This study compared postural responses and reaction times of arm movements in the frontal and sagittal planes, i.e. situations with different postural stability. Muscle activity (EMG) was recorded from prime movers (right & left deltoids) and muscles of the legs and trunk controlling postural stability (peroneus longus, tensor fascia latae, hamstrings, obliguus externus, rectus abdominis and ... [Show full abstract] erector spinae). Reaction time was measured (six male subjects) from the onset of a light signal to the onset of prime mover EMG. Reaction time of frontal arm rasing movements were significantly longer (21 ms, p<0.01) than sagittal arm raising movements. Both sagittal and frontal arm movements were preceded by an activation of the hamstring muscles (20-80 ms before and from 10 ms before to 40 ms after prime mover onset, respectively). During single arm raising to the side an associated movement was often seen as an involuntary raising of the contralateral arm accompanied by activity in the deltoideus muscle. Contralateral trunk muscles were activated almost simultaneously with the prime mover. It was concluded that preactivation of postural muscles and reaction time was less in frontal than in sagittal arm movements probably due to the better mechanical stability of the body in the frontal plane.