Integrated EMG index (IEMG) of the left and right neck flexor and extensor muscles (NFL, NFR, NEL, and NER, respectively) during upward and downward arm movements, and under head, trunk, and head and trunk perturbations (HP, TP, and HTP, respectively). Average time profiles across subjects and standard error bars are shown; * means p<0.05. Lines connecting the mean bars indicate significant differences across conditions.