency rTMS applied to the primary motor cortex (M1) may produce depression of motor-evoked potentials (MEPs). This depression is commonly assumed to reflect changes in cortical circuits. However, little is known about rTMS-induced effects on subcortical circuits. Therefore, the present study aimed to clarify whether rTMS influences corticospinal transmission by altering the efficiency of corticomotoneuronal (CM) synapses. The corticospinal transmission to soleus α-motoneurons was evaluated through conditioning of the soleus H-reflex by magnetic stimulation of either M1 (M1-conditioning) or the cervicomedullary junction (CMS-conditioning). The first facilitation of the H-reflex (early facilitation) was determined after M1- and CMS-conditioning. Comparison of the early facilitation before and after 20-min low-frequency (1 Hz) rTMS revealed suppression with M1- (-17 ± 4%; P = 0.001) and CMS-conditioning (-6 ± 2%; P = 0.04). The same rTMS protocol caused a significant depression of compound MEPs, whereas amplitudes of H-reflex and M-wave remained unaffected, indicating a steady level of motoneuronal excitability. Thus, the effects of rTMS are likely to occur at a premotoneuronal site-either at M1 and/or the CM synapse. As the early facilitation reflects activation of direct CM projections, the most likely site of action is the synapse of the CM neurons onto spinal motoneurons.

Changes in predictive motor control in drop-jumps based on uncertainties in task execution

Drop-jumps are controlled by predictive and reactive motor strategies which differ with respect to the utilization of sensory feedback. With reaction, sensory feedback is integrated while performing the task. With prediction, sensory information may be used prior to movement onset. Certainty about upcoming events is important for prediction. The present study aimed at investigating how uncertainties in the task execution affect predictive motor control in drop-jumps. Ten healthy subjects (22±1 years, M±SD) participated. The subjects performed either (i) drop-jumps by knowing that they might had to switch to a landing movement upon an auditory cue, which was sometimes elicited prior to touch-down (uncertainty). In (ii), subjects performed drop-jumps by knowing that there would be no auditory cue and consequently no switch of the movement (certainty). The m. soleus EMG prior to touch-down was higher when subjects knew there would be no auditory cue compared to when subjects performed the same task but switching from drop-jump to landing was possible (uncertainty). The EMG was reversed in the late concentric phase, meaning that it was higher in the high uncertainty task. The results of the present study showed that the muscular activity was predictively adjusted according to uncertainties in task execution. It is argued that tendomuscular stiffness was the variable responsible for the adjustment of muscular activity. The required tendomuscular stiffness was higher in drop-jumps than in landings. Consequently, when it was not certain whether to jump or to land, muscular activity and therefore tendomuscular stiffness was reduced.

Jump performance and augmented feedback: Immediate benefits and long-term training effects

Drop jumps and their adaptations to training have been extensively investigated. However, the influence of augmented feedback (aF) on stretch-shortening cycle (SSC) was not scrutinized so far despite the well-known positive effects of aF on motor performance and motor learning. The aim of the present study was therefore to investigate the effects of aF by evaluating immediate within-session effects and long-term adaptations. 34 participants were assigned to three groups that trained drop jumps with different relative frequencies of aF about their jump height: 100%, 50%, or 0%. A significant within-session effect of aF on jump height was observed before and also after the training period (pre: +4.6%; post: +2.6%). In the long-term (comparing pre- to post-measurement), the 100% group showed the greatest increase in jump height (+14%), followed by the 50% (+10%) and the 0% group (+6%). The importance of aF on drop jumps is therefore twofold: (i) to immediately increase jump performance and (ii) to improve long-term training efficacy. In contrast to the proposition of the guidance hypothesis, high frequency of aF seems to be beneficial when maximizing SSC-performance. As jump height cannot be quantified without objective technical measures it is recommended to include them into daily training.