Kinetic, kinematic and electromyographic study of the effects of reach velocity and postural set on postural adjustments associated with reaching in seated subjects

Postural adjustments in advance of voluntary movement are considered to anticipate and minimize disturbance to balance caused by voluntary movement. Based on kinetic, kinematic and electromyographical evidence, preparatory postural adjustments (PPA) in standing subjects who made voluntary arm movements have been associated with fast, unsupported and loaded arm movements. Postural muscle activity for slow and/or supported arm movements has been reported as delayed, reduced in amplitude and often absent. While the standing model is useful for evaluating postural control in normal persons and in persons with mild neurological deficits, a sitting model would permit study of postural control in subjects unable to stand and perform a dynamic voluntary movement. The focus of this research was development and validation of a protocol for assessing PPA in seated subjects, under clinically relevant conditions. Evidence from force platform analysis, motion analysis and surface electromyography (EMG) was investigated to provide validation for the existence of PPA in seated subjects.;Experiment 3 was designed to further investigate postural muscle activity in advance of reaching and to complete validation of PPA in seated subjects by documenting motion of postural body segments in advance of hand movement. In Experiment 3, reaches were made to a target placed just within arm's length, at shoulder height and in the parasaggital plane with the right shoulder. Evidence for PPA was evaluated for two series of reaches in which reach velocity, and 4 conditions of postural support (supported/chest restraint; supported/chair back only; unsupported/erect sitting; unsupported/slumped sitting) were manipulated.;Force changes from the steady state, were transmitted separately through the subject's chair and feet, 70 to 80 milliseconds in advance of reaching, regardless of reach velocity or postural support. Contralateral T8-level paraspinal activity was present in greater than 90% of trials regardless of reach velocity or postural support, and was significantly positively related to reach velocity, indicating a dynamic relationship between the postural and voluntary systems. Paraspinal onset preceeded onset of the prime mover (anterior deltoid) in 75% of all trials and therefore was considered "preparatory." Kinematic analysis did not produce any reliable evidence of trunk motion prior to movement of the arm. This study has implications for evaluation of postural and voluntary systems' coupling in persons unable to stand and perform a voluntary movement.;A series of three experiments was conducted. Experiment 1 resulted in a practical protocol for assessing PPA for reaching in seated subjects and provided evidence of forces being generated prior to initiation of reaching. Experiment 2 resulted in identification of postural muscle onsets in anterior and posterior trunk muscles in advance of reaching, but not in advance of onset of the prime mover for the voluntary movement.