The present study aimed to investigate the acute and chronic effects of hyperbaric oxygen therapy on blood volume (THb) and oxygen saturation (StO₂) of human Achilles tendon in vivo. Using red laser lights, we compared the effects of hyperbaric oxygen therapies with the three different conditions (1.3 atmospheres absolute + 50% oxygen, 1.3 atmospheres absolute + 21% oxygen, 1.0 atmospheres absolute + 50% oxygen) for 60 min on THb and StO₂ of tendon (Experiment-1) and to determine the time course of changes in THb and StO₂ of tendon at 1, 2, 3, 24, 48, and 72 hours after the hyperbaric oxygen therapy (Experiment-2). In addition, we determined the chronic effect of hyperbaric oxygen therapy (60 min, 2 times/week, 8 weeks) on THb and StO₂ of tendon (Experiment-3). In Experiment-1, there were no differences in the relative increases in THb and StO₂ among the three conditions. In Experimet-2, THb and StO₂ of tendon returned to the resting level after 3 hours of therapy. In Experiment-3, THb and StO₂ of tendon during the resting were significantly lower at the point of 6 and 8 weeks than at the point of beginning. These results suggested that 1) increases in blood volume and oxygen saturation of tendon during hyperbaric oxygen therapy were related to both atmospheric pressure and oxygen density, 2) effects of hyperbaric oxygen therapy disappeared after 3 hours of therapy, 3) blood volume and oxygen saturation of tendon during the resting decreased after 8 weeks of hyperbaric oxygen therapy.

DESCENTE SPORTS SCIENCE Vol.32/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE