Postural Control and Activity of Erector Supinae Muscles during Lateral Shift of Center of Gravity in Sitting

Abstract

Postural control during a lateral shift of center of gravity (COG) in sitting was studied for ten healthy subjects (age 22 ± 1 years, height 172.5 ± 4.7cm, weight 63.3 ± 6.1 kg). The electromyography (EMG), trunk alignment and center of pressure (COP) during the lateral shift of COG in sitting were measured. The EMG signals (TELEMG, BTS) were recorded bilaterally from four parts; (1) lower lumbar part (LL), (2) upper lumbar part (UL), (3) lower thoracic part (LT), and (4) gluteus maximus (GM). Bipolar surface electrodes were placed over the muscle belly. The date were sampled at 1000 Hz, and kinetic date were recorded using a force plate (9287, Kistler) at 1000 Hz of sampling frequency. Kinematics were recorded synchronously with the other signals using a four camera system (ELITE, BTS), and the date were then sampled at 50 Hz. The lateral shift in sitting was classified into three movements; (1) lateral bending of trunk (LB), (2) reverse lateral bending of trunk (RLB), and (3) combined lateral bending (CLB). The lateral shift movements were separated to five phases with cue signal and COP trajectory. The direction of the COG shift was right side at both fast and normal speed. The right anterior superior iliac spine (ASIS) trajectory was similar to the COP trajectory during fast speed movement; both the COP and the right ASIS trajectory moved to left side immediately after cue. Thereafter, these trajectories rapidly moved to right side that was the direction of COG shift. In contrast, the right acromion part didn't move to left side. On the other hand, neither the COP or ASIS tajectory moved to left side during normal speed movement for the half of subjects. The activity of left LL decreased at phase II during fast speed movement (p<0.01). Thereafter, the left LL activity increase again in the midst of phase III except for LB. On the other hand, the activity of right LL obviously increased at phase III for LB. The correlation between Al and V_max was significant during fast speed movement (p<0.01, r=0.628). The results indicated that the initial change in COP for fast speed movement was very important for producing the moment of gravity, and that the characteristic COP change was related to the pelvic motion and the activity of erector spinae muscles.