## 1 Introduction  
The contingent negative variation (CNV) is a measure of the electrical activity of the brain; specifically, it is an event-related potential (ERP) associated with motor preparation and attention [1, 2]. It is typically measured using electroencephalography (EEG) and is characterized by a negative slope beginning approximately 1-1.5 s prior to movement onset, a point of peak negativity around the time of movement onset, and then a positive slope [2]. The CNV has been used in a variety of research contexts, including studies of cognitive processes such as attention and decision-making [3], and studies of motor planning in populations with neurological [4] and psychiatric disorders [5]. Research on the CNV has found that it can provide valuable information about the integrity of cortical areas that connect with the corticospinal tract, which is the neural pathway that connects the cerebral cortex to the spinal cord and is responsible for voluntary movement [6]. In recent years, researchers have used the CNV to evaluate people with conditions such as locked-in syndrome [7], amyotrophic lateral sclerosis [8], stroke [9], and cerebral palsy [10]. Research has also shown that the CNV can be used to evaluate the efficacy of rehabilitation interventions such as physical therapy [3, 11]. The CNV is a potential tool in the field of stroke rehabilitation and neuromodulatory interventions. Studies in people with stroke have shown that movements, both attempted and executed, can be detected in the electroencephalogram up to 2 s before they occur [12; 13]. This makes the CNV useful as a control signal in brain-computer interface (BCI) applications that rely on early detection of movement intentions [14]. The CNV can be used to monitor the intention to move before the movement begins, as well as during the actual movement. Factors such as fatigue, level of attention, and movement type can all modulate the CNV [15; 16; 17]. One of the main benefits of using the CNV in stroke rehabilitation is the ability to detect movement intentions early, which may improve the effectiveness of BCI-based therapies. For example, if a patient with a stroke is able to generate a CNV signal indicating their intention to move a limb, a BCI system could provide feedback or assistance to help them execute the movement. This could be particularly useful for patients with severe motor impairments, who may have difficulty initiating or completing movements on their own. Recently, the CNV has been used to drive the delivery of an endogenous paired associative stimulation (ePAS) intervention, where peripheral electrical stimulation is timed to coincide with the peak negativity (PN) of the CNV [13; 18; 19]. The CNV is recorded as the participant performs a movement and the timing of the PN of the CNV is determined in relation to the onset of the CNV or the onset of movement [20; 21]. This timing is used to drive the delivery of electrical stimulation to the common peroneal nerve and is essential to the efficacy of the intervention [21]. This ePAS intervention has been shown to increase corticomotor excitability and improve lower limb impairment in people with stroke [13] and therefore has the potential to improve the long-term recovery of this population. The next phase of research should involve testing the effectiveness of the ePAS intervention within a stroke rehabilitation context; however, limited knowledge about the characteristics of the CNV is hindering the progression of this intervention towards clinical trials. The first limitation with applying ePAS into a stroke rehabilitation context is the movement task in which the CNV is recorded. To date, the ePAS intervention has largely been delivered using single-joint movements [13; 18; 19; 22]. These single-joint movements, also known as isolation exercises, focus on the movement of a single muscle group [23]. Examples include bicep curls at the elbow joint, leg extensions at the knee joint, and the most widely-used with ePAS, dorsiflexion of the ankle joint [13; 18; 19; 22].