Human quiet standing is often modeled as a single inverted pendulum rotating around the ankle joint, under the assumption that movement around the hip joint is quite small. However, several recent studies have shown that movement around the hip joint can play a significant role in the efficient maintenance of the center of body mass (COM) above the support area. The aim of this study was to investigate how coordination between the hip and ankle joints is controlled during human quiet standing. Subjects stood quietly for 30 s with their eyes either opened (EO) or closed (EC), and we measured subtle angular displacements around the ankle (thetaa) and hip (thetah) joints using three highly sensitive CCD laser displacement sensors. Reliable data were obtained for both angular displacement and angular velocity (the first derivative of the angular displacement). Further, measurement error was not predominant, even among the angular acceleration data, which were obtained by taking the second derivative of the angular displacement. The angular displacement, velocity, and acceleration of the hip were found to be significantly greater (P<0.001) than those of the ankle, confirming that hip-joint motion cannot be ignored, even during quiet standing. We also found that a consistent reciprocal relationship exists between the angular accelerations of the hip and ankle joints, namely positive or negative angular acceleration of ankle joint is compensated for by oppositely directed angular acceleration of the hip joint. Principal component analysis revealed that this relationship can be expressed as: thetah=gammathetaa with gamma=-3.15+/-1.24 and gamma=-3.12+/-1.46 (mean +/-SD) for EO and EC, respectively, where theta is the angular acceleration. There was no significant difference in the values of y for EO and EC, and these values were in agreement with the theoretical value calculated assuming the acceleration of COM was zero. On the other hand, such a consistent relationship was never observed for angular displacement itself. These results suggest that the angular motions around the hip and ankle joints are not to keep the COM at a constant position, but rather to minimize acceleration of the COM.

We compared the effect of recovery condition and durations on performance and muscle oxygenation during short-duration intermittent sprint exercise. 8 subjects performed a graded test and ten 5-s maximal sprints with 25-, 50-, and 100-s passive recovery (PR) or active recovery (AR) on a cycle ergometer. Peak power and percent decrease in power were determined. Oxygen uptake and blood lactate were measured during the sprint exercise. Oxyhemoglobin (O2Hb) and deoxyhemoglobin were measured using near-infrared spectroscopy. Peak power values were higher for PR than AR for the 25-s (2-9 sprints) and 50-s (2-6, 9, or 10 sprints) but not for the 100-s durations. Percentage decrease in peak power was lower for PR than AR in the 25-s (8.5±2.5 vs. 11.5±3.6%, P=0.008, ES=0.66) and 50-s (2.7±1.4 vs. 6.2±3.5%, P=0.007, ES=0.67) but not 100-s durations (2.1±1.3 vs. 3.1±2.6%, P>0.05). O2Hb variations were significantly higher for PR than AR for the 25-s and 50-s durations. AR was associated with reduced sprint performance and lower muscular reoxygenation. Performance was not affected over longer recovery durations regardless of recovery condition.

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