The aim of this study is to establish a new dynamic model for balance keeping control in upright standing; and to deduce the underlying possible control mechanism of central neuronal system with a special concern on the roles of pelvis and its muscles. The dynamic model including five joints, i. e. two ankles, two hips and one lumbosacral making up a multi-link system being driven by two pairs of muscles, the psoas major (PM) and glutaeus medius (GM). In coronal section, experimental data shows the ankle and lumbosacral sway in almost the same amplitude, whereas their phase difference is approximately equal to π. The results indicate that the trunk is keeping perpendicularly to horizon during the standing process. By defining the model's physical parameters, assuming that the corrective torque needed for balance keeping process is regulated by PID (stands for proportional, integral and derivative) control, the body sway can be simulated. The simulation result is quite consistent with the experimental data suggests that the pelvis is one of the most important structure in balance keeping, moreover, the dynamics of the present proposed balance keeping model is a quite useful model for analyzing the posture sway.

The purpose of this study was to identify the relationships between trunk sway (TS) and the motion of center of pressure (COP) during quiet upright stance. Eight young healthy subjects (averaged 24±6.7 years) including 3 females were recruited for this study. By comparing TS with COP, we found that TS is moving in phase with COP both in lateral and sagittal plane. On the basis of observations that the COP-TS error signal is very similar to the changes of friction between the feet and floor. We also found that the friction is the impetus of trunk sway obeying the Newton's law. Then, a dynamic model between TS and COP can be identified. The results showed that TS and the motion of COP are in accordance with a specific differential equation. Supporting TS is v and COPS is u, then u can be expressed as : u=-Hm/k v+hv, where H, k, h are constants, m is the body mass. The simulation results fitted the experimental findings very well. The results suggested that TS instead of COP is a promising index for human standing ability assessment.

Keeping upright stance is important to other complex motions like locomotion and running for human beings. The mechanism of balance-keeping control in upright standing is still unknown. This study was conducted to analyze the body sway by using a simple PID (proportional, integral, derivative) control model and to investigate the influence of vision on its gains. Ten healthy subjects took part in the study. The upright body was modeled as one-link inversed pendulum model. While determining the model parameters according to subject's physical statue, the gain of PID parameters, (K_P, K_D, K_I are gains of proportion, derivative, and integral respectively.) could be estimated. Four kinds of visual patterns, (three for central visual field stimulation, one is eyes closed) were designed for visual stimulation. The results showed that the gain of K_D was decreased significantly in eyes closed (131.5±37.6 Nms/rad in eyes open and 90.4±26.0 Nms/rad in eyes closed, p<0.001), and, K_P, K_I were not changed. The results suggested that the PID control model was a promising means for individual balance ability analysis and that the visual effect on balance-keeping control in upright standing was analogized to a damper in the mechanical system.

The purpose of this study was to investigate the effects of short-term living and training at an altitude of 1, 300 to 1, 800 m on physiological responses of high school elite endurance athletes. Fifteen male and seven female senior high school elite athletes, aged from 15 to 18, from three different sports (cross-country skiing, long-distance running and endurance cycling), participated in our study. The short-term (6 days) altitude exposure did not elicit abnormal responses of body tempera-ture, body weight, blood pressure or urine samples. There were also no significant changes in blood parameters examined before and after altitude exposure. Resting heart rate (HR) increased at altitude and presented an initial peak value followed by a steady decline on the following days of altitude exposure. Blood lactate concentration and exercise peak llR examined by submaximal 20-m shuttle run test decreased after the ascent to altitude and still showed lower values at postaltitude than at prealtitude. We conclude that 6-day living and training at an altitude of 1, 300 to 1, 800 m elicits positive decrements of exercise blood lactate and exercise peak HR as well as adaptive changes of resting IlR for these high school elite endurance athletes, which are probably related to an attenuation of muscle glycogen utilization and alterations in the autonomic neural system taken at altitude.