On electromyographic (EMG) gradients and movement-related brain activity: significance for motor control, cognitive functions, and certain psychopathologies.

Electromyographic (EMG) voltage that rises continuously during motor performance or mental activity and falls precipitately at the end is known as an EMG gradient. Our review is based on 55 studies of EMG gradients, which were published during the period 1937-1994. The extremely wide diversity of situations yielding EMG gradients suggests the possibility that these gradients may be universal accompaniments of organized goal-directed behavioral sequences, overt and covert. Motor tasks and cognitive tasks (without any requirements for motor output) were found to have this in common: they both produced EMG gradients. EMG gradients were not observed during simple, repetitive exercises. On the efferent side, we propose a dual model for the production of EMG gradients, which is based on empirical findings. This model is discussed in relation to current views on central and peripheral neural control of muscle contractions, and on the electrical properties of extrafusal and intrafusal muscle fibers, with particular reference to surface electromyography. The complex relations between EMG gradient steepness and mental effort seemed well represented by the dual model, which was also useful in the interpretation of certain EMG gradients that were found in patients with anxiety disorders, tension-type headaches, and auditory hallucinations, respectively. On the afferent side, drawing on data from human and animal studies, we consider the evidence for movement-related brain activity generated by proprioceptive input, in relation to different types of feedback to the central nervous system during tasks that produce EMG gradients. On electromyographic (EMG) gradients and movement-related brain activity: significance for motor control, cognitive functions, and certain psychopathologies.