Changes in the excitability of the cortical projections to muscles in the upper and lower limbs can be induced in the intact human by paired associative stimulation (PAS). An interstimulus interval (ISI) of 25 ms between peripheral nerve and transcranial magnetic stimuli has been found to be effective when targeting hand muscles. The optimal ISI to induce plasticity changes in the cortical projections to lower limbs is still not well established. The purpose of this study was twofold: first, to investigate the effect of PAS with four different ISIs based on the individual latency of the sensory evoked potential (SEP plus 6, 12, 18 and 24 ms) and second, to evaluate the repeatability of the established optimal ISI. Transcranial magnetic stimulation was used to measure changes in the motor evoked potentials (MEPs) of the soleus (SOL) muscle before and after the PAS interventions. Significant increases in the amplitude of SOL MEPs (88 %) were attained with an ISI of SEP latency plus 18 ms (P32 + 18 ms). The PAS effect was long-lasting, input-specific and supraspinal in origin. The intraclass correlation coefficient to test the repeatability of the PAS intervention with the optimal ISI was 0.85. The results show that the excitability of cortical projections to the soleus muscle can be repeatedly increased after PAS with an optimal ISI of SEP plus 18 ms.

Functionality of PAS intervention was not demonstrated in this study. However, the observed relationship between excitability and fatigue resistance suggests that PAS might have affected central fatigue during short maximal contractions.

Human leg muscles are often activated inhomogeneously, e.g. in standing. This may also occur in complex tasks like walking. Thus, bipolar surface electromyography (sEMG) may not accurately represent whole muscle activity. This study used 64-electrode high-density sEMG (HD-sEMG) to examine spatial variability of lateral gastrocnemius (LG) muscle activity during the stance phase of walking, maximal voluntary contractions (MVCs) and maximal M-waves, and determined the effects of different normalization approaches on spatial and inter-participant variability. Plantar flexion MVC, maximal electrically elicited M-waves and walking at self-selected speed were recorded in eight healthy males aged 24-34. sEMG signals were assessed in four ways: unnormalized, and normalized to MVC, M-wave or peak sEMG during the stance phase of walking. During walking, LG activity varied spatially, and was largest in the distal and lateral regions. Spatial variability fluctuated throughout the stance phase. Normalizing walking EMG signals to the peak value during stance reduced spatial variability within LG on average by 70%, and inter-participant variability by 67%. Normalizing to MVC reduced spatial variability by 17% but increased inter-participant variability by 230%. Normalizing to M-wave produced the greatest spatial variability (45% greater than unnormalized EMG) and increased inter-participant variability by 70%. Unnormalized bipolar LG sEMG may provide misleading results about representative muscle activity in walking due to spatial variability. For the peak value and MVC approaches, different electrode locations likely have minor effects on normalized results, whereas electrode location should be carefully considered when normalizing walking sEMG data to maximal M-waves.

The possible reason for differential motor cortex plasticity in skill and endurance groups is likely related to the different training-induced adaptations. The findings of the current study suggest that long-term skill training by skill group induced preferable adaptations in the task-related areas of the motor cortex because increased plasticity is known to enhance motor learning.