Corticospinal facilitation studied during voluntary contraction of human abdominal muscles

Transcranial magnetic stimulation (TMS) of the human motor cortex was used to study facilitation of motor-evoked potentials (MEPs) in the rectus abdom inis (RA) muscle, a trunk flexor, during voluntary activation. MEPs could b e produced in the relaxed RA muscles of all six normal subjects studied. Th e MEPs had short latencies (18-22 ms) which are consistent with other studi es suggesting a fast corticospinal input to the trunk muscles. Marked facil itation was observed in the MEPs when subjects mere asked to produce graded levels of voluntary contractions. The two tasks used to produce voluntary contractions were a forced expiration during a breath-holding task (FEBH) a nd bilateral trunk flexion (BTF). Maximal voluntary EMG activity during the BTF task produced around 4.2 times more integrated EMG than during the FEB H task. Similarly the MEP amplitude at MVC was 2.3 times greater during BTF than FEBH. The pattern of MEP facilitation with increasing voluntary EMG w as not linear and a maximal MEP amplitude was observed at a level of volunt ary contraction around 30% MVC in both tasks. There were some subtle differ ences in the pattern of facilitation in the two tasks. When TR IS was appli ed to the right cortex only, MEPs were seen in both left and right RA muscl es suggesting some ipsilateral corticospinal innervation. The latency of th e right (ipsilateral) response was approximately 2 ms longer than the left. Comparison with studies in hand and leg muscles suggests that the facilita tion pattern in RA may reflect a substantial degree of corticospinal innerv ation.