Do surface electrode recordings validly represent latissimus dorsi activation patterns during shoulder tasks?

Because of its superficial location surface electrodes are commonly used to record latissimus dorsi (LD) activity. Despite the fact that the recommended electrode placement is over the belly where LD is quite thin no studies have investigated the possibility of signal contamination from muscles lying deep to LD. Therefore, the aim of this study was to determine the validity of using surface electrodes to record activity from LD. Eight asymptomatic subjects performed ramped isometric (0-100% maximum load) and dynamic (70% maximum load) shoulder tasks. Intramuscular electrodes were inserted into LD and the adjacent erector spinae. Surface electrodes were placed over LD around the intramuscular electrodes. Results indicated that while there was no difference in activity level or activation pattern (ICC>0.94) recorded by the two electrode types during shoulder tasks in which LD would be expected to be active (extension and adduction), significantly lower (p<0.05) LD activity was recorded via intramuscular electrodes during the shoulder flexion and abduction tasks. Therefore, recordings of LD activity by surface electrodes overestimate LD activity during shoulder tasks when this muscle would be expected to be activated at minimum levels. Erector spinae immediately deep to LD was confirmed as a source of crosstalk contamination.