Table 1.

Summary of the techniques used to measure brain activity and clinical interpretations in ACL injury.

Technique		Measurement	Interpretation
EEG	Somatosensory-evoked potentials (SEPs)	Peaks of activity are measured by electroencephalography (EEG) electrodes in the somatosensory cortex after an external stimulus is delivered to the common peroneal nerve or to the ACL (i.e., via arthroscopy)	The ascending stimulus to the somatosensory cortex, following common peroneal nerve stimulation, is detected as P27 component which provides information about the afferent system. Literature shows contradictory information regarding the ability to reproduce SEPs in people with ACL deficiency or reconstruction
	Spectral analysis	EEG signals are measured during a movement (i.e., joint angle or force reproduction). The mean absolute EEG spectral power is divided into different frequencies: delta (0–4 Hz), theta (4.75–6.75 Hz), alpha 1 (7–9.5 Hz), alpha 2 (9.75–12.5 Hz), beta (12.75–18.5 Hz), and gamma (30–80 Hz), corresponding to different levels of activity in different areas of the cortex	It has been suggested that differences in theta power in the frontal cortex may be linked to differences in working memory and focused attention, whereas alpha power is typically inversely related to the neuronal activation. As such, increased alpha power recorded over parietal cortical areas may be interpreted as a deactivation of the somatosensory cortical areas
TMS	Motor-evoked potentials (MEPs)	Muscle response (measured by electromyography), following a transcranial magnetic stimulus (TMS) delivered at the motor cortex travelling down the motor pathways	Decreased MEPs represent less information travelling in the motor pathways to the target muscle
	Motor threshold	Minimum transcranial magnetic stimulation (TMS) intensity necessary to cause a response (MEP) in the target muscle—it is a measure of motor cortex excitability and can be measured at rest (i.e., resting motor threshold), or during an activity (i.e., active motor threshold (AMT))	Motor threshold is inversely related to motor cortex excitability, meaning that people with reduced corticomotor excitability would have a higher motor threshold. A reduction in motor cortex excitability may affect motor output
	Intracortical inhibition (SICI and LICI)	Paired TMS pulses (first, a conditioning subthreshold pulse, followed by a suprathreshold testing pulse) are delivered with varying interstimulus intervals. Short intervals (<5 ms) produce short-interval intracortical inhibition (SICI), while longer intervals (>50 ms) produce long-interval intracortical inhibition (LICI)	SICI is associated with GABAa activity, while LICI is associated with GABAb activity. Higher levels of intracortical inhibition may be associated to lower cortical excitability
	Cortical silent period (CSP)	The cortical silent period (CSP) corresponds to an interruption in voluntary electromyography (time from MEP onset to EMG activity resumption) in the target muscle following TMS. CSP is mediated by GABAb activity at a cortical level	Longer CSP represent higher levels of inhibition, which may lead to muscle inhibition. However, a link between CSP and MEP changes has not been established
	Intracortical facilitation	Similar to intracortical inhibition measurements, paired TMS pulses are used for measuring intracortical facilitation. In this case, a 7 to 30 ms interval between the conditioning and testing pulses is used	Cortical facilitation is mediated by neurotransmitter glutamate onto non-N-methyl-D-aspartate receptors. There is conflicting evidence on whether ICF is changed in people with ACL injury or reconstruction
MRI	Functional MRI during a task	The blood oxygen level-dependent signal is quantified through the blood hemodynamics during a specific task (e.g., knee flexion-extension cycles)	An increased BOLD signal is associated to a higher activity of the respective brain area, which may be associated to reduced efficiency of these cortical regions, in people with ACL injury