Crossed responses in human trapezius are not H re exes

Trapezius is commonly a ected in work-related musculoskeletal disorders. The H re ex may be a good tool to provide knowledge about the mechanisms involved in this motor dysfunction. In trapezius, previous ndings has described both ipsilateral and unusual crossed H re exes. Our study aimed to con rm these ndings by investigating the responses evoked in trapezius by stimulation of the C3/4 cervical nerves. Mmax and Hmax were measured in the ipsilateral trapezius in subjects (n=10) by percutaneous electrical stimulation of the accessory nerve and the cervical nerves of C3/4 respectively. Repeated stimulation of the C3/4 cervical nerves was performed during 3 di erent tasks (relaxation, contraction of ipsilateral side, contraction of contralateral side). Stimulation of the accessory nerve evoked M waves at an average latency of 2.9±0.2 ms for the upper part, 3.9±0.5 ms for the middle part, and 5.0±0.7 ms for the lower part of the ipsilateral trapezius. Stimulation of the C3/4 cervical nerves evoked ipsilateral re exes in the upper (n=8) middle (n=10) and in the lower (n=8) trapezius with an ipsilateral contraction. The latencies were 8.9±0.2 ms, 10.0±0.4 ms, and 10.9±0.4 ms respectively in the upper, middle, and lower parts of trapezius. These responses increased signi cantly with an increase in pre-stimulus EMG and decreased signi cantly with a decrease in pre-stimulus EMG in all ipsilateral parts of trapezius. Thus, they were considered H reexes. Contralaterally, responses with latencies corresponding to those observed ipsilaterally occurred in the upper (n=1), middle (n=9), and lower (n=6) part of trapezius. These potentials which were recorded from the left trapezius increased signi cantly more with contraction of the right trapezius compared with contraction on the left side. Thus, they were considered not to be H re exes but to be far eld EMG. This study found ipsilateral and contralateral responses consistent with previous ndings. However, we conclude that the contralateral response is not a crossed re ex. Further investigations are required to be able to fully dismiss that crossed re exes between the two trapezii are present.