Wolfgang Taube

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.

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.

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.

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.

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.

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).