Motor responses in the human trunk due to load perturbations

The trunk constitutes an integral part of the motor system and represents a large part of the total body mass. An adequate control of trunk movements and posture is therefore a prerequisite for the maintenance of the body equilibrium during various motor tasks. There is, however, little experimental data on how such a control is executed in different situations. During locomotion, phasic variations of the pressure within the abdominal cavity resulting from preprogrammed abdominal wall muscle activity occur and may be one important factor stabilizing the trunk (Grillner et al. 1978). Simultaneous bilateral phasic activation of different parts of the lumbar back muscles is another factor which may govern the stiffness of the spine in repetitive motor activities like walking and running (Carlson et al. 1979). In experiments on postural control associated with voluntary arm movements it has been found that different trunk muscles are activated in a specific order and prior to the execution of the actual movement of the extremity (Belenki et al. 1967, Weiss & Hayes 1979). This type of preactivation may give rise to a preparatory stabilization of the trunk.