Many polymer gels undergo reversible, discontinuous volume changes in response to changes in the balance between repulsive intermolecular forces that act to expand the polymer network and attractive forces that act to shrink it. Repulsive forces are usually electrostatic or hydrophobic in nature, whereas attraction is mediated by hydrogen bonding or van der Waals interactions. The competition between these counteracting forces, and hence the gel volume, can thus be controlled by subtle changes in parameters such as pH (ref. 4), temperature, solvent composition or gel composition. Here we describe a more direct influence on this balance of forces, by showing that the radiation force generated by a focused laser beam induces reversible shrinkage in polymer gels. Control experiments confirm that the laser-induced volume phase transitions are due to radiation forces, rather than local heating, modifying the weak interactions in the gels, in agreement with previous observations of light-induced chain association in polymer solutions. We find that, owing to shear-relaxation processes, gel shrinkage occurs up to several tens of micrometres away from the irradiation spot, raising the prospect that the combination of stimuli-responsive polymer gels and laser light might lead to new gel-based systems for applications such as actuating or sensing.

Continuous competitive baseball and swimming activities during youth may be associated with disk degeneration. Furthermore, the study indicates that the experience of severe low back pain might be a predictor of disk degeneration in youth. The authors hope that preventive measures and management to protect against disk degeneration and low back pain in athletes will be established by further studies based on these results.

We studied the effects of high intensity resistance exercise training on bone metabolism in 17 young adult Oriental males (23-31 years) by measuring sensitive biomarkers of bone formation and resorption. The subjects were assigned to a training group and a sedentary group. The training group followed a weight training program three times per week for 4 months. In the training group, serum osteocalcin concentration and serum bone-specific alkaline phosphatase activity were significantly increased within the first month after the beginning of resistance exercise training, and the elevated levels remained throughout the training period, while there was no significant change in plasma procollagen type-I C-terminal concentration. Urinary deoxypyridinoline excretion was transiently suppressed and returned to the initial value but was never stimulated during the 4 months. These results suggest that the resistance exercise training enhanced bone formation without prior bone resorption. In the sedentary group, there was no significant difference in bone metabolic markers except plasma procollagen type-I C-terminal, which continuously decreased during the experimental period. There were no significant changes in total and regional bone mineral density in either group. In conclusion, (1) resistance exercise training increased markers of bone formation, while it transiently suppressed a marker of bone resorption, and (2) such adaptive changes of bone metabolism to resistance exercise training occurred during the early period of the training, before changes in bone density were observable through densitometry. (J Bone Miner Res 1997;12:656-662)

It has been suggested that decreasing muscle pH because of the accumulation of lactic acid is a fatigue factor during high-intensity exercise [1][2][3]. Lactic acid production in muscle releases lactate (Lac Ϫ ) and hydrogen ions (H ϩ ) within the physiological pH range. Therefore the accumulation of lactic acid causes metabolic acidosis. The decrease of intracellular pH brings about muscle fatigue through several mechanisms such as decreasing skeletal muscle tension, relaxation [4], and the inhibition of phosphofructokinase activity [5]. However, the body has an ability to restrain the decrease of intracellular pH, which is known as buffering capacity. It has been suggested that a higher buffering capacity can better stabilize the intracellular pH and better enhance the capability for high-intensity exercise performance.Most intracellular buffering within the skeletal muscle is accomplished by proteins, dibasic inorganic phosphate, bicarbonate, and carnosine [6]. A large amount of the histidine-containing dipeptide carnosine (␤-alanyl-L-histidine) has been shown to be present within the skeletal muscle of most vertebrate species [7]. It has been suggested that the carnosine significantly contributes to the physicochemical Japanese Journal of Physiology, 52, 199-205, 2002 Key words: carnosine, buffering capacity, acid-base balance, fiber-type distribution, high-intensity exercise. Abstract:The histidine-containing dipeptide carnosine (␤-alanyl-L-histidine) has been shown to significantly contribute to the physicochemical buffering in skeletal muscles, which maintains acid-base balance when a large quantity of H ϩ is produced in association with lactic acid accumulation during high-intensity exercise. The purpose of the present study was to examine the relations among the skeletal muscle carnosine concentration, fiber-type distribution, and high-intensity exercise performance. The subjects were 11 healthy men. Muscle biopsy samples were taken from the vastus lateralis at rest. The carnosine concentration was determined by the use of an amino acid autoanalyzer. The fiber-type distribution was determined by the staining intensity of myosin adenosinetriphosphatase. The high-intensity exercise performance was assessed by the use of 30-s maximal cycle ergometer sprinting. A significant correlation was demonstrated between the carnosine concentration and the type IIX fiber composition (rϭ0.646, pϽ0.05). The carnosine concentration was significantly correlated with the mean power per body mass (rϭ0.785, pϽ0.01) during the 30-s sprinting. When dividing the sprinting into 6 phases (0-5, 6-10, 11-15, 16-20, 21-25, 26-30 s), significant correlations were observed between the carnosine concentration and the mean power per body mass of the final 2 phases (21-25 s: rϭ0.694, pϽ0.05; 26-30 s: rϭ0.660, pϽ0.05). These results indicated that the carnosine concentration could be an important factor in determining the high-intensity exercise performance.

Excessive exposure to competitive sports activities during youth was associated with low back pain and symptoms in the lower extremities, with the severity varying with the sport. To reduce low back pain in youth, factors that may be causing low back pain, such as sport-specific postures and motions, need to be investigated.