Cortical adaptations during muscle fatigue: the role of sensorimotor oscillations

During physical activity, fatigue arises not only from peripheral processes within the active spinal motor units but also from supraspinal mechanisms within the brain. Although the failure to generate a maximal output from the motor cortex is well established, the neural mechanisms of central fatigue remain unclear. In a recent study, Fry et al. investigate changes in oscillatory activity in sensorimotor cortex following a fatiguing intervention to identify the cortical adaptations to muscle fatigue. The ability of the brain to voluntarily generate the maximum drive to the muscles is reduced during and after maximal and submaximal exercise. The standard method to assess whether the corticospinal drive is the maximal output that the brain can generate is to stimulate the motor cortex with a single pulse of TMS while the participant performs a maximum voluntary contraction (MVC). If the TMS pulse generates a superimposed twitch, that is, a force increment above the maximal voluntary strength, this demonstrates that the maximal voluntary strength is below the true maximal muscle force. An exercise-related increase in the superimposed twitch provides a classic marker of central fatigue, indicating a supraspinal mechanism that prevents the use of all available resources to generate output from the motor cortex. Despite extensive research into central fatigue, the underlying mechanisms remain unclear. The reduced ability to generate maximal cortical drive may not necessarily indicate a reduction in motor-cortical excitability, as spinal motor neurones become less excitability and hence require more descending drive to keep force output constant when fatigue develops. In fact, TMS studies suggest that the excitability of the motor cortex increases during fatiguing contractions. For example, a TMS pulse elicits a larger motorevoked potential (MEP) in contracting muscles than in relaxed muscles and the size of the MEP further increases with continued contraction. During submaximal contractions, increases in MEP size at least partly reflect augmented cortical excitability associated with more voluntary cortical output. Hence, the reduced ability to generate cortical output does not simply reflect an overall reduction in cortical excitability. This seems also consistent with imaging studies showing that the fMRI BOLD signal increases progressively in the sensorimotor cortex during prolonged submaximal contractions. However, the BOLD signal is only an indirect measure of neural activity, and an increase in the BOLD signal in the motor cortex may not necessarily reflect increased cortical output. To better understand the neural changes in the sensorimotor cortex during muscle fatigue, Fry et al. acquired magnetoencephalography (MEG) data in 14 healthy participants while they performed a set of submaximal contractions before and after a fatiguing intervention. A set consisted of 10 isometric wrist flexions at 40% MVC, which were performed every 30 s to measure changes in oscillatory activity in the sensorimotor cortex. Electrophysiological studies have consistently shown that ongoing oscillations within alpha and beta frequency bands (8–13 Hz, known as mu rhythm, and 15–30 Hz, respectively) are attenuated in the sensorimotor cortex during muscle contractions (referred to as event-related desynchronization) and show a rebound after contraction offset (event-related synchronization). To investigate the effect of muscle fatigue on oscillatory activity in motor cortex, Fry et al. used a randomized crossover design in which MEG was recorded before and after a fatiguing or control intervention on two separated sessions, which were at least 7 days apart. Both interventions comprised a series of 40 contractions of 12-s duration, performed every 30 s at 55% MVC (fatiguing intervention) or 5% MVC (control intervention). The authors used beamforming, a source reconstruction technique, to estimate oscillatory activity in the sensorimotor cortex. A beamformer tries to reconstruct the contribution of a single source within the brain from the magnetic field that is measured outside the scalp. Using this technique, the authors confirmed that the motor-related oscillations in alpha and beta band were localized in the contralateral sensorimotor cortex. MEG is particularly suitable for source reconstruction, as it offers greater spatial resolution than EEG because the magnetic fields generated within the brain pass through skull and various tissues undistorted. The main outcome of the study was that after fatigue, beta-band desynchronization during the contractions was maintained, while it was reduced after the