Perturbed upper limb movements cause short-latency postural responses in trunk muscles

Addition of a load to a moving upper limb produces a perturbation of the tr unk due to transmission of mechanical forces. This experiment investigated the postural response of the trunk muscles in relation to unexpected limb l oading. Subjects performed rapid, bilateral shoulder flexion in response to a stimulus. In one third of trials, an unexpected load was added bilateral ly to the upper limbs in the first third of the movement. Trunk muscle elec tromyography, intra-abdominal pressure and upper limb and trunk motion were measured. A short-latency response of the erector spinae and transversus a bdominis muscles occurred similar to 50 ms after the onset of the limb pert urbation that resulted from addition of the load early in the movement and was coincident with the onset of the observed perturbation at the trunk. Th e results provide evidence of initiation of a complex postural response of the trunk muscles that is consistent with mediation by afferent input from a site distant to the lumbar spine, which may include afferents of the uppe r limb.