The 20 participants (13 men and 7 women) comprising the study cohort were the same participants used in 2 other studies,
15,16 which both reported analyses of data collected as part of a larger study. The mean (SD) age of participants was 25 (2) years; height, 177 (12) cm; weight, 74 (15) kg; and body mass index, 24 (4).
Participant 2 could not tolerate insertion of the electrodes, so none of the 36 planned trials for that participant could be collected. For the RCPm muscles, all data from participant 19 were excluded because he or she had high NHP voltage values. For participant 9, all right-side trials were excluded because of high NHP voltages; for participants 5, 6, 7, 8 and 10, all left-side trials were excluded because of high NHP voltages. Overall, 82 trials were excluded, as well as 1 participant from whom we collected EMG data, leaving 260 trials from 18 participants in the analysis (76% of the 342 RCPm trials actually collected).
For the RCPM muscles, all data from participants 7 and 10 were excluded because they either had high NHP voltage values or their PHP voltages consistently exceeded the MVIC voltage. For participant 9, all right-side trials were excluded because of high NHP voltages, for participants 11 and 19, all left-side trials were excluded because of high NHP voltages. Overall, 69 trials and 2 participants from whom we collected EMG data were excluded, leaving 273 trials from 17 participants in the analysis (80% of the 342 RCPM trials actually collected).
Head position strongly affected normalized levels of EMG activity observed in the RCPm and RCPM muscles of participants (OR, 5.04;
P<.001) (
Figure 3). In the RCPm muscles, mean activation was about 10% to 12% of MVIC in a self-defined NHP, but it increased to around 34% to 36% of MVIC in a PHP (OR, 5.04; 95% CI, 3.33-7.62). In the RCPM muscles, mean activation was about 11% to 18% of MVIC in a self-defined NHP, but it increased to around 35% to 42% of MVIC in a PHP (OR, 4.65; 95% CI, 3.51-6.16).
The effects of side of the body, sex, and BMI on mean RCPm muscle activation were not statistically significant. In addition, the head position × side of the body interaction terms were not statistically significant in either model, indicating that there is no evidence of lateral asymmetry in the size of the head position effect.